Frontline Core Ideas | ON Semiconductor SiC modules provide new inspiration for reliable and efficient fast charging circuit design of battery swap stations

1. PFC

The function and power range of the charging circuit in the charging station are similar to those of the DC charging pile. It uses the AC power of the grid to charge the car battery. The power is also high-power fast charging that exceeds the OBC power, usually greater than 50kW. The part that connects to the grid is a three-phase active power factor correction circuit, hereinafter referred to as PFC.

PFC can maintain the phase relationship between input current and voltage and minimize the total harmonic distortion (THD) in the line/grid current. New designs now increasingly require bidirectional operation to achieve bidirectional flow of energy between the battery and the grid. There are currently three popular three-phase PFC lines: 6 switches, T-NPC, and I-NPC.

Figure 1 6-switch three-phase PFC circuit

Figure 2 T-NPC three-phase PFC circuit

Figure 3 I-NPC three-phase PFC circuit

6 The switch circuit is simple and directly supports bidirectional operation, but requires power devices with higher voltage resistance.

T-NPC or I-NPC can select active switching devices with lower voltages, but if bidirectional working capability is required, some of the diodes in the figure need to be replaced with active switching devices MOS or IGBT.

Reliability is of vital importance in automotive circuit design. The 6-switch three-phase PFC circuit has the simplest circuit design, which is even more obvious if it is a bidirectional design.

With 1200V SIC MOSFET already being used in automotive circuit design, this architecture is undoubtedly very attractive, especially in bidirectional design. The circuit design in the charging station needs to consider efficiency more, because the circuit operation time of its entire life cycle is very long, and the design that prioritizes efficiency can save more electricity and reduce electricity costs.

Therefore, silicon carbide devices are the preferred choice, especially for the PFC part that has high requirements for switch performance. Considering that the power of charging stations is often relatively large, the single-tube parallel solution faces relatively large design challenges, and its overall reliability design is also quite difficult. The module solution simplifies the overall design and has a huge reliability advantage.

Figure 4 ON Semiconductor silicon carbide module

ON Semiconductor's silicon carbide module has a relatively complete range of internal resistance options . The internal resistance range of 3mΩ-40mΩ is suitable for design selection of different powers. The half-bridge architecture is convenient for application in PFC and DC-DC circuits. Its excellent thermal management provides users with excellent overall heat dissipation performance , and the heat dissipation part has its own isolation characteristics, which greatly facilitates design and manufacturing. Compared with the discrete device solution, the reliability of the whole machine is greatly improved and the manufacturing difficulty is greatly reduced.

NXH003P120 and NXH004P120 are leading products using the new generation of M3S technology. Their extremely low internal resistance and parasitic parameters are very suitable for PFC needs, and they also have excellent switching performance and low on-resistance. They can help users achieve higher power output in non-parallel designs.

2. LLC and CLLC Between the PFC circuit and the battery is the DC-DC part, which usually needs to be isolated. The unidirectional design mostly uses LLC resonant power supply or phase shifted full bridge PSFB (Phase Shifted Full Bridge). The bidirectional design mostly uses CLLC or DAB (dual active bridge) . Today, taking LLC and CLLC as examples, let's talk about the application advantages of silicon carbide modules in the DC-DC circuit of charging stations.

The LLC circuit has good performance in all aspects, especially when it is near the optimal working point, it can achieve a very ideal working state, thereby obtaining a very high operating efficiency. LLC itself has bidirectional working capability (the secondary side uses active rectifier devices such as SIC MOSFET), but it cannot work in LLC mode when working in reverse, and can only work in a poor working point (LC series resonance mode), and the overall efficiency is low. CLLC (adding L2, C2 on the secondary side) can take into account both the forward and reverse working point designs.

Figure 5 Bidirectional CLLC circuit structure

Charging stations are generally required to be compatible with 800V and 400V batteries. The DC-DC secondary can be divided into two coils to correspond to 800V or 400V batteries through series or parallel connection. This can greatly reduce the designed load voltage range and achieve operation closer to the ideal working point, thereby improving efficiency and saving electricity costs.

Although LLC and CLLC are resonant power supplies, and their switching performance requirements for switching devices are not as strict as those of the PFC level, SIC MOSFET still has great advantages. First, the battery voltage is constantly changing during the charging process, and the primary side of the LLC cannot always operate near the ideal operating point, so the switching capability of the SIC MOSFET still has advantages. Secondly, the rectifier tube on the rectifier side requires good switching performance. In the circuit design where the charger works bidirectionally, both the primary side and the secondary side will be used as rectifier tubes in a certain mode. In the unidirectional design, the secondary side is always in the rectification mode. The body diode of the SIC MOSFET has good switching performance, which can better meet the needs of rectification work and reduce the impact of reverse recovery current during shutdown.

Figure 6 M3S technology brings significant efficiency improvement

ON Semiconductor's SIC MOSFET modules, especially the NXH003P120 and NXH004P120 developed based on the new generation M3S technology, provide excellent internal resistance and switching characteristics, as well as excellent body diode switching characteristics, which are very suitable for the DC-DC part of the charging station, especially the bidirectional design circuit. It can help users achieve high power output capability in a single module non-parallel design.

In short, in the design of high-power charging circuits, silicon carbide technology can achieve the goal of high efficiency, energy saving and high reliability (higher temperature resistance). Silicon carbide modules are more convenient for design and manufacturing, further improving the overall reliability of the product. In high-intensity commercial application scenarios such as charging stations, high efficiency and high reliability are even more important.

Figure 7 Simplification of design using 1200V SIC MOSFET

ON Semiconductor has different types of silicon carbide module products to adapt to the design of different circuit structures. The three-phase 6-switch PFC and LLC, CLLC structures introduced today are very suitable for 1200V half-bridge SIC MOSFET modules . It provides a simple circuit structure and natural bidirectional working ability. The third-generation M3S SIC MOSFET technology gives it efficient performance. It includes two models (NXH003P120M3F2PTHG/NXH004P120M3F2PTHG), which adopt standard F2 package and have excellent Rds(on). M3S technology is specially developed for high-speed switching applications and has excellent quality in switching loss, Coss and Eoss.

In addition, ON Semiconductor provides a new Elite Power simulation tool, which has achieved a technological breakthrough through the innovative PLECS model, which is applicable to both hard-switching and soft-switching applications (such as LLC and CLLC resonance, dual active bridge and phase-shifted full bridge, etc.). This tool can accurately present the working conditions of the circuit when using our EliteSiC product series.

Below is the internal structure and appearance of the F2 packaged SIC MOSFET module and the selection table of the half-bridge 1200V SIC module. These products can cover the power range of 25KW to 100KW, providing a simple design and high-efficiency, high-reliability fast charging performance.

Figure 8 SIC M3 MOS module F2 package half bridge example

Figure 9 SICMOS half-bridge module selection

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