Thermal pads for power supplies and inverters
Electrically insulating thermally conductive silicone pads are a common solution for transferring heat from MOSFETs, IGBTs, diodes and transformers to the heat sink housing in power supplies and inverters. The material conducts heat well while ensuring electrical insulation between high-voltage components and the metal frame.
As equipment power increases while size keeps shrinking, choosing the right thermal pad directly affects the service life of power components.
A dual requirement: thermal conductivity and electrical insulation
Power components in inverters can operate at voltages from a few hundred to over a thousand volts. The pad between the component and the heat sink must have a sufficiently high breakdown voltage, typically 4 kV or more, while also having low thermal resistance so that the junction temperature does not exceed 125–150°C.
Common types of thermal pads
Fiberglass-reinforced silicone pads
0.15–0.45 mm thick, with a fiberglass mesh layer that resists puncture when screws are tightened, used in place of mica and thermal grease for TO-220 and TO-247 components.
Soft gap pads
0.5–5 mm thick, soft and compressible, filling the gap between multiple components of different heights and the enclosure.
Silicone-coated polyimide pads
Thin and with very high breakdown voltage thanks to the polyimide film layer, suitable for high-voltage equipment.
| Pad type | Thickness | Thermal conductivity | Breakdown voltage |
|---|---|---|---|
| Fiberglass silicone | 0.2–0.45 mm | 1.0–3.0 W/m·K | 4–6 kV |
| Soft gap pad | 0.5–5 mm | 1.5–6.0 W/m·K | Above 5 kV/mm |
| Silicone-coated polyimide | 0.1–0.25 mm | 0.8–1.5 W/m·K | 6–10 kV |
Assembly notes
- Tighten screws to the recommended torque so the pad is compressed evenly, avoiding localized increases in thermal resistance.
- Check for burrs on aluminum heat sinks, as sharp edges can puncture the pad.
- Consider pads with a single-sided adhesive layer for quick positioning on the line.
- For gap pads, choose a thickness 10–20% greater than the actual gap to ensure contact.
Evaluating performance after changing materials
The simplest method is to attach a thermocouple to the power component case and run at rated load for a few hours until the temperature stabilizes. Comparing the temperature difference between the component case and the heat sink before and after changing the pad shows the actual thermal resistance, which is more accurate than relying only on datasheet values. A withstand voltage test on the finished product is also recommended to confirm insulation.
AMS supplies Bergquist and Shin-Etsu thermal pads and offers die-cutting to drawing. Contact AMS Vietnam for material advice for power supplies and inverters.
