Soft-Switching dc-dc Converters

A class of dc-dc converters, known in the literature as soft-switching resonant converters, has been thoroughly investigated in recent years for its various attractive features. Soft switching means that one or more power switches in a dc-dc converter have either the turn-on or turn-off switching losses eliminated. This is in contrast to hard switching, where both turn-on and turnoff of the power switches are done at high current and high voltage levels. One approach is to create a full-resonance phenomenon within the converter through series or parallel combinations of resonant components. Such converters are generally known as resonant converters. Another approach is to use a conventional PWM buck converter, boost, buck-boost, Cuk, and SEPIC and replace the switch with a resonant switch that accomplishes the loss elimination. Because of the nature of the PWM circuit, resonance occurs for a shorter time interval compared to the full-resonance case. This class of converters, combining resonance and PWM, is appropriately known as quasi-resonance converters. In this chapter, our focus will be on the latter method, mainly using the resonance PWM switch to achieve soft switching. For simplicity, here we use the term soft switching to refer to dc-dc converters, quasi-resonance converters, and other topologies that employ resonance to reduce switching losses. Two major techniques are employed to achieve soft switching: zero-current switching (ZCS) and zero-voltage switching (ZVS). This chapter will focus on ZCS and ZVS types of PWM dc-dc resonant switches and their steady-state analyses.