Study on Zero Crossing Detection in CRM Totem-pole PFC Converter

Abstract

The totem-pole PFC is a promising candidate for achieving high efficiency and high-power-density power factor correction. Critical conduction mode (CRM) or triangular current mode (TCM) operation of totem-pole PFCs has become popular for achieving zero-voltage switching (ZVS). This enables pushing the converter's switching frequency into the megahertz range to achieve high power density while maintaining high efficiency. Zero-crossing detection (ZCD) of inductor current is important for CRM/TCM operation. The ZCD signal helps synchronize the gate signals and turn off the synchronous switch at the appropriate current to achieve ZVS. A sensing resistor is typically used in conjunction with an amplifier and comparator to generate the ZCD signal trigger. The key issues associated with zero-crossing detection are detection delay and noise immunity. A delay in turning off the synchronous switch due to ZCD delay results in a large negative current and an overall increase in ripple current, reducing efficiency. ZCD delay is also variable, as it depends on the operating point, making it difficult to compensate for in the control algorithm. One of the main challenges of the ZCD circuit is managing common-mode noise caused by fast-switching GaN devices, which exhibit high dv/dt and di/dt. This noise can lead to false ZCD triggers, affecting PFC operation. First, this thesis investigates and characterizes sources of delay in ZCD, identifies the sources of variable delay, and examines the effect of sensing parasitic inductance. Based on an understanding of the delay mechanism, ZCD delay compensation techniques are discussed and compared. Second, the coupling mechanism of dv/dt switching noise as common-mode noise in the ZCD circuit is analyzed. Upon understanding this coupling mechanism, a shielding technique is discussed and demonstrated to mitigate common-mode noise issues in the ZCD circuit.