How hot are your key components getting? There’s a good chance you’ve built a project and thought: “Well I guess I better slap a heat sink in there to be safe”. But when working on a more refined build you really need to calculate heat dissipation to ensure reliability. This is actually not tough at all. The numbers are right there in the datasheet. Yes, that datasheet packed with number, figures, tables, graphs, slogans, marketing statements, order numbers… you know right where to look, don’t you?
Hackaday has you covered on this one. In under 10 minutes [Bil Herd] will not only show how easy these calculations are, he’ll tell you where to look in the datasheets to get the info you need quickly.
Above, [Bil] used his bench as a whiteboard to illustrate the thermal resistance equation. In this case each resistor symbol represents part of the heat dissipation. You must consider all places where heat can be transferred: (from left to right) the component die (junction) to the component case, the component case to a heat sink, and the heat sink to ambient air. He illustrates each of these dissipation points in the video.
An example of the junction-to-case is shown to the right. This is a TO-3 case which has had the lid cut off. It’s a much simpler way to look at a chip die than trying to decap a component with a plastic case.
Make with the Math Already!
Okay, okay, we’re getting there. The math is not hard… just multiplication and addition, so hang on a minute more.
Gather the following values: maximum power you plan to use with this component, maximum heat rating of the part, maximum ambient air temperature in which this component will be used, and the theta values from the datasheets. Theta, which is a measure of degrees per watt, is often listed as a symbol: Θ Multiply theta by the max wattage and you will know how much temperature to add to your equation
Datasheets: Finding Θ and Temperature
Because [Bil] does such a great job in the video we’re giving you the quick version here. Temperature generating components will include a maximum operating temperature like the one shown below (click through for full datasheet) which is for a linear regulator:
The theta for “Juntion-to-Case” is found a bit further down the same datasheet in the Electrical Characteristics table. Datasheets will also provide a “Junction-to-Ambient” value (also shown below but not used in our calculations) used to calculate how much power you can use without any type of active or passive cooling. This answers the question of: “do I need a heat sink?”.
Finally, you want to look at values from the heat sink being used. [Bil] looks at the datasheet of a heat sink which lists a thermal resistance of 25.8Θ with the chart below on the left showing how that number may be altered with moving air (a fan). The chart to the right covers the use of interface agents like thermal grease, and a mica pad (for electrical insulation) with thermal grease. Both of those values are circled but only one will be used in the calculation.
Putting It All Together
If we assume an ambient air temperature of 38 C (100 F) and a maximum power of 2 W all of the numbers we need have been collected.
Max Temp = Junction + Mica/Grease + Heat Sink + Ambient
Max Temp = (4Θ * 2W)º + (0.4Θ * 2W)º + (25Θ * 2W)º + 38º
Max Temp = 8º + 0.8º + 50º + 38º
Max Temp = 96.8º
The maximum temperature rating for this part is 125 C, which means that this part is being properly cooled. [Bil] goes one step further in the video, showing how to calculate how much more reliable the properly cooled part will be.