Calculation of the Heat sink
For the calculation of the heat sink, three thermal resistances must be considered:
- The junction between the component and its case. Its value is specified in data sheets of manufacturers.
- The one between the housing component and the heat sink. It depends on the housing component, and is generally between a few tenths of ° C / W and a few tens of ° C / W. This value is also specified in the manufacturers’ data sheets (but unfortunately not always).
- between the heat sink and the ambient air. This value is given by the manufacturer of the heat sink, and is even lower than the heat sink large.
For a total thermal resistance (junction between the component and ambient air), simply add the three thermal resistances above. It would be unwise to be satisfied with this simple addition, without considering some small safety margins. We must indeed avoid the component work continuously too close to the limit, and consider some cases of “overheating” natural (equipment exposed to the sun) or less controllable (failure of a fan). Remember that when it comes to room temperature, refers to the air surrounding the component or the radiator, and if it is enclosed in a box, its temperature will not be the one you think.
Example heat sink calculation 1
We want to cooling a 2N3055 type transistor in which case TO3 must dissipate a power of 20 W at room temperature. First recall the simplified formula announced at the beginning of the article:
Rth = (T1 – T2) / P
Now replace T1 by Tj (junction temperature), T2 Ta (ambient temperature) and P with Pd (power dissipation). this gives
Rth = (Tj – Ta) / Pd
First thing, take a safety margin. We will base the calculation on a 25% higher power dissipation, or 25 W instead of 20 W. Room temperature is estimated at 55 ° C (it sounds a lot, but it happens more often than one thinks), the component and heat sink are enclosed in a box. The manufacturer indicates for the 2N3055, resistance junction-case of 1.5 ° C / W, and a resistance heater box-0.5 ° C / W with silicone grease. According to the manufacturer, the maximum temperature junction is 200 ° C. By applying the above formula, we get this:
Rth = (200-55) / 25 = 5.8 ° C / W
Thermal resistance of 5.8 ° C / W which should be deducted resistance junction-case of 1.5 ° C / W resistance heater box-0.5 ° C / W. Finally, we find that we need a radiator thermal resistance of 3.8 ° C / W. A radiator of thermal resistance but also lower should take more space for you to find a mechanical model that suits the application.
