The terms “hard switching” and “soft switching” refer to the methods of switching based on the relationship of current and voltage during the turn-on and turn-off of the IGBT. Hard switching is a switching method that simply uses a device’s own ability.
Figure (a) shows a typical hard-switching current, voltage waveforms and its operating locus. During on-off transitions, both voltage and current are applied to the device. With hard switching, collector current and collector-emitter voltage change sharply, causing switching noise and loss. Hard switching is used for simple switch, motor drive inverter, and switched-mode power supply applications.
In contrast, soft switching uses an LC resonant circuit to turn on and off a device at zero current or voltage. Or the voltage and current switching timing is controlled in order to minimize the intersection of their waveforms. Figure (b) shows typical current and voltage waveforms of a soft-switched device and its operating locus. Soft switching helps reduce the switching noise and loss because switching devices turn on and off at zero or nearly zero voltage or current. Soft switching is commonly used for induction rice cookers, induction cooktops, and microwave ovens.
Soft switching has an added advantage over hard switching in terms of a safe operating area (SOA) as shown below.