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Mosfet heatsink calculations ?

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curry87

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What calculations do i need to perform to find the maximum amount of power a IRF510 mosfet can safely dissipate before needing to add a heatshink to keep within safe temperature limits?
 
According to this the junction-to-air thermal resistance of a TO-220 case is 62.5 degrees C/W. According to the IRF510 data sheet the maximum junction temperature is 175 degrees C. Thus you subtract the maximum expected ambient temperature from 175C and then divide this value by 62.5 C/W to get the absolute maximum allowed transistor dissipation.

For example, for a 30C maximum ambient temperature (next to the transistor), the maximum transistor power would be (175-30)/62.5 = 2.32W.

But for maximum reliability I would keep the junction temperature below 125C (that's still well above the boiling point of water and can readily burn you if you touch the case). Thus practically you probably don't want to dissipate much more than a watt or so in the transistor without a heat sink.
 
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Tmax > T + P × Rt

Tmax- maximum junction temperature
T - maximum expected ambient temperature

Rt -thermal resistance (Celsius/Watt or Kelvin/Watt) - the sum of all the thermal resistances between junction and ambient (heatsink and thermal grease included if applicable). Treating this variable as multiple terms and solving for the one representing the heatsink is how you size the heatsink. If you have multiple thermal paths to ambient, you sum up the thermal resistances in each path separately (like series resistors) and then run those sums through the same equation you use to find the equivalent of in parallel parallel resistors to get a single equivalent thermal resistance. One example might be if you have a top-side heatsink on your IC but it also has significant heatsinking to ambient through the PCB.

P - power dissipation - I^2 x R if it's juts constant current with no high frequency PWM switching. If high frequency switching is involved then you have account for that power dissipation and heating as well. It's rather complicated to calculate though. You'll have to do some reading if you are interested in that. Also, do not forget to account for R increasing as you heat up. There is a graph that shows you the resistance vs temperature. To figure it out exactly you have to start with any temperature (a guess really) and use the corresponding resistance in the calculation to get a new temperature, then go back to the chart to see if the new temperature matches the one you started with. If it doesn't you go through the process again using the corresponding resistance for the new temperature. Or...you could just assume worst case and use the resistance when the FET is at maximum temperature (which is usually 1.5x to 2x the room temperature resistance). You can look at the chart for a more accurate number.
 
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