capteur de courant à haute température

RapportèFinalèdeèTravailèdeèFinèd'études

Tuteurs:

Integrated Power Electronics SystemsSafran -Ampère Joint Research Laboratory

ALLES,hJoan

Perrin,hRémiBergogne,hDominiqueh

ÉcoleèCentraleèdeèLyonTFEè2014

CapteurhdehCouranthàhHautehTempérature

Optionè:

Filièreè:è

Métierè:

ECL:

Enterprise:Beroual,hAbderrahmane

Énergiehd-Infrastructure

Énergieh

Eco-ConceptionhethInnovation

IPESLyon,èFrance

Page: 2 of 78 TFE/FYI- Laboratoire Ampere/IPES, 2014


ALLES, Joan / TFE / 2014

Version: 17 August 2014

FYI

Academic year 2013-‐2014

-‐13-‐

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Abstract and keywords

ABSTRACTIn this document an indirect measurement high temperature probe based on tunnel magne-toresistance is presented.

The first part discusses the commercial available devices in the market and which one isbest suited for the specifications. The second part is a theoretical introduction to magne-toresistance and its behavior at high temperature and finally the last part is the analysis ofthe experimental data obtained. The sensor, which operates in open-loop and can be easilyintegrated, is able to operate up 250 degrees Celsius.

Keywords: current probe; magnetoresistance; magnetic sensor; high temperaturesensing

RESUMECe memoire presente un capteur de courant electrique pour la haute temperature. Il s’agitd’une mesure indirecte et basee sur la magnetoresistance a effet tunnel.

La premiere partie est une presentation de l’etat de l’art traitant des solutions commerciale-ment disponibles pour la mesure de courant. Le choix des solutions repondant au cahier descharges est discute a la fin de cette premiere partie. La deuxieme partie est une introductiontheorique a la magnetoresistance. Enfin, dans la derniere partie une analyse des donneesexperimentales est realisee.

Le capteur, fonctionne en boucle ouverte et peut etre facilement integre, tout en etantcapable de travailler jusqu’a 250oC.

Mots-Cles :capteur de courant ; magnetoresistance; capteur magnetique ;electroniquede haute temperature.




Contents

1 Introduction 13

2 Specifications 15

2.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 Specifications Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3 Direct Measurements 17

3.1 Direct Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.1 Direct Measurements with very independent temperature resistors . . 17

3.1.2 Direct measurements with resistance correction using temperature . . 18

4 Indirect Measurements 21

4.1 Hall-effect probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.1.1 Open Loop Hall-effect probe . . . . . . . . . . . . . . . . . . . . . . . 22

4.1.2 Closed Loop Hall-effect probes . . . . . . . . . . . . . . . . . . . . . . 22

4.2 Flux Gate probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.3 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3.1 Brief Physical Introduction . . . . . . . . . . . . . . . . . . . . . . . . 26

4.3.2 Anisotropic Magneto Resistance . . . . . . . . . . . . . . . . . . . . . 26

4.3.3 Giant Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.3.4 Tunnel Magneto Resistance . . . . . . . . . . . . . . . . . . . . . . . . 30

4.3.5 Colossal Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . 32

4.3.6 Extraordinary Magnetoresistance . . . . . . . . . . . . . . . . . . . . . 32

4.3.7 Ballistic Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . 32

4.3.8 A temperature model for the TMJ . . . . . . . . . . . . . . . . . . . . 33

4.3.9 Temperature models for GMR-SV and GMR-multilayer . . . . . . . . 36

4.4 Final comparison and decision . . . . . . . . . . . . . . . . . . . . . . . . . . . 37




5 Device Implementation 39

5.1 What is a Wheatstone bridge? . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.2 Temperature Coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

5.2.1 Temperature Dependent Resistive Components (TDRC) . . . . . . . . 41

5.2.2 Current Source Compensation . . . . . . . . . . . . . . . . . . . . . . 42

5.2.3 |TCS| ≤ |TCR| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.3 Manufacturers and models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.3.1 Measurement Specialities-KMY20S . . . . . . . . . . . . . . . . . . . . 44

5.3.2 NVE Tech.-AA002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.3.3 NVE Tech.-AA005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.3.4 Dowaytech-Multidimension-MMLP57 . . . . . . . . . . . . . . . . . . 45

5.3.5 Sensitec-GF705 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.4 Temperature Compensation at output side . . . . . . . . . . . . . . . . . . . . 46

5.4.1 Digital Temperature Compensation at output side . . . . . . . . . . . 46

5.5 Geometrical analysis using numerical methods: COMSOL . . . . . . . . . . . 46

6 Prototyping 51

6.1 Measurements Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.2 Prototypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

7 Experimental Results and Analysis 55

7.1 Measurements with first prototype : ambient temperature . . . . . . . . . . . 55

7.1.1 Model MMLP57 results at ambient temperature . . . . . . . . . . . . 55

7.1.2 Model KMY20S results at ambient temperature . . . . . . . . . . . . . 56

7.1.3 Model AA002 results at ambient temperature . . . . . . . . . . . . . . 56

7.1.4 Model AA005 results at ambient temperature . . . . . . . . . . . . . . 56

7.1.5 Model Comparison at ambient temperature . . . . . . . . . . . . . . . 57

7.2 Measurements with third prototype: 50oC to 250oC . . . . . . . . . . . . . . . 58

7.2.1 Test at constant voltage . . . . . . . . . . . . . . . . . . . . . . . . . . 58

7.2.2 Tests at constant current . . . . . . . . . . . . . . . . . . . . . . . . . 62

7.2.3 High temperature problems of electromigration . . . . . . . . . . . . . 64

8 Conclusions 65

8.1 Pedagogical Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

8.2 Project Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

9 Future Work Lines 67



TFE/FYI- Laboratoire Ampere/IPES, 2014 Page: 9 of 78

10 Acknowledgements 69

11 Bibliography 71

12 Appendix 1 73

12.1 Brief Physical Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

12.1.1 Electron spin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

12.1.2 Ferromagnetism, ferrimagnetism, antiferromagnetism and paramag-netism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

12.1.3 Magnetization of a material . . . . . . . . . . . . . . . . . . . . . . . . 74

12.1.4 Electron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

12.1.5 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

12.1.6 Anisotropy field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

12.1.7 Easy axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

12.1.8 Demagnetization field . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

12.1.9 Exchange anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

13 Appendix 2 77

13.1 Constant Current Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77




About SAFRAN andLaboratory Ampere

This section is dedicated to the presentation of SAFRAN and laboratory Ampere.

SAFRAN is a holding company founded in 2005 from the fusion of Snecma and Sagem.Since 2011 the company is present in the stock exchange of Paris, among the first 40 com-panies in value.

SAFRAN is well known internationally as an important french industrial group in the do-mains of aeronautics, astronautics, defense and security. This translates in the design, con-ception and production of motors for planes, helicopters and even rockets. One of the mostwell known milestones of the group is the design of the cryogenic motor Vulcain for theARIANE 5. The group also focuses in wiring and electronic systems.

The holding contains a large number of subsidiaries. The more important of them are:Snecma,Turbomeca, Herakles, Techspace Aero, Messier-Bugatti-Dowty, Aircelle, LabinalPowerSystems, Hispano-Suiza, Sagem Defense and Security and Morpho.

The revenue of the company is 13.5 billion euros. The french governement controls 30 percentof the total shares.

Labinal Power Systems is one of the parts of the aircraft equipment branch of SAFRANand where this project took place. The responsibles of the project inside Labinal are M. RegisMeuret et M. Ali Marwan.

Labinal Systems was born in 1921 and will became Precision Mecanique Labinal after itsmerger with Precision Mecanique. From its beginning is related with the aircraft manufac-turing world.

In 2014 Labinal Power takes all the activities of the group SAFRAN related with electricalsystems inside aircrafts.

Laboratory Ampere , or Laboratoire Ampere in its initial name, was founded in 2007from the union of CEGELY, LAI and the Environnemental Microbial Genomics group ofUCBL. Its objective is to manage and use energy in a rational manner from the point ofview of the relationship between the systems and its environment.

The laboratory has 4 main partners: CNRS, the Ecole Centrale de Lyon, INSA-Lyon andUCBL. The main 3 field of research are: Automatic,Electrical engineering/Electromagnetismand Environmental Microbiology.




In 2007 the laboratory consisted in 145 people (45 of them professors or associate professorsand 7 members of CNRS). This human resources were divided in 3 departments and 7research groups. At 2009 the number of professors and associate professors was 50 and atotal of 9 members of CNRS. A total of 80 Ph.D students complete the staff.

The laboratory is member of the Centre de competence europeen en electronique de puissanceand the reseau d’excellence du Fluid Power.

The facilities of the laboratory are spread among the campus of ECL and the campusUCBL-INSA.




Chapter 1

Introduction

“developement of a power converted taking in account the safety aspectsand the environment and supplied from a +-270VDC network...currentand temperature measurement integrated inside the powermodule.

MOET Public Technical ReportDecember 2009”In 2002 the European Commission decided to create the Power Optimised Aircraft (POA)

project, a technology platform to create and improve technological solutions to reduce theconsumption of the non-propulsive parts of aircraft. The measures were focused on largeand medium commercial aircraft and extended in different domains such as electronics,mechanics and control.

The program, which was constituted by a 45 company consortium had a budget of 100million euros and a 4 year horizon.

In July 2006, at the very moment of the dissolution of the POA and after analyzing theaccomplishment of its goals and its contributions to the european industry, the EuropeanCommission decided the launch of the More Open Electrical Technologies or MOET. MOETwas conceived to establish new regulations and protocols for the electrical systems insideairplanes. The concept ”fly-by-wire” is one of its most interesting results.

In the final conclusions presented in 2009, Safran, as one of its participants, is chargedwith the objectives of developing a power converter in which the current and temperaturemeasurement are integrated inside the power module.

All these efforts would not terminate there, in 2008 the EC had already in mind the contin-uation of this campaign of projects with a third initiative: Clean Sky.

The natural continuation of MOET was the Clean Sky initiative, which has been correctlydescribed as by far the most ambitious and well financed technological program regardingthe aircraft world.

Clean Sky was born in 2008 and awarded with one and a half billion budget and a 10 yearcalendar. Its goal is to develop breakthrough technologies for reduction of fuel consumptionand noise emissions.




It is in the very same milieu which has been described above where Safran-Labinal Power andLaboratoire Ampere founded, in 2013, an institute dubbed ”Integration de Puissanceen Environnement Severe” or IPES to promote and encourage the research anddevelopment of Power Electronics in the aircraft domain.

The present document is part of IPES and pretends to be a very small contribution to thischain of events. It searches to accomplish the objective stated at the MOET Public TechnicalReport to create an integrated current sensor able to work in an environment up to 250oC.

The author hereby analyses the different sensing options and compare them to find the onethat best suits the specifications. Hereinafter, different prototypes are created to verify thespecifications and the results analyzed.




Chapter 2

Specifications

Hereinafter follows the specifications and constraints that the resulting device must accom-plish. Specifications were provided by Mr. Bergogne and SAFRAN, represented by M.Meuretand M.Marwan, at the beginning of the design phase in March 2014.

2.1 Specifications

Design of a current sensor able to determinate the current in an HVDC inverter’s outputconductor within a range of 0-30 A, peak amplitude, and within the spectrum of 0-15 kHz.

The sensor will experience non-predictable temperature changes from -50oC to 250oC. Theerror on the measurements cannot exceed 3 percent.

The transducer will be positioned on the output of a power converter DC/AC of half-bridgetype and the chain of measurements must be able to isolate noises of common mode typeand differential mode generated by wide-bandgap power switches.

The solution adopted must be compact in the sense that the displacement of circuit elementsoutside the ”hot zone” is not allowed. All parts of system will be under the constriction ofhigh temperature. Integrability of the final solution is also required in order to integrate itinside the inverter.

The analog output voltage of the sensor must be adapted to an ADC in such a way thata null current generates half the maximal output transducer voltage, and the signalingmust be single ended (not differential output unless the ADC is compatible). The maximumresolution of the transducer is 0,01 A.

The sensor can be powered by the high side gate driver power supply, a built-in power supplyor be self-powered.




?

tr1

tr2

DC bus AC motor

L RE

540V

0-X Vlow pass filtered

D

D

High Side Sensing

0V

X V

30A

0A

Resolution:0.01A

-30A

X V2

Figure 2.1: Electrical scheme for the specifications. High side current option, low-side notallowed.

2.2 Specifications Summary

Table 2.1: Summary table with specifications that must be accomplished

Parameter Nominal

Type of transducer Bidirectional Current ProbeBandwidth of transducer 0(DC) — 15 kHzDynamic Range 0 — 30 ANominal Current(min. error) 5 AMax.Over Current 35 ATemperature Range -50 oC(223.2K) — 250 oC(523.2K)Current Consumption 10 mASupply Voltage 5 VType of Output Signaling Single ended(Non differential)Gain error to Supply Voltage 5 %/1 %

Resolution 0.01 AError at all range of temp. 3 per cent of range




Chapter 3

Direct Measurements

There are two different types of current measurement: direct and indirect measurements.

Direct measurements are based on Ohm’s law which states that voltage established at theterminals of a resistor is equal to the product between the current and the resistance of theresistor.

This fact allows an easy conversion from current to voltage if the value of the resistoris known[12]. The problematic arises when the non-predictable variations of temperatureaffect the resistance of the resistor.

3.1 Direct Measurements

In order to solve the problem related with temperature variation two strategies are proposed:

i Very independent temperature resistor, i.e. a stable temperature coefficient of resistiv-ity.

ii Resistance correction using temperature measurements.

3.1.1 Direct Measurements with very independent temperatureresistors

The resistance is defined as R = ρlS (a.k.a Pouillet’s law). The parameter ρ is called resistivity

and is a propriety of each material. The other values are longitude(l) and section of conductor(S).

The resistivity is measured in Ωm and is a value dependent of the temperature and of eachmaterial. Table 3.1 shows values of resistivity according to different materials.

There exists a linear approximation of resistivity change according to temperature based onthe formula:

ρ(T ) = ρ0[1 + α(T − T0)]

Where T represents temperature in Kelvin and α is the temperature coefficient of resistivity.This approximation of the temperature dependence does not take into account the effects




Table 3.1: Table of resistivities depending of material.Source:[4]

Material Resistivity[Ωm] Conductivity[S/m] Temperature Coefficient

Silver 1.59× 10−8 6.30× 107 0.0038Copper 1.68× 10−8 5.96× 107 0.003862Annealed copper 1.72× 10−8 5.80× 107 0.00393Gold 2.44× 10−8 4.10× 107 0.0034Aluminium 2.82× 10−8 3.5× 107 0.0039

of dilatation in the conductor, which also affect the final value of resistance. The values forthe coefficient are given in the table 3.1.

There exists other approximations for describing the dependence as the Bloch–Gruneisenformula, more accurate at higher temperatures.

The variation of ρ of copper from 20oC to 250oC using linear approximation gives:

ρ(250) = (1.68× 10−8)[1 + 0.003862(250− 20)] = 3.172× 10−8 (3.1)

which supposes a difference of 1,88 times more of resistivity and therefore induces a mistakein the current for every ampere that circulates the resistor. Errors relative to dilatations arenot included and have to be added.

The materials like constantan or manganin have temperature coefficients of resistivity muchlower (around ×10−6). These materials will be suitable for the measurement but their resis-tivities are ten times bigger than copper or silver.

AC motor

L RE

tr1

tr2DC bus

175V

D

D

Low pass filter

OUTPUTVEEOUT

6100 F04

LT6100

81

VS– VS

+

A42

VCC

A23

4

7

C20.1µF3V

6

5

FIL

+

– +

Figure 3.1: Scheme for measuring voltage drop on shunt using the LT6100 from AnalogDevices.

A table with Pros and Cons is also provided in table 4.1.

3.1.2 Direct measurements with resistance correction using tem-perature

The idea behind this theory of operation is to measure the voltage drop at the terminalsof the shunt resistor and correct the obtained value according to a measurement of its




Pros Cons

• High Integrability• A very temperature independent shunt mustbe used

• High sensibility •Electrical circuit altered

•The system does not require softwarecorrection

•With ageing the system can createnon detectable skews

•Fast changing dynamics of temperatureare not a problem

• Non electric isolated

Table 3.2: Pros and cons of shunt devices

temperature. This correction relies in the fact that the resistance of the shunt is well-knowat any temperature in the range -50oC to 250oC. The correction should be done at software-side after the measurement.

There exists different ways of measuring temperature in the specified range. The most wellknown are: thermocouples, thermistors and RTD’s.

Thermocouples

are based on the Seebeck effect and provide an inexpensive temperature measurement witha resolution within the degree. There are different types of thermocouples but most of themhave a range bigger than the range of the specifications. Resolution is around 100µV/oCand quite constant in the range of temperature.[1]

Thermistors

are resistors with a huge resistance variation over temperature. There are two kinds ofthermistors: Positive Temperature Coefficient(PTC) thermistors and Negative TemperatureCoefficient thermistors(NTC). The former are thermistors whose resistance increases withthe increase of temperature and the latter are thermistors with a decrease of resistivity forincreases of temperature.

The range of thermistors can be in some cases large enough for the present specifications.

Resistance Temperature detectors

or RTD are as thermistors used to measure temperature from a change in resistivity. The dif-ference is the material used for the resistors, whereas for thermistors ceramics and polymersare used, pure elements as platinum or copper constitute the body of the RTD’s.

The presence of a pure and unique element provokes a more predictable change in temper-ature. The range of RTD extends over the specifications of the present application.

A table of Pros and Cons is also provided for this method:




Pros Cons

• Thermocouples do not need external power source • Need of two measurements to obtain current value

• Prices are very competitive• Need of software calculations to correct resistancefrom temperature

• Easy integrability •Possibility of error due to aging

Table 3.3: Pros and cons of resistance correction with temperature measurement.




Chapter 4

Indirect Measurements

The present chapter describes the possible methods for current sensing based on indirectmeasurements. As said in previous chapters indirect measurements do not interfere with thecurrent path but use the magnetic field associated with any current to deduce its value.

There are three grand types of indirect measurements: Hall-effect sensors, Flux-Gate sensorsand Magnetoresistance[12].

4.1 Hall-effect probe

Hall-effect probes are based on the Hall effect discovered by Edwin Hall in 1879. If a thinconductive layer is situated inside a magnetic field and a current passes through the former,a differential of potential is created in the layer. Figure 4.1 shows the physical set-up.

This potential is perpendicular to the current direction and explained because of the Lorentz’sForce: ~F = q( ~E + ~v × ~B).

Where ~E is the electrical field, ~B the magnetic field, ~v the speed of the charge and q itscharge.

The voltage generated by the Hall effect is equal to VH = KIcBd + VOH where K is called

the Hall constant, Ic the current that passes through the conductor, B the magnitude of themagnetic field and d the thickness.

Figure 4.1: Heuristic diagram of how the Hall effect produces a difference of voltage(VH)when a magnetic field B is applied and a current (Ic) flows [11].




The value VOH refers to an offset present in the final voltage that is undesired and must bereduced. This parameter is temperature dependent.

There are at least three different types of Hall-effect probes: Open-loop Hall-effect probe,closed loop, and eta probe.

4.1.1 Open Loop Hall-effect probe

Open loop Hall effects current transducers are based on the following scheme:

Figure 4.2: Figure shows typical structure of a open loop Hall probe [11].

A current IP creates a magnetic flux B which is driven and amplified by the magneticcore. The magnetic circuit is not continuous but contains a gap within the hall sensors ispositioned.

The output voltage of the Hall sensor is linear dependent with current as much as the coreis not saturated.

In case of an excursion of current the core can be magnetized and the resulting values ofmeasurement can be incorrect.

The output of an open-loop architecture is a voltage. According to LEM [11] the bandwidthis limited because electronic circuits and the core heating due to eddy losses.

4.1.2 Closed Loop Hall-effect probes

The operation of a closed loop transducer searches to improve the performance of the probeand extending the bandwidth. The principle of operation is very similar:

The voltage of the Hall probe is used to drive a current source which assures a current thatgenerates the same magnetic flux (ampere turns) than the Iprim. Due this secondary currentthrough a secondary winding the flux in the core is maintained to zero.

At the same time the same secondary winding assures the operation at high frequencies asa transformer, where the Hall probe cannot operate.

Operating the Hall probe near zero flux permits eliminating the drift gain with temperaturedue to magnetic core.

The main limitation of the closed loop circuit is that electronic components must supplyimportant currents and that prevents low voltage secondary supplies.




The output of the closed loop Hall-probe is current and can be easily converted to voltageby a resistor.

A table of Pros and Cons is provided:

Pros Cons

• Large bandwidth up to 200KHz achievable [11]• Presence of magnetic core, preventingits integration

• Medium-high precision•Magnetic core properties depending oftemperature

• Low power consumption for eta andopen-loop

• With time the system can create nondetectable skews

• If core used the output is unaffected byposition of current line

• Variable stress on sensor chip transformsvia piezo-electric into drift parameter

• Possible elimination of temperature dependenceusing self-calibrated integrate coil

• Offset difficults low frequencymeasurements

Table 4.1: Pros and cons of Hall effect probes




4.2 Flux Gate probe

The principle of operation of a flux-gate technology is a piece of magnetic material of easysaturation (a.k.a sensing head) which is driven through its B-H loop by an electronic circuit.

The presence of an external current that generates a magnetic flux can be deduced accordingto variations of the B-H loop given the fact that an increase in B in the sensing head willallow its saturation easier than without the presence of the external field.

A secondary winding around the core is usually added in order to assure a zero field in thegap of the magnetic material where the sensing head is situated.

The following figure shows the scheme:

Figure 4.3: Gate flush structure. The saturated magnetic material is in the gap of themagnetic circuit [11].

The saturable inductor is designed in such a manner that any change in the external fieldaffects its saturation level, hence affecting its inductance.

If the saturable inductor is driven with a square voltage wave the currents are differentdepending of the presence of external field.

The way to extract from the plot of the current the value of the external magnetic field isusing the 0-th harmonic(DC component) or the 2nd harmonic [10].

According to the response the current in the secondary winding is changed in order to achievea zero flux in the magnetic core.

For increasing the bandwidth of the fluxgate a separated core is used to work as a transformerfollowing the same principle as the eta-Hall-probe.

There are other implementations of the fluxgate system but they require the use of big-ger cores in order to improve bandwidth or resolution. This clearly violates the restrictionimposed by the specifications related to the integration inside an IC.




Pros Cons

• Large bandwidth up to 500KHz achievable [10]• Existence of a magnetic core that preventsthe integration

• Better accuracy than Hall-effect•Limited bandwidth if special architecturesare not used

• Large dynamic range(both smalland large currents)

•Magnetic core properties depending oftemperature

• Large temperature range • High Power Consumption

• Robustness

• If core used the output is unaffected byposition of current line

Table 4.2: Pros and cons of fluxgate devices.




4.3 Magnetoresistance

The use of magnetoresistance as a measuring method fits within the spintronics develop-ments of the latter years of the 20th century and these are related to the ability of depositionmethods of very thin layers (ultra vacuum deposition). Whereas, natural phenomena asso-ciated with the material property of magnetoresistance is known since its discovery in 1857by Lord Kelvin (aka William Thomson).

Lord Kelvin discovered during his essays that the electrical resistance of the material changesdue to the application of a magnetic field. He discovered that such variation of resistancedepends of the angle between the magnetization of the material and the electric currentwhich experiments the change of resistivity, this effect was dubbed anisotropic magne-toresistance (AMR).

The fact is that Lord Kelvin did experimentation with iron before the use of nickel in 1857and discovered magnetoresistance but with a very small of change in resistance. That wascalled OMR(Ordinary Magneto Resistance) [8].

Before introducing the differents mechanism of magnetoresistance: OMR[8], AMR [8] , TMR[8] [19] [15] [6],GMR [16] [20], CMR [12] and BMR [15] a brief remembrance of certainphysical concepts must be made.

4.3.1 Brief Physical Introduction

A brief physical review is attached in Appendix 1 follows for the benefit of the readerswithout a usual handling of the phenomena related to spin.(See appendix1.)

4.3.2 Anisotropic Magneto Resistance

If a very thin film of FM material is used, the magnetization inside the layer can be attributedto a unique magnetic domain. The magnetization of this domain is parallel to the layer. Theelectrical resistance of this thin layer is dependent of the angle between the current passingthrough the film and direction of magnetization, and has the form of R = R0 + ∆Rcos2(θ).

This kind of magnetoresistance is called anisotropic magnetoresistance because resistanceis dependent of orientation. When magnetization and current are parallel the resistancereaches its maximum, and minimum when they are perpendicular. The formula presents alinear change of resistance with magnetic field around 45o(π4 ).

Magnetic film (Permalloy)

Current I

Field HHy

R

Figure 4.4: Image of the different parameters that influence the AMR effect on a thin film[13].

The value ∆RR0

is called magneto resistive coefficient and in some materials such coefficient isnegative, i.e low resistivity levels are reached when magnetization and current are parallel.

The fact that AMR is non linear and the impossibility to know angle of magnetization ofthe material difficulties its use. Even, it has to be noted as caveat that if the magnetizationrotates in the direction of the applied field, it does not align perfectly [12].




4.3.3 Giant Magnetoresistance

The Giant Magneto Resistance (GMR) was discovered in 1988 by Grinberg and Fert [6] andthe mechanism that allows the change in resistivity differs from that of AMR.

The discovery is tight related to the possibility of creating very thin layers (few atoms) of aferromagnetic material.

If in the case of ORM and AMR there was only one kind of material, for the exhibition ofGMR a two layer system is needed. The two layers of FM material are separated by a thinlayer of non-magnetic material.

The understanding of the mechanism that governs the coupling of a FM-AFM system is notcompletely established and a plateau of theories dwell into the scientific literature awaitingfor a complete explanation of all experiments.

In GMR the variation of resistance of the system is proportional to cos(β) where β is theangle formed between magnetization of the adjacent FM layers. Therefore and against themechanism in AMR the direction of current and the angle it forms with other parametersdoes not interact.

The previous mentioned thin layer of non-magnetic material can be: Cu, Pt or Au. Thematerial of this intermediate layer is extremely important because if the AFM layer has abig anisotropic field the MR coefficient is low.

In a GMR union two situations are possible regarding the direction of magnetization of thetwo FM layers: parallel or anti-parallel.

M

Negligible Scattering

M

Parallel Configuration

Figure 4.5: Heuristic figure for the case for parallel magnetic moments. Electron passesbecause no scattering center exists

If a electron traverses the interlayer of the nonmagnetical metal is because is a currentelectron (in the Fermi level) and therefore as we previously said in FM materials the majorityof electrons in Fermi levels have a determinate spin direction. When this electron arrives atthe other side no problem is found because it encounters spaces and other electrons of thesame spin (the magnetic moment of the other FM layer is the same)[6].




M

Strong Scattering

M

Anti-Parallel Configuration

Figure 4.6: Case for antiparallel magnetic moments. Electron encounters non-uniformity andcannot pass. Resistivity increases.

But when the two layers are magnetized differently the electron that departs from the lowerFM layer finds a scattering center. This electron has not the same spin direction than themajority of electrons in the Fermi levels.

In order to create a well operating GMR detector is necessary to assure that in absence ofexternal magnetic field the two FM layers have antiparallel states. This way only in casea external field is applied the two layers became the same spin orientation and resistancedecreases.

In a GMR probe the thickness of the AFM layer must be smaller than the coherence length,so the electron can remember its spin orientation while it travels through the system.

Sandwich GMR detector

The typical sandwich GMR probe is composed of two thin layers of FM material(NiFeCoalloys) and an intermediary layer of Cu or Au.

The layers are presented in long strips creating large surfaces of material. When a biasingcurrent is applied to the resistor the magnetization directions of the two FM layers becameanti parallel. In this state the resistance is maximum [16].

M

M

Bias Current

M

M

Bias CurrentB

External field

Low resistance stateHigh Resistance State

Figure 4.7: Image showing the strucutre of a sandwich GMR device. A current between theFM layers creates a field that makes them to be in antiparallel.

This bias current assures that in case of absence of external magnetic field the layers areantiparallel and the resistance is high.

When an external field is applied the field created by the bias current is not big enough andthe two FM layers are magnetized in the same direction than the external field. This way alow resistance state is achieved.




The typical change in resistivity of a sandwich GMR probe is within the limits of 4-9 percent.

A part of the biasing current a permanent magnet is required to move the operating pointto the center of the linear range.

Multilayer GMR detector

The materials used in this type of sensor are the same than the previous case but now thethickness of the layers is crucial. The multilayer is based on the coupling of the different FMlayers with the AFM layers.

The coupling will allow for the elimination of the biasing current[16].

The structure can be repeated as many times as necessary in order to modulate the overallnominal resistance. In addition the presence of more layers creates a bigger percent changein resistivity.

B

External field

Low resistance stateHigh Resistance State

B

Figure 4.8: The natural coupling between FM layers avoids the use of biasing current formultilayer.

Multilayer GMR probes have a much larger linear rage and no permanent magnets arerequired to assure linearity. Figure 4.8 shows the structure.

Spin-valve GMR detector

The spin-valve detector (aka pinned sandwich) has a structure similar to the sandwich GMRbut with an extra layer of FeMn or NiO on one side. The spin-valve instead of being basedon the coupling between FM layers is based on the pinning of one layer due to exchangebias. This pinning permits creating a polarized probe that responds different according tothe direction of the external magnetic field applied.

The extra layer is heated over Neel Temperature and then cooled and exposed to a magneticfield so as to create a permanent magnetization of the layer.

This layer permits to the adjacent FM layer to couple with the extra layer and assure theantiparallel state in absence of external magnetic field. The other FM layers is free to rotateits magnetization and hence is dubbed free layer.




The structure of a spin valve probe can be simplified as:

150

25

150

100

Copper

AFM Layer

FM Layer(Pinned)

FM Layer(free)A

A

A

A

exchange bias

Figure 4.9: Scheme of layers of a Spin Valve GMR probe

Cobalt in a layer of 5Amstrong is added as diffusion barrier stopping the copper-permeabilitydiffusion.

4.3.4 Tunnel Magneto Resistance

This new kind of magnetoresistance exhibits a new mechanism based on tunnel effect. Theyare the newest MR sensors available in the market, being first fabricated in 1995. Its ar-chitecture is similar to spin-valves GMR sensors but the layer that separates the two FMmaterials is extremely thin (around 1 nm) and the typical composition is Al2O3.

The intermediate layer is not any more a conductor but an insulator and the electronspass through it thanks to quantum tunneling. A magnetic tunnel junction (MTJ) is hencecreated.

The facility with which the electrons pass through the insulator depends of the angle betweenthe magnetization of the two layers.

In fact such resistance can be expressed as :

R(θ) =

(RP −RAP

2

)cos(θ) +

RP +RAP2

Where θ is the angle between the magnetizations. This process is known as spin-dependent-tunneling (SDT) and the ease is greater if the 2 FM layers have parallel magnetization andminimum if magnetizations are anti-parallel.

In contrast with GMR, the responsible for resistance are not the scattering centers at theother side but the probability for the electron to find space for itself at the other side.

In a simple tunnel magnetic junction the Julliere model, based on the Hamiltonian theory,is the one with easiest explanation of the process.

When the system is in antiparallel state, as the figure 4.10, the majority of electrons of theemitting side (which has the biggest energy at Fermi level) have not space at the other sidebecause that spin orientation is not majority at the other side.




Figure 4.10: Figure shows both sides of a tunnel barrier. Is possible to see that the majorityspin direction in the emitting side is not the majority at the other side due to magneticorientation.

By contrast when the system is in parallel state, as figure 4.11, there is a majority of electronswith certain spin and a majority of states at the other side of the same spin. Electrons cantravel and resistivity is low.

Figure 4.11: Figure shows how if the magnetizations are parallel the resistance can decreasebecause the majority of electrons can cross due to the equivalent states at the other side.

The simple model of a TMR sensor is shown below. A more detailed structure will beprovided in the next chapters.

Insulator

AFM Layer

FM Layer(Pinned)

FM Layer(free)

exchange bias

Figure 4.12: Structure of TMR probe




This structure share the characteristic of spin-valve GMR detectors of having a pinnedmagnetic layer and a free magnetic layer.

If no external field is provided the two FM layers have opposite magnetization.

The device is an applied magnetic field-polarized sensor and the thin insulator can easily bedestroyed if a strong field is applied.

4.3.5 Colossal Magnetoresistance

CMR is merited to Bell Labs, while investigating about manganese films in 1994 discovered amagneto resistance effect much bigger than GMR, and hence was called colossal. The CMRpresent some problematic as the need of applying a very high magnetic filed which adviseagainst its use in current measurement.

CMR does not have a widely accepted theory that explains its nature.

4.3.6 Extraordinary Magnetoresistance

Extraordinary Magnetoresistance is known since year 2000 and showed the biggest variationsof resistance at room temperature. This kind of MR for is based on the Van der Pauw diskand it could improve magnetic probes in certain fields.

No technological implementation has been found during the present bibliographical researchand hence it will not be considered as a too immature technology.

4.3.7 Ballistic Magnetoresistance

This magnetoresistance is much larger than the observed for TMR or GMR and of the orderof 300 % or even bigger. It occurs only if the spin-flip mean free path is long in proportionto the magnetic domain wall.

The most recent literature regarding this king of magnetoresistance is Chopra et al.[5] whoused Ni nanocontacts in order to eliminate the presence of extraneous chemical layers.

As extraordinary magnetoresistance is restricted to basic research and due to the absenceof industrial applications is discarded as an option

Pros Cons

• Highly integrability• Not as extended as current probeas the other solutions

• Enormous sensibility • Operability not proven at 250oC

• Very low consumption

Table 4.3: Pros and cons of magnetoresistance devices.




4.3.8 A temperature model for the TMJ

The resistance of a TMJ and therefore its TMR coefficient variates in temperature. Thisnatural behavior cannot be understood within the model given by Julliere.

In the present subsection the Moodera model [19] will be implemented in order to give anapproximation of the behavior. In this simple model the dependence of the resistance ontemperature is explained by elastic and inelastic tunnelling. The values of conductance forthe TMR in parallel and anti-parallel state are:

GP = GT [1 + P1P2]︸︷︷︸elastic

+ sT 1.33︸︷︷︸inelastic

(4.1)

GAP = GT [1− P1P2] + sT 1.33 (4.2)

with GT = G0CT

sin(CT ) and where C = 1.387× 10−4 t√Φ

with t the value of thickness in Aand

Φ the value of the barrier height in eV[19].

From all the previous it can be deduced that

TMR =P1P2 − 1

(1 + sT 1.33)

This formula resumes the behavior of the junction with temperature: the bigger the polar-isation of both FM layers the more TMR coefficient, i.e the bigger difference in resistancebetween the parallel and antiparallel states.

It can be roughly said that the more polarized the layers the more parallel or alignedthey are and hence more conductivity. The same equation also explains that an increase intemperature decreases the TMR. This decrease is inversely proportional with T 1.33.

In order to develop a model in case a TMJ sensor is used it is better to avoid the previousequation and use the equations of conductivities GP and GAP . It is thence necessary toexpress the polarizations. Bloch law will be used :

P (T ) = P0(1− αT 32 )

The paper [19] provides values of α,P0,s ,t,G0 and Φ for the parallel and antiparallel statesof two different heads. Even it is only an example, this permits to create a range of valuesfor each parameter, variate them and observe the degree of change in the GMR coefficient.A table with parameters and correspondent values is provided:

Table 4.4: Table with parameters for the equations of Moodera model. Source[19]

Parameter TMJ Head 1 TMJ Head 2

tP (A) 11.3 6.7tAP (A) 12.7 7.2ΦP (eV) 1.28 3.46ΦAP (eV) 1.03 3.28G0 (Ωµm2)−1 5.1× 10−9 1.7× 10−8

P0 % 34 59α (K−3/2) 2.1× 10−5 1.4× 10−5

s (Ωµm2)−1(K−4/3) 1.1× 10−12 1.5× 10−12




0 100 200 300 400 500 6000.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4x 108

Temperature in K

Values of resistance of a single TMJ. In red case for parallel states

Figure 4.13: Plot showing the resistance of the parallel and antiparallel states of a TMRhead using Moodera model, units in Ω. Red for parallel and blue for anti-parallel.

If the free layer and the pinned layer are of the same material and similar thickness, andsputtered under similar conditions, they have same polarization and α parameter [18][19].

Once the equations are stated and also the parameters, it is possible to plot the variationof resistance (inverse of conductance) versus temperature. Figure 4.13 displays the variationaccording to temperature.

The values decrease in temperature as expected. The curve obtained is practically equal tothe appeared in [19].

The coefficient decreases in temperature, which is a bad notice for sensors operating at hightemperatures.

In the previous image obtained through the use of Moodera model in Matlab, it is possibleto appreciate how an increase in temperature produces a decrease in both resistances, butwhat is worst, a decrease in the difference between the resistance of parallel and antiparalleland hence the MR coefficient is reduced. This will obligate to increase amplification in orderto achieve same resolution in all order of temperatures.

Nothing can be said from the slope of the magnetic loop in the linear range from this theory,hence it can change as in figure 4.14b or as in figure 4.14c




AA'

(a) Typical plot of ∆RR

versus magneticfield of a TMR head.

AA'

(b) The image shows how the curvecould evolve(in red) in temperature.Real behaviour is unknown.

AA'

(c) The image shows another option ashow could evaluate the curve in temper-ature.

Figure 4.14: Figures show the plot TMR coefficient vs. H for a TMJ head and its evolutionin temperature [3].

The Moodera model here deployed does not consider the polarization as a function of theexternal field and only the extreme cases of parallel and antiparallel states can be studied.

Although, it would be logical to think that if there is a practically linear decreaseof RAP−RP

RP, there would also be a linear decrease of Rx−RP

RPwhere Rx is the

resistance for a polarization of the free layer inferior than the polarization whenantiparallel states occur. From this it can be deduced that figure 4.14c is true.

The slope in the linear range is called sensitivity and is given in %Oersted . Manufacturers

usually provide this value but not the TMR coefficient.

According to Ralinowski the blocking temperature of IrMn is 255oC. The model does nottake into account that fact and therefore according to the materials of the head the maximumtemperature varies and the model does not apply.

The model does not take into account the possible diffusions of materials at high tempera-tures neither.

It has also been studied the sensibility of the different parameters inside the range givenby [19].

The t (barrier thickness) and the Φ (barrier height) have been jittered and no real change




has been observed apart of a slight curve variation at high temperature. Parameters α ands had also been changed inside the range without huge changes.

4.3.9 Temperature models for GMR-SV and GMR-multilayer

Given the fact that the physical explanations for magnetoresistance in GMR and TMR arenot the same, it is reasonable to expect that the behavior with temperature is not also thesame.

Thermal behavior of SV-GMR heads is explained at [17] and [9]. Unfortunately no equationsare given for this kind of devices and only a qualitative approach can be considered.

The main affectation in SV-devices due to temperature increase are: spin independent tun-neling, phonon scattering and the modification of the exchange bias coupling at the proximityof Neel Temperature.

In Portier et al. [17] is found that at room temperature the exchange-field value of the pinnedlayer varies with temperature.This is exactly the same that we found for the TMRdevices, i.e a decrease in the extension of the linear range for an increase oftemperature. One example at [17] shows that at room temperature it has a exchange-fieldof the pinned layer about 120 Oe , at 100oC becames 80 Oe, and it falls until 37 Oe at200oC.

The article of [20] states : ”The AFM layer has been called the Achille’s heel of the spinvalve head, since it frequently fails to provide exchange pinning with sufficient strength andstability at elevated operating temperatures.”

In [15] it is stated that the temperature effect of SV-GMR devices is due to: intermixing ofthe spin-up and spin-down due to the scattering in the inners of the FM layers and becausephonon scattering.

It also states that the higher the Curie temperature, the less the thermal variation of theSV-GMR: ” Structures based on Co or Fe give less thermal variation of the spin valve MRthan Ni-rich allows”[15]

In [2] a linear decrease of ∆RR is also found.

It has been also reported a problem of layer interdiffusion in NiFe, Co and Cu based spin-valves is in the 250oC-300oC range

This theory also fits with the GMR-multilayer experimental results from datasheets, shownin figure 4.15 where it can be seen how the curves in GMR-heads are smaller for highertemperatures.

The elbow of each curve that separates from the linear range and the very planar responsediminishes with temperature, reducing the linear range and hence difficulting a measure.

But the most interesting this time is that it can be observed how the slope of thelinear range also changes with temperature, fact not reported before with TMJor GMR-SV.

In the image of NVE catalog, figure 4.15, it is possible to see how for 25oC the ratio is about0.2V

10Oersted but for 125oC it is 0.1V10Oersted .

It can be seen how the mechanism of TMR and GMR-SV are not so different regarding theirtemperature behavior. For both a decrease of the linear range (related with the exchangefield of the pinned layer) is seen at higher temperatures.




Figure 4.15: Image from NVE catalog that shows how output variates depending of temper-ature for a device. The device is a Wheatstone bridge of four multilayer GMR type.

4.4 Final comparison and decision

A final table of comparison is provided. Only key figures are provided and the comparisonis done in a very qualitative manner in order to simplify the decision.

A checkmark as Xsignifies that such column/property is well accomplish by that kind ofsensor. By contrast a X ,represents that such column does not accomplish the specifications.

Table 4.5: Comparison table for indirect measurements

Method Economical Cost Sensitivity Integrability Temperature adaptat.

Direct Measurements-shunt XX XX X XHall-effect XX X X XFluxgate X X X XMagnetoresistivity X XX X ?SQUID X X X XXX X X X

The shunt option represents a very good option as its historical use confirms but given thetemperature problems associated with the present application difficults its implementation.

Hall-effect and Fluxgate sensors present the problematic of a very difficult integrability,a key aspect of specifications and therefore have to be eliminated. It must be noted thatefforts are reported for the creation of an integrated fluxgate sensor, but they are limited toacademical research and no market offers can be found [7].

SQUID and other methods are or too new or too expensive to be implemented and aretherefore discarded.




Magnetoresistance peers as the best adapted solution to the problem with a veryhigh sensibility and a excellent integrability. It is still necessary to check if the tem-perature fluctuations can be compensated but an initial approach indicates that it seemsplausible.

For all the previous, magnetoresistance seems the best option for the implementation of thecurrent sensor. Next chapters discuss and develop a possible implementation of a prototypeusing a commercial IC.




Chapter 5

Device Implementation

The present section is dedicated to the first stage of the solution implementation. For thisfirst approach a layout in a PCB, not in an integrated circuit, will be used.

This section presents the different models available in the market, shows the different tem-perature compensation architectures and presents geometrical considerations of layout.

The more practical consideration of how the 30 A will be generated, how to heat the device,how to measure the real current, how to solder the device to the PCB and maintain structuralstrength at 250oC and what measure need to be done are provided in the next chapter.

5.1 What is a Wheatstone bridge?

Most of the current sensing applications do not present a single MR head but a system offour in a Wheatstone bridge configuration.

Two of these heads are shielded from magnetic fields using ferromagnetic materials, in sucha way that magnetic fields do not affect these. This is done in order to obtain a output moreindependent of temperature than the output of a voltage divider (two resistors) or a simpleMR head (one resistor).

α

δ β

γ

R1

R4

R3

R2

Figure 5.1: Electrical circuit of a Wheatstone bridge. The shielded resistors are R2 and R3and they variate only with temperature.

A Wheatstone bridge as the one of figure 5.1 has two inputs, labeled as α and γ, and twooutputs labeled as δ and β.

If the equationR2

R1=R4

R3




is satisfied, the bridge is said to be balanced and the differential voltage between δ and β isexactly zero.

The Wheatstone bridge can be driven by current or voltage, if it is driven by a constantsupply voltage the value of differential voltage is :

Vβδ =

(R4

R3 +R4− R2

R1 +R2

)Vs

where Vs = Vα − Vγ is a constant voltage supply.

It can be easily deduced from the equation that if the shielded MR heads are 3 and 1, or 4and 2, the output is always zero because the ratios R2

R1and R4

R3remain constant.

Given the fact that manufacturers try and usually obtain very similar devices in the pro-duction line, the values of resistance of the two unshielded resistors are very similar, i.eR1 ≈ R4, and the same for the two shielded resistors.

α

δ β

γ

R+ΔR

R+ΔR

R

R

Figure 5.2: Electrical scheme of a Wheatstone bridge with a different formulation.

From previous image it can be deduced that:

Vδ =R

2R+ ∆R(Vα − Vγ) + Vγ (5.1)

Vβ =R+ ∆R

2R+ ∆R(Vα − Vγ) + Vγ (5.2)

Vβδ ≈∆R

2R(Vs) (5.3)

which is another expression more simplified than the previously presented.

5.2 Temperature Coefficients

This section is based on the application note of Sensor Technics called : ”Understandingconstant voltage and constant current excitation for pressure sensors”.

From before it is clear that the differential output of the bridge is proportional to: Vo =S ×H × VB where S is sensitivity and expressed in mV/V , H is the magnetic field and VB(B for bridge) is the supply voltage.




To the previous equation a offset voltage can be applied, this voltage is ignored in thefollowing parts for the sake of simplicity.

From the previous equation no need of temperature compensation seems to be needed, butreality is that S, sensitivity, varies enormously with temperature. This variation withtemperature can be described by two parameters: TCS and TCR.

TCS stands for Temperature Coefficient of Span and introduces to the equations the decreasein sensitivity for every increase of temperature.

TCR stands for Temperature Coefficient of Resistance and describes the increase in the totalresistance of the bridge for an increase of temperature.

All manufacturers except NVE provide information about these two parameters.

From the previous equation of the differential output of the bridge Vo, we can calculate theexpression for the temperature derivative ( Vo

dT = Vo):

Vo = H(SVB + SVB)

In the case where the sensor is perfectly compensated with temperature the variation of thedifferential output with temperature is zero, i.e Vo = 0, and thence:

VBVB

= − SS

If the previous condition is achieved that implies a perfect temperature compensation.

One of the two different strategies for temperature compensation is based on variating thesupply voltage and compensate the temperature drift. The other option is compensate thedifferential output after being measured in the Wheatstone bridge.

There exist three main ways of executing temperature correction at the supply side: tem-perature dependent resistive components, current source compensation and the method forTCS ≤ TCR.

5.2.1 Temperature Dependent Resistive Components (TDRC)

In this method a set of elements (resistors mainly) that variate with temperature are includedbetween the power supply and the Vα of the Wheatstone bridge, so a temperature affectationmodifies the voltage applied to the Wheatstone bridge in such a way that the output remainsconstant with temperature.

The following figure resumes the operation principle:




RB

Vout

+

-

Vs

RvCompensationNetwork

Vb

Figure 5.3: Electrical scheme of the Wheatstone bridge with temperature compensation atsupply side by TDRC strategy.

In mathematical terms it can be described as :

VB =VS

(RB +RV )RB (5.4)

VB = VS

(RN RB −RBRN

(RB +RN )2

)(5.5)

VBVB

=RB +RN

RB

(RN RB −RBRN

(RB +RN )2

)(5.6)

if compensation is reached the following must be true: (5.7)

S

S= − 1

RB

(RN RB −RBRN

(RB +RN )

)(5.8)

(5.9)

5.2.2 Current Source Compensation

This is by far the most elegant method of temperature compensation. Unfortunately itrequires a condition on the TCS and TCR coefficients: TCR = −TCS. Such characteristicis not strange and it is reached in at least one of the commercial available MR heads.




RB

Vout

+

-

Vs

Vb

Current Source

Figure 5.4: Electrical scheme of a the Wheatstone bridge with temperature compensationat supply side by current source strategy.

If such a condition applies and current is constant IB = 0, we have that :

VB = IBRB (5.10)

IB = 0 (5.11)

VB = IbRB (5.12)

˙VBVB

=RBRB

(5.13)

and because TCR=-TCS :RBRB

= − SS

(5.14)

thence :VBVB

= − SS

(5.15)

(5.16)

That is, because there is a decrease of S, but this change is perfectly compensated by theincrement of resistivity, if the device is driven by current, the device itself without anyexternal component is able to compensate the temperature drift.

Unfortunately this method has limited benefits because second order effects cannot be omit-ted for a large temperature range.

5.2.3 |TCS| ≤ |TCR|

There exists a third method if the condition |TCS| ≤ |TCR| rules, but this condition is notcommon with MR heads. The method is based on introducing a fixed resistor (variation lessthan 50 ppm/oC) in series with the Wheatstone bridge (if is driven by voltage) or a resistorin parallel if is driven by current. This strategy allows reducing the TCR of the device.

5.3 Manufacturers and models

The plateau of manufacturers of MR devices suitable for current measuring is not veryextense, there exists a larger number of manufacturers for MR heads to be used in memoryapplications but this cannot be used for the present purpose.




At date of this report (2014) there are seven manufacturers of MR devices suitable forcurrent measurement:

1. Honeywell

2. ZETEX Semiconductors

3. Measuremnt Specialities

4. Sensitec

5. NVE Corporation

6. Dowaytech-MultiDimension

7. NXP

Advanced Microsensors was an american company that in the past provided MR heads butis now closed.

Only a few devices have the qualities for being used with the required specifications. Here-inafter follow a commentary of each one of the devices considered to be adequated for theapplications.

5.3.1 Measurement Specialities-KMY20S

The device KMY20S of Measurement Specialities is a bidirectional device based on AMRtechnology. The IC is able to measure magnetic fields in both directions. The axis of sensi-tivity is a plane parallel to the package.

Table 5.1: Table of characteristics of device KMY20S from Measurement Specialities

Parameter Max Value Value Min Value Unit

Max. Storage Temp. 150oC oCSensitivity 3.7 4.7 5.7 mV/V /kA/mSensitivity 0.29 0.374 0.453 mV/V /OerstedLinear Range -2 +2 kA/mTCS -0.36 -0.32 -0.28 % /KTCR 0.27 0.32 0.37 %/KPackage available - SOT223,E-LINE4 - -Offset Voltage -1 +1 mV/V

5.3.2 NVE Tech.-AA002

The device AA002 of NVE Technology is an unidirectional device based on GMR-multilayertechnology. NVE Tech. is one of the few companies that do not provide values for TCS.Instead they provide coefficients for calculating how the output of the devices variates ac-cording to temperature if the device is driven by current or voltage.




Table 5.2: Table of characteristics of device AA002 from NVE Inc.


Max. Storage Temp. 125 oCSensitivity 3.0 - 4.2 mV/V /OerstedLinear Range 1.5 10.5 OerstedTCS - - - % /KTCR - 0.14 - %/oCPackage avaibles - SOIC8 - -Offset Voltage -4 +4 mV/V

5.3.3 NVE Tech.-AA005

The device AA005 of NVE Technology is a unidirectional device based on GMR-multilayertechnolgy. NVE Tech. is one of the few companies that do not provide values for TCS. Insteadthey provide coefficients for calculating how the output of the devices variates according totemperature if the device is driven by current or voltage.

Table 5.3: Table of characteristics of device AA005 from NVE Inc.


Max. Storage Temp. 125 oCSensitivity 0.45 - 0.65 mV/V /OerstedLinear Range 10 70 OerstedTCS - - - % /KTCR - 0.14 - %/oCPackage available - SOIC8 - -Offset Voltage -4 +4 mV/V

5.3.4 Dowaytech-Multidimension-MMLP57

The device MMLP57 from Dowaytech-MultiDimension (a Chinese company) is a bidirec-tional device based on TMR technology.

Table 5.4: Table of characteristics of device MMLP57 from Dowaytech-Multidimension


Max. Storage Temp. 150 oCSensitivity - 4.9 - mV/V /OerstedLinear Range -30 +30 OerstedTCS - -1160 - PPM/oCTCR - -820 - PPM/oCPackage avaibles - SOP8 - -Offset Voltage -20 - +20 mV/V




5.3.5 Sensitec-GF705

The device GF705 of Sensitec GmbH, german group from the group LTiGroup, is a unidi-rectional device based on GMR-multilayer technology.

This device presents very good characteristics for being implemented by unfortunately theavailable packages were very unsuitable for a PCB prototyping. The main good characteristicwas a very large linear range, which permits to have a still decent linear range at very hightemperatures.

Table 5.5: Table of characteristics of device GF705 from Sensitec.


Max. Storage Temp. 150 oCSensitivity 8 10 13 mV/V /mTLinear Range 1.8 (18) 8 (80) mT ( Oersted)TCS -0.26 -0.22 -0.18 % /KTCR 0.17 0.20 0.23 %/oCPackage avaibles - bare dice, FlipChip,LGA6S - -Offset Voltage -5 +5 mV/V

5.4 Temperature Compensation at output side

There exists another option of temperature compensation that is based on the modificationof the differential output of the Wheatstone bridge. This strategy has two approaches: adigital correction based on a look-up table and temperature measurement or an analogcorrection based on NTC or RTD and Operational Amplifiers.

The analog method has the same strategy as stated before with O.A.

5.4.1 Digital Temperature Compensation at output side

This method is based on temperature measurement and a posterior digital correction of theoutput using a look-up table. The operation principle is the creation of a table that containsa correction for every output according to the temperature.

The previous table must be created experimentally before in-field deployment of the sensorand loaded into permanent memory of the micro-controller.

A preliminary approach shows that the look-up table must have a resolution of two degreesCelsius to obtain desired resolutions and hence for the range -50oC to 250oC about 150values must be stored, i.e around 600 bytes.

5.5 Geometrical analysis using numerical methods: COM-SOL

In order to provide a better understanding of the magnetic field distribution around the ge-ometry of the prototype and the analysis of the results a simulation with COMSOL softwarehas been determined necessary.




The simulation relies in a very simple geometry and looks forward the confirmation orrejection of the formula used before to determinate the magnetic field around a conductor.

In the model 4 entities are created: air, PCB(FR-4), copper conductor, and IC. The magneticproperties assigned to each area are listed below.

Table 5.6: Table of characteristics of domains

Domain (in mm) Relative Permeability(µr) Relative Permittivity(εr) Conductivity (S/m)

Copper(10x0.07) 1 1 5.99× 107

Air (20x20) 1 1 0PCB(FR-4) (20x1.5) 1 1 0IC (3x1.5) 1 4,5 0,004

The simulation is in the magnetic domain and stationary in time. A current density of42.86 A

mm2 is assigned to the copper conductor. The geometry can be seen in figure 5.5.

PCBCopper Trace

IC

Figure 5.5: Geometry of the simulation. The direction perpendicular to the image is the zaxis.

Once the meshing of the geometry is done and the electromagnetic equations applied (mag-netic vector potential formulation) , the interpretation of results is possible:




Figure 5.6: Distribution of the magnetic field in the x direction. The nearer the field to theconductor the higher the intensity. Remark that at the lateral edges of the conductor thefield is practically zero because the x component is 0 there.

Figure 5.7 shows the norm of the magnetic field, both x and y component.

Figure 5.7: Image of the total magnetic field (x and y component). The two maximums aredue mainly to vertical component of the field at the edges of the conductor.

Figure 5.8 plots the values of the x component for the magnetic field in a vertical line passingthrough the device.




Figure 5.8: Image shows the values of the x component of the field for a vertical line passingthrough the center. The IC is situated between 9.5 and 12 values of the horizontal axis.

Detail of the previous image is find in 5.9 for the interesting values. We can observe thatthe value found is less that the one using the analytic formula presented before.

Figure 5.9: Image shows the values of the x component of the field for a vertical line passingthrough the center.

This was expected, because an increment in the distance that the magnetic lines mustdescribe implies an increment in the reluctance for the same current and hence less value ofOersteds.




It can be found that if before for every ampere at a distance of 2mm we had a value of 1Oersteds ( 0.002

2/1000 = 1), with this geometry for every ampere we obtain 0.45 Oersteds.

These results clearly state that the simplifications needed for applying the formula of chapter6.4.1 are wrong.




Chapter 6

Prototyping

It is necessary for the sake of the validity of the theories the validation of these throughexperimentation. The creation of a system to test the chosen MR heads arises as a needimpossible to avoid.

The generation of the 30 A in DC current is done using the Agilent 6681A DC Power Source,capable of a maximum of 580 A. The total resistance of the circuit is about 0.02 Ohm andtherefore a very low voltage is necessary to generate the 30 A.

The measurements are taken with two Keithel Multimeters with a resolution of 0,001 mVand the voltage source for the Wheatstone bridge is a BKPrecision Power Supply model9110 .

The shunt used to calculate the main current has a value of 0.001035Ω and is from TMResearch Products.

6.1 Measurements Protocol

As the scientific method states and on order to achieve precision and accuracy, all measure-ments are done in the same manner.

First the Wheatstone bridge is powered up and the measurement tools are switched on.If everything is correct the Offset voltage is notated and the main current generator isactivated.

We proceed to jot one measurements for every ampere for the first 15 A and one measurementfor every two amperes for the rest of measurements until 30 A. A total of 22 measurementsshould be obtained(44 for both directions).

Above 15 A and between each measurement a discharge of the main current is done in orderto avoid the heating due to the joule effect.

In each measurement the value of the differential output voltage and the shunt voltage arenotated alongside the temperature..

Once all measurements are done we reduce the output current of the source and switch itoff. The terminals at the rear of the DC Power Supply are flipped and the configuration forthe other direction of main current is done.

In this new configuration the same protocol is followed and in total 90 measurements areobtained (each one consisting of 3 values).




The values jotted are all the significant figures that remain stable at naked eye in order tobe written down.

6.2 Prototypes

In this section photos of the three prototypes are shown. The reason for the constructionof three prototypes is that at first the authors needed a cheap solution to test all devicesselected from the market. This first cheap solution does not offer the possibility to test athigh temperature.

The second and the third prototypes are suited for high temperature testing. The secondprototype is done with a ceramic and the third with polyamide.

The first prototype consists of four MR devices on one side of a PCB-prototyping boardand a conductor made of layers of copper at the other side. The soldering is done usinghigh-temperature lead (300oC).

The second prototype is used to test the MMLP57F and the AA005. The last PCB, withnames and date, only tests two devices of the MMLP57F.

Figure 6.1: Image of the first prototype. At the other side of the PCB there is a copperconductor for generating the magnetic field. The BNC connector is the shunt for measuringthe main current.




Figure 6.2: Image of the second prototype, made of alumina. At the other side of the PCBthere is a copper conductor for generating the magnetic field. The shunt for measuring thecurrent with high precision is outside the oven in another PCB.

The third prototype serves only to the MMLP57F.

Figure 6.3: Image of the third prototype. The third prototype is made of polyamide, thismaterial able to operate up to 250oC.

In Appendix 2 it is shown how the constant voltage supply method is implemented.




Chapter 7

Experimental Results andAnalysis

The objective of the prototypes in no other than obtaining experimental results. This partshows the data obtained from the different devices and analyze it in order to obtain thecharacteristics of the measure.

7.1 Measurements with first prototype : ambient tem-perature

7.1.1 Model MMLP57 results at ambient temperature

In order to contrast the validity of the model and effectuate a first approach to the device,the latter has been tested at the specified conditions.

Table 7.1: Table of conditions for measurements.

Parameter Max Value

Device MMLP57Temperature 23,4oCSupply Constant Voltage: 5.00 VApparatus BKPRECISION+ 2X Keithel MultimeterShunt value 0.001035 ohm

In order to obtain values with the same temperature the measurements taken with morethan 15 A as main current are separated between each other by a discharge in main currentto cool the device. The heating produced by the main current alters the results.

We can confront the value obtained in the linear regression with the theoretical from thedatasheet, shown in table 7.1) . If the value 0.45OeA obtained at the section of COMSOLsimulation is used at a 5 V supply:




4.9mV/V

Oersted· 5 V · 0.45

Oersted

Ampere= 11.02 ≈ | − 10.4|mV

A

Because the regression has a very high R coefficient, and hence all dispersion of values isexplained by the current variable, a more detailed analysis with the software R is done.

7.1.2 Model KMY20S results at ambient temperature

The author proceeds in the same manner with the KMY20S device. The table of conditionsduring the measurements follows:


Parameter Max Value

Device KMY20STemperature 22.6oCSupply Constant Voltage:5,00VApapratus BKPRECISION+2x Keitherl MultimeterShunt value 0.001035 ohm

This device presents a good linear response but have the problem of operating at its limits.It should be studied the possibility to use the version with internal magnet that allowsoperating in a safe area.

The problem with the version with magnet is its integrability in the power module.

During the experimentation a test was performed: the maximum current (30 A) was providedfrom a cool sate and the evolution of the output was observed. For a device not sensibleto temperature the output must remain constant, but reality shows that after 3 minutes(when a steady state is reached) the output is the same than the output for a current of oneampere more.

7.1.3 Model AA002 results at ambient temperature

The device from NVE Corp. AA002 is also tested in a similar manner. The table of conditionsis shown :

The device presents an important hysteresis and is not adequate to measure low currents.Besides the device is not able to distinguish between the two opposite directions of themagnetic field.

7.1.4 Model AA005 results at ambient temperature

In order to contrast the validity of the model and effectuate a first approach to the devicethe device the device has been tested at the specified conditions.

As before the device presents an important hysteresis and this can lead to problems formeasuring low magnetic fields. Neither the AA002 or the AA002 is able to distinguishbetween the two opposite directions of the magnetic field.




The AA005 was chosen for its high linear range and its best performance must be expectedduring the high temperature analysis.

7.1.5 Model Comparison at ambient temperature

We can compare between the different devices plotting them all together, as in figure 7.1.

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−200

0

200

−20 0 20 −20 0 20 −20 0 20 −20 0 20Current[A]

Output[mV]

Differential Output of Wheatstone Bridge for 5V supply

1 2 3 4

Figure 7.1: Figure with all the outputs for an easier comparison.(1)=KMY20S,(2)=MMLP57F,(3)=AA002, (4)=AA005.

The comparison image permits to verify how the device with more output range and sen-sibility (slope) is the MMLP57F and its linearity. This first impression will be contrastedwith data at high temperature.




7.2 Measurements with third prototype: 50oC to 250oC

Once again the device MMLP57F is tested. The tests are realized with the third proto-type, made of polyamide, up to 250oC. Some problems have been encountered during hightemperature testing. The results of the test are the linear regressions hereafter found.

By contrast with the previous cases the multivariate regression will be done with current asindependent variable, in order to simulate a real current sensing application.

7.2.1 Test at constant voltage

The conditions of the test are shown in table 7.3.


Parameter Max Value

Device MMLP57FTemperature 50oC to 250oCSupply Constant Voltage: 5.00V+ResistorApapratus BKPRECISION + 2X Keithel MultimeterShunt value 0.001035 ohm

In this occasion the regression is done with the electric current as independent variable. Thereason is to simulate a real application where the current is the unknown variable.

The plotting of the output in mV against the main current to be measured at differenttemperatures is shown in figure 7.2.




−30 −20 −10 0 10 20 30

−20

020

40

MMLP57 % Vb=5

Vshunt (mV)

Vou

t(m

V)

Figure 7.2: Figure showing the output of the MMLP57F for five different temperaturesaccording to main current.

As predicted and stated before in previous occasions the higher the temperature the lower theslope. The linearity observed at high temperature is the same than the other temperatures.

The saturation of the device is not shown in order to avoid altering the algorithm for MLR,althought it is about 31 A at 250oC.

The results for the residuals are shown in figure 7.3. The residual standard error is 0.138 ,i.e. about 0.14 A, which implies that the final resolution achieved is of 0.3 A, and hence theerror is 0.5 percent in the range of 60 A.




−30 −20 −10 0 10 20 30

−0.

50.

00.

5

Fitted values

Res

idua

ls

Residuals vs Fitted

254

196

195

−3 −2 −1 0 1 2 3

−4

−2

02

46

Theoretical Quantiles

Sta

ndar

dize

d re

sidu

als

Normal Q−Q

254

196

195

−30 −20 −10 0 10 20 30

0.0

0.5

1.0

1.5

2.0

2.5

Fitted values

Sta

ndar

dize

d re

sidu

als

Scale−Location254

196

195

0.00 0.05 0.10 0.15 0.20

−4

−2

02

46

Leverage

Sta

ndar

dize

d re

sidu

als

Cook's distance 0.5

0.5

1

Residuals vs Leverage

254

196

55

Figure 7.3: Figure of the statistical residuals graphics for the linear multivariate regression.




The equation that satisfies the regression is :

Call:

lm(formula = Current ~ Output + Temp + Temppow + Outputpow +

OutputxTemp + Aux50 + Aux100 + Aux150 + Aux200 + Aux250,

data = datos)

Residuals:

Min 1Q Median 3Q Max

-0.55258 -0.09956 0.00396 0.09887 0.77072

Coefficients: (1 not defined because of singularities)

Estimate Std. Error t value Pr(>|t|)

(Intercept) 9.774e-02 4.388e-02 2.227 0.02674 *

Output 8.160e-01 1.039e-01 7.857 9.28e-14 ***

Temp -1.458e-02 6.220e-04 -23.437 < 2e-16 ***

Temppow 4.985e-05 1.988e-06 25.082 < 2e-16 ***

Outputpow -1.435e-04 3.046e-05 -4.712 3.92e-06 ***

OutputxTemp 4.442e-03 4.092e-04 10.855 < 2e-16 ***

Aux50 -2.639e-01 8.198e-02 -3.219 0.00144 **

Aux100 -3.915e-01 6.173e-02 -6.342 9.46e-10 ***

Aux150 -4.895e-01 4.110e-02 -11.912 < 2e-16 ***

Aux200 -4.569e-01 2.085e-02 -21.914 < 2e-16 ***

Aux250 NA NA NA NA

---

Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.138 on 271 degrees of freedom

Multiple R-squared: 0.9999, Adjusted R-squared: 0.9999

F-statistic: 3.783e+05 on 9 and 271 DF, p-value: < 2.2e-16

The last parameters are used to create the formulae for any temperature. The final equationresults:

I = 0.097 + 0.816Vout − 0.01458T + 0.00004985T 2 − 0.0001435V 2out+

+ 0.0044TVout − Vout(0.00002T 2 − 0.0054T − 0.03)(7.1)

The resolution is worst than the following case, constant current supply.




−30 −20 −10 0 10 20 30

−10

0−

500

5010

0

MMLP57 @ Const. Current

Vshunt [mV]

Vou

t(m

V)

250ºC

200ºC

150ºC

100ºC50ºC

Figure 7.4: Figure showing the output of the MMLP57F for five different temperaturesaccording to main current. Case for constant current supply.

7.2.2 Tests at constant current

The conditions of the test at constant current are shown in table ??.


Parameter Max Value

Device MMLP57FTemperature 50oC to 250oCSupply Constant Current:0.015mAApapratus BKPRECISION+2x Keithel MultimeterShunt value 0.001035

The plotting of the output in mV against the main current to be measured at differenttemperatures is shown in figure 7.4.

The results for the residuals are shown in figure 7.5. The residual standard error is 0.096,i.e. about 0.1 A, which implies that the final resolution achieved is of 0.2 A, and hence theerror is 0.33 percent in the range of 60 A.




−30 −20 −10 0 10 20 30

−0.

40.

00.

20.

4

Fitted values

Res

idua

ls

Residuals vs Fitted

111

246

30

−3 −2 −1 0 1 2 3

−4

−2

02

4

Theoretical Quantiles

Sta

ndar

dize

d re

sidu

als

Normal Q−Q

111

246

30

−30 −20 −10 0 10 20 30

0.0

0.5

1.0

1.5

2.0

Fitted values

Sta

ndar

dize

d re

sidu

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Scale−Location111

246

30

0.00 0.05 0.10 0.15 0.20

−4

−2

02

4

Leverage

Sta

ndar

dize

d re

sidu

als

Cook's distance 0.5

0.5

Residuals vs Leverage

111

31

30

Figure 7.5: Figure of the statistical residuals graphics for the linear multivariate regression.Case for constant current supply.

The equation that satisfies the regression is:

Call:

lm(formula = Current ~ Output + Temp + Temppow + Outputpow +

OutputxTemp + Aux50 + Aux100 + Aux150 + Aux200 + Aux250,

data = datos)

Residuals:

Min 1Q Median 3Q Max

-0.39202 -0.04909 -0.00330 0.06121 0.40898

Coefficients: (1 not defined because of singularities)

Estimate Std. Error t value Pr(>|t|)

(Intercept) -3.224e-01 2.980e-02 -10.818 < 2e-16 ***

Output 2.324e-01 2.534e-02 9.171 < 2e-16 ***

Temp -3.772e-03 4.350e-04 -8.671 4.56e-16 ***

Temppow 1.014e-05 1.409e-06 7.198 6.46e-12 ***

Outputpow -1.206e-05 2.467e-06 -4.889 1.77e-06 ***

OutputxTemp 1.157e-03 9.987e-05 11.585 < 2e-16 ***

Aux50 -4.656e-02 2.003e-02 -2.324 0.0209 *

Aux100 -7.690e-02 1.500e-02 -5.128 5.70e-07 ***

Aux150 -9.542e-02 1.000e-02 -9.540 < 2e-16 ***

Aux200 -8.179e-02 5.035e-03 -16.246 < 2e-16 ***




Aux250 NA NA NA NA

---

Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.09693 on 262 degrees of freedom

Multiple R-squared: 1, Adjusted R-squared: 1

F-statistic: 6.606e+05 on 9 and 262 DF, p-value: < 2.2e-16

Or expressed in a more proper manner:

I = −0.3224 + 0.2324Vout − 0.003772T + 0.00001014T 2 − 0.00001206V 2out

+ 0.001157TVout + Vout(0.000004T 2 − 0.0013T + 0.0108)(7.2)

The method of constant current supply offers more range of output and better performancein front of noise.

7.2.3 High temperature problems of electromigration

During the first phases of experimentation at constant voltage supply and at high tempera-ture, with devices not rated for high temperature operation, the authors discovered an effectwhich seems to affect the offset voltage of various different MR mechanisms. This affectationprevents the measurement of the magnetic field at high temperatures.

The solution proposed, at the expense of the sensibility, is to reduce by an order of magni-tude the typical supply current of the Wheatstone bridge at ambient temperature. This isimplemented in constant voltage supply by applying a resistor of high value in series withthe Wheatstone bridge.

It turns out that the parameter current × temperature must be conserved in some way,obligating to operate with low currents in the Wheatstone bridge at high temperatures,which leads to believe that some kind of electromigration problem can be the cause.




Chapter 8

Conclusions

Project AMA, which started on the 31st of March 2014 and finished on the 14th of September2014, had as a main objective to demonstrate the viability of using magnetoresistivity as acurrent sensing solution at high temperatures(250oC).

Even though there existed a secondary objective: the development of skills and capacitiesof the author, still at the beginnings of his learning curve. Conclusions must be dividedinto these regarding the pedagogical part, indeed the most important for the author, andinto these regarding the industrial objective, the most important for the industrial partner:SAFRAN.

8.1 Pedagogical Conclusion

Even though the pedagogical side is part of the entire project and cannot be separated andtreated independently, this part describes the capacities and abilities developed.

At the beginning of the project a bibliographic study ensued. This exercise required thesearch and comprehensive lecture of master theses, papers and some chapters of books. Atthe end, an analysis of the factual data was needed in order to decide which was the besttechnique to implement the current sensor.

The author also had to initialize himself with COMSOL, LTSpice and the statistical soft-ware R. This software presented different levels of difficulty, with COMSOL being the mostchallenging.

At the prelude of the experimental part, the author had to create the Gerber files of thePCB for the prototype. This has been done with ALTIUM software, already known to theauthor.

During the experimental part the author had to prepare the setup to heat up the system,measure current and output signals and record the data. The second and third prototypesare built by other parties.

Hence the author developed and reinforced his skills with basic software tools and his ca-pacities of critical thinking.




8.2 Project Conclusion

In its initial chapters the report describes the state-of-the art of the current sensing solutions,both direct and indirect measurements. It follows a discussion of which option, between theproposed, appears to fit the specifications.

In the second main part, the theoretical principles of magnetoresistivity are exposed and itsbehaviour at high temperature discussed and analyzed. In the third part of the report, theexperimental results, are maybe the most important and fructiferous from the point of viewof a future industrialization. The available devices on the market are used to test each oneof them and against its specifications. Up to three prototypes are created, only two suitedfor high temperature operation, and the results interpreted.

Results show that it is possible with temperature measurement to determinate the currentthrough the circuit with a resolution on the order of magnitude of 0.1 A , and with abandwidth supposed to be up to 1 MHz.

Lots of historical technological battles show that the ”best” option (here more resolution ordynamical range) does not impose itself if it is not the more ”suited” solution. The needof temperature sensing to correct the output deviation in the magnetoresistance solutionsupposes a handicap.

There exists the possibility of reducing resolution and dynamical range in the proposed solu-tion to avoid temperature sensing, or to implement an analogical correction of temperature.These methods, and the resolutions that can achieve, are yet to be tested.




Chapter 9

Future Work Lines

During the development of the project, and due to reasoning or experimentation, differentquestions and strategies have arisen. Also at the end of the project new future work lineshave appeared towards the final design of the sensor.

Due to lack of time and knowledge not all the questions have been answered or all thestrategies tested. This section exist as a manner to write, for the record, what could be thefuture work lines and the problems to check again. A future product designer or intern couldfind this chapter interesting.

As for unanswered questions and strategies a total of four can be stated:

i magnetoresistive devices are very new and different models appear each year. It seemsto have been changes in the web page of Honywell that could led to future devices totest.

ii in the last years new types of magnetoresistance have appeared. Although they arenot industrially available they should be considered more carefully.

iii an analog temperature correction could be an excellent solution if a circuit with a suitedresponse could be designed. Reality is that the market of NTC or Pt1000 is small andthe desired response is very precise, therefore creating a circuit of compensation is verydifficult with a limited set of parameters.

iv test at high frequency should be executed and proper geometries of the trace designed.

As future lines of work there are various that must be highlighted:

i the implementation of a shield for external magnetic fields at high frequency (with apossible amplification of the signal inside the shield).

ii the utilization of two sensors for two different ranges of current with different resolu-tions and the mixing of the two signals in order to facilitate a unique signal to theuser.

iii the study of the ratios current/temperature for derating operation that have put intotrouble the author during the experimental set-ups.




Chapter 10

Acknowledgements

This document would have been impossible without lots of people, this section pretends tolist them as a form of recognition.

I would like to thank Christian Vollaire, director of IPES, for giving me the opportunityof this stage. Also Dominique Bergogne and Abderrahmane Beroual for their role assupervisors and markers.

Remi Perrin is without any doubt the person who has been next to me this months and hadto cope with all my ideas, confabulations, nightmares and others. Thanks for his patienceand comprehension.

Patrick Denis played and important role with his photos of the dice and its recommen-dations for derating. Raphael Riva also contributed with the creation of a PCB for hightemperature.

All the previous would have been impossible without SAFRAN, represented with Ali Mar-wan and Regis Meuret

I cannot forget neither that lots of abilities that I put in practice during the internship arein fact fruit of my time in CITCEA, to all of them: thanks.




Chapter 11

Bibliography

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[2] B. Dieny, V. S. Speriosu, and S. Metin. Thermal variation of the magnetoresistance ofsoft spin-valve multilayers. EPL (Europhysics Letters), 15(2):227, 1991. 36

[3] P. P. Freitas, R. Ferreira, S. Cardoso, and F. Cardoso. Magnetoresistive sensors. Journalof Physics: Condensed Matter, 19(16):165221, 2007. 35

[4] Oriol Gomis i Bellmunt. Design rules for actuators in active mechanical systems.Springer. 18

[5] S.Z. Hua H.D. Chopra. Very large ballistic mr. Phys Rev. B.,66,020403-1. 32

[6] Michel Hehn, Francois Montaigne, and Alain Schuhl. Magnetoresistance geante etelectronique de spin. Techniques de l’Ingenieur,traite Electronique. 26, 27

[7] Wen-Sheng Huang, Chih-Cheng Lu, and Jen-Tzong Jeng. A novel 3d cmos micro-fluxgate magnetic sensor for low magnetic field detection. In Sensors, 2010 IEEE,pages 1791–1794, Nov 2010. 37

[8] Albrecht Jander, Carl Smith, and Robert Schneider. Magnetoresistive sensors for non-destructive evaluation. 10th SPIE Symposium. 26

[9] Yuan L. and S.H. Liou. Temperature dependence of magnetoresistance in magnetictunnel junctions with different free layers structures. Dep. of Physics and Astronomy.University of Nebraska-Lincoln. 26, 33, 34, 35

[10] Chan-Gyu Lee, Jung-Gyu Jung, R.D. McMichael, R.A. Fry, Andrew Chen, W.F. Egel-hoff, and V.S. Gornakov. Structural, magnetic, and thermal stability of irmn exchangebiased layers. Journal of Applied Physics, 91(10):8566–8568, May 2002. 36

[11] LEM. Catalog high precision current transducers. Application Note. 24, 25

[12] LEM. Lem current and voltage transducers, 3rd ed. Application Note. 21, 22, 23, 24

[13] J. Lenz and Alan S. Edelstein. Magnetic sensors and their applications. Sensors Journal,IEEE, 6(3):631–649, June 2006. 17, 21, 26

[14] Gregory Malinowski. Transport dependant du spin et couplage d’echange: de la jonc-tion tunnel au capteur magnetique integre. These pour obtenir le titre de Docteur del’Universite Henri Poncaire, Nancy I. 26




[15] John C. Mallinson. Magneto-resistive and spin valve heads: Fundamentals and appli-cations. second ed. Academic Press. 74

[16] Deborah Morecroft. In situ magnetoresistance measurements during patterning of spinvalve devices. Thesis-Downing College- Cambridge. 26, 36

[17] Erik R. Olson and Robert D. Lorenz. Design of integrated magnetoresistive currentsensors for ipems. Technical report, University of Wisconsin-Madison. 26, 28, 29

[18] X. Portier, A.K. Petford-Long, T.C. Anthony, and J.A. Brug. Temperature dependenceof the reversal mechanism in spin-valve films. Applied Physics Letters, 75(9):1290–1292,Aug 1999. 36

[19] Dexin Wang, C. Nordman, J.M. Daughton, Zhenghong Qian, and J. Fink. 70free andreference layers. Magnetics, IEEE Transactions on, 40(4):2269–2271, July 2004. 34

[20] Y.B. Zhang, P.A.A. van der Heijden, J. Nozieres, K. Pentek, T. K. Chin, T. Tuchscherer,A.M. Zeltser, H.-R. Blank, S. Trotter, S. Jaren, and V.S. Speriosu. Thermal stabilityof nimn spin valve heads. Magnetics, IEEE Transactions on, 36(3):586–590, May 2000.26, 36




Chapter 12

Appendix 1

12.1 Brief Physical Introduction

A brief physical review follows for the benefit of the readers without a usual handling of thephenomena related to spin.

12.1.1 Electron spin

Spin can be defined as a magnetic moment which is carried by electrons, elementary sub-particles. Spin can be interpreted as the magnetic moment associated with kinetic rotationalmoment due to the rotation of the electron around its own axis. The discovery of spin canbe attributed to Stern and Gerlach in 1922.

The spin of a electron only have two possible states (−~2 ,+~2 ), and its heavily influenced by

the local magnetic intensity.

In 1936 Mott discovered the influence of the intrinsic property of spin into the mobility insideferromagnetic materials. In ferromagnetic materials the electrons in the Fermi level are ina majority of one kind of spin; this disproportion, non-existing in non magnetic materials,is the cause of the spontaneous magnetization of ferromagnetics. Electric conductivity isrelated to the electrons in the Fermi Level and the disproportion in the presence of the twokind of electrons produces different mobility for each type of electron.

Even the spin of an electron is a intrinsic property it can change during time. The coherencelength (aka penetration length, aka spin diffusion distance) is a measure of the distance thatan electron can travel before changing its spin.

In order to change the spin the electron must pass through the process of transformation(aka diffusion), at the end of this process the spin of the electron is the opposite.

The probability for this transformation to take places varies if the spin of the electron isparallel(aligned) or antiparallel with the local magnetic moment.

12.1.2 Ferromagnetism, ferrimagnetism, antiferromagnetism and para-magnetism

The previous define different states of magnetic materials. Normally every magnetic materialcan achieve only two of this states.




Table 12.1: Table of Neel Temperature and Curie Temperature of few materials. Source[14]

Substance Curie Temperature Neel Temperature.

Iron 1043K -Cobalt 1400K -Nickel 631K -MnO - 116KMnTe - 307KNiO - 525K

Ferromagnetism is the state in which all magnetic moments are ordered without theneed of applying a magnetic field. In the application of such a field the material magnetizesitself according to its permeability.

Ferrimagnetism is the state where magnetic moments are aligned in two groups. Thesetwo groups are in opposite directions and one is stronger than the other and hence a netmagnetic moment is generated.

Antiferromagnetism is a state where as in ferrimagnetism there exists two groups insidethe magnetic material but in this case there is no net magnetic moment because the twogroups cancel each other.

(a) Image of magnetic mo-ments inside a ferromagneticmaterial with no external fieldapplied

(b) Image of magnetic mo-ments inside a ferrimagneticmaterial with no external fieldapplied

(c) Image of magnetic mo-ments inside an antiferromag-netic material with no exter-nal field applied

Figure 12.1: Figure showing two different plots of current depending the existance of externalfield. Source: Wikipedia Commons

Paramagnetism is the state by which a material have all its magnetic moments in totaldisorder and no net magnetic moment results from the total sum.

Ferromagnetic and ferrimagnetic materials become paramagnetic above Curie Temperature.Antiferromagnetic materials become paramagnetic above Neels temperature.

12.1.3 Magnetization of a material

The magnetization of a material is defined as the state where a majority of internal magneticsources are aligned so as a net total magnetic field is generated.




When a magnetic material becomes magnetized by the application of a magnetic field, itreacts by generation within its volume an opposing field that resists further increases in themagnetization.

This opposing field is called the demagnetization field (aka stray field) because it tends to

reduce or decrease magnetization. Inside a magnetic material the ~H field is contrary to Band to ~M , which created the poles in first place.

12.1.4 Electron scattering

Electron scattering is the process by which the path of a electron is modified by the presenceof an uniformity in the medium. This concept is used to explain resistivity of the materials.When an electron encounters a scattering center (non uniformity in the material) it changespath due to the electric field and resistivity increases.

12.1.5 Annealing

Annealing is the process of heating a MR system and then cool it slowly. The magnetore-sistance ratio can be greatly improved due to thermal annealing. Postdeposition thermalannealing is critical to achieving high tunnelling magnetoresistance. The process is occursin a vacuum furnaces where a magnetic of various Tesla is applied.

If a non-vacumm oven in used a Ru layer can be used to avoid the oxidation of the rest ofthe layers.

12.1.6 Anisotropy field

Field needed to saturate the magnetization in the hard direction, i.e a measure of the strengthof the magnetocrystalline anisotropy

12.1.7 Easy axis

Direction where the application of a magnetic field results in the rotation of the domainmagnetization but not wall motion ensues.

12.1.8 Demagnetization field

If the magnetic poles model of a magnet is chosen (in contraposition of the Ampere modelof atomic currents) it can be observed that the lines of field try to use the minimum pathfrom north poles to south poles. In the interior of the magnet such lines are contrary to theapplied external field ~H and ~M , ~B = µ0( ~H + ~M)

The magnetization of this demagnetizing field depends upon the form of the magnet.

There exists certain directions inside the material where ~Hdemag. is larger, and thence aneasy axis of magnetization can be defined along the longer axis . In a thin film the easy axisis parallel to the plane of the film.




12.1.9 Exchange anisotropy

The exchange anisotropy (aka exchange bias) is an effect that is key to understand certainkinds of MR. When a FM material is in direct contact with an antiferromagnetic materialcertain properties of the ferromagnetic material change.

This effect was discovered because the hysteresis loop of an FM material alone is centered atzero of the external field axis, but an FM-AFM system exhibits a hysteresis loop displacedfrom zero by a certain amount in one direction.

This implies that when trying to switch magnetization back from this preferential direction,a higher energy is needed that to switch it back. This directional anisotropy in the magneti-zation of the FM layer is called exchange anisotropy and this name is due the anisotropyis inducted by interaction with the AFM layer.

Exchange bias happens in multilayer because the AFM material has a hard magnetizationbehavior and the ferromagnetic film presents a soft magnetization curve. Only soft ferro-magnetic materials that are strongly exchange-couple will have its interfacing spins pinned.

This process that takes place between FM and AFM materials permit to speak about acouple between materials,i.e AFM materials can couple to ferromagnets.

During the process of exchange bias the surface atoms of the ferromagnet align with thesurface atoms of the antiferromagnet. This way the ferromagnetic material pins its orienta-tion.

Ferromagnet

Antiferromagnet

Near epitaxial Interface

Figure 12.2: Heuristic figure of AFM coupling into a FM layer. Even the AFM layer has total0 magnetization it contains magnetized and ordered lines of atoms that induce by proximitythe magnetization.

The blocking temperature is defined as the temperature above which the AFM materialloses its ability to pin the magnetization direction of the contiguous FM material. Neeltemperature and blocking temperature are related to grain size and can be for large grainsize very similar.

If an external magnetic field is applied in order to reverse the magnetic moments, extraenergy will be needed because Neel domain walls will must have to be created inside theAFM layer. This extra energy is the deformation of the H-B curve.

The fact that Curie temperatures are usually bigger than Neel temperatures aids to theconstruction of these multilayers. Usually the multilayer system is heated between the Curietemperature of FM materials and the Neel temperature of the AFM material, then cooledwithin a magnetic field that fixes the orientation of the AFM layer. These layers will maintainthis orientation permanently(even with the application of external magnetic fields) if the Neeltemperature is not surpassed again.




Chapter 13

Appendix 2

13.1 Constant Current Supply

Beforehand it has been explained the differences between the constant voltage supply methodand the constant current supply method. Supplying constant voltage requires only a DCvoltage source, whereas for a constant current an IC has been implemented.

The devices used are the PSSI2021 from NXP and the REF200 from Texas Instruments.The electrical circuit implemented for its use are represented in figure 13.1.




Iout= 0.6178ARext +158uA

45

1 2 3

Vss(5V)

R1

R4

R3

R2

0.02mA=0.617/123400+0.015mA

PSSI

2021

NXP

124K

R1

R4

R3

R2

Vss(5V)

1 2 3 4

8 7 56

Vdd(-5V)

Vdd(-5V)

REF200TI

Figure 13.1: Electrical scheme for the constant current supply. Two options are presentedfor two available devices in the market (one of NXP and another of TexasInstruments).



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