Thermal materials for on-board charger modules
Thermal pads and thermally conductive adhesives keep an EV's on-board charger (OBC) at a safe operating temperature, even during continuous fast charging. Choosing the right thermal interface material improves efficiency, extends component life and reduces heat sink size.
The on-board charger converts alternating current from the grid into direct current to charge the battery, with typical power ratings from 6.6 kW to 22 kW. An efficiency of about 94–96% means several hundred watts of heat are continuously generated in a cramped space, mainly in the power transistors, inductors and transformers.
Hot spots in the charging module
- MOSFET, IGBT or SiC transistors: the highest heat density.
- Inductors and transformers: large components with irregular shapes.
- Capacitors: heat-sensitive, with service life halved for every 10°C increase.
- Control board: needs to be thermally isolated from the power section.
Material selection by location
| Location | Recommended material | Thermal conductivity | Special requirements |
|---|---|---|---|
| Transistor to heat sink | Thermally conductive insulating pad or phase change material | 3–8 W/m.K | Insulation above 4 kV |
| Inductors, transformers | Thermally conductive potting compound | 1–2 W/m.K | Vibration resistance, insulation |
| Circuit board to aluminum housing | Liquid gap filler | 2–4 W/m.K | Low compression force |
| Power SMD components | Soft gap pad | 3–6 W/m.K | Compensates for height tolerances |
Factors to consider
Beyond thermal conductivity, engineers need to consider total thermal resistance at the actual compressed thickness, dielectric strength, silicone outgassing that can affect relay contacts, and stability after thermal cycling. For 800 V systems, insulation clearances and materials need to be re-evaluated.
Production automation
Liquid gap fillers applied with metering equipment are gradually replacing pre-cut pads because they reduce manual handling and optimize the amount of material for each location. However, a grade with suitable viscosity must be chosen so the bead holds its shape and does not flow before the housing is fitted.
To confirm the selection, actual temperatures should be measured with thermocouples attached to the transistor case and capacitor leads during continuous charging at maximum power and an ambient temperature of 65°C. The results should be compared against the component manufacturer's limits, leaving a minimum safety margin of 10–15°C. Thermal images taken with the housing open are for reference only, since the heat flow differs from that in the sealed unit.
For advice on thermal management solutions for chargers and power modules, please contact AMS Vietnam.
