Estimation of Radiated Emissions of an Automotive

HV-Inverter in a Distributed System

D. Schneider1, M. Böttcher1, S. Tenbohlen1, W. Köhler1 1Institute of Power Transmission and High Voltage Technology (IEH)

University of Stuttgart, Germany

Abstract—EMC measurements according to CISPR 25 during

a component’s development process in the automotive industry

are expensive and time-consuming. In order to avoid these

disadvantages alternative measurement methods are desired.

One of such pre-compliance methods for estimating radiated

emissions is the transfer function method. With this method

radiated emissions can be calculated based on disturbance

current measurements on a component’s harness. This is possible

due to the correlation between disturbance currents and radiated

field. In this contribution the transfer function method is applied

to a distributed automotive system comprising an automotive

high voltage inverter, line impedance stabilization networks and

an electric machine emulation. In order to characterize the

radiated emissions of such a system a combination of two transfer

functions is used. The method of combined transfer functions will

be introduced and applied to a simplified setup showing its

applicability. In addition, an extended setup will be analyzed for

meeting specifications of the automotive industry.

Keywords—Automotive Inverter; CISPR 25; Distributed

System; Pre-Compliance; Radiated Emissions; Transfer Functions

I. INTRODUCTION

Cost reduction during product development processes is a main business goal. This also applies to the EMC development of automotive components. Due to the increasing density of electronic devices in modern cars more EMC measurements have to be carried out regarding susceptibility and radiation. Consecutive measurements are needed if limit values are exceeded. Measurements according to standards are mainly expensive, time-consuming and the availability of measurement facilities is limited. Hence, cost-efficient and early applicable EMC tools are needed during the development process. This demand can be satisfied by pre-compliance methods, which are more economically regarding measurement instruments and facilities. At the end of a development a validation according to CISPR 25 is still mandatory.

Electric drive systems bring a new challenge into the automotive EMC. Applied automotive high voltage (HV) inverters convert the HVDC of the traction network from the car’s HV battery into a three phase AC on the phase network for the drive machine. Due to fast slew rates of the IGBTs, those inverters are critical disturbance sources. This contribution introduces a pre-compliance test method for radiated emissions of an automotive HV inverter setup. The presented method is based on disturbance current measurements within the HV cables in a laboratory setup. It allows an estimation of the expected radiated emissions occurring during a component test in an absorber lined shielded enclosure (ALSE) according to CISPR 25 [1]. For this purpose

transfer functions (TFs) gained by measurement are used. TFs represent the correlation between disturbance currents in an ALSE setup and the related measurable electric field strength [2]. Needed TFs are maybe available from previous similar projects or have to be generated during the first measurement of a component in an ALSE. This procedure leads to a minimization of needed measurement time for iterative test series within the development process in an ALSE, which is directly linked to cost reduction. Previous investigations on the TF method in the automotive area use only none-distributed and unshielded setups comprising a device under test (DUT), a two wire cable harness and two line impedance stabilization networks (LISNs) as loads [3], [4], [5].

In the following, the TF method for distributed systems is explained and applied to a simplified setup with an automotive HV inverter showing its applicability in the frequency range up to 30 MHz. The system has shielded coaxial HV cables for the traction and the phase network. Furthermore, the setup includes an HV source, an HV LISN, a 12 V source and an electric machine emulation (EME), see Fig. 1. To comply with automotive industry specifications an extended system will be examined also.

Fig. 1. Block diagram of a simplified distributed system for an inverter test

setup based on CISPR 25

II. COMBINED TRANSFER FUNCTIONS

A. Principle of Transfer Functions

The radiated electric field can be calculated on base of field dominating common mode currents [6] with analytic models or TFs. Analytic models are only usable for simple setups. Modeling of complex setups with the coupling between setup and antenna, ALSE-characteristics, spatial current distributions along the whole measurement setup as well as diffraction and scattering is beyond practicability if performed analytically. Those aspects can be integrated into a calculation by TFs. A TF includes the correlation between electric field strength E(f) and

Inverter EME

Measurement

Table

EMI Test

Receiver

Traction

Network (TN)

Phase

Network (PN)

12 V

Monopole

HV

Source

HV LISN

EMC’14/Tokyo

Copyright 2014 IEICE

15P-H2

457

the according disturbance current I(f). TFs can be generated by current probe measurements on a cable harness and a field strength measurement with an antenna [5]. Therefore, the measurement setup of a component inside of an ALSE is excited by a tracking generator. The tracking generator is coupled to the harness instead of the DUT feeding the harness with a common mode signal. TFs can be calculated as:

( ) ( )

( ) ( )

( ) ( ) ( )

with the antenna voltage Uant(f), the current probe voltage UCP(f), the antenna factor AF(f) and the transimpedance ZT(f) of the current probe.

The estimation of a component’s radiated emission can be performed in a laboratory setup without an ALSE. For this purpose, the DUT is attached to the same setup as for the TF generation replacing the tracking generator. The disturbance current IDUT(f) has to be measured at the same position as for the TF determination. With the corresponding TF the radiated field strength Ecalc DUT(f), which appears in an ALSE, can be estimated by:

( ) ( )

B. Generation of two Combined Transfer Functions

This contribution focuses on an HV inverter setup consisting of two main branches: The traction network (TN), connecting the inverter with the high voltage source. And the phase network (PN), connecting the inverter with the electric drive machine. To represent this system with the TF method two independent TFs are needed, one for each branch. An inverter is modified in such a way that the traction network can be excited over its terminals T+ and T- as well as the phase network over its terminals U, V, W with the common mode signals ITN(f) and IPN(f) from a tracking generator, see Fig. 2 and Fig. 3.

Fig. 2. Setup for the generation of the TF for the traction network (TN)

For this purpose, the inner conductor of the feeding cable from the tracking generator can be connected over a feeding connector to bus bars, compare Fig. 4. The feeding connector is attached to the grounded housing of the inverter. No voltage sources are connected to the inverter during the generation process. Also, internal inverter electronics are insulated and not connected to the terminals T+, T-, U, V and W. Successive measurements of the resulting disturbance currents ITN(f) and IPN(f) plus the according field strength ETN(f) and EPN(f) have to be performed. With this information the TFs can be calculated as:

( ) ( )

( ) ( ) ( )

( ) ( )

( ) ( ) ( )

Fig. 3. Setup for the generation of the TF for the phase network (PN)

The radiated electric field of an active inverter setup is predictable on base of only two disturbance current measurements, one on the traction and one on the phase network. The application of the superposition principle in (5) leads to the calculated field strength Ecalc DUT(f). Due to the fact, the setup is electrically small [6] in the considered frequency range up to 30 MHz, phase information for the TFs and the disturbance currents is neglected. The low voltage (LV) cable harness is not yet considered by an own TF because of its observed minimal influence on the overall field strength.

( ) ( ) ( ) ( ) ( )

Fig. 4. Top view of the inverter: Interconnection with a bus bar using the

example for the phase network excitation over U, V, W

C. Verification of Combined Transfer Functions

For verification purpose of the method using two combined TFs, the traction and the phase network have to be excited simultaneously with a tracking generator (TG) as DUT. This is possible by connecting T+, T-, U, V and W over one bus bar to the inner conductor of the feeding connector. Again, no voltage sources are connected to the setup and internal inverter electronics are insulated from the terminals. This leads to the disturbance currents ITN TG(f) and IPN TG(f), see Fig. 5. The measurements of the occurring disturbance currents are carried out sequentially with a current probe on the same positions as during the TF generation. A monopole measurement of the electric field strength Emeas TG(f) serves as reference. Equation (5) and the currents ITN TG(f) and IPN TG(f) provide the verification result Ecalc TG(f), which has to be compared to the reference Emeas TG(f).

Feeding Connector

Bus Bar

V

U

W

T-T+

Measurement Table

EMI Test

Receiver

MonopoleTracking

Generator

ITN

EMEHV LISN

Inverter

ETN

T+

T-

Measurement Table

EMI Test

Receiver

MonopoleTracking

Generator

IPN

EMEHV LISN

Inverter

EPN

U

WV

EMC’14/Tokyo

Copyright 2014 IEICE

15P-H2

458

Fig. 5. Setup for the verification of two combined transfer functions (TF)

III. SIMPLIFIED TEST SETUP

The main element of the measurement setup is a modified pre-production sample of an inverter, see Fig. 6. The inverter operates usually with 280 V, but is adjusted by software to a measurement facility without an HV source. Only 12 V are needed for the internal control unit and a minimum of 20 V for the intermediate circuit voltage, which is applied at T+ and T-. Under those conditions the IGBTs switch without any additional communication being the main noise source. The operation mode of the inverter is set to a zero torque mode at an electric drive machine. For the power supply of the control unit a 12 V car battery is placed under the measurement table and is directly connected to the terminals 15, 30 and 31 of the inverter. This LV harness has a length of 1500 mm. It is placed directly on the metallic table perpendicular to the table edge for minimizing its influence onto the electric field strength. The inverter’s intermediate circuit is supplied from an HV LISN connected to a 24 V automotive battery placed under the measurement table. The inverter is equipped with N-type adaptors at its terminals T+, T-, U, V, and W. This allows measurements for the modeling of an emulation [7]. Another N-type connector is mounted to the body housing serving as feeding connector for an external signal source, as already mentioned. This is needed for the TF generation and verification. The used HV LISN and the EME are placed in shielded housings. Those and the used HV cables are equipped with N-type connectors for easy laboratory use. The shielded HV cables have a length of 1000 mm for the traction and 500 mm for the phase network. They are positioned 50 mm above the table and 100 mm away from its edge. The HV LISN consists of a printed circuit board comprising two LISNs according to CISPR 25. The EME is set up on a printed circuit board with three star connected 5 µH inductors. For the emulation of the parasitic capacitances between U, V, W and ground there are additional 10 nF capacitors at the inputs of the emulation to ground. The setup is placed in a shielded enclosure (SE) for gaining a good ground concept. It is used for a first test of the combined TF method on a simple setup.

As described in chapter II the traction and the phase network of the modified inverter are excited simultaneously with a tracking generator for verification. Fig. 7 shows the comparison between measurement and calculation. The curve shapes are almost identical. There are only minor deviations of less than 5 dB below 0.4 MHz and between 5 and 12 MHz. This result shows the applicability of the combined TFs for distributed systems in the frequency range up to 30 MHz.

Fig. 6. Simplified inverter setup in a shielded enclosure (SE)

Fig. 7. Comparison of reference measurement Emeas TG(f) to Ecalc TG(f) for

verification purpose in a shielded enclosure (SE)

After verification of the TFs, the method is applied to the active inverter supplied with 12 V on the LV harness and 24 V on the traction network. Therefore, the internal power electronic of the inverter is connected with the terminals of T+ and T- and the terminals U, V, W. The occurring disturbance currents are measured on the traction and the phase network as during the TF generation. As reference, the measured electric field strength Emeas Inv(f) is available. With the verified TFs and (5) the electric field Ecalc Inv(f) can be calculated as estimation. Fig. 8 shows the result. The calculation reproduces the reference measurement very good. Except between 0.3 and 2 MHz deviations of up to 10 dB can be observed. In this region internal structures of the inverter affect the result. Those structures are not yet implemented in the TFs. This result is sufficient for a pre-compliance test and can indicate critical frequencies.

Fig. 8. Comparison of reference measurement Emeas Inv(f) of the inverter in a

shielded enclosure (SE) to the calculation Ecalc Inv(f)

IV. EXTENDED TEST SETUP

The simplified setup is extended and placed into an ALSE to bring it close to today’s specifications of the automotive industry. Hence, the LV harness is positioned 100 mm away from and parallel to the table’s edge as well as 50 mm above the table [1]. Additionally, the LV harness is loaded with two LISNs, which are connected to a 12 V car battery. The used

HV LISNInverter

EME

Traction Network (TN)

Phase Network (PN)

LV-Harness

Measurement Table

EMI Test

Receiver

MonopoleTracking

Generator

(TG)

IPN TG

EMEHV LISN

ITN TG

Inverter

Emeas TG

0.15 1 10 300

10

20

30

40

50

60

70

f / MHz

E / d

Bµ

V/m

E

calc TG

Emeas TG

Enoise

0.15 1 10 300

10

20

30

40

50

60

70

f / MHz

E / d

Bµ

V/m

E

calc Inv

Emeas Inv

Enoise

EMC’14/Tokyo

Copyright 2014 IEICE

15P-H2

459

battery is situated on the table. The HV LISN consists of a shielding box including two LISNs according to CISPR 25. Due to the observed influence of the cables between HV LISN and the 24 V car battery on to the TF generation, the battery is placed into the shielding box of the HV LISN. Fig. 9 shows the realization of the extended measurement setup in an ALSE.

Fig. 9. Realization of the extended setup in an (ALSE)

The generation and verification of the needed TFs for this setup is performed as described in chapter II. For the generation an additional 30 dB amplifier is used after the tracking generator’s output. This is required to gain electric field strengths above noise floor below 1 MHz for each network. Fig. 10 shows the verification result of the extended setup. The curve trend of Emeas TG(f) is well matched by Ecalc TG(f). In the region between 4 to 20 MHz an offset of around 5 dB can be observed. Fig. 11 shows the calculation of the radiated emissions Ecalc Inv(f) of the active inverter compared to the reference measurement Emeas Inv(f). The calculation reproduces the tendency of the reference. An overestimation of up to 15 dB occurs between 0.15 and 3.5 MHz. The reason for this has to be clarified in future work. Beginning from 3.5 MHz, approximately 5 dB higher values are calculated. Nevertheless, the prediction gives a good overall impression of the disturbance, which can be expected.

Fig. 10. Comparison of reference measurement Emeas TG(f) to Ecalc TG(f) for

verification purpose in an absorber lined shielded enclosure (ALSE)

Fig. 11. Comparison of reference measurement Emeas Inv(f) of the inverter in an

absorber lined shielded enclosure (ALSE) to the calculation Ecalc Inv(f)

V. CONCLUSION AND OUTLOOK

The demand for alternative EMC measurement methods is constantly rising. One reason is cost reduction. Another is the request for measurements on a bench as early as possible. In this contribution a method is presented being able to calculate the electric field strength, which can be observed during a standard CISPR 25 measurement for automotive components. Here, the TF method is investigated and applied to a distributed system consisting of an automotive inverter, LISNs and EME. The TF method uses the correlation between the disturbance currents on the HV networks and the electric field strength. The method works on a simplified inverter setup with a deviation of mainly below 3 dB between reference measurement and calculation. Only between 0.3 and 2 MHz an offset up to 10 dB can be observed due to neglected internal elements of the inverter. These elements have to be considered in the future. The radiated emissions of an extended setup, close to specifications of the automotive industry, can be estimated as well. Deviations of 15 dB in the frequency range of 0.15 to 3.5 MHz occur. Above 3.5 MHz an offset of 5 dB appears. The presented measurements show the general utilizability of the introduced pre-compliance method with potential for cost reduction in the EMC development. A good estimation of the radiated disturbances can be obtained without the need of an ALSE. For a practical use of this method there has to be an examination whether applied EMC solutions like filtering or changed operation modes can be represented.

ACKNOWLEDGMENT

The authors would like to thank the Robert Bosch GmbH for funding this work and for the supply with measurement equipment and components. Special thanks to R. Eidher and S. Nishizawa from the Robert Bosch GmbH for their support.

REFERENCES EMC OLÉ

[1] I. E. Comission, "CISPR 25 Ed.3: Vehicles, boats and internal combustion

engines - Radio disturbance characteristics - Limits and methods of

measurement for the protection of on-board receivers," 2008.

[2] W. Smith and R. Frazier, "Prediction of anechoic chamber radiated

emissions measurements through use of empirically-derived transfer

functions and laboratory common-mode current measurements,"

Electromagnetic Compatibility, 1998. 1998 IEEE International

Symposium on, vol. 1, pp. 387-392, 1998.

[3] D. Schneider, M. Böttcher, B. Schoch, S. Hurst, S. Tenbohlen and W.

Köhler, "Transfer Functions and Current Distribution Algorithm for the

Calculation of Radiated Emissions of Automotive Components,"

Electromagnetic Compatibility (EMC EUROPE), 2013 International

Symposium on, pp. 443-448, Sept. 2013.

[4] D. Schneider, M. Böttcher, S. Tenbohlen and W. Köhler, "Pre-

Compliance Test Method for Radiated Emissions with Multiple Segment

Transfer Functions," Electromagnetic Compatibility, 2013 IEEE

International Symposium on, pp. 605-610, Aug. 2013.

[5] D. Schneider, S. Tenbohlen and W. Köhler, "Pre-compliance test method

for radiated emissions of automotive components using scattering

parameter transfer functions," Electromagnetic Compatibility (EMC

EUROPE), 2012 International Symposium on, pp. 1-6, Sept. 2012.

[6] C. Paul, Introduction to Electromagnetic Compatibility, New York: John

Wiley & Sons, Inc., 1992.

[7] M. Reuter, T. Friedl, S. Tenbohlen and W. Köhler, "Emulation of

Conducted Emissions of an Automotive Inverter for Filter Development in

HV Networks," Electromagnetic Compatibility, 2013 IEEE International

Symposium on, pp. 236-241, Aug. 2013.

HV LISN + 24V

Inverter

EME

Phase Network (PN)

LV Harness

LISNs + 12V

Traction Network (TN)

0.15 1 10 300

10

20

30

40

50

60

70

f / MHz

E / d

Bµ

V/m

E

calc TG

Emeas TG

Enoise

0.15 1 10 300

10

20

30

40

50

60

70

f / MHz

E / d

Bµ

V/m

E

calc Inv

Emeas Inv

Enoise

EMC’14/Tokyo

Copyright 2014 IEICE

15P-H2

460