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Reply To: Power Electronic Riddle No.10 – Power inverter using IGBT


    A half-bridge voltage-fed IGBT inverter is shown in Figure, whereas Figure below shows the switching voltage and current waves when no snubbers are used (hard switched). Assume that the load is highly inductive and initially Q1 is off so that the lower diode D2 is carrying the full load current IL. When Q1 is turned on at t = 0, after a short delay time it starts to pick up the load current at full supply voltage (with a small leakage inductance drop) diverting D2 current. After D2 forward current goes to zero, current in Q1 consists of IL and D2 reverse recovery current (shown by the hump). When the recovery current is near the peak, the voltage of Q1 (VCE) falls to zero. When the turn-off gate signal is applied to Q1, its collector voltage begins to build up with a short delay at full collector current. When full voltage is built up across the device, D2 begins to pick up the load current. The short fall time (tf) and relatively long tail time (tt) of IC due to minority carrier storage of Q1 is shown in the figure. The SOA of the device is thermally limited like that of a MOSFET and there is no second breakdown effect. The conduction and switching loss curves, shown at the bottom of Figure, indicate that average switching loss will be high at high switching frequency. Note that the diode recovery current contributes significantly to the turn-on loss. Snubberless operation is possible but will cause high dv/dt and di/dt induced EMI problems. With a snubber, the turn-on di/dt and turn-off dv/dt will be slowed down, causing diversion of switching loss from the device to the snubber. A power integrated circuit is basically a monolithically integrated power and control circuit, sometimes with protection elements. Figure below, in contrast, illustrates a hybrid integrated circuit. Sometimes, a PIC is defined as an