Small duty cycle concept reducing the conduction losses in synchronous rectifier and its implementations

The small duty cycle concept can help reduce the RMS current of the synchronous rectifier for less conduction loss in the isolated DC/DC converters. However, the primary side conduction losses will increase with duty cycle decreasing in the conventional PWM DC/DC converter. Therefore, the parallel resonant in the secondary side is proposed to get small duty cycle while keeping high turns ratio for small primary current and losses. A family of quasi-resonant synchronous rectifier DC/DC converters implements the small duty cycle concept. A prototype of 1.8 MHz 48 V-to-1.2 V/25A DC-DC converter achieves 78% efficiency.