Adaptive Driving Scheme for ZVS and Minimizing Circulating Current in MHz CRM Converters

The synchronous rectifier (SR) turn-off time should be accurately controlled to achieve zero voltage switching (ZVS) while minimizing circulating current in pulsewidth modulation converters operating at critical conduction mode. Traditionally, the zero-crossing of the inductor current needs to be detected, and the optimal SR turn-off time is set with reference to the zero-crossing signal. However, problems such as nonlinearity and time delay need to be dealt with, and these make the tuning process very time-consuming at high frequencies. Moreover, accurate zero-current detection is also challenging at high frequencies. This letter proposes an adaptive SR turn-off time tuning method for MHz power converters. The body diode conduction of active switch is detected to adjust the SR turn-off time. With the proposed method, the SR turn-off time can be tuned under various working conditions so that ZVS is achieved while circulating current is minimized. The proposed scheme is easy to implement and not sensitive to switching noise at high frequencies. A 500-W gallium nitride based boost converter is built to verify the proposed method, and the peak efficiency reaches 99% at 1 MHz.