A synchronous rectification featured soft-switching inverter using CoolMOS

For high voltage high power inverter applications, power MOSFET has not been widely used due to its slow body diode reverse recovery and high on-state voltage drop. This paper explores coupled magnetic type soft-switching technique to solve the body diode "hard" turn-off problem and uses the low on resistance CoolMOS to give low on-state voltage drop as well as synchronous rectification behaviour for reverse on-state period to achieve high efficiency power conversion. Detailed design consideration and optimization are discussed. A 2.5-kW CoolMOS based soft-switching inverter prototype is designed, fabricated and tested. Test results verified the above features.

I. Introduction

MOSFET has been mostly used in dc-dc converters in low-voltage applications. This is based on two distinct features. The first feature is that MOSFET on-drop is resistive, which is superior to IGBT in low voltages because its current related on-state voltage drop can be very low given sufficient chip area. The second feature is that the low voltage rating Schottky diode can be connected in anti-parallel to help solve the slow reverse recovery of body diode because Schottky diode has lower forward voltage drop and much faster reverse recovery characteristic than those of body diode. For high voltage high power applications, however, MOSFET is not favourable because of its high on-state voltage drop and unavailability of low voltage drop Schottky diode with high voltage rating. Therefore, high power application needs to find a way to reduce the MOSFET body diode reverse recovery loss.