The simple power factor correction (PFC) topology for a single-phase input AC system (Figure 1) is a conventional single-channel boost converter. This scheme includes a diode full-bridge for input AC rectification and a PFC controller to increase the power factor of the load, thereby improving energy efficiency and reducing harmonics applied to the AC input power supply. The advantages of this popular PFC boost topology are its simple design, low implementation cost, and reliable performance. However, the conduction losses of the diode bridge rectifier are unavoidable, and this would not support bidirectional operation of the vehicle supplying power to the AC grid. Using a multi-channel interleaved conventional boost converter, iterating the boost circuit multiple times, can improve some system performance parameters, but it cannot eliminate the input diode bridge.
Figure 1: Traditional PFC
Simulation data (Figure 2) shows that in the PFC block, the power loss of the input diode bridge is greater than that of all other components.
Figure 2: Power loss distribution in PFC
To improve the energy efficiency of OBC systems, different PFC topologies have been studied, including traditional PFC, semi-bridgeless PFC, bidirectional bridgeless PFC, and totem pole bridgeless PFC. Among them, totem pole PFC (Figure 3) is widely popular due to its reduced component count, lower conduction losses, and high energy efficiency.
Figure 3: Bridgeless Totem PFC
Traditional silicon (Si) MOSFETs struggle to operate in continuous conduction mode (CCM) in totem-pole PFC topologies due to poor reverse recovery characteristics of the body diode. Silicon carbide (SiC) MOSFETs, employing novel technology, offer superior switching performance, extremely short reverse recovery time, low on-resistance RDS(on), and higher reliability compared to Si MOSFETs. Furthermore, their compact chip size ensures low capacitance and low gate charge (QG).
Another challenge in designing OBCs is the limited space allocated to modules within a vehicle. With ever-increasing power requirements and battery voltages, designing an OBC that meets both mechanical size requirements and delivers the necessary output power is becoming increasingly difficult. Using current OBC technologies, engineers have had to make trade-offs between power, size, and energy efficiency, but SiC is breaking down these design barriers. Engineers using SiC with higher switching frequencies can use smaller inductors while still achieving the same inductor ripple current requirements as before.
The advantages of using SiC MOSFETs in OBC systems include higher switching frequencies, higher power density, higher energy efficiency, improved EMI performance, and smaller system size. SiC is now widely used, and engineers can use totem-pole PFCs in their designs to enhance performance.
ON Semiconductor's newly released OBC evaluation board with a 6.6 kW totem-pole PFC provides a reference design for a multi-channel interleaved bridgeless totem-pole PFC topology. The design includes an isolated high-current, high-efficiency IGBT driver (NCV57000DWR2G) and two high-performance SiC MOSFETs (NVHL060N090SC1) in each high-speed branch. Additionally, the low-speed branch utilizes two 650 V N-channel power MOSFETs SUPERFET III (NVHL025N65S3) controlled by a monolithic high-side and low-side gate driver IC (FAN7191_F085).
Figure 4: 6.6 kW staggered totem pole PFC evaluation board
Employing these high-performance SiC MOSFETs in a totem-pole topology, the system achieves 97% system efficiency (typical). The design includes hardware overcurrent protection (OCP), hardware overvoltage protection (OVP), and an auxiliary power distribution system (non-isolated) to power every circuit on the PFC board and control board without requiring additional DC sources. A flexible control interface accommodates a variety of control boards.
Figure 5: 6.6 kW staggered totem pole PFC evaluation board block diagram
