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Hello,

ia have one question regarding heatsink temperature, so i have two IRFP460 mosfet on pretty big heatsink en testing on 4A current and i measured temperature on heatsink after one hour arround 80 degrees, so high. I made calculation and says:

P_Dmax=5A*(12V-(5A*0.1Ω)-5V)=5A*6.5V=32.5W

So max power dissipation is one transistor is 32.5W, thermal resistance THj-c is 0.45C/W. why then so high temperature, i select pretty bigger heatsink that it sould be regarding my calculations??

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Simple arithmetic error.

5A x 0.1 ohms= 0.5V. 12V - 0.5V= 11.5V. 5A x 11.5V= 57.5W.The heatsink thermal resistance might be 1 degree C per Watt and the thermal grease might be 0.5 degrees C per Watt. The ambient might be 30 degrees C. Then the heatsink will be very hot and the chips inside the Mosfets might be hotter than allowed. 

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Yes, but what with 5V voltage drop on 0.5E dischatge resistor? Is this nedd to take into account, or? Now i tasted with 4A, so 2A only through each mosfet and i measured 75degree per C on heatsink so from calculation is junstion temperature is allowed range.

Is that not truth?

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