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For my circuit, I have mounted heatsinks to my mosfets to dissipate heat. I have below a crude drawing of how the heatsink/mosfet is placed on the PCB

enter image description here

I was thinking to integrate a LM35 to measure temperature, and to turn on a fan when the heat of the mosfet/heatsink gets too high, so that the air may cool down the mosfet/heatsink.

My question is, can I place a fan in some way that would cool off the mosfet/heatsink? If so, how? I was thinking to place the fan on top to suck air from the heatsink and blow the heat upwards?enter image description here

Would this work? I don't think placing the fan to blow at the mosfet/heatsink would work since the heat would just dissipate into the board?

Would my mosfet/heatsink placement even allow for an effective fan to be placed?

UPDATE: So my design for placing the fan above the heatsink should work from what I'm gathering. Though I am still unsure of where I should direct airflow.

@Swonkie: If I enclosed the bottom of my fan into a shroud like WhatRoughBeast suggested, would the airflow be "directed"? If I implemented a shroud, would the air be drawn directly from the mosfet/heatsink strictly? If I did direct airflow to the mosfet/heatsink, wouldn't the heat dissipate through to my PCB board?

ALSO: As I am using an LM35 for temperature sensing, should I place the LM35 so that it's touching the heatsink or the mosfet itself? Which would be more effective implementation for temperature reading?

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    \$\begingroup\$ If your heatsink has an "open" design like in your drawing, it will be much more effective to blow air into it, forcing it over the heatsink. Otherwise the fan just sucks in a lot of air from the side, which never passes the heatsink. The air intake isn't directed - air comes from everywhere. The stream of air leaving the fan instead is somewhat directed. \$\endgroup\$
    – user70682
    Mar 23, 2015 at 22:37
  • \$\begingroup\$ I think you have the right idea. I'd try a shroud which completely encloses the fan, and which extends about 2/3 of the way down the heatsink. That way, all of the cool air will be drawn up along the heatsink, maximizing the chance to transfer heat to the air. Note that there's a sort of tradeoff. If you don't use much shroud extension, a lot of air enters the fan from the side and doesn't do any good. If you extend the shroud too close to the pcb you get good channelling, but the opening for the air becomes small and the total air volume is restricted. \$\endgroup\$
    – WhatRoughBeast
    Mar 4, 2018 at 14:30

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These things actually exist.

http://www.ebay.com/bhp/mosfet-heatsink

And well, roughly you could say that the amount of surface area will allow better cooling. In combination with the amount of air flowing through/against it.

So that it will have the most possible contact with cool air. I'm not sure what your implementation is. But I'd suggest to try passive cooling. It really helps a lot, won't cause any noise or 'waste' energy. (Cooling with a fan might increase the current over the mosfet and thus backfire you?) Also, if it gets too hot for passive cooling (probably) something is wrong with your implementation.

So yes, it will work, it's effectiveness will vary on the exact implementation. But it 'should' not be neccesary to use active cooling (with a fan). The mosfet heatsinks on ebay look pretty promising and are way easier to implement and cheaper than an extra fan+temperature sensor.

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That will certainly work. Just how WELL it will work depends on the heatsink and the amount of airflow you can generate. If you have the ability, a shroud to concentrate the airflow and produce a higher air speed at the heat sink will improve performance.

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