Impact of dynamic avalanching of reverse conducting diode in an IGBT module on optimum power converter design

This paper presents a study of an ultimate limit on the performance of power converters imposed by a freewheeling diode in an IGBT module, in hard switching applications. The main focus is on studying the reverse recovery of a freewheeling diode and to evaluate its impact on the performance of IGBT, diode itself and the circuit. An improved understanding of the internal charge dynamics is obtained by studying the reverse recovery of the freewheeling diode in an advanced two dimensional finite element device and circuit simulator. Simulated results, along with extensive experimental characterization provide the knowledge of the ultimate stress that can be imposed on the IGBT module, which determines the safe operating area of the device. It is shown that, in a hard switching circuit; the freewheeling diode may fail to recover instantaneously because of excess electric field generated inside the device. This large electric field generates an excess amount of carriers in the diode by impact ionization, which delays its reverse recovery. Under extremely stressful conditions, the diode may fail to recover leading to thermal runaway of the device, which implies that the IGBT module can be used only up to certain bus voltage that will keep the power dissipation in the module, under control.