IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 6, NOVEMBER

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1 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 6, NOVEMBER DV =DT Related Spurious Gate Turn-On of Bidirectional Switches in a High-Frequency Cycloconverter Rajni Kant Burra, Student Member, IEEE, Sudip K. Mazumder, Senior Member, IEEE, and Rongjun Huang Abstract We identified a failure mode in a two stage dc/ac converter, comprising a high-frequency dc/ac inverter followed by an ac/ac cycloconverter, both operating at the same switching frequency. The failure-mode is a short-circuit condition, which is a combined effect of the reverse recovery of the MOSFET body diode and simultaneous spurious turn-on of the bidirectional switches of the cycloconverter, owing to a significantly high ( V/ns). A high causes appreciable current to flow through the gate-to-drain (Miller) capacitance, thereby producing a significant amount of voltage drop across the external gate resistance. Consequently, the gate-to-source voltage of the power MOSFET may exceed the threshold voltage of the device, which turns the device on. We explain the mechanism for the -related gate turn-on and present experimental results to validate the explanation. We also demonstrate, how a two-fold increase in the value of external gate resistance of the inverter switches (to reduce the applied to the cycloconverter) reduces the periodicity of the short-circuit condition. Index Terms Body diode reverse recovery, cycloconverter, failure mode, shoot-through, spurious gate turn-on. I. INTRODUCTION RECENTLY, forced cycloconverter based fuel-cell residential power systems have been proposed [1] [4] following the topological concepts outlined earlier in [5] [7]. Such a residential power system typically comprises a dc/dc boost converter (which interfaces to the fuel-cell stack to step up the stack voltage), followed by a dc/ac converter comprising a high-frequency (HF) inverter, a high-frequency transformer (for galvanic isolation), and a single-/split-phase ac/ac cycloconverter. The circuit configuration for one such single-phase ac/ac converter is shown in Fig. 1, where the HF inverter may have a full-bridge or a multilevel arrangement [4]. The advantages and operation of the dc/ac converter comprising the forced ac/ac cycloconverter and HF inverter are outlined in [1] [6]. In a recent experiment, while operating the dc/ac converter we observed a shoot-through mechanism in the ac/ac cycloconverter due to simultaneous body diode reverse recovery and induced spurious gate turn-on of one of its bidirectional switches. It results in a short-circuit condition across the secondary of the high-frequency transformer. Manuscript received May 18, 2004; revised March 11, This work was supported by the U.S. Department of Energy (DOE) under Award DE-FC2602NT Recommended by Associate Editor E. Santi. The authors are with the Laboratory for Energy and Switching-Electronic Systems, Department of Electrical and Computer Engineering, University of Illinois, Chicago, IL USA ( mazumder@ece.uic.edu). Digital Object Identifier /TPEL Fig. 1. Circuit configuration of the dc/ac converter comprising a HF inverter and ac/ac cycloconverter. The HF inverter can have a full-bridge or a multilevel arrangement [1] [6] of switches. Shoot-through during commutation, which is a common inverter-failure mode, usually occurs in voltage-source and current-source inverters. However, such a failure mode does not appear in cycloconverter-based inverters because of the reverse biased diode of the four-quadrant bidirectional switch [4]. We describe the experimentally-observed shoot through and present an explanation for this mechanism. Using the circuit model, we obtain simple expressions for the and gate-voltage, using which a designer can compute optimum values for device parameters, which ensure safe operation of the converter. Finally, we experimentally demonstrate the validity of our explanation. II. NORMAL MODE OF OPERATION OF THE AC/AC CYCLOCONVERTER Control signals and topologies for the four operating modes (for negative inductor current 1 ) of the cyclconverter are shown in Figs. 2(a), and 3, [4], [5]. The switches of the HF inverter are excluded for the sake of simplicity. The normal modes of operation discussed in this section assume that the MOSFET body diode is ideal. Mode 1 : During this interval, transformer primary voltage has a positive polarity (the drain of Q1 is positive with respect to the drain of Q4), the primary current is negative and the current in the filter inductor is negative. The power transfer to the load is negative. The current flows from the filter capacitor 1 Four extra modes exist for positive filter inductor current. Relevant waveforms are shown in Fig. 2(b) /$ IEEE

2 1238 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 6, NOVEMBER 2005 Fig. 2. Schematic waveforms showing the operation of the PCS. It shows Modes 1 through 4 (a) for negative output current and (b) for positive current. filter inductor current flow is same as in Mode 1. Therefore, the drain of Q1 is positive with respect to the drain of Q4 and the power flow to the load is positive during this interval. Mode 3 : During this interval, transformer primary voltage has the same polarity as in Mode 2 and the primary current is positive. The filter inductor current flows from the filter capacitor, through a path comprising the inductor, channel of the Q3, diode D4 and transformer secondary winding and back to the capacitor through the center-tap of the transformer. The power flow to the load is negative during this interval. Mode 4 : During this interval transformer primary voltage and current have a positive polarity and the inductor current flow path is same as in Mode 3. The power flow to the load is positive during this interval. III. SPURIOUS GATE TURN-ON Fig. 3. Four topological modes of the ac/ac cycloconverter., through a path comprising the inductor, channel of the Q2, diode D1 and transformer secondary winding, and back to the capacitor through the center-tap of the transformer. Mode 2 : During this interval transformer primary voltage and current have a negative polarity and the path of the During the course of experimentation, a spurious gate turn-on is observed when the cycloconverter transitions from Mode 3 to Mode 4. Fig. 4 illustrates the spurious turn-on of the bidirectional switch pair Q1 Q2 and using topological diagram we illustrate the failure mode. For clarity, the transition region is exaggerated. Interval : At the beginning of this interval Q2 is turned off and Q3 is turned on and the current in diode D1 abruptly falls to zero. Therefore, the load current now flows through Q3 and Q4. Meanwhile, the excess stored charge in the drift region of the body diode D1, diffuse out as a result of the carrier concentration gradient. The amount of excess charge remaining in the drift region at the end of this interval is a function of: a) the magnitude of the diode forward current; b) the length of the time

3 BURRA et al.: RELATED SPURIOUS GATE TURN-ON OF BIDIRECTIONAL SWITCHES 1239 shown in the Fig. 4(b). The current splits into and thereby charging the gate to source capacitance. During this process, if the gate voltage of Q1 exceeds its threshold voltage, the channel turns on thereby creating a conducting path through the diode D2 and the switch pair Q3 and Q4. This interval ends (at ) when the value of reduces to zero. Fig. 4(d), is an equivalent circuit of the converter topology during the interval. The input to this circuit is the. The bidirectional pair of conducting switches are represented by a short circuit and the nonconducting pair is represented by its off-state capacitances and resistances as shown in the figure. The values of instantaneous currents and voltages are obtained using the following equations: (1) The effective gate-source capacitance is usually an order of magnitude larger than and, therefore, could be neglected for all practical purposes. As such, (2) can be simplified to obtain the following: (2) (3) Fig. 4. (a) Schematic waveforms showing the dv=dt related spurious gate turn-on. The transition from Mode 3 to Mode 4 is shown exaggerated. (b) (d) Topologies showing the abnormal operating modes (b) body diode recovery as result of diffusion of stored charges (Mode 3)-interval (t 0 t ), (c) body diode reverse recovery phase because of reverse bias-interval (t 0 t ), and (d) equivalent circuit for the topology during spurious gate turn-on-interval (t 0 t ). interval ; and c) the minority carrier lifetime and recombination rate. For minority carrier lifetimes 100 ns, most of the stored charges recombine or diffuse out of the drift region. Interval : At time, commutation of the HF-inverter switches takes place. This results in a positive at the primary of the transformer and a reversal of voltage polarity at the secondary of the transformer. If, negligible amount of charge is present in the drift region then the length of the interval is very short and hence the charge dynamics can be ignored. However, if significant amount of charge is present then the MOSFET Q1 will not able to support the applied voltage and a huge reverse current will flow out of diode D1. At the peak of the reverse current sufficient amount of charge is swept out of the drift region and the body diode junction is capable of supporting the externally applied voltage. Interval : At the beginning of this interval, bidirectional switches Q3 and Q4 are on and Q1 and Q2 are off, and the secondary voltage of the transformer is negative, as shown in the Fig. 4(a). The positive results in capacitive charging currents and flowing into and, respectively. While charges, it discharges (and as such, D2 is ready to conduct by the end of this interval), as where is the gate-to-source voltage at which a MOSFET starts conducting and is the gate resistance of MOSFET Q1. In terms of the transformer secondary voltage, (3) can be rewritten as follows: where describes the equation for the HF-inverter switches S1 and S2 (during turn-off) and is the gate resistance, and is the magnitude of gate voltage in the Miller region at turn-off [8]. Interval : As shown in Fig. 4(d), at the beginning of this interval, the switch pair Q3-Q4 are conducting, the channel of Q1 is open, diode D2 is ON, the polarity of transformer secondary is positive and the capacitances and are charged. At the commencement of this time interval the charged capacitances and are rapidly discharged and short circuit current flows from top rail to bottom rail. A similar short circuit condition is also produced when the transformer secondary voltage polarity changes form positive to negative, resulting in spurious gate turn-on of Q4. Turn-on of Q4 results in short circuit current flowing from top rail to bottom rail. IV. FINITE ELEMENT SIMULATIONS In order to understand the internal MOSFET charge dynamics of the cycloconverter switches, a two-dimensional (2-D) finite element (FE) MOSFET simulation model is developed in ATLAS (SILVACO) [9]. The developed model is tuned achieve similar (4) (5)

4 1240 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 6, NOVEMBER 2005 Fig. 5. MOSFET structure used for simulation (a) schematic with a split source and (b) FE half-cell simulation model (showing the net doping concentration) used for mixed-mode SILVACO simulations. static and dynamic electrical performance as the switch used in the actual prototype. The tuned model is subjected to similar current, voltage, and conditions as observed in measurements. In order to observe the current contribution from the MOSFET channel and the body diode separately a split source structure is used. Such a structure is necessary to accurately comment on the real nature of the spurious turn-on mechanism. Fig. 5(a) is the schematic of the split source structure and Fig. 5(b) is the FE model used in the simulations. In the simulations, two extreme values of minority carrier lifetimes (100 ns and 1 s) were chosen in order to study the effect of reverse recovery on the spurious turn on mechanism. From Fig. 6 it can be seen that the reverse recovery definitely contributes to the shoot through current and the magnitude of the recovery current increases with increase in lifetime. The magnitude of the spurious turn on current is large in both the cases and is more pronounced withincreaseintherecoverycurrent(becausethereverserecovery increases). Also, MOSFET structures are stored at three different intervals during the course of simulation, to observe the current sharing mechanism between the MOSFET body diode and the channel. Fig. 7(c) is the simulation structure showing the shoot through current being shared by the body diode and the MOSFET channel. Such large reverse currents accompanied by rising can turn-on the parasitic bipolar transistor resulting in the destructive failure of the MOSFET. V. EXPERIMENTAL RESULTS AND DISCUSSION Fig. 8 is the experimentally observed shoot-through mechanism during the transition from mode3 to mode4. The gate voltage and current waveforms are shown for four switching cycles. Voltage and current spikes are observed for the second, third, and fourth switching cycles because the gate-voltage magnitude is greater than the threshold voltage. In the first switching

5 BURRA et al.: RELATED SPURIOUS GATE TURN-ON OF BIDIRECTIONAL SWITCHES 1241 Fig. 6. Simulation results showing the current contributions from the reverse recovery of the body diode and the shoot through current because of the MOSFET channel spurious turn-on (a) for a minority carrier lifetime of 100 ns and (b) for a minority carrier lifetime of 1 s. cycle, the gate voltage is lower than the threshold voltage; therefore, no current spike is observed. To verify, that the 2 was the cause of the short-circuit current, a simple test was conducted to study its effect of on the magnitude of the current spike. When the gate resistance ) of the HF switches is changed to 10 from 5, the short-circuit current spikes are eliminated to a great extent [as shown in Fig. 9(b)]. This establishes a clear link between the spurious gate turn on and the magnitude of. Table I, is a list of parameters for load conditions shown in Fig. 9, which is used to verify the spurious gate turn-on. The known parameters are obtained from the data-sheet and the circuit parameters used. The calculated parameters are obtained from the expressions developed in Section III. In Table I, 2 The dv =dt is controlled by controlling the switching speed of the HF switches. is the HF switch current, is the magnitude of gate voltage due to spurious turn-on, and is the value of the gate-source voltage in the Miller region during HF turn-off. Apart from the above mentioned reasons, if the gate driver circuit is not stiff enough the spurious turn-on mechanism can be aggravated.typically,theuseofattlbasedgatedriveroutputstages are known to induce a spurious turn-on. However, in our experimental design a gate driver with a CMOS output stage is used and the gate drivers are perfectly isolated from each other. Therefore, the gate driver induced spurious turn-on is not the reason for the shoot-through (as evident from the simulation results presented). VI. CONCLUSION Forced commuted cycloconverter based fuel-cell residential power systems have been proposed as a viable solution by several researchers. For the modulation scheme discussed

6 1242 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 6, NOVEMBER 2005 Fig. 7. MOSFET simulation structure showing the current density at three different intervals: (a) body diode (D1) forward conduction during Mode 1; (b) body diode (D1) reverse recovering during the beginning of Mode 3 [point ta Fig. 6(a)]; and (c) simultaneous reverse current contributions from the body diode and the shoot through current due to spurious channel turn-on [point tb Fig. 6(a)]. Fig. 8. Experimental results with circuit parameters v = 100 V, R = 5, and R = 20. (a) (top) i and (bottom) v for Q2. (b) (top) i and (bottom) v. Section II, a failure mode resulting in a shoot through current in the cycloconverter is observed. The observed shoot through current has current contributions from reverse recovery of the anti-parallel body diode and spurious turn-on of the MOSFET channel owing to a significantly high ( 2 10 V/ns). The reverse recovery current can be minimized to a large extent by using lifetime control techniques and shunting the MOSFET channel, during body diode forward conduction. However, the

7 BURRA et al.: RELATED SPURIOUS GATE TURN-ON OF BIDIRECTIONAL SWITCHES 1243 Fig. 9. Experimental results showing the effect of dv =dt on the short circuit current. (Top) transformer secondary current (i ) (a) (Top) transformer secondary current (i ) and (Bottom) output voltage. The parameters for (a) and (b) are v = 100 V, R = 5 ;R = 20 and v = 100 V, R = 10 ;R = 20, respectively. TABLE I LIST OF DEVICEPARAMETERS FOR THE LOAD CONDITIONS SHOWN IN FIG. 9 spurious gate turn-on can only be prevented by limiting the. Also, for high power applications, as the current rating of the cycloconverter switches increases, the effect of the Miller capacitance cannot be ignored and reduction in the applied becomes inevitable. REFERENCES [1] P. T. Krein and R. Balog, Low cost inverter suitable for medium-power fuel cell sources, in Proc. IEEE PESC 02, vol. 1, 2002, pp [2] Department of Energy. SECA Review. Tech. Rep. [Online]. Available: http.// [3] University of California at Irvine. Power Electronics. Tech. Rep.. [Online]. Available: http.// [4] R. K. Burra and S. K. Mazumder. Fuel cell power conditioner for stationary power system: towards optimal design from reliability, efficiency, and cost standpoint. presented at ASME 3nd Int. Conf. Fuel Cell Science, Engineering Technology. [Online] Available: [5] T. Kawabata, K. Honjo, N. Sashida, K. Sanada, and M. Koyama, High frequency link dc/ac converter with PWM cycloconverter, in Proc. IEEE Industry Applications Soc. Annu. Meeting, vol. 2, 1990, pp [6] K. Tazume, T. Aoki, and T. Yamashita, Novel method for controlling a high-frequency link inverter using cycloconverter techniques, in Proc. IEEE PESC 98, vol. 1, 1998, pp [7] I. Yamamoto, N. Tokunaga, Y. Matsuda, and Y. Suzuki, New conversion system for UPS using high-frequency link, in Proc. IEEE PESC 88, 1988, pp [8] R. Burra, MOSFET Switching under ZVS/ZCS, in Proc. Power Electronics Reliability Group 5th QuarterlyMeeting, Chicago, IL, Nov [9] ATLAS User Manual, Silvaco International, Santa Clara, CA. Rajni Kant Burra (S 03) received the B.Tech. degree in electrical engineering from the Indian Institute of Technology (IIT), Kharagpur, in 2000 and the M.S. degree in electrical engineering from the University of Illinois at Chicago (UIC), in 2003 where he is currently pursuing the Ph.D. degree. He is a Research Assistant with the Laboratory for Energy and Switching-Electronics Systems (LESES), and has published 13 papers in IEEE and ASME conferences. Since 2004, he has actively been involved in design and fabrication of inverters for fuel cell energy systems, and ripple mitigation techniques. His research interests include nonconventional energy systems, active power filtering, and power semiconductors. Sudip K. Mazumder (SM 02) is the Director of Laboratory for Energy and Switching-Electronics Systems (LESES) and an Assistant Professor in the Department of Electrical and Computer Engineering at the University of Illinois, Chicago. He has over 10 years of professional experience and has held R&D and design positions in leading industrial organizations. He has published over 50 refereed and invited journal and conference papers and is a reviewer for 6 International Journals. His current areas of interests are interactive power-electronics/power networks, renewable and alternate energy systems, and new device and systems-on-chip enabled higher power density. Dr. Mazumder received the IEEE PELS Award in 2002, the DOE SECA Award in 2002, the NSF CAREER Award in 2003, the Prize Paper Award from the IEEE TRANSACTIONS ON POWER ELECTRONICS, and the the ONR Young Investigator Award in He is an Associate Editor for the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS and the IEEE POWER ELECTRONICS LETTERS. Rongjun Huang received the B.E. and the M.E. degrees in electrical engineering from Jiaotong University, Beijing, China, in 1998 and 2001, respectively, and is currently pursuing Ph.D. degree at the Laboratory for Energy and Switching-Electronics Systems (LESES), Department of Electrical and Computer Engineering, University of Illinois, Chicago. In 2001, he was an R&D Engineer with Huawei Technologies, China. His research interests include wireless sensor, energy harvesting, and power electronics for renewable energy systems.