2. Test setup for transfer function measurement on IGBT. 1. Introduction

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1 th Nordic Insulation Symposium on Materials, omponents and Diagnostics Wideband Transfer Function Measurements on IGBTs for Active Gate Driver Design and Transient Studies Tonny Wederberg Rasmussen Associate Professor /LO Technical University of Denmark Abstract IGBTs (Insulated Gate Bipolar Transistor) are used for power converters. For medium voltages about 3-6kV stacking of IGBTs is an interesting issue but lag of information from the data sheet makes it difficult to design active gate drivers [], []. For these reason measurements of transfer functions has to be done for different conditions in voltages and currents. In relation to this also the IGBTs reaction to applied frequencies and transients is investigated in different states. With the achieved information s a model of the IGBT and hereby converters for transient studies can be made. With these studies parasitic components and their behavior can be included in the models. Studies on passive components like D capacitors have been done in [3]. The paper describes the component theory in relation to higher frequencies. A measurement system (Hz -MHz) is designed and described to being used for lab measurements in order to verify the theory. The IGBT SKMGB3D from SMIKRON is used for the investigation. Results from the measurements are given and analyzes with respect to the theory are done.. Introduction The common used symbol of an IGBT is seen in fig. left. The collector current I is controlled by the gate emitter voltage as described in []. G U G Fig. Schematic and two models of an IGBT If U G is less than the threshold voltage U T no current flow in the component. For a larger voltage the current is given as () where the IGBT acts as a voltage controlled current generator fig. mid. II = GG (UU GGGG UU TT ) () In () G is the trans conductance. When U G is larger than the matching load current the IGBT goes into saturation mode. Here it behaves as a diode fig. right. The IGBT has also parasitic capacitance between the terminals as described for the diode. The high voltage diode is a three layer semiconductor as shown in fig.. The terminals for the Anode and I athode are connected with metal contacts to the semiconductor. This structure forms a capacitor with the semiconductor as dielectric material. A p + n_ n+ Metal contact Fig. Schematic of the diode with parasitic capacitor As seen in [] the capacitance highly depends on the voltage across the diode. As an example it changes between pf and pf for a voltage U RA between.3v and V for the diode FFHUS6S FAIRHILD. RURG6 from FAIRHILD is used in the rectifier setup for this paper.. Test setup for transfer function measurement on IGBT A principle of the schematic for the transfer function measurement on the chosen IGBT is seen in fig. 3. A D S U G A A3 R G I SH T R IM IM A I U D Fig. 3 Schematic of the measurement system T is the IGBT under test SKMGB3D from SMIKRON. It is driven from an analogue gate driver named A in series with R G = Ω the recommended gate resistor from the manufactory. The gate driver is build up around LM77 with an additional output stage.the input capacitance ies = 6.6nF for the IGBT forms a low pas filter with R G where the cut off frequency f =.6MHz is above the range of measurements. As well the U G is measured directly on the gate to eliminate the changes due to the cut off frequency. The D input gives a U G gate emitter voltage above U T the threes hold voltage of the IGBT and some voltage extra to set the collector current needed for the investigation. A is an applied voltage with given amplitude and a frequency that change between Hz and.mhz. This U G reate a D current together with an A current in the emitter. The U G is measured with the amplifier A3. The emitter current I pass a current shunt I SH. The current create a voltage across the shunt where the shunt forms an RL circuit. A compensation circuit R IM, IM with a time Technical University of Denmark, openhagen, Denmark. -7. June,

2 3 th Nordic Insulation Symposium on Materials, omponents and Diagnostics constant matching the RL in the shunt as seen in () correct the impedance. 3 measurement of current gain ττ = LL RR = RR IIII IIII () A voltage proportional with the current is picked up by A. A and A3 are designed with the LM77 and they need a similar transfer function in the operation area. The trans conductance can then be find by (3). GG(ff) = II (ff) UU GGGG (ff) (3) The calculation in (3) is done by a FRA frequencyresponse-analyzer from NL model PSM73. The instrument is set up as a phase gain measurement so the phase difference comes in degree and the trans conductance G in db as seen in () GG dddd = llllll II (ff) UU GGGG (ff) () This is recalculated and a correction to the current measurement is done by the verification from paragraph.... Verify amplifier A and A3 By a test amplifier similar voltages are supplied to A and A3. The difference of the two output voltage is seen in fig.. Deelta Gain / db measurement of difference in A and A Fig. Difference of output voltage from A and A3 Fig. shows that the difference between the two output voltages is less than 3mdB... Verify the current measurement A current generated with the amplifier model LPA from NL and a front resistor is feted through the compensated current shunt. A measurement of the current is done with TPA3, TP3 from TKTRONIX. This is supplied to the FRA together with the output voltage from A. Due to the shunt impedance R SH = mω a gain of G = db is given to the shunt current measurement. The transfer function is seen in fig.. Gain / db Fig. Transfer function of the current measurement It is seen in fig. that the measurement is correct within +.8dB -.db. This result is OK and is used to make correction of the current measurement from the IGBT. In order to save space the phase measurement is not shown for the description of the set up. 3. Measurement of transfer function The Transfer function of the IGBT is found from different settings of the U D voltage and the I D current in such a way that the loss in the IGBT does not become too large. 3.. hanges in collector emitter voltage For this measurement where the dependency of U is investigated a constant D current of I = 6.A is used. Table shows the used U voltage together with the low frequency trans conductance G low and the cut off frequency f. Table U G low f V.S 6.9kHz V 7.S.kHz V 9.S.kHz V 8.S.kHz 3V.8S.kHz In table G low is the low frequency trans conductance and f is the frequency where G is reduced to -3dB as described in () GG 3dddd = GG llllll () In the datasheet G =.7S at U = V. This match well the result G low from the measurement. G as a function of frequency is seen in fig. 6 below. Technical University of Denmark, openhagen, Denmark. -7. June,

3 th Nordic Insulation Symposium on Materials, omponents and Diagnostics Angle / degree G / S Fig. 6 Trance conductance G with U as a parameter and I = 6A Above khz the conductance decrease but independent on the voltage level. 3.. hanges in collector current For this measurement where the dependency of I,D is investigated a constant D voltage of U = 3V is used. Table shows the used I,D current together with the low frequency trans conductance G low and the cut off frequency f. Table I,D G low f.a.8s.3khz.a.6s 8.kHz 3.A 3.S 8.kHz.A 36.S.kHz.A.S 8.kHz 6.A.3S 6.9kHz As expected from the datasheet G increase with increasing emitter current I,D for small currents. G / S Angle / degree Fig. 7 Trance conductance G with I,D as a parameter and U = 3V For frequencies above khz the trans conductance decrease. This match the expected switching frequency of khz to khz for the IGBT D trans conductance G D Measurement of the trans conductance G D by using a triangle voltage to the gate is seen in fig. 8. The measurement is done by capture U G and I in time and then plot I against U G. Fig. 8 show the result for a triangle frequency of f tr = Hz. Ie(A) 8 6 Transconduktans Hz Uge(V) Fig. 8 Trance conductance G D at Hz. Again G D is increasing with increasing current. It is also seen that there are symmetry with increasing and decreasing current. In fig. 9 G D is seen with a triangle frequency of f tr = khz. Ie(A) 8 6 Transconduktans.kHz Uge(V) Fig. 9 Trance conductance G at.khz. When the frequency is increased to khz a hysteresis phenomenon is observed. The I is seen in fig.. Ie(A) 8 6 mitter current.khz x - Fig. urrent I e for a triangle U ge voltage. The slope of the current is different for increasing and decreasing current. From the data sheet rise time t r = 7ns and fall time t f = 7ns for I = 7A and gate driver voltages U g = ±V. R g and eis forms an R circuit means that the change in voltage is three times larger for turn off than for turn on means that G is 3 times less for turn off than for turn on. 3.. Discussion The investigation shows that the IGBT behave different trans conductance G for low and high frequencies where in general the trans conductance G decrease with high frequencies. Also it is shown that G at higher frequencies has different values for turning on and off. Technical University of Denmark, openhagen, Denmark. -7. June,

4 th Nordic Insulation Symposium on Materials, omponents and Diagnostics When stacking IGBTs the U have to be controlled [] but this only involved small change in current that can be controlled with an active gate driver even for a trans conductance G = -S. There is a tendency that G increase for frequencies above f = khz but this have to be investigated further.. Test setup for measurement of transients in a diode rectifier The diagram of the rectifier used for the transient investigation is seen in fig.. A A I RHRG6 I I3 I BH u V +U I WIMA FKP n 6V = Fig. Single phase rectifier with fast diodes and a high quality smoothing capacitors The rectifier is built of four diodes type RHRG6 from FAIRHILD two capacitors one fast capacitor of nf type FKP from WIMA and an electrolytic capacitor of µf V from BH. To create transients discharging of a fast capacitor is used. This is done with a relay as shown in fig.. kw R Re U D Out WIMA MKP 3*. mf 3V Fig. apacitor with a relay to create a transient The capacitance is made with a parallel connection of three.µf series MKP from WIMA... A input voltage and currents With the rectifier loaded with 3Ω at the D side input voltage and current are delivered from the amplifier LPA. The result of input voltage U A input current I A and the dc voltage U D is seen in figure 3. -U D D Iac(A) Uac(V) Idc(A) Fig. 3 Voltage U A read U D green and current I A blue input for a frequency of Hz. Fig. 3 shows a normal operation for the low frequency of Hz. This is expected especially with these fast diodes. With a frequency of f = khz the operation changes as seen in fig.. Iac(A) Uac(V) Idc(A) x - Fig. Voltage U A read U D green and current I A blue input for a frequency of khz. The inductive behavior is now seen in the current and a larger forward voltage drop across the diode can be observed. Also a resonance is seen at the input voltage when the current becomes zero. The resonance is formed by the inductance between the amplifier and rectifier together with the capacitance of the diodes... Transients from A and D side Fig. shoves the result of discharging the WIMA capacitors from fig.. The voltage at the capacitor is V. The D voltage is V. I(A) I(A) x -6 Uac(V) Udc(V) x -6 Fig. Transient from the A side. Top input current I (blue) lectrolyte current I (green). Bottom input voltage (blue) and D voltage (green). V at the D side. Variable names come from fig.. Technical University of Denmark, openhagen, Denmark. -7. June,

5 th Nordic Insulation Symposium on Materials, omponents and Diagnostics 6 From fig. it is estimated that the current increase with 3A/µs. The voltage at the input increase fast to about 7V. A voltage of V gives an inductance given by (6) LL = UU dddd/dddd = VV 3AA/μμμμ = 333nnnn (6) Surprisingly nearly all current runs in the electrolytic capacitor. Dynamically the D voltage increase with V chowing an inductance of about L S = 66nH. Then the D side is pre charged to V. The result of the transient is seen in fig. 6. I(A) I(A) Uac(V) Udc(V) x x -6 Fig. 6 Transient from the A side. Top input current I (blue) lectrolyte current I (green). Bottom input voltage (blue) and D voltage (green). U = V at the D side. A similar result as in fig. is seen. The slope and the peak value of the current change do to the pre charging of the D side. Then the transient is given at the D side with a short circuited A side. The result is seen in fig. 7. Idc(A) Iac(A) Udc(V) Time(S) x -6 Input voltage. Input and output current Time(S) x -6 Fig. 7 Transient from the D side. Top input current D (blue) output current A (green). Bottom input voltage (blue) and D voltage (green). Short circuit A side. From the D side there are no forward voltages so the current slope di/dt increase to 6A/µs. With this frequency the inductance can with (7) be found to ff = LL = ππ LL (ff ππ) = (67kkkkkk ππ).μμμμ = nnnn (7) Here the frequency with damping is used. The estimation of the inductance is correct within % due to the tolerance of the capacitor. The reason for the small inductance is that an exact di/dt for the equation (7) is difficult to get from fig.. It is also seen from fig. 7 that no current is running in the A side. With components of a good quality the du/dt at the D side is very small this means that the parasitic capacitances in the diodes are so small that they do not contribute to a current transmission in the diodes. This is valid even that there are no voltages across the diodes so the capacitances have the maximum value..3. Discussion In the previous section it is seen that transients pass the diode rectifier without larger forward voltage drop and the belonging charge is stored at the capacitors. There is no significant voltage rise at the D side means that this is low inductive. A transient from the D side of the rectifier gives no contribution to the current at the A side for the reason that the parasitic capacitors in the diodes are small compared with du/dt at the D side when the current pulse charge the capacitors.. onclusions Measurements of the transfer function equivalent with the trans conductance G of an IGBT has been done. It is shown that G depends on the current level but do not depend on the voltage level. It is shown that G decrease for frequencies above f = khz. The G for high frequencies f = khz tends to increase but this has to be investigated further. The G for high frequencies is large enough for active gate drivers. At last it is shown that G is different for positive and negative current slopes at higher frequencies f = khz. For rectifiers with fast diodes it is shown that transients in the range of MHz can pass the rectifier. With quality capacitors transients do not affect the D voltage and hereby this kind of disturbances does not pass to the A side through the parasitic capacitors in the diodes. 6. References [] T.W. Rasmussen, Active Gate Driver for dv/dt ontrol and Active Voltage lamping in and IGBT Stack, Proceeding on D, P, Dresten Germany, - September, 89 Technical University of Denmark, openhagen, Denmark. -7. June,

6 7 th Nordic Insulation Symposium on Materials, omponents and Diagnostics [] R. Voigt, K. Handt, M. ckert, Fully Digitized Quasi-continuous Working Gate-drive Unit for V-IGBTs, Proceeding on USB, P, Lille France, 3. September 3, number 78. [3] W. Z. l-khatib, J. Holbøll, T. W. Rasmussen, apacitor Performance Limitation in High Power Applications, 3dr Nordic insulation symposium, Tromdheim Norway 3. [] FFHUS6S FAIRHILD, Technical University of Denmark, openhagen, Denmark. -7. June,