The maximum operating frequency depends on the type of hard or soft switching used. The high Eon losses of the switching devices keep the hard switching frequency below 100-200 kHz. For the soft-switch circuit, a 650 V SiC cascode FET is used at 500 kHz. A 1200 V FET can also be used from 200 to 500 kHz, although most high efficiency circuits use lower frequencies.