The half-bridge series resonant converter (SRC) has many advantages such as clamped voltage stress on the semiconductor devices, zero-voltage-switching of primary switches and small magnetizing current due to large magnetizing inductance. However, there is a severe problem that it has very high switching frequency to regulate output voltage, which results in high switching loss. Therefore, the conventional SRC has low conversion efficiency under light load conditions. To overcome these problems, this paper proposes a new SRC with
additional unidirectional switch and pulse width modulation (PWM). It uses frequency control under heavy load conditions like the conventional SRC. On the other hand, when the switching frequency reaches its maximum value under light load conditions, the duty ratio of main switches is reduced and one of additional switch is increased to regulate output voltage. As a result, low operating frequency leads to low switching loss. Moreover, the proposed converter features low conduction loss due to large magnetizing inductance and low core loss of transformer due to small peak-to-peak flux density. Therefore, the proposed converter has high efficiency under light load conditions. To confirm the operation, features, and validity of the proposed converter, an output laboratory prototype converter with 320-385V input and 55V/750W is built and tested.