Heatsinks and Thermal Budget
(1) Heatsinks
Overall heatsink objectives are:
(1.1) Conduct the heat away from the transistor mounting area by a thermal conducting material (ally or copper but stick with ally for explanation) with a low thermal resistance. This means thick for a sheet of ally.
(1.2) Once the heat has been conducted away from the transistor mounting area, present a large surface area to the air to efficiently transfer the heat to the air by convection. This is normally done by fins. The best fins are vertical because heated air rises, but there must be a free flow for the air through the fins (open top and bottom)
(1.3) The heatsink surface area must be in free flowing air as cold as possible. The transfer of heat from a heat sink to air is proportional to the difference between the heatsink surface area, the temperature of the heatsink, and the temperature of the air.
For example, if the heatsink surface is 60 deg C and the air in the area of the heatsink is also 60 deg C there will be no convection and no cooling of the heatsink. 60 deg C is not unusual inside equipments.
If the air was 70 deg C, again not uncommon in an equipment cabinet, the air would actually heat the heatsink up instead of cooling it!
There is stacks of data on the net and elsewhere about heatsinks and also about making heatsinks
Heatsink are specified by their thermal resistance in deg/CW, to free air This is a measurement of how much heat the heatsink can dissipate into the air A small heatsink would typically be 10 deg/CW and a large one 0.5 deg/C W
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(2)
Thermal Budget
Take an example:
A transistor has a maximum junction temperature of 170 deg C, as shown on the data sheet. That temperature must not be exceeded or the transistor will be destroyed.
The transistor is dissipating 50W. The air temperature in the vicinity of the heat sink is 50 deg C.
Thus, the temperature difference between the transistor maximum junction temperature and the air is:
170- 50 = 120 deg C
The transistor dissipation is 50W so the maximum thermal resistance of the heatsink is 120/50= 2.4 deg C W, a perfectly achievable figure.
It would not be wise to operate the transistor with a maximum junction temperature. You only do that if you absolutely have no option.
A reasonable max junction temp design aim would be 90% of data sheet Tjmax.
So now, Tjmax_design = 0.9 * Tjmax_data_sheet = 0.9 * 170 = 153 deg C. That is now the design maximum junction temperature.
Doing the heatsink calculations again:
Tjmax_design- Tamb = 153- 50 = 103 deg
Now the heatsink thermal resistance is, 103 deg C/50W = 2.06 deg C W, a lower figure you will notice.
For an actual design, the thermal budget calculations are exactly the same priciple, excpt you would also have to take into account the thermal resistances between the junction and case, and case to heatsink. When you include these thermal resistances the situation becomes much more critical.
A typical thermal resistances from junction to case for a high power TO-3 transistor, is around 0.8 deg C W and, using a mica washer between the case and heatsink, the thermal resiatance between the case and heatsink is 1.1 deg C W. Mica has the lowest thermal resistance out of the freely available and low cost group of insulating washers. It is also the most stable and has the best dialectric performance. Aluminium oxide has an even lower thermal resistance but is expensive, while Beryllium oxide is the best, but is very expensive and also toxic, so it is now banned.
Finally, the total thermal resistance between the junction and the air is, 0.8 + 1.1 + heatsink thermal resistance.
I will leave you to work out, if you like, the maximum heatsink thermal resistance that would do the job!