Analysis of on-board charger application in new energy vehicle disassembly

Consumers are concerned about two types of experiences with new energy vehicles: driving experience (power, comfort, entertainment, safety) and charging (speed).

1

Introduction to Vehicle OBC

From the perspective of products/systems, we look at OBC and its role in new energy vehicles. VMAX's OBC onboard charger, which was just launched this year, is a leading company in OBC and DC/DC.

The on-board charger OBC (On-Board Charger) is a component installed in new energy electric vehicles. It converts the AC power output by the AC charging pile into high-voltage DC power to charge the high-voltage power battery of the entire vehicle.

Image source: Mobility Foresight

The core parts of new energy vehicles can be divided into three parts: power battery, electric drive (controller, motor, reducer), and small three electrics (PDU+DC-DC+OBC). OBC is also one of the most critical aspects of electric vehicle design and its performance.

Image source: "Technical Research on Small Three-Electric System"

The OBC of new energy vehicles is divided into unidirectional OBC and bidirectional OBC. The circuit consists of a power circuit (PFC + phase-shifted full bridge/LLC) and a control circuit. The unidirectional OBC can only charge the power battery, while the bidirectional OBC can invert the DC power of the power battery into 220V AC power for household use.

Product Features:

• Rated output power: 6.6kW

• AC input voltage: 85Vac ~ 265Vac

• Maximum AC current: 32A

• DC output voltage: 230Vdc ~ 450Vdc

• Maximum output current: 22A

• Power factor: ≥ 0.99

• Peak efficiency: ≥ 94%

2

Vehicle OBC indicators

The components of OBC mainly have the following technical indicators:

Image source: OBC technical indicators, from the School of Electrical Engineering, Zhejiang University

Power level: The charging power of new energy vehicles in China and overseas is different. Common OBC charging powers are 3.3 kW, 6.6 kW, 11 kW and 22 kW. An 11kW OBC means that it takes 6 hours to fully charge a 66kWh power battery.

Conversion efficiency: Efficiency is an important goal and is closely related to how the entire unit dissipates heat.

Figure: Main design parameters of a 6.6KW bidirectional OBC from wolfspeed.

Volume, weight, and power density: Automobiles have strict requirements on the volume and weight of components, and the design requirements are relatively high. The current trend is to integrate DC/DC and OBC two-in-one, or to integrate DC/DC, OBC, and PDU three-in-one, which greatly improves the power density and reduces the volume.

Cooling method: There are two methods: active air cooling and liquid cooling. When the power increases to 11KW, the demand for liquid cooling increases.

Cost: The cost requirements for components are relatively high. Currently, the three small electric appliances are in a competitive market, and the general gross profit margin of the three small electric appliances suppliers is between 15% and 20%.

3

Vehicle OBC topology

OBC is an AC-DC converter composed of PFC (Power Factor Correction) + isolated DC-DC. PFC (Power Factor Correction) realizes the conversion of grid AC voltage into DC voltage and ensures that the input AC current is in phase with the input AC voltage. According to the actual design power requirements, multi-stage Boost circuits can be connected in parallel for capacity expansion; DC/DC realizes the conversion of PFC-level output DC voltage into the required charging voltage, realizes constant current/constant voltage charging function, and ensures electrical insulation between the AC high-voltage side and the DC high-voltage side. Similarly, according to the actual design power requirements, multi-stage DC/DC circuits can be connected in parallel for capacity expansion. In addition, the more common DC/DC-level circuit topologies are phase-shifted full-bridge and LLC.

Figure: OBC circuit overview.

The system block diagram of the OBC component using SiC MOS devices is as follows:

Figure: System block diagram of a 6.6KW bidirectional OBC from wolfspeed.

Figure: System block diagram of ON Semiconductor’s 1200V silicon carbide-based unidirectional OBC.

Figure: System block diagram of ST's OBC.

Figure: System block diagram of Infineon’s OBC.

4

Disassembly diagram of vehicle-mounted OBC

The physical picture of OBC is as follows:

Figure: Wolfspeed's 6.6KW OBC physical picture (based on TO-247 650V SiC MOSFET solution).

Figure: Integration of magnetic components and power semiconductors in Wolfspeed’s OBC.

Figure: BYD's OBC disassembly.