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Adding heatsink with electrical isolation

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EEEnthusiast:
Folks,
What is the best way to mount a power transistor (TO-220 non isolated tab) to the metal case of the power supply in order to act as a good heat sink. It should be electrically isolate from the case as well.

It should be assembly friendly as the TO-220 package is PCB mount. The transistor cannot be connected to the board using wires. I was thinking of using two heatsinks, one on the transistor and the other on the case. After the case is closed, the two heat sinks would sit close to each other acting like a small heat exchanger. Like below. Any experience with this, or a better solution?

BravoV:
Air gap is one of the best heat ... insulator.

Whats wrong with thermal pad ?

EEEnthusiast:
The thermal pad on the PCB would mean more heat dissipation on the PCB, causing temperature fluctuations. I need to keep the PCB at the most stable temperature and hence need to take the heat off the board.
Air gap could be a good insulator, but with enough fins interacting with each other, should the thermal gap place a big restriction in the heat exchange?

BravoV:
Thermal pad to the metal body, then secure the TO220 on it.

brabus:
Linn Amplifiers use the case as heatsink, by screwing the transistors to an anodized block of alluminium. The thick anodization acts as electrical insulator and good heat conductor at the same time.

In your case I would definitely use a traditional sil-pad, it's an industry standard. I don't see how you could induce temperature fluctuations on the PCB with a sil pad: if it's correctly placed you will notice no difference in comparison with a direct alluminium mount.

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