Why components need a heat sink
Transistors, regulators and power LEDs turn part of their power into heat. If the internal junction gets too hot, usually above 125 to 175 °C, the part degrades or fails. A heat sink spreads the heat into the air so the junction stays within limits.
The thermal resistance chain
Heat flows from the junction to the air through resistances in series, measured in °C per watt:
- Rjc: junction to case, from the datasheet;
- Rcs: case to heat sink, set by the thermal pad, grease or insulating washer (about 0.1 to 1 °C/W);
- Rsa: heat sink to ambient, the value you choose.
Junction temperature: Tj = Ta + P × (Rjc + Rcs + Rsa). Solving for the heat sink:
Rsa max = (Tj max − Ta) ÷ P − Rjc − Rcs
Example
A regulator dissipates 10 W, its Tj max is 125 °C, the air inside the enclosure is 40 °C, Rjc = 1.5 °C/W and the insulating pad adds 0.5 °C/W:
- total allowed: (125 − 40) ÷ 10 = 8.5 °C/W;
- heat sink: 8.5 − 1.5 − 0.5 = 6.5 °C/W or less.
Without a heat sink, with a junction-to-ambient resistance of 62 °C/W (a typical TO-220), the junction would try to reach 40 + 10 × 62 = 660 °C. In practice the part would shut down or fail. The package alone can handle only about 1.4 W here.
Choosing the heat sink
- Pick a heat sink with a rating below the result, and keep a margin of 20% or more.
- Ratings are measured in specific conditions, often vertical fins in still air. Horizontal mounting, enclosures and dust make them worse.
- Forced air from a fan can divide the thermal resistance by two to five.
- Use the ambient temperature inside the enclosure, which is often 10 to 20 °C above room temperature.
Reducing the heat
- For a linear regulator, P = (Vin − Vout) × I. Lowering the input voltage, or switching to a switching regulator, cuts the heat dramatically.
- Use thermal grease and the correct mounting torque; a dry joint can add 1 °C/W or more.
- On a circuit board, copper area and thermal vias act as a heat sink for surface-mount parts.