A simple THD improving method for CCM boost PFC converter under mixed conduction mode operation

In this paper, a simple THD improving method for a CCM boost PFC converter is proposed. Due to its small inductor current ripple, small conduction loss, and small input filter, the conventional CCM boost PFC converter is widely used in medium to high power applications. But, although it is designed to operate with CCM, it starts to operate with DCM in light load condition. This mode is called the mixed conduction mode (MCM), and it degrades THD and PF. To solve this problem, the proposed method increases the switching frequency gradually near the zero-crossing of the line voltage to reduce the DCM region. Also, by decreasing the switching frequency near the peak of the line voltage the method minimizes the switching loss. The biggest advantage of the proposed method is that it is very simple and available to analog IC. The effectiveness is verified with a 60Hz/230Vac input and 800W/400V output prototype.