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Measurement 39 (2006) 137–146 www.elsevier.com/locate/measurement

Measurement of the magnetic ﬁeld radiating by electrostatic discharges using commercial ESD generators G.P. Fotis, I.F. Gonos *, I.A. Stathopulos School of Electrical and Computer Engineering, Electric Power Department, High Voltage Laboratory, 9, Iroon Politechniou Street, 15780 Zografou, Athens, Greece Received 2 February 2005; accepted 17 October 2005 Available online 5 December 2005

Abstract The aim of this work is the investigation of the transient magnetic ﬁeld radiating by two diﬀerent commercial generators of electrostatic discharges. Measurements of the magnetic ﬁeld generated by contact electrostatic discharges have been conducted a few centimeters away from the discharge point. In this paper the current transducer, which is used for the measurement of the discharge current is not mounted on a grounded metal plate, but instead it is on an insulating material. With this aberration to the Standard a closer simulation to the magnetic ﬁeld produced by the electrostatic discharge generators on the equipment under test is obtained. It is proved by the measurements that each generator produces a diﬀerent transient magnetic ﬁeld, which results in a diﬀerent way on the equipment that is tested. Also, comparisons of the magnetic ﬁeld between the two generators and useful conclusions for the decrease of the magnetic ﬁeld are presented. 2005 Elsevier Ltd. All rights reserved. Keywords: Electrostatic discharge (ESD); ESD generators; Field probes; IEC 61000-4-2; Magnetic ﬁeld; Pellegrini target

1. Introduction Electrostatic discharge (ESD) is very common in our lives. The human body can be charged up to a few kV simply by walking on a carpet. The triboelectric eﬀect, as it is known creates electrostatic charge by contact and separation of materials. When the discharge takes place the discharge current may come up to a few Amperes. This makes clear that the electrostatic discharge may be destruc-

*

Corresponding author. Tel.: +30 2107723603; fax: +30 2107723504. E-mail addresses: [email protected] (G.P. Fotis), igonos@ieee. org (I.F. Gonos).

tive for electronic or integrated circuits, which are very sensitive to these currents although the ESD phenomenon lasts a few hundred nanoseconds. Therefore, the International Electrotechnical Committee (IEC) prescribed the 61000-4-2 [1] in order to deﬁne the procedure, which must be followed for the tests on electrical or electronic equipment against electrostatic discharges. A considerable amount of eﬀort has been made to study the current waveforms deriving from ESD and it has been proven that they are aﬀected by various factors. David Pommerenke [2,3] found that the current waveform is depending on the relative arc length, the charging voltage and the geometric characteristics of the metal piece through which the discharge takes place. Also, Masugi [4] has

0263-2241/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2005.10.009

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studied the stability of the ESD current waveforms for various speed discharges using multiresolution analysis. Fujiwara in a recent publication [5] proposed an equivalent circuit model for the analysis of the discharge current and useful conclusions for the factors that govern the currentÕs waveforms have been derived. To the measurement of the electromagnetic ﬁeld less attention had been paid until the end of 80Õs. Wilson and Ma [6] were the ﬁrst, who simultaneously measured the current and the electric ﬁeld during electrostatic discharges at a distance of 1.5 m, using a broadband, TEM horn antenna. During the last years many researchers have conducted the measurement of the electromagnetic ﬁelds associated with the ESD event. David Pommerenke [2] measured the electric and the magnetic ﬁeld at a distance between 0.1 and 1 m, for both air and contact discharges. He found that the magnitude of the magnetic ﬁeld strongly depends on the 1/R factor (R being the distance from the point where the ESD occurs), while the magnitude of the electric ﬁeld is decreasing for a time period after which increases. There have been also studies [7,8], where the ESD current waveform can be calculated by measuring the electromagnetic ﬁeld. In [9,10] the current waveforms and the produced electromagnetic ﬁelds have been investigated by taking into consideration correlated parameters to the ESD event, in order the repeatability of the ESD generators to be improved. At the High Voltage Laboratory of the National Technical University of Athens an important observation was made during the ESD tests that they were conducted the last decade. During the veriﬁcations of the ESD generators for the investigation of the discharge current they produce, it was noted that though their discharge current was in the limits that are deﬁned by the IEC 61000-4-2 these generators could give diﬀerent results at the same equipment under test (EUT). That was a sign that various EUTs could be aﬀected not only by the discharge current, but also by something else, which had a dramatic eﬀect on them. The measurement of the magnetic ﬁeld proved that each generator was producing diﬀerent waveforms not only at the maximum values, but also at the slope. Therefore, each generator was producing a diﬀerent electromagnetic ﬁeld, which was the reason for diﬀerent induced voltages to be produced, and the same EUT to pass the test with one ESD generator and to fail with another.

This fact was the reason that the electromagnetic ﬁeld produced by each generator had to be measured. The comparisons between diﬀerent ESD generators led to conclusions and to the need of limits for the values of the electromagnetic ﬁeld to be adopted, in order the reproducibility of the ESD tests to be possible. This also means that the construction of the ESD generators, with respect to the electromagnetic ﬁeld they produce must be uniﬁed and these limits to be followed. The aim of the present study is to investigate the magnetic ﬁeld radiating by contact electrostatic discharges for two diﬀerent types of commercial ESD generators, when the current transducer (Pellegrini target) is not mounted on a grounded metal plate, but it is instead on an insulating material. The purpose of this is a closer to reality investigation of the radiating magnetic ﬁeld, since the current transducer aﬀects to the electromagnetic ﬁeld, when it is mounted on a metal plate. The produced electromagnetic ﬁelds and their induced voltages on the equipment that is usually tested are produced without the presence of grounded metal plates, the presence of which alters the radiating electromagnetic ﬁelds. 2. The International Standard IEC 61000-4-2 The IEC 61000-4-2 [1] relates to equipment, systems, sub-systems and peripherals, which may be involved in static electricity discharges owing to environmental and installation conditions, such as low relative humidity, use of low conductivity carpets, etc. According to the IEC 61000-4-2 the electrostatic discharges can occur either as contact discharges or as air discharges, but the application of contact discharges is the preferred test method. The range of the test level voltages for the contact discharges is 2–8 kV and for the air discharges is 2–15 kV. It must be underlined that for the veriﬁcation of the ESD generators the discharges are contact discharges and not air discharges. The ESD generator must produce a Human Body Model (HBM) pulse [11] as it is shown in Fig. 1. The pulse of Fig. 1 is divided into two parts: A ﬁrst peak called as ‘‘Initial Peak’’, caused by a discharge of the hand (where there is the maximum current Imax) and a second peak, which is caused by a discharge of the body. The rise time of the initial peak is between 0.7 and 1 ns and its amplitude depends on the charging voltage of the ESD simulator.

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switch opens and the second closes and so the electrostatic discharge on the EUT occurs. According to the speciﬁcations of the Standard for the veriﬁcation of the ESD generators there are four parameters whose values have to be in speciﬁed limits. These parameters are: the rise time (tr), the maximum discharge current (Imax), and the current at 30 and 60 ns. As it is shown in Fig. 1 these two current values are calculated for a time period of 30 and 60 ns, respectively, starting from the time point, when the current equals to 10% of the maximum current. The limits of these parameters are shown in Table 1 and are valid for contact discharges only. 3. The dipole source model

Fig. 1. Typical waveform of the output current of the ESD generator [1].

Wilson and Ma [6] developed a model for the ESD discharge and they gave the analytical equations for both the electric and the magnetic ﬁeld. The ESD discharge was modeled by an electrically short time dependent linear source (dipole-dz), as it is shown in Fig. 3.

Fig. 2. Simpliﬁed diagram of the ESD generator [1].

Fig. 2 shows a simpliﬁed diagram of the ESD generator [1]. According to the Standard it consists of the charging resistor Rc (50–100 MX), the energystorage capacitor CS (150 pF ± 10%), the discharge resistor Rd (330 X ± 10%) and the EUT. The values mentioned have been selected according to the Human Body Model. The value of the energy-storage capacitor CS is representative of the electrostatic capacitance of the human body, while the resistance of 330 X has been chosen to represent the skin resistance of the human body. Also, in Fig. 2 two switches are depicted. When the ﬁrst switch is closed, the second is opened in order the capacitor to be charged. After the capacitorÕs charge, the ﬁrst

Fig. 3. The dipole source model.

Table 1 Waveform parameters Voltage (kV)

Imax (A)

Rise time (tr) (ns)

Current at 30 ns (A)

Current at 60 ns (A)

2 4 6 8

6.75–8.25 13.50–16.50 20.25–24.75 27.00–33.00

0.7–1 0.7–1 0.7–1 0.7–1

2.8–5.2 5.6–10.4 8.4–15.6 11.2–20.8

1.4–2.6 2.8–5.2 4.2–7.8 5.6–10.4

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The magnetic ﬁeld from an electric dipole source of length dz 0 , can be calculated by Eq. (1) in cylindrical coordinates (q, u, z): l dz0 iðz0 ; t RcÞ 1 oiðz0 ; t RcÞ þ dBu ðR; tÞ ¼ 0 sin h. ot cR 2p R2

the early stages of the development of the phenomenon. 4. Measurement system 4.1. Experimental set-up

ð1Þ By integrating Eq. (1) the magnetic ﬁeld strength is given by Eq. (2): H u ðq; z; tÞ ¼

d q 2p R 0 iðz ; t RcÞ 1 oiðz0 ; t RcÞ þ . ot cR R2 ð2Þ

R is the variable representing the distance from the source to the observation point, c is the speed of light, t is the time, d is the total length of the current path and l0 is the permeability of free space. Eq. (2) suggests that the magnetic ﬁeld is depending on two factors: (a) the current i(t), which dominates in the near ﬁeld zone and (b) the current derivative oiðtÞ , which dominates in the ot far ﬁeld zone. Pommerenke [12] analyzed the distance dependence of the magnetic ﬁeld. oi If the current derivative term does not dominate ot ¼ 0 the magnetic ﬁeld at the ground plane can be calculated by Eq. (3): H u ðR; tÞ ¼

I d pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ . 2pr 2 r þ d2

ð3Þ

Eq. (3) is valid when r and d have similar lengths. If r d or r d then the AmpereÕs Law is valid and Eq. (3) can be simpliﬁed as it is shown in Eq. (4): H u ðR; tÞ ¼

I . 2pr

ð4Þ

The explanation of the electromagnetic process with the charged human body in front of an EUT is quite easy to be explained. Before the breakdown an electrostatic ﬁeld between the ﬁngertip and the EUT has been developed. This ﬁeld depends on the distances, the geometry of the conducting bodies and the environmental conditions. Part of the concentrated electric energy is used for the ionization of the air. Due to the high electric ﬁeld the free electrons are accelerated and a discharge current occurs. Consequently, a part of the electric ﬁeld is converted into magnetic energy due to the existence of the current. An increase in the magnetic energy and a simultaneous decrease in the electric ﬁeld happen at

Fig. 4 shows the ESD current experimental setup. The current and the magnetic ﬁeld (H-ﬁeld) for various charging voltage levels were measured simultaneously, by the 4-channel Tektronix oscilloscope model TDS 7254B, whose bandwidth ranged from DC to 2.5 GHz. The electrostatic discharges were contact discharges and they were conducted using two SchaﬀnerÕs ESD generators. The experiment was made only for contact discharges, because there is a reproducibility problem for the air discharges; during the air discharges the produced electric arcs are diﬀerent and therefore the produced magnetic ﬁelds can be compared only if the electric arcs of the air discharges are the same. The ESD generators used were the NSG-433 and the NSG-438. In order the measurement set-up to be unaﬀected by surrounding systems, the experiment was conducted in an anechoic chamber. The generatorÕs capacitance was charged at +4 kV and the discharge electrode of the ESD generator used for the contact discharge measurements had a sharp point. The temperature and relative humidity were measured and found in the ranges 23 ± 2 C and 40 ± 5%, respectively. In order the current to be measured a resistive load was used, as the IEC deﬁnes. This resistive load (Pellegrini target MD 101) [13] was designed to measure discharge currents by ESD events on the target area and its bandwidth is ranged from DC to above 1 GHz. The probe that was used for the experiment was the loop probe of 3 cm in diameter of the HZ-11 set

Fig. 4. Experimental set-up.

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The discharge current can be given by the following equations: CVR ; Z0 C ¼ C CT C A ; I ESD RL þ Rb þ Z 0 C CT ¼ ¼ ; I0 RL

I ESD ¼

Fig. 5. The measurement points where the H-ﬁeld probe was placed.

of Rohde and Schwarz, which is consisted of ﬁve passive near ﬁeld probes. The probe was placed at various distances · (10, 20 30, 40 and 50 cm) and in two perpendicular directions at the horizontal plane from the discharge point, as it can be seen in Figs. 4 and 5. 4.2. Current’s reconstruction The equivalent circuit of the measurement system at DC analysis is illustrated in Fig. 6 and it includes the ESD generator, the current transducer and the oscilloscope [14]. In this ﬁgure RL, Rb and Z0 are the load resistance of the current transducer (CT), the backward matching resistance of the CT and the nominal input impedance (50 X) of the measurement system including the oscilloscope, respectively.

Fig. 6. The equivalent circuit of the ESD generator at DC analysis.

ð5Þ ð6Þ ð7Þ

where IESD is the amplitude of the discharge current, VR is the voltage measured by the oscilloscope due to the output current I0. C is a current conversion factor, CCT and CA are the conversion factors of the CT and the attenuator, respectively. The attenuator was 20 dB, which means that it attenuates the signal 10 times (CA = 10). Measuring the DC resistance of the Pellegrini target the values of RL and Rb can be found. Although available data of the target could be used this was avoided in order the measurement results to be more accurate. The DC load resistance of the target (RL) is the resistance between the inner electrode (disc) and the outer electrode of the CT. RL was found 2.005 X. The DC backward matching resistance of the CT (Rb) is the resistance between the input and the output of the inner electrode of the target. It was found that Rb was 48.246 X. The calculation of these two values (RL and Rb) was made by taking the average value of 20 measurements in order to minimize the measurement uncertainty. Taking all the above into consideration the voltage reading of 1 V at the oscilloscope corresponds to the discharge current of approximately 10 A. 4.3. Measurement reconstruction for the electromagnetic ﬁeld using the H-ﬁeld probes The H-ﬁeld probe, which was used for the measurement of the magnetic ﬁeld, was capable of measuring signals up to 1.5 GHz. The probe was a loop probe of 3 cm in diameter and it was containing a single turn, shorted loop inside a balanced E-ﬁeld shield. The loop is constructed by taking a single piece of 50 X, semi-rigid coax from the connector and turning it into the loop. When the end of the coax meets the shaft of the probe, both the center conductor and the shield are 360 soldered to the shield of the shaft as it is shown in Fig. 7. Thus a single, shorted turn is formed. A notch is then cut at the high point of the loop. This notch creates a balanced E-ﬁeld shield of the coax shield. The loops are thus matched to 50 X by the characteristic

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Fig. 7. H-ﬁeld probe of the HZ-11 set of Rohde and Schwarz.

impedance of the loop and shield structure. The loops highly reject E-ﬁeld signals due to the balanced shield. The probes for the H-ﬁeld measurement measure the derivative of the magnetic ﬁeld dB according to dt the Eq. (8): V 0 ¼ Aeq

dB ; dt

Fig. 8. Comparison of the time derivative of the B-ﬁeld for two diﬀerent ESD generators at 30 cm from the discharge point (charging voltage = +4 kV).

ð8Þ

where V0 is the sensorÕs output (in Volts) and Aeq is the equivalent sensorÕs area (in m2). Consequently, a measured voltage of 1 V with the loop probe of 3 cm in diameter equals to 1 ¼ 1414:71 T=s. By integrating the deriva7:0686104 tive of the magnetic ﬁeld either using the equation editor function of the Tektronix oscilloscope or the Matlab program the magnetic ﬁeld (Bu) or the magnetic ﬁeld strength (Hu) is obtained, where u is the azimuthal coordinate around the tip axis.

Fig. 9. Comparison of the time derivative of the B-ﬁeld for the NSG-438 ESD generator and for three diﬀerent distances from the discharge point (charging voltage = +4 kV).

5. Experimental results The magnetic ﬁeld was measured following the procedure as it was described previously, using the two ESD generators. The time derivative of the magnetic ﬁeld (B-ﬁeld or magnetic ﬂux density) for the two ESD generators at the same distance of 30 cm is illustrated in Fig. 8. It can be seen that the peak values of the time derivative of the B-ﬁeld are greater for the NSG-438. In Fig. 9, the time derivative of the B-ﬁeld for the NSG-438 ESD generator and for three diﬀerent distances is depicted. As the distance from the point, where the ESD takes place increases the time derivative of the B-ﬁeld decreases.

Integration of the time derivative of the B-ﬁeld gives the B-ﬁeld. Figs. 10–12 depict the magnetic ﬂux density for the two diﬀerent ESD generators at 10, 30 and 50 cm from the discharge point, respectively. Observing the graphs two useful conclusions can derive. The ﬁrst is that the magnetic ﬂux density decreases as the distance from the discharge point increases, something that is known from the theory of the electromagnetic ﬁelds. The second one is that by comparing the produced transient magnetic ﬁelds diﬀerences can be found. The NSG-438 produces higher B-ﬁeld than this one of the NSG-433. This means that the induced voltages from each one generator are diﬀerent and therefore,

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Fig. 10. B-ﬁeld produced at 10 cm from the discharge point by the two diﬀerent ESD generators (charging voltage = +4 kV).

Fig. 12. B-ﬁeld produced at 50 cm from the discharge point by the two diﬀerent ESD generators (charging voltage = +4 kV).

Fig. 11. B-ﬁeld produced at 30 cm from the discharge point by the two diﬀerent ESD generators (charging voltage = +4 kV).

Fig. 13. ESD current and H-ﬁeld for the NSG-433 ESD generator at 10 cm from the discharge point (charging voltage = +4 kV).

when an EUT is tested it may pass the test using one ESD generator and fail using another for the same charging voltage and following the same experimental procedure. As it was expected the magnetic ﬂux density is proportional to the current. Figs. 13 and 14 verify this for the ESD currents and the magnetic ﬁeld strength (H-ﬁeld) that the NSG-433 and NSG-438 ESD generators produce, respectively. The H-ﬁeld has been calculated by the equation: B ¼ l0 H ;

ð9Þ

where l0 is the magnetic permeability and it is equal to 4p 107 T m A1 . It can be concluded in both of the two ﬁgures that the H-ﬁeld follows the alterations of the discharge current with a slight delay, which can be explained as the time needed for the magnetic wave to travel to space. The ESD current has diﬀerent waveforms depending on the material and its dimensions, where it is mounted. Experiments where the Pellegrini target has been mounted on diﬀerent materials have been conducted at the High Voltage Laboratory of NTUA and in Fig. 15 three diﬀerent ESD currents using the

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Fig. 14. ESD current and H-ﬁeld for the NSG-438 ESD generator at 10 cm from the discharge point (charging voltage = +4 kV).

Fig. 16. Peak of H-ﬁeld for various distances from the discharge point using the ESD generators NSG-433 and NSG-438 (charging voltage = +4 kV).

the magnetic ﬂux density is proportional to the ESD current the magnetic ﬁeld in the case, where the target is mounted on the insulating material is lower than this when it is mounted on a grounded metal plane. The maximum values of the magnetic ﬁeld strength (H-ﬁeld) for both the NSG-433 and the NSG-438 are presented in Fig. 16. The distances that have been chosen for both of the two ESD generators are 10, 20, 30, 40 and 50 cm. The magnetic ﬁeld strength of the NSG-438 is higher than this of the NSG-433. It is obvious that Hmax decreases

Fig. 15. Comparison of the ESD current for 2 kV charging voltage and for three diﬀerent cases, where the target is mounted (contact discharge).

NSG-438 are depicted depending on the material, where the target is mounted for 2 kV charging voltage. These diﬀerent materials are a 1 m · 1 m grounded metal plate (horizontal position), a 0.5 m · 0.5 m grounded metal plate on the wall of the anechoic chamber (vertical position) and an insulating material. Observing Fig. 15, it can be concluded that when the target is mounted on an insulating material the currentÕs values are lower than these, when the target is mounted on a metal plane (either on the vertical or the horizontal position). This explains why the currentÕs values in Figs. 13 and 14 for 4 kV charging voltage are diﬀerent than these deﬁned by the Standard. Consequently, since

Fig. 17. Magnetic ﬁeld strength for the NSG-433 ESD generator at 20 cm from the discharge point but for two perpendicular directions on the horizontal plane (charging voltage = +4 kV).

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Fig. 18. Magnetic ﬁeld strength for the NSG-438 ESD generator at 20 cm from the discharge point but for two perpendicular, directions on the horizontal plane (charging voltage = +4 kV).

hyperbolically with the increase of the distance. It is proportional to the 1/R factor where R is the distance. During the measurements of the magnetic ﬁelds produced by the two ESD generators it was found that for the same horizontal plane, the same charging voltage and the same distance but at perpendicular directions from the ESD generator the produced magnetic ﬁeld was diﬀerent. This can be seen in Figs. 17 and 18 for the NSG-433 and the NSG-438, respectively. A possible cause for the differences of the produced magnetic ﬁeld at diﬀerent directions may be the circuitÕs construction, which produces diﬀerent magnetic ﬁeld around it. 6. Conclusions An experimental approach has been carried out in order the transient magnetic ﬁeld of the electrostatic discharge to be investigated. The transient magnetic ﬁeld produced by two diﬀerent ESD generators for positive discharges was measured, when the Pellegrini target was mounted on an insulating material. The comparisons showed that each generator produces a diﬀerent magnetic ﬁeld, depending on the ESD current. Due to this fact there are diﬀerent results when EUT are tested. Therefore, there is a need the next revision of the IEC 61000-4-2 to take into consideration this remark, in order the limits of the produced transient ﬁelds to be deﬁned and uniﬁed. Also, it was found that each ESD generator produces diﬀerent magnetic ﬁelds depending

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on the direction that the measurement is carried out. This means that there are diﬀerences at the produced magnetic ﬁeld not only from generator to generator but at the same generator as well. This means that depending on the orientation of the ESD generator the induced voltages are diﬀerent and therefore an EUT may pass the test with one orientation of the ESD generator and fail with another. It was also concluded that the magnetic ﬁeld is decreased as the distance from the distance point increases. This decrease is proportional to the 1/R factor where R is the distance from the discharge point. The measurement of the magnetic ﬁled produced by electrostatic discharges may have a very useful application. It is known that many explosions at the industry may come from ESD discharges. Therefore, a future development of a systems which will detect the number and the magnitude of the electrostatic discharges could be developed based on the measurement of the magnetic ﬁeld, since its behavior is proportional to the ESD current. Future work must include measurements for both the magnetic and electric ﬁeld for contact discharges as well for air discharges, in order useful conclusions to derive for the produced ﬁelds by the ESD generators. Furthermore, a computing method for the calculation of the electromagnetic ﬁeld radiated by electrostatic discharges must be applied and a comparison with the measured data to be made. Acknowledgements The project is co-funded by the European Social Fund (75%) and National Resources (25%)(EPEAEK II)-PYTHAGORAS II. G.P. Fotis is supported by a Ph.D study scholarship from the State Scholarships Foundation of Greece. References [1] IEC 61000-4-2: Electromagnetic Compatibility (EMC), Part 4: Testing and measurement techniques, Section 2: Electrostatic discharge immunity test – Basic EMC Publication. [2] D. Pommerenke, ESD: transient ﬁelds, arc simulation and rise time limit, Journal of Electrostatics 36 (1995) 31–54. [3] D. Pommerenke, M. Aidam, ESD: waveform calculation, ﬁeld and current of human and simulator ESD, Journal of Electrostatics (38) (1996) 33–51. [4] M. Masugi, Multiresolution analysis of electrostatic discharge current from electromagnetic interference aspects, IEEE Transactions on Electromagnetic Compatibility 45 (3) (2003) 393–403.

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