PFC efficency improvement and THD reduction at light loads with ZVS and valley switching

A PFC circuit's boost inductor and power MOSFET output capacitance resonate inevitably when the inductor current becomes discontinuous. The resonant current can be big enough to distort the PFC current significantly. Depending on the ac input voltage and the PFC output voltage, the MOSFET's Vds can resonate as low as zero volts. Proper control of the MOSFET to turn on at a Vds valley or zero voltage Vds can minimize switching loss and reduce PFC current distortion. A synchronous signal with a predicted noise blocking period is generated at the point where Vds rises from zero voltage. The synchronous signal is used to synchronize the PFC's PWM for reliable valley switching or ZVS. A 360W PFC circuit is used to validate the concept. Test results show at light loads a.c. current distortion is eliminated and overall efficiency is improved.