Totem-pole PFC and CLLLC topology for onboard chargers

Figure 2 shows a 3.3-kW OBC with a totem-pole PFC stage for the OBC PFC and a capacitor-inductor-inductor-inductor-capacitor (CLLLC) stage for the OBC DC/DC converter. A totem-pole bridgeless PFC improves efficiency by lowering the number of power devices in the current path, while enabling bidirectional operation (compared to a conventional bridge-based PFC). Implementations of totem-pole bridgeless PFC were previously limited to lower power levels only because the inherent body diode in silicon power metal-oxide semiconductor field-effect transistors was susceptible to high reverse-recovery losses under hard switching. With no such body diode within its structure, GaN power switches such as the LMG3410R050 from Texas Instruments have now made it practically feasible to implement multikilowatt totem-pole bridgeless PFC power supplies. Since GaN devices feature low output capacitance (Coss), they can be operated at high frequencies (100 to 200 kHz), which further allows using smaller inductor and thus shrinks the size of the passive components required in the totem-pole PFC converter.

During dead time, however, third-quadrant operation in GaN switches results in additional losses that a real-time MCU needs to optimize by regulating the dead time precisely. C2000 real-time MCU type 4 PWMs enable features such as high-resolution dead time, which can regulate the deadband to 150 ps of resolution. As an example, for a 100-kHz totem-pole PFC, the loss savings with dead-time optimization is 1 W. As the designers reduce the inductor size further by increasing the switching frequency to 1MHz and employing control techniques such as Critical Mode PFC, the power loss savings can be as high as 10W, thus making optimization of third quadrant losses a critical feature, for which precise and accurate control of the dead time if required.