Need of heat sink; thermal Ohm's law

Why a heat sink is needed

A power transistor dissipates a large amount of power as heat at its collector junction (PD ≈ VCE · IC). If this heat is not removed, the junction temperature rises. Above the maximum rated junction temperature (about 150 to 200 °C for silicon), the transistor can be permanently damaged. Also, higher temperature increases leakage current ICBO, which increases collector current and heating further (thermal runaway).

The case of the transistor has a small area, so it cannot lose heat quickly to the air. A heat sink (a finned aluminium block attached to the case) provides a large surface area, and so lowers the thermal resistance between the junction and the ambient air. This keeps the junction temperature within the safe limit and allows the transistor to handle a higher power.

Thermal Ohm's law

Heat flow is analogous to current flow in a resistor:

TJ − TA = PD · θJA

The path from junction to ambient has three thermal resistances in series:

θJA = θJC + θCS + θSA

 Junction --[θJC]-- Case --[θCS]-- Sink --[θSA]-- Ambient
   TJ                TC               TS              TA

• θJC: junction to case (device data)
• θCS: case to heat sink (insulator or grease)
• θSA: heat sink to ambient (decreases with larger sink)

Example: TJ(max) = 150 °C, TA = 25 °C, θJC = 1.5, θCS = 0.5 °C/W, PD = 20 W. Required total θJA = (150 − 25)/20 = 6.25 °C/W, so the sink must have θSA ≤ 6.25 − 2 = 4.25 °C/W. Without the sink, θJA is typically 40 to 60 °C/W and the allowed power would be only about 2 to 3 W.