Advanced Role of Thermal Pads in Power Supply Design

Power supply design has evolved dramatically with the push toward higher power density, lower losses, and stricter reliability requirements. Whether in data center rectifiers, industrial motor drives, or consumer adapters, thermal performance is now a primary constraint on design, not just an afterthought. Thermal interface pads (TIMs) are one of the most effective solutions for managing these challenges.

Thermal Stress in Modern Power Supplies

The thermal environment inside a switching power supply is severe:

• MOSFETs / IGBTs in half-bridge or full-bridge topologies can dissipate 5–15 W per device depending on conduction and switching losses. Junction temperatures exceeding 150 °C dramatically reduce MTTF (mean time to failure).

• Magnetic components (inductors, transformers) generate heat from core losses and copper winding resistance. Local hot spots of 120–140 °C are common in compact designs.

• Rectifiers and synchronous rectification controllers see high thermal cycling due to current surges.

• Compact PCB layouts increase coupling between hot zones, stressing electrolytic capacitors, which are especially vulnerable to ripple-current-induced heating.

Without adequate heat transfer, thermal impedance from junction to ambient (θJA) becomes the limiting factor in overall power density.

Why Thermal Pads Outperform Alternatives

While thermal grease or mica insulators were common in legacy designs, thermal pads offer a repeatable, manufacturable, and electrically robust solution.

• Thermal Conductivity: Pads range from 1 W/m·K (low-power supplies) up to 12+ W/m·K (high-performance silicone or ceramic-filled composites).

• Thermal Impedance: Defined as °C·cm²/W, lower values enable faster heat spreading into heatsinks or chassis. Pads compensate for uneven pressure, unlike greases that thin out under load.

• Dielectric Breakdown Voltage: Many silicone-based thermal gap fillers provide >5 kV/mm, allowing designers to mount high-voltage FETs or rectifiers directly to grounded chassis while maintaining creepage/clearance safety.

• Mechanical Compliance: Compression deflection force is critical. A softer pad (Shore 00 ~40–70) accommodates warpage in PCBs or metal baseplates without inducing mechanical stress on solder joints.

Application-Specific Use Cases

1. Switching Devices (MOSFETs/IGBTs):

   • Pads reduce junction-to-case thermal resistance (RθJC).

   • High-conductivity (>5 W/m·K) pads placed between TO-220/TO-247 packages and extruded aluminum heatsinks can drop case temperatures by 10–15 °C compared to air gaps.

2. Planar Magnetics / Transformers:

   • Gap fillers between winding surfaces and enclosure walls spread hot spots into chassis mass.

   • Thicknesses of 1–3 mm are common for bridging irregular coil surfaces.

3. Rectifier Bridges / Diodes:

   • Using pads with both thermal conductivity, and dielectric strength prevents leakage currents into baseplates.

   • Useful in PFC stages, where rectifiers operate with large repetitive surge currents.

4. Baseplate-Cooled Designs:

   • In sealed or fanless PSUs, thermal pads conduct directly to the aluminum baseplate for conduction cooling.

   • This reduces reliance on airflow, making designs suitable for military, aerospace, and ruggedized industrial applications.

Design Parameters to Optimize

When specifying thermal pads in power supply design, engineers should evaluate:

• Pad Thickness (t): Too thick increases thermal resistance; too thin may not accommodate mechanical tolerances.

• Compression Set: Ensures long-term contact pressure over thousands of thermal cycles.

• Flame Rating (UL94 V-0): Required for compliance in high-voltage power supplies.

• Operating Temperature Range: Many pads remain stable from -40 °C to 200 °C.

• Outgassing and Pump-Out Resistance: Especially critical in sealed supplies, where volatiles can contaminate optics or sensitive surfaces.

Conclusion for Thermal Pads in Power Supply Designs

Thermal pads are more than just a convenience—they’re an enabling technology for next-generation power supply designs. By reducing junction temperatures, improving dielectric isolation, and ensuring long-term reliability under harsh thermal cycling, they allow engineers to:

• Push higher power density without compromising safety.

• Improve efficiency by keeping semiconductors within optimal operating ranges.

• Extend lifetime reliability, especially in industrial and data center environments.

As switching frequencies increase and wide bandgap devices (SiC, GaN) enter mainstream power supply design, the role of advanced thermal pads will only grow more critical. NEDC does custom die cutting for thermal gap filler pads.