Research on voltage controlled variable capacitor in the passive lossless snubber for soft-switching circuits

A voltage controlled variable capacitor in the passive lossless snubber for soft-switching circuits is presented in this paper to further reduce the turn-off loss of IGBT. This voltage controlled variable capacitor consists of two snubber capacitors and an auxiliary MOSFET, which is controlled by the voltage across the snubber capacitor and the auxiliary voltage source. Before IGBT is turned off, the control signal of MOSFET is high and it is turned on automatically. So both two snubber capacitors are connected in parallel with IGBT and the voltage controlled variable capacitor represents large capacitance. Then the voltage across IGBT can be greatly suppressed. When the control signal of MOSFET falls to the turn-off threshold, it is turned off and the voltage controlled variable capacitor represents small capacitance. The working principles and additional loss are analyzed in detail in this paper. At last, the prototype of Buck converter is built to verify these theories. ©, 2015, Chinese Machine Press. All right reserved.