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Can I use CPU heat-sinks for cooling transistors?

CPU heat-sinks are low volume and they have been made to cooling a sensitive hot Chip and if the datasheet has said that it requires to hold on the transistor less than 150 C,I thought maybe it will be possible and good way instead of massive heat-sinks. the CPU heat-sinks are always available in the markets But maybe finding the heatsinks with special shape and enough size are slightly difficult. Are these fans and heat-sinks suitable and normative for transistor? If so, How should I choose heat-sink?(for example:for one 100 watt transistor Is the 100 Watt CPU heat-sinks suitable and enough?)

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Sizing heat sinks is actually a function of the thermal characteristics of your device. To appropriately decide on a heat sink you have to consider these parameters from your datasheet:

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From what I've read but never tried, you can even figure out necessary parameters using a circuit theory analog of sorts. Check this this link.

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The simple answer is yes. I do this, as CPU heatsinks are cheap and big. But, to get a proper thermal coupling between the semiconductor package and the heatsink, you may need to drill and tap a hole in the heatsink to screw them together, as CPU heatsinks tend to clip down to the motherboard. Also, a couple of holes to bolt it to some support are probably essential.

As to the thermal conductivity, CPU heatsinks are cheap enough to massively overspecify, but then I am not trying to miniaturise anything.

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Yes and no.

Yes, because ultimately all heatsinks serve same purpose: move heat from an element into surrounding air. As long as it can move enough power, it's fine.

No, because a power transistor works in a different environment than a CPU. First, the mounting doesn't match, both the transistor to the heatsink and the heatsink to the PCB. Second, a transistor heatsink usually operates passively at relatively high temperature (often over 100 C), while a CPU heatsink is designed to keep the temperature as close to ambient as possible at the cost of noise and dust accumulation from the fan. So you can't remove a fan from narrow-finned CPU cooler, because in passive mode it won't work as good as a dedicated transistor heatsink with large and sparse fins. If you keep the fan, it becomes a point of failure, because CPU can handle fan failure gracefully by throttling down, while your circuit is unlikely to have temperature sensors and shutdown-when-overheating procedure. For prototyping you can overcome some of those limitations (eg. by drilling new mounting holes), and just ignore the others (eg. fan failure and dust accumulation), but if you want to have a design that'll keep running reliably, you'd better buy (or design) a heatsink dedicated for the job.

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