A Development of Electro-Static Discharge Evaluation Circuit for Varistors and Zener Diodes

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1 A Development of Electro-Static Discharge Evaluation Circuit for s and Diodes Hitoshi Kijima, Koji Ochi Polytechnic University Ogawanshi Kodaira Tokyo JAPAN hkijima@uitec.ac.jp Abstract: - Surge Protective Devices (SPDs) such as varistors and zener diodes are used for both lightning protection and Electro Static Discharge (ESD) control. SPDs evaluations in the case of using for lighting protection are required to comply with IEC standard. When testing the devices for lighting protection purpose, a current having long rise time in the range of 1 to 10 µs are used. On the other hand, since there is no reference standard for SPD in the case of using for ESD control, SPDs are tested on the basis of IEC standard defines equipment testing only. However, surges caused by ESD produce current waveforms having an extremely short rise time of several ns. As a result, the testing of SPDs according to IEC is affected by the wiring between the surge generator and measuring circuit, making it difficult to evaluate the true characteristics of SPDs. To take into account this issue, we thought out our own ESD evaluation circuit consisting of a surge generator, a measurement circuit and SPD. Using this circuit, we performed investigations on the characteristics of the varistors and zener diodes for ESD control purpose. Key-Words: - SPDs (Serge protective devices) diode lightning protection ESD (Electro Static Discharge) 1 Introduction Surge Protective Devices (SPDs) such as varistors and zener diodes are used for both lightning protection and Electro Static Discharge (ESD) control [1]-[3]. SPDs evaluations in the case of using for lighting protection are required to comply with IEC standards [4]-[6]. When testing the devices for lighting protection purpose, a current having long rise time in the range of 1 to 10 µs are used. On the other hand, since there is no reference standard for SPD in the case of using for ESD control, SPDs are tested on the basis of IEC standard defines equipment testing only [7]-[16]. However, surges caused by ESD produce current waveforms having an extremely short rise time of several ns. As a result, the testing of SPDs according to IEC is affected by the wiring between the surge generator and measuring circuit, making it difficult to evaluate the true characteristics of SPDs. To take into account this issue, we thought out our own ESD evaluation circuit consisting of a surge generator, a measurement circuit and SPD. Using this circuit, we performed investigations on the characteristics of the varistors and zener diodes for ESD control purpose. 2 ESD testing circuit The countermeasure method against ESD can be divided roughly into two kinds. One is the method of preventing the excessive current pulse produced by ESD flowing into equipment through a circuit pattern etc. This method is to insert high impedance components such as a ferrite bead in series. Another countermeasure method is to eliminate the excessive voltage caused by ESD by installing low impedance SPDs such as varistors and zener diodes. When the electric discharge pulse by ESD is impressed, it is not indicated to the manufacture's ISBN:

2 data sheet of SPDs to what value overvoltage can be held down. Moreover, even when the data sheet is offered, based on IEC standard, it is measured using commercial ESD test equipment. By this standard, the impedance of a system of measurement is only 50-ohm regularity, and we have to measure the characteristics of the SPDS by soldering them to the connector of coaxial attenuator. However, a test result is influenced by the wiring between SPDs and ESD test equipment. And also, there are many possibilities to use impedance of not 50 ohms. Therefore, the circuit block was made by us in order to establish the test method which is not influenced by wiring etc. 2.1 The first trial production circuit From a viewpoint of reproducibility, the surge generating part, the device under test, and the measurement part was integrated. The first circuit made is shown in Fig. 1. In order to make the inductance component of a substrate small, wiring is taken as the shortest distance. However, a noise and overshooting had occurred in measurement of this circuit. V 95pF Fig. 1. First trial production circuit V out I out By inserting a low pass filter, it used decreasing a noise and overshooting. Cutoff frequency was calculated using equation (1). X 2ns/div, Y 10V/div, 40A/div Fig. 2. Voltage and current waveform 1 f = 2π RC (1) Frequency Band restriction was set to 21.14MHz considering a circuit constant. The final circuit which inserted high resistance and a noise filter is shown in Fig. 3. As shown in the photograph of Fig. 4, in order to reduce the inductance component of a capacitor, the stray capacity of the printed circuit board itself was used as a capacitor. Moreover, by insertion of a capacitor, the overshooting was made to reduce. It had been 16.6ns as a result of calculating rise time by equation (2). 2.2 The final circuit made as an experiment (1) Insertion of impedance High resistance value of 1k ohms was inserted in series, and it was considered as high impedance so that actually near measurement might be possible. Measurement voltage and an example of a current waveform are shown in Fig. 2. In order that high resistance might work as dumping resistance, the noise decreased from the measurement voltage waveform. However, the noise and overshooting of measurement current did not decrease. (2) Insertion of a noise filter V f 30M T r 85pF = k (2) pF 47pF 2 2 Fig. 3 Final circuit 1k V out I out ISBN:

3 decrease rapidly if the current value increases the threshold level. α I = ( V / C) (3) C: constant, α : voltage nonlinear coefficient. Table 1. Measurement apparatus used High-voltage generator NoiseKen ESS-2002 Electrostatic Discharge Simulator Low-voltage power supply Kikusui Regulated DC Power Supply Pal135-10D Fig. 4. Photograph of final circuit The voltage and the current waveform at this time are shown in Fig. 5. As results of various measures, noise on voltage and a current waveform have been removed. X 400ns/div, Y 10V/div, 20A/div Fig. 5. Voltage and current waveform 3. Examination of Evaluation to Electric Static Discharge (1) Measurement apparatus used Measurement apparatus used is listed in Table 1. (2) Measurement samples Measurement samples are listed in Table 2. A varistor's electrical property is an exponential function. Until it reaches a fixed voltage value, its resistance value is very high. Then resistance value Oscilloscope Curve Tracer Tektronix TDS MHz 5Gs/s Four Channel Collar Digital Phosphor Oscilloscope IWATSU TT-507 Curve Tracer The varistor voltage listed in Table 2 is the voltage between terminals. voltage is defined when current value of 1mA flows. In the case of using a varistor, even current value increases, a voltage value remains almost constant value. Therefore we have to use this constant voltage corresponding to the peak current instead of varistor voltage as a residual voltage. The data sheet of a varistor's catalog indicates a residual voltage against lightning surge (several 10 microseconds), and does not support ESD (several ns). A varistor is divided roughly into two kinds. One is a ceramic varistor made from a metal oxide. The other is silicon varistors using PN junction of silicon. A varistor does not have polarity. It is predominant to a miniaturization and a price is cheap. Since a varistor's surge response is fast and the surge current capability is large, it is well used for the countermeasures against lightning surge. A B diode A diode B Table 2. Measurement samples () voltage Catalog Actual value (V) measurement (V) Capacitance Value (pf) ISBN:

4 voltage is the voltage between terminals in the steady state, after the current value of several mill amperes usually flows. The response time of a diode to ESD is fast. In order that zener voltage may start the tunnel effect or an electron avalanche to reverse bias, this voltage is considered to be residual voltage as it is. On the other hand, the disadvantages of a diode are as follows. 1 It is comparatively weak to ESD. 2 Since it has a polarity, it may become change of a substrate pattern. 3 The miniaturization is not progressing. Surge voltage and current waveform impressed in the case of using 's A and 's B have response delay. In the case of using varitor, resistance value has gradually decreases. Therefore the rise time of a varistor is late. As a result bigger voltage still remains. From the experimental result as listed Table 2, both varister voltage and zener voltage obserbed are indicated low values comprising with the values shown in catalog. (3) The contents of an experiment Using the experiment circuit made by us, 1.5kV is impressed to samples, and both the voltage and the current waveform are observed with an oscilloscope. From these waveforms, the current value and voltage are measured. 4. Experimental Result An output waveform in the case of no load is shown in Fig. 6. Peak voltage is 187V and rise time is 38 ns. Both a noise and an overshooting are not generated. X 400ns/div, Y 20V/div, 40A/div Fig. 7. Surge voltage and current waveform 's A X 400ns/div, Y 20V/div, 40A/div X 400ns/div, Y 50V/div, 40A/div Fig. 6. Surge voltage and current waveform impressed in the case of no load Fig. 8. Surge voltage and current waveform 's B ISBN:

5 Surge voltage and current waveform impressed in the case of using diode A and diode B have no response delay. In the case of using zener diode, the resistance value becomes very low when voltage exceeds zener voltage. Therefore the rise time of a zener diode is fast. These experimental results are collectively listed in Table 3. The residual voltages of varistors are in the range of V. The current values which flow into varistors are in the range of A. The rise times of varistors are in the range of ns. The residual voltages of diodes are in the range of V. The current values which flow into diodes are in the range of A. The rise time of diodes is 2 ns. It became clear that the residual voltages of diodes are smaller than those of varistors. And the rise time of diodes is faster than those of varistors. It means that performances of zener diode for ESD control are much better than those of varistors. X 400ns/div, Y 50V/div, 40A/div Fig. 9. Surge voltage and current waveform diode A A B diode A diode B Table 3. Experimental results Maximum voltage (V) Residual voltage (V) Rise time (ns) Current (A) which flows into samples (A) Fig. 10. Surge voltage and current waveform diode B 5. Conclusion Surge Protective Devices (SPDs) such as varistors and zener diodes are used for both lightning protection and Electro Static Discharge (ESD) control. Even the test method for lightning protection using these devices was established, the test method for ESD control has not yet being established. To take into account this issue, we thought out our own ESD evaluation circuit consisting of a surge generator, a measurement circuit and SPD. Using this circuit, we performed investigations on the characteristics of the varistors and zener diodes for ESD control purpose. Major results on these topics are as follows. (1) High resistance value of 1 k ohms was inserted in 50 ohms in series, and it was considered as high impedance so that actually near measurement might be possible. ISBN:

6 (2) As it is not influenced by the inductance component of a wiring, true characteristics evaluation for ESD can be achieved. (3) The rise time of a zener diode is fast and a varistor is late. In the case of using zener diode, the resistance value becomes very low when voltage exceeds zener voltage. Therefore the rise time of a zener diode is fast. On the other hand, in the case of using varitor, resistance value has gradually decreases. Therefore the rise time of a varistor is late. (4) When the performances of varistors for ESD control were compared with those of the zener diodes, it became clear that the zener diodes are much better devices for ESD control comprising with those of varistors. References: [1] H. kijima, Lightning surge response improvement by combinations of varistor and GDT. WSEAS Transactions on Power Systems, Issue 2,Vol.7 pp.60-69, 2012 [2] H. Kijima, T. Hasegawa, Electrical force analyzed results on switchgear, WSEAS Transactions on power systems, Issue 1, vol. 5, pp32-41, 2010 [3] H. Kijima, M. Shibayama, Circuit breaker type disconnector for SPD, WSEAS Transactions on power systems, Issue 5, vol. 4, pp , 2009 [4] IEC , Low-voltage surge protective devices - Part 11: Surge protective devices connected to low-voltage power systems - Requirements and test methods, 2011 [5] IEC , Low-voltage surge protective devices - Part 12: Surge protective devices connected to low-voltage power distribution systems - Selection and application principles, 2008 [6] IEC ,Components for low-voltage surge protective devices - Part 331: Specification for metal oxide varistors (MOV), 2003 [7] IEC Ed. 2.0:(b), Electromagnetic compatibility (EMC) - Part 4-2: Testing and measurement techniques - Electrostatic discharge immunity test, 2008 [8] White Paper 1: A Case for Lowering Component Level HBM/MM ESD Specifications and Requirements, August 2008, [9] White Paper 2: A Case for Lowering Component Level CDM ESD Specifications and Requirements, March 2009, [10] K. Wang, D. Pommerenke, and R. Chundru, Numerical modeling of electrostatic discharge generators, IEEE Trans. EMC., vol. 45, pp , May [11] W. G. Traa, Approach to improve ESDgenerator calibration and the realization of a simple discharge device for very wide band measurements, Int. Symp. Electromagnetic Compatibility, Zurich, Switzerland, [12] The JEOL material industrial meeting on semi conductive-ceramics, sectional meeting technical committee,:"how to use a varistor" Part 1-3" 2010 [13] Hirofumi Tanaka, Japan Printed Circuit Association: "A guide to a printed wired board for a new employee" 2010 [14] EMAS8301: The general rules about the ceramic varistor for electric devices [15] EMA8302: A disk form zinc oxide varistor's test method [16] Michael Chaine, James Davis, Al Keamey TLP Analysys of 0.125μm CMOS ESD Input Protection Circuit. EOS/ESD Symposium, 2003 ISBN: