OBC moves towards 22kW, how to choose the design solution?

OBC is an onboard charger. Whether it is a plug-in hybrid or pure electric vehicle, as long as there is a slow charging interface, an OBC is needed to perform slow charging, converting the AC power input from the AC charging pile into the DC power required for charging the power battery.

With the development of new energy vehicles in recent years, including the increase in battery pack capacity and the 800V voltage platform, OBC also faces new demands.

New demands facing OBC

In pure electric vehicles, the OBC power generally supports 3kW and 7kW in the past. This is because most AC charging piles use single-phase 220V input, and the power of mainstream AC charging pile products on the market is 7kW or less. Of course, there are also some 380V three-phase AC piles with a charging power of up to 40kW. BYD's early E5, E6, electric minibus and other models support 40kW AC charging, but 40kW AC piles are basically eliminated at present, and high-power public piles basically use DC.

On the other hand, the battery capacity of new energy vehicles is also getting bigger and bigger. For example, from the perspective of plug-in hybrid models, the battery pack capacity of plug-in hybrid models in the past was less than 10kWh, and the pure electric range was only tens of kilometers; now some plug-in hybrid or extended-range models can have a battery pack capacity of more than 50kWh, and the pure electric range exceeds 300 kilometers.

Pure electric models have also developed from battery packs of about 50kWh a few years ago to a level of more than 100kWh in exchange for a range of more than 700 kilometers.

Therefore, if 7kW AC slow charging is used, the charging time will be so long that it is unacceptable on some electric vehicles with large batteries. In fact, the OBC of new energy models on the market currently generally has a power of 6.6kW, 7.2kW or 3.3kW, which can only be used for emergency charging, or for long-term parking and charging under the condition of being equipped with home charging piles.

On the other hand, the battery pack voltage of electric vehicles is developing from 400V to 800V, and 800V battery packs are accelerating to popularize mid-range models. For OBC, in addition to converting AC power into DC power, it also needs to boost the voltage to charge the 400V battery pack. The 650V rated voltage power devices and other chips used in the DC-DC during the boosting process cannot be directly applied to the 800V architecture, so OBC also needs a new round of upgrades in the 800V era.

Some car companies also choose to abandon OBC directly, cancel the AC charging interface, and only support DC charging. This is considering that in the domestic charging environment, the penetration rate of DC charging piles in public piles is already quite high. According to data from the China Charging Alliance, as of July 2024, member units in the alliance have reported a total of 3.209 million public charging piles, including 1.431 million DC charging piles and 1.778 million AC charging piles.

However, except for mainland China and some European countries with a high penetration rate of electric vehicles, the coverage rate of DC charging piles in most parts of the world is low, which limits its widespread application. In addition, the cost of AC piles in home charging piles is also low. For the same 7kW AC home charging pile and DC home charging pile, the price of AC piles is nearly 50% cheaper.

This is because the DC pile essentially moves the AC/DC converter part from the OBC to the charging pile end, so this part of the extra cost is also transferred to the charging pile. Therefore, when users choose a home charging pile, unless the model does not have an AC charging port, they will inevitably choose a lower-priced AC pile.

Therefore, OBC is still the standard configuration for most new energy vehicles. In addition to the need for charging, OBC is also needed for external discharge. For example, when camping, it is necessary to output from the electric car to some electrical equipment, and the OBC needs to work in reverse to convert the DC output of the battery into 220V AC used by household appliances. OBC has

a high power density and high voltage trend, and the third-generation semiconductor is the first choice.

As mentioned earlier, as the battery capacity of electric vehicles increases, traditional OBCs below 7kW can no longer meet the demand. Therefore, some manufacturers are currently developing OBCs with a power of 11kW to 22kW for pure electric models with large batteries.

As the power increases, the size of the OBC will naturally increase accordingly, but in the space of the car, how to increase the power density of the OBC and reduce the size of the OBC is also one of the keys. In addition, while supporting power of 11kW to 22kW, it is also necessary to support battery voltages above 800V and support bidirectional output and other functions. In practical applications, heat dissipation management, device cost, electromagnetic compatibility, etc. are all problems that high-power OBCs need to face.

Especially with the trend of 800V batteries, the device selection of OBCs must first be able to operate safely under 800V voltage conditions, and secondly, the devices need to have higher redundancy to ensure that the operating conditions are below the highest withstand voltage or current of the device to ensure long-term reliability.

At IEDM 2023 last December, Infineon gave its technical roadmap in the OBC field: in 2020, the power density of OBC will be about 2kW/L, mainly using silicon-based power semiconductors; by 2024, it will turn to SiC on a large scale, and the power density will be increased to 4kW/L; after 2025, GaN will be promoted to enter OBC, and the power density will be increased to more than 6kW/L.

Infineon's white paper mentioned that OBCs that support three-phase AC grid input and 800V battery voltage can use 1200V SiC MOSFETs and three-phase PFC with CLLC DC/DC resonant converters. SiC MOSFETs support the use of higher switching frequencies, which helps to make designs more compact and lighter. Innovative packaging that simplifies thermal management helps improve efficiency and heat dissipation, allowing designers to be more flexible in overall design. Like single-phase designs, three-phase designs can also be used in parallel systems to support higher power output, thereby helping to shorten charging time.

ON Semiconductor has also launched a 11kW-22kW bidirectional OBC solution, which also uses SiC devices. ON Semiconductor's OBC solution includes a boost-type three-phase PFC and a bidirectional CLLC full-bridge converter, using the EliteSiC 1200V APM32 power module, which is optimized for 800 V battery architecture and is more suitable for high voltage and power level OBCs. The APM32 series includes three-phase bridge modules for the power factor correction (PFC) stage, such as the NVXK2VR40WDT2 with 1200 V 40 mΩ EliteSiC MOSFET integrated temperature sensing.

Compared with discrete solutions, the APM32 module is smaller in size, has better heat dissipation design, lower stray inductance, lower internal bonding resistance, stronger current capability, better EMC performance, and higher reliability, which helps to design high-performance bidirectional OBCs.

In addition to SiC, OBC solutions using GaN devices can achieve higher power density, and many manufacturers are currently actively developing related products. According to TI data, using its GaN power devices can achieve CLLLC switching frequencies of more than 500kHz and PFC switching frequencies of 120kHz. At the same time, the integrated gate driver simplifies system-level design. The power density of OBCs using GaN can be higher than that of OBCs using SiC, and the system conversion efficiency is as high as 96.5%.

In conclusion:

Before charging infrastructure is widely used around the world, OBC will continue to exist in new energy vehicles for a long time, so it will still be a market with huge room for development. In this process, OBC will continue to develop in the direction of higher power density and support higher voltage systems.