Power electronic converters serve as the standard interface between source and load in almost any electrical equipment. They have power semiconductor devices that are operated as ON/OFF switches at high switching frequencies. These switching devices are not ideal, however, and generate a considerable amount of switching losses that reduce converter efficiency. The main cause of these losses is the overlap of voltage and current that occurs whenever a switch transitions from being fully on or fully off, or vice versa. A significant reduction of these switching losses can be achieved if a converter switch is made to operate with “soft-switching,” with switching transitions that are gradual as opposed to sudden or hard. Soft-switching methods can generally be classified as being either zero-voltage switching (ZVS) with the switch voltage made to be zero during a switching transition or zero-current switching (ZCS) with the switch voltage made to be zero during a switching transition. Making either the voltage or current of a switch zero during a switching transition ensures the reduction of any overlap of voltage and current and thus the reduction of switching losses.

In this article, the basic principles of soft-switching for dc-dc power converters operating with high switching frequencies (>50 kHz) are reviewed. A number of soft-switching methods for simple single-switch converters such as the boost and buck converters are presented along with methods for more sophisticated dc-dc converters such as the forward, flyback, and full-bridge converters. Each method is described in some detail and its strengths and weaknesses are discussed. The article also presents some brief discussion of soft-switching methods for ac-dc converters and dc-ac inverters as well.