# understanding power dissipation

Discussion in 'General Electronics Chat' started by ian123, Sep 19, 2011.

1. ### ian123 Thread Starter Member

Aug 24, 2011
73
2
hi there

I have a mosfet h bridge design which works with a passive load of 3 amps @300vdc.

therefore p=v x i = 300 x 3 = 900 watts

so the whole circuits power disspation = 900 watts

how do i work out the power going through the mosfets?

is it 900 watts or is it P=I² x Rdson = 3² x 0.27 = 2.43 watts

which means that 2.43 watts is going through the mosfets and the rest in the passive load??

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
You have 2.43 Watts being dissipated in a MOSFET. Since you say it's an H-bridge, and if all of the MOSFETs are identical, you will have 2.43 Watts dissipated in one high side and another 2.43 Watts in one low side MOSFET, for a total of 4.86 Watts. Then subtract that from the 900 Watts to get 895.14 Watts power being dissipated in the load.

3. ### ian123 Thread Starter Member

Aug 24, 2011
73
2
So in order to calculate the heatsink for the mosfets i can get a heatsink 200°c per watt per meter

therefore a 60mm heatsink would do for the two mosfets and a 12mm lenght for four mosfets?