Heat sink selection and thermal calculation.

Question

Do you have available app. notes on heat sink calculation for through-hole packages like TO-220 ? 

Answer

Please see Application Note AN-1057 which discusses heatsinks.

The limiting factor in whether a heat sink is needed and if so what thermal rating is required, will be determined by the temperature of the silicon junction in the device in your application. This will depend on the power dissipated in the device, the local ambient temperature adjacent to the part and the overall thermal resistance of the device and its mounting in the application.

The equation to use is known as the Thermal Equilibrium Equation:-

Tj = Pd (RthJC + RthCS + RthSA) + Tamb for discrete parts mounted on heatsink, or
Tj = Pd (RthJA) + Tamb for discrete devices without heatsink.

Where Tj = junction temperature
Pd = dissipated power (RMS current through the part x voltage drop) + switching losses if applicable.
RthJC = thermal resistance, junction to case (from the device data sheet).
RthCS = thermal resistance, case to heat sink (from the device data sheet).
RthSA = thermal resistance of the heatsink to air.
RthJA = thermal resistance, junction to ambient for surface mount devices or through hole parts not on heatsinks.
Tamb = ambient temperature immediately adjacent to the module in your application.

In the equation fill in the maximum temperature you are happy with provided it is below the data sheet stated maximum. Remember for every 10*C the junction is below the maximum permitted , you will approximately double the inherent reliability of the part in your application. Add in the thermal resistances, except that of the heatsink, include the expected ambient temperature and solve the equation for Rth for the heatsink to air. This will determine if a heatsink is needed and if so what the minimum Rth for it will be.