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Old 13th December 2011, 02:15 PM   #1
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Default When does a MOSFET need a fan to cool?

Hello,
I beg to know what is the maximum dissipation (Watts) of a TO220 or TO247 Mosfet attached to a heatsink of any size , where a fan is NOT needed?

In other words, what is the dissipation level in a mosfet, where any size increase of heatsink provides no further cooling improvement , and a fan becomes necessary?

I am speaking of a mosfet in a well ventilated enclosure with 30 degrees C ambient.
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Old 13th December 2011, 02:32 PM   #2
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How long is a piece of string? Similar question, similar answer: it all depends on which piece of string and how much you stretch it.

Specify: ambient temperature, natural convection cooling rate, thermal resistances (chip-case, case-heatsink, heatsink-ambient) then do a heat flow calculation/simulation.
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Old 13th December 2011, 02:42 PM   #3
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Is there a rule-of-thumb for max temp? Clearly, more watts dissipated means larger heat sink, but if one where to measure the case temp, is there a reasonable target temp to stay below?
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Old 13th December 2011, 02:48 PM   #4
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For TO220 typical junction to case thermal resistance is 3 degrees per watt.

Usually, the golden rule (given to us by the chief engineer at a big smps company) was that the case should stay below 109C.
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Old 13th December 2011, 02:50 PM   #5
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Quote:
Originally Posted by eem2am View Post
For TO220 typical junction to case thermal resistance is 3 degrees per watt.

Usually, the golden rule (given to us by the chief engineer at a big smps company) was that the case should stay below 109C.
It depends on how reliable you want your project to be.

The cooler the better.

The maximum temperature of the silicon junction itself is about 150 Degrees C.

For good long reliable life I try to keep the case temperature below 80 degrees C, normally much lower, in the order of 50 degrees C.

You also have to consider the cooking effect on the other components - Electrolytics will rapidly dry out if they are overheated.

Last edited by KatieandDad; 13th December 2011 at 02:59 PM.
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Old 13th December 2011, 05:04 PM   #6
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I would strive to keep junction temperature below 100degC. This Leaves 50Cdegrees of margin for peak temperatures etc.
Further, I would check for a variety of worst case operating conditions to determine that the circuit can survive these hopefully rare moments.

As for infinite heatsink. Just substitute Rth s-a = 0 in the equation to see what effect Rth j-c and Rth c-s have on the operating temperatures of the device/s being examined.
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Old 13th December 2011, 05:18 PM   #7
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thanks, i see,

Rth c-s is the difficult one to know.....because its through that heat paste.

Supposing there was an infinite heatsink, and Rth j-c was 3 degC per Watt.......

...then i presume that would mean the maximum dissipation before a fan was needed would be.......23 Watts? (taking 30C ampbient).
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Old 13th December 2011, 07:24 PM   #8
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The IRFP244 happilly runs dissipating 40 Watts on a large heatsink.
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Old 13th December 2011, 07:32 PM   #9
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Originally Posted by KatieandDad View Post
It depends on how reliable you want your project to be.

The cooler the better.

The maximum temperature of the silicon junction itself is about 150 Degrees C.

For good long reliable life I try to keep the case temperature below 80 degrees C, normally much lower, in the order of 50 degrees C.

You also have to consider the cooking effect on the other components - Electrolytics will rapidly dry out if they are overheated.

I spent many years as a TV repair engineer. Most failures were down to Electrolytics being placed close to inadequate heatsinks. The caps would fail due to cooking and then they would take out the regulator ICs as they went down.

If you keep everything cool, this wont happen. If you must run semiconductors at their limits then keep heat sensitive components away from the heat.
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Old 13th December 2011, 08:03 PM   #10
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Be very sparing with the heatsink compound. Its only there to fill in the microscopic holes in the pads and the metal. Too much and you are defeating the object. You only need the meerest smear. In the ideal world you dont need it. The metal of the semiconductor should sit immediately next to the mica and the mica should sit immediately next to the heatsink. The world isnt perfect so the paste just fills in the microscopic gaps.
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