An auxiliary quasi-resonant tank soft-switching inverter

An auxiliary quasi-resonant DC tank (AQRDCT) inverter is presented for achieving soft-switching. An AQRDCT circuit is coupled directly across a DC bus to the inverter to generate a quasi-resonant DC bus voltage. It consists of two auxiliary switches connected in the common phase leg fashion as an auxiliary switch leg, two resonant capacitors connected in series as a resonant leg, a pair of DC capacitors connected in series as a DC tank, and a resonant inductor in series with the lower device of the auxiliary switch leg. The AQRDCT circuit resonantly brings the DC bus voltage to zero in order to provide a zero-voltage switching interval for the main inverter to switch, then quickly rebounds back to the DC tank voltage after the main inverter changes state. The auxiliary switches are turned on and off softly as well. The AQRDCT circuit absorbs only ripples of the inverter DC bus current, thus having less current stress. In addition, since the AQRDCT circuit is coupled in parallel with the DC power supply and the inverter for merely assisting soft-switching of the inverter without participating in real DC power transmission and power conversion, malfunction and failure of the tank circuit will not affect the functional operation of the inverter. Thus a highly reliable inverter system is expected. A 100 kW (150 kVA) continuous power AQRDCT inverter has been built, fully tested, and put into a 22-foot electric bus for traction drive applications. Tested results showed good efficiency, reduced EMI, and reliable operation. This paper describes the operating principle and control and reports the test results.