Variable frequency and constant frequency modulation techniques for GaN based MHz H-bridge PFC

Abstract

This paper explores two modulation techniques for power factor corrector (PFC) based on critical conduction mode (CRM) and proposes a new modulation technique which has the benefits of CRM and allows quasi constant switching frequency also. The converter is designed for MHz range switching frequency as high frequency reduces the size of the EMI filter. However at high frequency, the switching losses become the dominant losses making soft switching a necessity. CRM allows zero current switching (ZCS) turn-on but it's not able to achieve zero voltage switching (ZVS) turn-on when the input voltage is greater than half of the output voltage. To achieve ZVS turn-on over the entire mains cycle, triangular current mode (TCM) was proposed by Marxgut, C. et al.[1] but both these methods have the drawback of variable switching frequency. The new method proposed modifies TCM so that the switching frequency is quasi constant and ZVS turn-on is also achieved over the entire mains cycle. Based on analytical loss model of Cascode GaN transistor, the efficiency of the three modulation techniques is compared. Also, the parameters of the EMI filter required are compared based on simulated noise measurement. As variable switching frequency is not preferred in three phase systems, the quasi constant frequency approach finds its benefits in three phase PFC.