Selecting the Most EffectiveCurrent Sensing TechnologyWhen faced with selecting the most suitable method of accurate current measurement, a designengineer is faced with a number of possible alternative technologies. The selection of the mostappropriate technology can often appear a little daunting. This article will assist the designer to the bestchoice for their specific application. Warren Pettigrew, CTO Raztec Sensors, Christchurch, NewZealand

Firstly, what are the necessary qualitiesthat need to be considered? Of courseproduct cost and the desired performance.The latter covers accuracy over currentrange (linearity, calibration, hysteresis);desired current range; acceptable outputsignal level; stability of offset overtemperature range; stability of gain overtemperature range; required frequency andtransient response; satisfactory steady statesignal to noise ratio and noise rejectionfrom the primary circuit; stability over time;and satisfactory physical size. Other factorsinclude compliance with appropriatestandards; rated for application temperaturerange; acceptable level of heat generation,voltage and insulation rating and creepagedistance of desired galvanic isolation,quiescent current, supply voltage, influenceon the measuring circuit (impedance), andacceptable output resistance.When considering the extensive

(although, not exhaustive) list of qualitiesnecessary, it becomes obvious that thedesign of current sensing is non-trivial andis often underestimated. This is furtherconfounded by the number of availablesensor options.

The most popular of the availabletechnologiesThere are numerous technologies

available such as shunts, PCB trackresistance, FET on-resistance, currenttransformer, Rogowski coil, open/closed-loop Hall effect sensors, or closed loop flux-gate. The task boils down to matching atechnology to the need.Something a designer really cannot

afford to ignore is the time taken to designa product. If production volumes are small,normally a short design time is importantso the selection of a simple, sure to worksolution would be useful. To assist with theselection of appropriate technology, itshould be possible to draw up a matrixmatching qualities against the differenttechnologies. However, such a matrix

would be far too unwieldy. It is alsocomplicated by the fact that appropriatetechnology changes with the level ofcurrent to be measured. Instead, the meritsof each available technology, in turn, will beexamined.

ShuntsIf no galvanic isolation is needed, shunts

are often the technology of choice formeasuring ground referenced currentsbelow approximately 20A. Performance canbe improved by increasing the voltageacross them (which, in turn, increases the

need for heat dissipation), using a 4terminal configuration, using a lowinductance winding if high (100KHz)frequencies are to be measured, orinterfacing with an op-amp which has lowand stable offset voltage and highfrequency response and high commonmode rejection. If galvanic isolation isneeded, opto-isolation can be incorporated,but this would then probably push costshigher than that of Hall sensors for allcurrent ratings.If the shunt is not ground referenced

and is switched between ground and the

Figure 1: Basic shunt interface circuit

Figure 2: Various current sensing positions

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supply voltage, it is a distinct challenge forany operational amplifier to reliably seemillivolts across the shunt with a relativelyhigh common mode voltage - especially ifthe common mode voltage has PWMsuperimposed on it. One would be muchbetter off choosing a galvanically isolatedcurrent sensor technology (see Figure 1).For example, using the classical

differential amplifier circuit, the shuntvoltage can swing high R1>>R6 andR3>>R5 so that the op-amp is kept withinits specified input voltage. However, ifR1>>R6 and R3>>R5, the gain of thecircuit is <<1 for a signal which is alreadysmall, it can be a challenge to re-amplify.Very stable, low noise amplifiers andresistors are needed. The amplification andnoise suppression circuitry is likely to harmthe frequency response and introducesignificant phase shifts which, in turn cancreate instability. A Hall effect sensor ismuch easier to implement and is very likelyto be cheaper than the combinedshunt/amplifier configuration.As the current gets higher, dissipation

increases with an increase of footprint andcost.For example, if we need to measure

100A and the minimum practical voltagethat can be sensed is 0.2mV (noiselimited) and we choose a 1mΩ shunt; itsdissipation is 10W @ 100A and theminimum sensed current is 0.2A. Anopen-loop Hall effect current sensor wouldout-perform the shunt for size and cost,especially if the cost of the shunt’s interfacecircuit is included. For any shunts, it isessential that thermocouple inducedvoltages are taken into effect – these WILLsubstantially limit dynamic range. Figure 2shows various current sensing positions.For non-critical applications and lower

current applications, a piece of PCB tracecan be used as a shunt. Its actualresistance would have to be calibrated, asthere will be substantial batch to batchvariation. Temperature compensationwould probably also be necessary. Thetrace will be inductive, thus reducing its

application for lower frequencymeasurements.

FET on-resistanceIf your application allows you to get away

with it, the cheapest current sensor is FETon-resistance. A FET is a resistor whenturned on. However, there is a dauntingarray of variables such as gate-sourcevoltage; junction temperature which isdependant on time, junction to heat-sinkthermal resistance (device mountingpressure), ambient temperature, current(non-linear), or device characteristics.Despite all these issues, this technique isuseful for detecting overload ordesaturation of the FET.

Current transformersThese devices can only measure AC

currents, but can do so very precisely overa wide dynamic range and up to highfrequencies (>100KHz). Small units arecheap, offer galvanic isolation and are easy

to interface to. They have good immunityto short-term overload, give negligibleloading to the primary circuit and are self-powered. So what are their weak points?They don’t like a DC component within

the AC current that they are measuring.This causes saturation. However, there arespecial DC immune transformers, but thisthen often makes them more susceptibleto stray magnetic fields. They can displaynon-linear phase shifts. Again, if specialcore material is chosen this effect can bereduced, but this then results in increasedcost. Larger current devices are bulky, muchmore bulky than certain Hall effect baseddevices. There could be cost advantages inthese sizes as well. Unless electrostaticshielding is fitted, primary voltage noise willinevitably appear on the output. Outputvoltage overshoot protection may beneeded to protect the monitoring circuit.

Rogowski coilA Rogowski coil is an air-cored current

transformer. Its output is proportional todi/dt, so for it to generate a useful outputan integrator is required to recover thecurrent. By definition, they can’t measureDC current, but are immune to DC currentin the primary circuit. The coils themselvesare cheap. They are often flexible, so theycan be wrapped around a conductor. Theyare excellent for measuring high ACcurrents or high frequency currents.So what are the downsides of using

Rogowski coils? When required to measurelow mains frequency currents, their outputsare minute. This results in the need forexpensive amplifiers and an integrator torecover a useful signal. Dedicated interface

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Figure 3: Simplistic DCCT

Figure 4: Hall sensingwithout magneticcircuit

circuits are available which claim operationover a wide dynamic range of current.Since signals are small, careful electrostaticscreening will often be needed.

Direct current current-transformerThese sensors work on the principle that

when a magnetic core saturates, it loses isinductance. A typical DCCT has anexcitation winding wound onto a torroidalcore which has enough excitation to justdrive it into saturation. A primary conductoris passed through the core. The conductorscurrent will modulate the core saturationwhich, in turn, will modulate the secondharmonic of the excitation current. This isthen the monitored output. For mainsfrequency applications, they can be excitedusing a sinusoidal mains frequency voltage.Figure 3 shows the DCCT principle.As the name suggests, DCCTs can

measure DC and AC currents with galvanicisolation. The strengths of these devices aresimplicity and ruggedness against overload;if an appropriate core material is selected,they are sensitive to low (<20mA) currents;their offset is stable over a wide temperaturerange; and they offer good immunity to straymagnetic fields. But they don’t have verygood linearity; their frequency response islow (~500Hz) for a basic unit; they havelimited dynamic range of current; and theoutput may carry excitation noise.A single core design injects noise into

the primary circuit.

Open-loop hall effect currenttransducersThese fundamentally simple devices

can, with the selection of appropriate

components, exhibit admirableperformance. There are two distinctfamilies:- units with a magnetic circuit andunits without. The latter technology simplyuses a magnetic field strength sensor to

measure the flux surrounding a currentcarrying conductor (Figure 4).This is very simple and generally low cost

technology and is appropriate for someapplications. Strong points are i.e. simplicityand low cost, providing galvanic isolationfor measuring AC and DC currents;compact design especially when high(~150A) currents are to be measured;reasonable frequency response up to50kHz; and good signal to noise ratio forhigh current sensing.Weak points i.e. are very sensitive to

stray magnetic fields. Even the earth’smagnetic field gives a 0.5A error. Strayfields could arise from nearby conductorsor contactor coils. Needs to be locatedvery close to the current carryingconductor, this can then lead to insulationand electrostatic screening issues.Controlling the drift of offset voltage withtemperature can be quite a challenge.Gain changes with temperature can be anissue.The gain calibration of the devices will be

very loose, as their output is verydependant on conductor position. Thismeans that in-process calibration probablywill be required.This technology may be worth a look if it

is necessary to measure reasonably high

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Figure 5: Hall sensingwith magnetic circuit

Figure 6: Effect of conductor passes through core

current cheaply and can tolerateimprecision.Open-loop Hall devices with magnetic

circuit (Figure 5) work on the sameprinciple as the above sensors but, veryimportantly, include a magnetic circuit

which concentrates the flux through theHall magnetic field sensor. This actionincreases the relative precision by afactor 5 to 10 and greatly improvesimmunity to stray magnetic fields.Advances in magnetic materials allow

the selection of core materials that offersharp saturation with little remanence andhigh saturation flux density. This can oftenresult in better than 1% linearity over therated current range. The selection ofthermally stable Hall devices allow apractical dynamic current range of 2.5decades, meaning a 100A sensor will beable to accurately measure down to200mA.In order to measure low currents, the

primary can be passed multiple timesaround the magnetic circuit (Figure 6).Strengths of this technology i.e. are

compactness with very low loss method ofmeasuring AC and DC currents;fundamentally simple low cost design;good immunity to stray magnetic fields;moderate quiescent current; good linearity;and temperature stable products. Mostopen-loop sensors have a moderate~25KHz frequency response, but deviceswith bandwidths up to 350KHz areavailable providing reaction times of~ 300ns and 90% primary current in~1µs. These sensors feature a very easyinterface with near rail to output voltage.Selected products have good signal tonoise ratio and are rated for the automotivetemperature range (-40 to 120°C).Weaknesses i.e. are thermal drift of the

quiescent voltage, and hysteresis may be aproblem for some high precision

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Figure 7: Closed-loopHall effect currentsensor

Raztec and the Solar FernThe Panasonic World Solar Challenge is abiennial, friendly competition of solar andenergy-efficient vehicles crossing betweenOctober 21 to 28 the Australian continentfrom Darwin to Adelaide in a 3000km trek.Now in its 20th anniversary year, teams camefrom across the globe, having researched,designed and built their own vehicles. Halleffect current transducers helped NewZealand’s challenger for the World SolarChallenge.

Energy management and conservation arecritical for the performance of solar poweredvehicles. Energy flow is measured bymonitoring voltage and current flow andRaztec Hall effect current transducersmeasure the current flow, as no loss isinduced and readings are not disturbed byvoltage in the current carrying conductor. Forexample, the preferred way to control motorpower is to operate under torque modecontrol. Depressing the accelerator pedaldemands an increase in torque from themotor in proportion to the angle of depression. To ensure that this process works consistently, the torque controllermust monitor motor current quickly and accurately, and compare this with the demand from the accelerator - motortorque being directly proportional to current flow.

applications. In some low current applications, Hall sensors cost more thanshunts or Cts. Some sensors may overheat if subjected to high frequencycurrents. Gain can change with temperature. A negative shift with increasingtemperature could cause problems if the sensor was used for overloadprotection. Low cost sensors may require in-process calibration as their gainand offsets are commonly not tightly specified.Open-loop Hall effect current sensors come in a variety of flavours. It is

important that specifications are carefully reviewed so that the mostappropriate sensor is selected. A reputable supplier can assist with theirimplementation.There is a good chance that there will be an open-loop sensor that fits all

your needs, but there are more options. It is possible to increase the currentrange of current sensors by shunting some current away from the sensor.However, this kills the frequency response as the high frequency componentof the current passes through the shunt, bypassing the Hall sensor.

Closed-loop hall effect current sensorsThese sensors work on a principle similar to the open-loop Hall sensors,

but include a feedback winding to null the flux in the core (Figure 7). GMRsensors may be used in place of the Hall element. Because of the servoeffect, excellent linearity results and gain become very independent oftemperature. The feedback coil and associated amplifier does add some costhowever.The strong points i.e. are good DC and AC accuracy over a reasonable

dynamic range. Low insertion loss; good frequency response (~100kHz);reasonable cost if a commitment made to ASICs; good immunity to straymagnetic fields and electric fields with suitable screening.Weaknesses, in turn, are dynamic range limited in practice by core hysteresis

and null drift with temperature; physically larger and costlier than open loopdevices. Also, high quiescent current particular when the primary current ishigh. This makes for self-heating which, in turn, limits their use to lowerambient temperatures. These factors make them less attractive for automotiveapplications.The performance of closed-loop sensors has stabilised, while that of open-

loop continues to progress with the introduction of new materials andcomponents. There are now few instances where closed-loop sensors areneeded rather than their less costly open-loop counterparts.

Closed-loop flux-gate current sensorsIf a compensating coil is added to a DCCT, its maximum operating range

is no longer limited by core saturation; the compensating winding beingexcited to maintain the mean flux in the core to zero. However, two flux-gate cores working in opposition are needed to prevent excitation energybeing transferred to the compensating winding (and the primary). If highfrequency performance is needed, a CT can be added and its output addedto the low frequency component from the flux-gate sensor. All this addscost and complexity to the sensor, but these sensors may offer the mostaccurate current sensing available.Strong points are i.e. very low hysteresis means that the sensors can

measure very low (~50mA) currents with a single primary turn; with aclosed loop compensating winding, hundreds of amps can be measured;very linear output over a reasonable dynamic range; very good signal tonoise ratio; and if a high frequency winding is included, their frequencyresponse is good.Weaknesses i.e. are cost driven by complex magnetics and electronics;

bulky; high quiescent current, particularly with high primary current, whichmakes for high self-heating which limits that applications to lower temperatureenvironments; the compensating winding carries a lot of copper so it isexpensive.

ConclusionSelecting appropriate current sensing technology is complex and there are a

great many options. Typical of engineering, most are imperfect. Continualprogress in open-loop Hall effect current sensor technology means that thepossible applications of these versatile devices continues to grow and displaceother more traditional devices.

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