“Tighter carbon dioxide emission standards and changing public and corporate opinions are accelerating the development of electric vehicles (EV) worldwide. This will bring huge growth to on-board chargers (OBC) in the next few years. According to the latest trends in the 2024 calendar year, the compound annual growth rate (CAGR(TAM)) is expected to be 37.6% or higher. For global cars that are designing OBC modules, improving system efficiency or defining new highly reliable topologies has become an urgent challenge.
Tighter carbon dioxide emission standards and changing public and corporate opinions are accelerating the development of electric vehicles (EV) worldwide. This will bring huge growth to on-board chargers (OBC) in the next few years. According to the latest trends in the 2024 calendar year, the compound annual growth rate (CAGR(TAM)) is expected to be 37.6% or higher. For global cars that are designing OBC modules, improving system efficiency or defining new highly reliable topologies has become an urgent challenge.
The simple power factor correction (PFC) topology for single-phase input AC systems (Figure 1) is a traditional single-channel boost converter. The solution includes a diode full bridge for input AC rectification and a PFC controller for improving the power factor of the load, thereby improving efficiency and reducing the harmonics imposed on the AC input power supply. The advantages of this popular PFC boost topology are easy design, low implementation cost and reliable performance. However, the conduction loss of the diode bridge rectifier is inevitable, which will not support bidirectional operation that allows vehicles to supply power to the AC grid.
Figure 1 Traditional PFC
The simulation data (Figure 2) shows that the input diode bridge dominates the power loss through the loss of all other components in the PFC block.
Figure 2 Power loss distribution in PFC
In order to improve the efficiency of the OBC system, 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 becoming more and more popular due to its reduced number of components, low conduction loss and high efficiency.
Figure 3 Bridgeless totem pole PFC
Due to the poor reverse recovery characteristics of the body diode, traditional silicon (Si) MOSFETs are difficult to operate in continuous conduction mode (CCM) in the totem-pole PFC topology. Silicon Carbide (SiC) MOSFET adopts a brand-new technology. Compared with Si MOSFET, it can provide excellent switching performance, shortest reverse recovery time, low RDS(on) and higher reliability. In addition, the compact chip size ensures low capacitance and low gate charge (QG) of the device.
Another challenge in designing OBC is the limited space allocated for modules in the vehicle. As power requirements and battery voltage increase, it becomes more and more difficult to design an OBC that meets the mechanical size requirements while providing the required output power. Engineers have to endure the trade-offs between power, size, and efficiency of the current technology used for OBC, but SiC is breaking down these design barriers. Engineers using SiC with higher switching frequencies can use smaller inductors and still meet their previous Inductor ripple current requirements.
The benefits of using SiC MOSFETs in OBC systems are the ability to switch at a higher frequency, increase power density, increase efficiency, improve EMI performance, and reduce system size. Now that SiC is widely used, engineers can use totem pole PFC in their designs to improve performance.
The newly released 6.6kW totem pole PFC for OBC evaluation board provides a reference design for the multi-channel interleaved bridgeless totem pole PFC topology. The design consists of an isolated high-current, high-efficiency IGBT driver (NCV57000DWR2G) and two high-performance SiC MOSFETs (NVHL060N090SC1) in each high-speed branch. In addition, the low-speed branch uses two 650V N-channel power MOSFET SUPERFET® III (NVHL025N65S3) devices, which are controlled by a single high-side and low-side gate driver IC (FAN7191_F085).
Figure 4. 6.6kW staggered totem pole PFC evaluation board
By configuring these high-performance SiC MOSFETs in a totem pole topology, the system can achieve 97% efficiency (typical value). The design includes hardware over-current protection (OCP), hardware over-voltage protection (OVP), and auxiliary power distribution system (non-isolated), without the need for another DC power supply to supply power to each circuit on the PFC board and control board. Provide a flexible control interface to adapt to a variety of control panels.
Figure 5. Block diagram of 6.6kW staggered totem pole PFC evaluation board