Dynamic avalanching considerations in optimization of reverse conducting diode in IGBT modules

This paper presents a study of the optimization of reverse conducting diode in an IGBT module, in hard switching applications. The main focus is on studying the reverse recovery of various freewheeling diodes and to evaluate the performance of each diode to make an optimum selection of the device which will ensure safe operation of the IGBT module. Results of 2-D simulations, along with extensive experimental characterization are used to study the ultimate stress that can be imposed on a commercially available IGBT module. Simulations are used to theoretically predict the performance of three more types of diodes and a comparison is made with the performance of the commercially available diode, to make an optimum choice. 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 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.