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Power dissipation in mosfet

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chris414

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I'm running an h-bridge made of N-channel mosfets (**broken link removed**). There is hardly any voltage dropped between the drain and source and I'm drawing about 10 amps. Therefore at most I am only dissipating 1 or 2 watts when the mosfet is rated to dissipate 79watts. Is heatsinking still necessary?
 
Is the gate drive on your high side switches going at least 10V above the drain voltage? This is a requirement for using these devices.

You can get the following information from the datasheet: With 10V Vgs, the ON resistance should be 40 milliohms max. With 10A passing through the transistor, it will dissipate up to 4 watts. The thermal resistance from junction to ambient is 62.5 deg C/W. At 25C ambient, without a heat sink, the junction (chip) temperature will reach 25+(62.5*4)=275C. The maximum allowable junction temperature is 175C, so the device will be destroyed without a heat sink.
How much heat sink do you need? I would try to keep junction temp below 125C, for greater reliability. This means you need total thermal resistance from junction to ambient of less than (125-25)deg/4W=25deg C/W. Thermal resistance from junction to case is 1.9 deg C/W, and thermal resistance from case to sink is 0.5 deg C/W (I think this includes a thermal grease of some sort). The heat sink thermal resistance must therefore be less than (25-1.9-0.5)=22.6 deg C/W. Heat sinks are specified for thermal resistance.
 
The on-resistance of a Mosfet rises when it heats so it heats much more than you think.

If you are using PWM then the switching time will also cause the Mosfets to heat.
 
Thanks for the info. The heatsinks I currently have are rated "18 K/W" for thermal resistance- is that kelvin/watt? This would be fine wouldn't it?
 
Thanks for the info. The heatsinks I currently have are rated "18 K/W" for thermal resistance- is that kelvin/watt? This would be fine wouldn't it?
That should work, assuming you don't get significantly more heating due to the factors Audioguru mentioned.
 
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