Full-wave Investigation of EFT Injection Clamp Calibration Setup

Spartaco Caniggia

EMC Consultant Viale Moranti 7,

20010 Bareggio (MI), Italy spartaco.caniggia@ieee.org

Eric Dudenhoeffer TESEQ AG

Nordstrasse 11F, CH-4542 Luterbach Switzerland

eric.dudenhoeffer@teseq.com

Francescaromana Maradei Dept. of Electrical Eng., Sapienza Univ. of Rome

Via Eudossiana 18, 00184 Rome, Italy fr.maradei@ieee.org

Abstract The paper deals with a full-wave investigation of the capa-citive coupling clamp that is used in electrical fast transient (EFT) immunity test to inject disturbances to nominal sig-nals. The analysis is performed by using the software tool MicroWave Studio (MWS) based on the finite integration technique. The model of the injection clamp is validated by comparison with measurements and with the results ob-tained by SPICE. The full-wave model is used to investi-gate EFT injection clamp calibration setup, such as that proposed in section “6.4.2 Calibration of the capacitive coupling clamp” of the IEC 61000-4-4 new Edition (i.e., the 3rd).

Keywords Electrical Fast Transient/burst (EFT/B), finite integration technique (FIT), full-wave analysis, immunity, Microwave Studio, SPICE.

INTRODUCTION Electrical Fast Transient (EFT) immunity of electrical and electronic equipment is part of the EMC testing as regu-lated by the IEC 61000-4-4 Standard [1]. In this edition of the standard, the calibration of the EFT generator is pre-scribed, but the specifications provided for the coupling clamp are too general, and the wide range of values set for the injection clamp capacitance doesn’t assure a good re-producibility of measurements. At the moment, the sub-committee MT12 of 77B is work-ing on the preparation of a third edition [2]. This new edi-tion is taking care of two major issues that are not consi-dered in the version that is currently in charge: the calibra-tion of the coupling clamp, and the definition of a suitable cable layout for the test setup. The motivations that brought those two issues to the attention, are related to the need of reducing measurement uncertainties and move towards a good reproducibility of test results. In the past, SPICE circuits were proposed to model the EFT immunity test configuration of a cable inside the capacitive coupling clamp [3]-[4]. In this paper a full-wave model of the capacitive coupling clamp is used to investigate different EFT test configura-tions. The investigation is performed by the software tool MicroWave Studio (MWS) based on the finite integration

technique. The model of the injection clamp is validated by comparison with measurements and with the results ob-tained by SPICE. The numerical prediction tool is suitable for the following tasks:

1. Calibration of the capacitive coupling clamp; 2. Investigation of layout of cables in a test setup; 3. Investigation of immunity of different cables in the

design phase. In this paper, a full-wave model of the coupling injection clamp is used to investigate possible calibration configura-tions.

IEC 61000-4-4 SETUP FOR EFT IMMUNITY TEST ON I/O CABLES The setup configuration for testing the immunity of an equipment under test (EUT) when subjected to EFT on I/O data cables is described in [1] and shown in Fig.1. The EFT simulator generates repetitive fast transient pulses which are injected on the clamp. The EFT disturbance is coupled to the EUT by the capacitive clamp housing the cable. The coupling capacitance of the clamp depends on several cable parameters such as diameter, material, and shield (if any). The clamp unit is a test fixture consisting of two hinged metallic plates that is designed to provide a fixed and re-peatable capacitance between the output of the EFT genera-tor and the cable connecting the EUT to auxiliary equip-ment. The clamp shall be placed on a ground reference plane of minimum area of 1 m2 that is not shown in Fig. 1. The ground (reference) plane shall extend beyond the clamp by at least 0.1 m on all sides. The clamp shall be provided at both ends with a high-voltage coaxial connector for the connection of the test generator at either ends. To provide maximum coupling capacitance between the cable and the clamp, the clamp itself shall be closed as much as possible.

IDEAL WAVEFORM OF A SINGLE EFT PULSE The new edition of the standard that is still under discus-sion [2], will provide a mathematical formula for modeling the normalized ideal waveform of a single EFT pulse into a 50 Ω load. This ideal waveform has rise time tr=5 ns and pulse width tw=50 ns as nominal parameters. This will cer-tainly be a benefit for calibration labs as well as test gene-rator manufacturers.

978-1-4244-6307-7/10/$26.00 ©2010 IEEE 602

EUT Capacitive coupling clamp

Fig.1 – Test setup for testing the immunity of EUT subjected to EFT according to IEC 61000-4-4 [1].

The mathematical expression of a single pulse is:

EFT

2

EFT

11

EFT vEFT

1

( )

1

n

t

n

tvv t k e

k t

−τ

⎡ ⎤⎛ ⎞⎢ ⎥⎜ ⎟τ⎢ ⎥⎝ ⎠= ⋅ ⋅⎢ ⎥⎛ ⎞⎢ ⎥+ ⎜ ⎟⎢ ⎥τ⎝ ⎠⎣ ⎦

(1)

where kv is the maximum or peak value of the open-circuit voltage (kv =1 means normalized voltage), v1=0.92, τ1=3.7 ns, τ2=51 ns, nEFT=1.8, and kEFT is given by:

EFT1/1 EFT 2

2 1

EFT

nn

k e⎛ ⎞τ ⋅τ

− ⋅⎜ ⎟τ τ⎝ ⎠= (2)

The normalized voltage (1) is shown in Fig. 2.

FULL-WAVE MODEL of COUPLING CLAMP by MWS The configuration of the EFT setup for immunity testing is modeled by the software tool Microwave Studio (MWS) [5], based on the finite integration technique. The MWS model of the capacitive coupling clamp housing a coaxial cable is shown in Fig. 3. For simplicity, only the shield of the cable is included in the model. The disturbance on the inner wire can be then derived using the transfer impedance concept [3]. The near-end (NE) and far-end (FE) termina-tions of the cable shield, as well as those of the clamp are modeled by lumped elements. The EFT generator is mod-eled as a voltage source whose wave shape can be set equal to the mathematical expression (1) or to the measured EFT voltage at the output of the generator.

SPICE CIRCUIT MODEL of COUPLING CLAMP A simple SPICE model was developed and experimentally verified in the past to model the test setup shown in Fig. 1 [3]. The clamp equivalent circuit is obtained decomposing the structure into N cells, and cascading the corresponding N circuits which are given by constant-values RLC lumped elements. The values of the parameters can be derived by measurements, or by analytical and/or numerical calcula-tions [3]-[4]. This circuit model is valid as long as the cell

physical dimension is much lower than the minimum wave-length of interest.

Time (ns)

Normalized voltage

tw

tr

Fig. 2 – Ideal waveform of a single pulse into a 50 Ω load with nominal parameters (tr=5 ns and tw=50 ns).

Open

RNE

EFT voltage source 50 Ω

RFE

IShield

IEFT

Cable shield Cable shield

Clamp

Clamp Clamp

Fig. 3 – MWS model of capacitive coupling clamp housing a coaxial cable.

603

50 Ω CAPACITIVE COUPLING CLAMP

housing a COAXIAL CABLE

RNE

Open

Ccable,clamp=13.125 pF

EFT source

RFE

Cascade of N cells

Cable shield

Clamp

Ground

IEFT

IS,FE

Cclamp,ground=5.375 pF Lclamp=43.875 nH

Lcable=61.875 nH

Mcable,clamp=39.375 nH

IS,NE

Fig. 4 – SPICE circuit model of capacitive coupling clamp housing a coaxial cable.

VALIDATION OF MWS SIMULATION MODEL The full-wave model of the EFT capacitive coupling clamp is validated by comparing the results with those obtained by the SPICE circuit model described in [3], and by mea-surements. In the SPICE circuit shown in Fig. 4, the clamp is modeled by cascading 10 cells of length 12.5 cm, so that the model validity extends up to fmax=240 MHz. The two lossless TLs in Fig. 4 have characteristic impedance 150Ω and delay time 0.6 ns. The currents IEFT injected on the clamp, and IS,FE induced on the far-end cable shield, obtained in the frequency do-main when the cable shield is open at the near-end (i.e., RNE=∞) and short circuited at the far-end (i.e., RFE=0), are shown in Figs. 5-6. The comparison of simulations with measurements shows a good agreement of MWS results over the whole frequency range, while the circuit model gives satisfactory accuracy only up to about 200 MHz. A critical aspect when dealing with EFT injection clamp calibration and immunity test, is the closing of clamp: to provide maximum coupling capacitance between the cable and the clamp, the clamp itself shall be closed as much as possible. An improper closing provides lower coupling and an underestimation of EFT disturbances. How an improper closing affects EFT test is shown considering the two clamp geometries shown in Fig. 7. The shield current in frequency domain for the two clamp geometries obtained by MWS are shown in Fig. 8, where it is evident the influ-ence produced by the closing. The transient currents IEFT and IS,NE obtained by MWS and SPICE models when RNE=0 and RFE=∞ are shown in Fig. 9. These load values simulate a real condition where the EUT (shield of the cable connected to ground at 360º) is near the EFT source as shown in Fig.1. Note that the waveforms are

slightly different in maximum values because the circuit model reproduces the commercial clamp used in [3], while the MWS model of the clamp has not exactly the same ge-ometry since it represents a typical coupling clamp to be used for investigation.

90

70

50

400 100 200 300 400 500

Frequency (MHz)

Current IEFT (dBμA)

Measurements MWS SPICE

60

80

Fig. 5 – EFT current injected on the clamp.

90

70

50

400 100 200 300 400 500

Frequency (MHz)

Current IS,FE (dBμA)

Measurements MWS SPICE

60

80

Fig. 6 – Current on the cable shield.

(a)

(b)

Fig. 7 – Clamp geometries: improper closing with low coupl-ing (a), and suitable closing with high coupling (b).

604

90

70

50

400 100 200 300 400 500

Frequency (MHz)

Current IS,FE (dBμA)

High coupling Low coupling

60

80

Fig. 8 – Cable shield current for different clamp geometries.

-5

SPICE MWS

Current IEFT (A)

0 20 40 60 80 100 Time (ns)

7.5

5.0

2.5

0.0

-2.5

-5.0

(a)

-5

Current IS,NE (A)

0 20 40 60 80 100 Time (ns)

7.5

5.0

2.5

0.0

-2.5

-5.0

SPICE MWS

(b)

Fig. 9 – Transient EFT current IEFT injected on the clamp (a) and near-end shield current IS,NE obtained by SPICE and MWS (b).

DISCUSSION on SPICE CIRCUIT MODELS Neglecting the inductances of the clamp a simplified circuit model can be derived as shown in Fig. 10. The two capaci-tances are equal to the value of one section of the lumped

model of Fig. 4 multiplied by 10. The results given by the simplified circuit are compared with those obtained by the circuit model of Fig. 4. Transient currents IEFT and IS,NE are shown in Fig. 11. It can be noted that nevertheless the ca-pacitive coupling between clamp and cable dominates the EFT coupling mechanism, the results given by the two cir-cuits are significantly different. For an accurate modeling of the injection clamp, inductive effect must be taken into account and transmission line theory must be used. Howev-er, the simplified circuit can be useful to show quickly how the currents change with the value of the coupling capaci-tance. Coupling

capacitanceCcable,clamp

RNERFE

IS,FE50 Ω

OpenEFT source

IEFT

Cclamp,ground

IS,NE

Fig. 10 – Simplified SPICE circuit obtained neglecting clamp inductances.

-5

Current IEFT (A)

0 20 40 60 80 100 Time (ns)

7.5

5.0

2.5

0.0

-2.5

-5.0

SPICE Simplified SPICE

(a)

-5

Current IS,NE (A)

0 20 40 60 80 100 Time (ns)

7.5

5.0

2.5

0.0

-2.5

-5.0

SPICE Simplified SPICE

(b)

Fig. 11 – Transient EFT current IEFT injected on the clamp (a) and the near-end shield current IS,NE obtained by different SPICE models of coupling clamp (b).

605

Fig. 12 – Setup for calibrating the capacitive coupling clamp using a transducer plate.

INVESTIGATION of CALIBRATION SETUP of COUPLING CLAMP The definition of a calibration setup for the coupling clamp is an important issue that will certainly lead to a reduction of measurement uncertainties and a better reproducibility of test results. To this aim, the full-wave model of the coupl-ing clamp has been used to investigate possible calibration setups. One of the main issues that has taken the attention of the Working Group charged with definition of the new edition (the 3rd) of IEC 61000-4-4 [2], has been the choice of the cable (i.e., round or flat) to be used in the coupling clamp calibration. As previously shown by the example of Fig. 4, the use of round cables makes EFT test strongly dependent on a suit-able closing of the clamp. On the contrary, flat cables allow to improve the capacitive coupling with the clamp and re-duce the problems related to the suitable closing. A further advantage in using flat cable will be highlighted later in this section.

The MWS model is here used to investigate the calibration setup proposed in section “6.4.2 Calibration of the capaci-tive coupling clamp” of the new IEC 61000-4-4 edition [2]. The calibration setup shown in Fig. 12 consists in a trans-ducer plate of rectangular shape (i.e., flat cable), inserted into the coupling clamp, and a proper adapter is used as connection to the measurement terminator/attenuator. The transducer plate consists in a metallic sheet of 120 mm × 1050 mm of max 0.5 mm thickness, isolated on top and bottom by a dielectric foil of 0.5 mm. Isolation for 5 kV on all sides must be guaranteed in order to avoid the clamp to contact the transducer plate. At one end it is terminated by a 30 mm long wire to be contacted to the 50 Ω measure-ment terminator/attenuator.

The mathematical formula (1) adopting kv=4 kV, and the waveform measured by Teseq are used as excitation source in MWS model to compute the induced voltage on a 50-Ω load. The measurement setup at Teseq and the MWS model of the calibration configuration are shown in Figs. 13 and 14, respectively. The load voltage obtained numerically and experimentally is shown in Fig. 15, and the following comments can be done:

• Measurements and simulations highlight that the 50-Ω load voltage has a waveform that is very similar to that of the source, with a peak lowered by about 90%.

• To have very good agreement between measurements and simulations, the measured output waveform of the EFT source must be used as input (ASCII format) in MWS numerical model.

• Mathematical waveform for the EFT source can be used as reference for the clamp calibration also.

Fig.13–Measurement setup at Teseq.

Distance between clamp and flat cable=0.5 mm Thickness of all metallic parts= 0.5 mm Clamp:1m x 0.14mx0.10m Flat cable=1.050mx0.12m

Fig. 14 – MWS model of capacitive coupling clamp housing a transducer plate as in calibration setup configuration [2].

606

2000

1000

0

-5000 50 100 150 200

Time (ns)

Load voltage (V)

Measurements MWS with analytical EFT source MWS with measured EFT source

Fig. 15 – Load voltage obtained by measurements and MWS model in calibration setup configuration [2].

If a round cable is used in the calibration setup, the load voltage is that shown in Fig. 16. It can be noted that, in this case the waveshape is not so similar to the EFT generator, as in case of a flat cable. This provides one more good rea-son in favor of flat cable. In this case, in fact, the same ideal waveform defined for the EFT generator can be adopted as reference for the clamp calibration, with the on-ly exception to consider the amplitude reduced to about the 90%.

The advantages of using a flat cable in the clamp calibra-tion setup can be summarized as follows:

– The complete chain made of generator, cable, coupling clamp and coupling path is calibrated;

– The waveform specification are the same as at the gene-rator output.

– Maximum amplitude of the measured waveform on the 50-Ω load must be 900±90 V when the output level of the source is 2 kV without load, as required in [2].

The main disadvantage of the described calibration setup is that additional accessories must be defined: flat calibration cable and BNC adapter.

CONCLUSIONS A full-wave investigation has been proposed to study the capacitive coupling clamp that is used in electrical fast

2000

1000

0

-5000 50 100 150 200

Time (ns)

Load voltage (V)

SPICE MWS

Fig. 16 – Load voltage obtained by SPICE and MWS models in calibration setup configuration adopting a round cable.

transient (EFT) immunity test to inject disturbances to no-minal signals. The analysis has been performed by using the software tool MicroWave Studio (MWS). The model of the injection clamp has been validated by comparison with results obtained by SPICE, and with measurements. The full-wave model has finally been used to investigate EFT injection clamp calibration setup, such as that proposed in section “6.4.2 Calibration of the capacitive coupling clamp” of the IEC 61000-4-4 new Edition (i.e., 3rd).

REFERENCES

[1] IEC 61000-4-4 Standard – Electromagnetic Compatibility (EMC) – Part 4.4: Testing and Measurement Techniques – Electrical Fast Transient/Burst Immunity Test, 2nd edition, 2004.

[2] IEC 61000-4-4 Standard – Electromagnetic Compatibility (EMC) – Part 4.4: Testing and Measurement Techniques – Electrical Fast Transient/Burst Immunity Test, 77B/631/CD, 2010.

[3] S. Caniggia, L. Vitucci, M. Acquaroli, A. Giordano, “Measurement and SPICE Models for Data Signal Lines Under Electrical Fast-Transient Test”, 4th European Symp. on Electromag. Compat. - EMC EUROPE 2000, 11-15 Sep. 2000, Brugge (Belgium).

[4] F. Musolino, F. Fiori, "Modeling the IEC 61000-4-4 EFT Injection Clamp", IEEE Trans. Electromag. Compat., vol.50, no. 4, pp. 869-875, Nov. 2008.

[5] MicroWave Studio (MWS), Computer Simulation Technology, http://www.cst.com/.

[6] SPICE, Spectrum Software, www.spectrum-soft.com.

607