How does SiC promote on-board charging technology to move towards 800V?

Author: Kevin Keller, Product Line Manager, ON Semiconductor

There are a variety of vehicles on the market today that use different propulsion systems, including those powered solely by internal combustion engines (ICE), hybrid electric vehicles (xHEV) and pure electric vehicles (xEV) that use a combination of ICE and electric power systems . xHEV includes two different types of vehicles, namely mild hybrid electric vehicles (MHEV) and full hybrid electric vehicles (FHEV) .

MHEVs rely primarily on an internal combustion engine while integrating a small battery (usually 48V). However, MHEVs cannot run on electricity alone, and the electric motors are designed to help moderately reduce fuel consumption.

In contrast, FHEV offers greater flexibility as it can seamlessly combine an internal combustion engine with an electric motor, where the electric motor is powered by a battery (typically operating in the 100-300V range). FHEV can also use brake energy recovery technology to recharge the battery, using energy captured during braking to improve efficiency.

All xEVs, including plug-in hybrid electric vehicles and battery electric vehicles (BEVs), are equipped with regenerative braking systems. However, due to their larger battery capacity, these cars rely heavily on onboard chargers for charging.

Figure 1: A wide variety of electric vehicles exist today, including MHEV, FHEV, PHEV and BEV

The simplest way to charge is just about connecting an EV on-board charger to a wall outlet via a cable (usually requiring ground fault protection). Although this charging method is very convenient, most residential Level 1 systems (or SAE AC Level 1 as defined in the J1772 standard) operate at about 1.2kW and only add 5 miles of range per hour of charging ^[1] . Class 2 systems (or SAE AC Class 2) typically use polyphase AC power from the grid and are most commonly found in public buildings and commercial facilities. The power can reach up to 22kW, and one hour of charging can add 90 miles of range.

Whether it is a Level 1 or Level 2 charger, it provides AC power to electric vehicles, so the on-board charger is the key to converting AC input into DC output to charge the battery. Currently, most chargers deployed on the market are Level 2 chargers.

High-power DC chargers, often called Level 3, SAE Level 1 and 2 DC chargers, or IEC Mode 4 chargers, output a DC voltage that can charge batteries directly without the need for an on-board charger. These DC chargers range in power from 50kW to over 350kW and can charge to 80% of battery capacity in approximately 15-20 minutes. Although the number of fast charging stations is increasing rapidly, it is still relatively limited given the high power levels and the need for modifications to the grid infrastructure.

Many car manufacturers are currently changing from 400V batteries to 800V batteries. This shift is designed to extend the range of electric vehicles by increasing system efficiency, boosting performance, faster charging and reducing cable and battery weight.

On-board chargers are typically two-stage power converters , consisting of a power factor correction (PFC) stage and an isolated DC-DC converter stage. It is important to note that although non-isolated configurations are possible, they are rarely used. The power factor correction stage rectifies the AC supply to maintain the power factor above 0.9 and generates a regulated bus voltage for the DC-DC stage.

The market demand for two-way systems has increased significantly over the past few years. Bidirectional systems enable electric vehicles to provide reverse power flow from the battery to the power source to support various uses such as dynamically balancing grid loads (V2G: Vehicle to Grid) or managing grid outages (V2L: Vehicle to Load).

Traditional power factor correction methods involve using a diode bridge rectifier in conjunction with a boost converter. The rectifier bridge converts AC voltage to DC voltage, while the boost converter steps up the voltage. An enhanced version of this basic circuit uses an interleaved boost topology by paralleling multiple converter stages to reduce ripple current and increase efficiency. These power factor correction topologies typically employ silicon technologies such as superjunction MOSFETs and low Vf diodes.

With the advent of wide bandgap (WBG) power switches, especially SiC power switches, new design methods are enabled. This type of power switch has the advantages of lower switching losses, lower R _DS(on) and low reverse recovery body diode.

Bridgeless totem pole topologies are becoming increasingly popular in power factor correction applications at medium to high power (typically 6.6kW and above). As shown in Figure 2, in this topology, the slow bridge arm (Q5-Q6) switches at the grid frequency (50-60Hz), while the fast bridge arm (Q1-Q4) performs current shaping and voltage boosting, and Hard switching mode operates at higher frequencies (typically 65-110kHz). Although the bridgeless totem pole topology significantly increases efficiency and reduces the number of power components, it increases control complexity.

Figure 2: Bridgeless totem pole topology

The DC-DC stage usually adopts an isolated topology, using a transformer to provide isolation . The main purpose is to adjust the output voltage according to the battery's state of charge. Although a half-bridge topology is possible, dual active bridge (DAB) converter solutions are currently mainly used, such as resonant converters (such as LLC, CLLC) or phase-shifted full bridge (PSFB) converters. Recently, resonant converters, especially LLCs and CLLCs, have received much attention due to their several advantages, including wide soft-switching operating range, bidirectional operation capability, and the convenience of integrating the resonant inductor and transformer into a single power transformer .

Figure 3: Bidirectional DC-DC allows power to be returned to the grid during peak demand periods

For 400V battery packs, SiC 650V devices are usually preferred . However, for 800V construction, the higher voltage requirements require the use of devices rated for 1200V .

The reason why SiC is used in the field of vehicle chargers is its excellent performance in various figures of merit (FOM). SiC has advantages in terms of specific R _DS(on) per unit area , switching losses, reverse recovery diodes and breakdown voltage. These advantages allow SiC-based solutions to operate reliably at higher temperatures. Taking advantage of these outstanding performance features enables more efficient and lightweight designs. As a result, the system can achieve higher power levels (up to 22kW) that are difficult to achieve using traditional silicon-based solutions such as IGBTs or superjunctions.

Although the use of higher-power on-board chargers for electric vehicles may not directly affect the vehicle's driving range, it can significantly shorten the charging time and help solve the problem of range anxiety. In order to achieve faster charging speeds, the power of on-board chargers is constantly increasing. SiC technology plays a vital role in making these systems more efficient, ensuring efficient conversion of grid power and avoiding wastage of energy. The technology enables the design of more compact, lightweight and reliable on-board charger systems.

^[1] The mileage increased by one hour of charging is estimated based on the car’s energy consumption of 0.21kWh/mile or 13kWh/100km.