Heatsink Sizing
The process of calculating the required sink-to-ambient thermal resistance and selecting a profile that delivers that value. The permitted temperature rise is divided by the total power loss to find the target thermal resistance; subtracting the semiconductor and interface resistances from it leaves the value expected from the heatsink. If the result comes out negative, a fan or a lower current is needed.
The steps of the calculation are as follows: first you set the target junction temperature (70–80% of the limit, for example 120 °C). You take the ambient temperature according to the worst case (55 °C in a closed cabinet). The 65 °C difference between the two, divided by the total loss, gives the total permitted thermal resistance.
Example: at 10 W total loss the permitted total resistance is 6.5 °C/W. If the semiconductor's junction-to-case resistance is 1.5 °C/W and the insulating pad is 0.8 °C/W, the value expected from the heatsink comes down to about 4.2 °C/W.
- Catalogue condition: Profile catalogues give the thermal resistance for a certain temperature rise and natural convection condition; under different conditions the value changes.
- Leave headroom: Choosing a profile at least 20% below the calculated result covers the allowances for dust, mounting imperfections and ageing.
- Alternative: If the target thermal resistance falls below 2 °C/W, in practice you have to move to a fan-cooled solution.
The context in which this term is used: DC Motor Driver Selection Guide