Protection Strategy for Flying Capacitor Totem-Pole PFC under the AC Drop Transient

The AC line transients are inevitable during the operation of the power factor correction (PFC). For 2-level PFCs, such transients can often pass smoothly without protection, as the large inductance of input filters can store enough energy to withstand short-term abnormalities. However, for the flying capacitor multilevel (FCML) PFC with a significantly smaller inductance, AC line transients could lead to rapid over-current issues and ruin the voltage balance of flying capacitors (FCs), resulting in over-voltage across switching devices and endangering the integrity of the converter. Among all different kinds of AC line transients, the AC drop transient is the most challenging one, in which the grid-side voltage suddenly drops to 0 and last for a certain period of time. In light of this issue, this paper presents a strategy for fast protection and quick recovery of the FCML PFC under the AC drop transient. It also analyzes the oscillation dynamics of the flying capacitor voltages (FCVs) during the recovery process and subsequently calculates the safe operating threshold of the switching devices. Finally, through a five-level FCML totem-pole prototype, this paper experimentally validates the feasibility of proposed PFC protection and recovery strategy. Furthermore, the oscillation dynamics of FCV is also tested under dc/dc boost mode.