About Mosfet Heat sink

Thread Starter

sonda

Joined Oct 30, 2012
10
Hi,

i am using a N channel mosfet IRF 1405 to drive a 19A fan motor. the mosfet is controlled by a 10K potentiometer & 50K NTC. my question is what type of a heat sink do i need to use for this Mosfet. thanks In advance.
 

#12

Joined Nov 30, 2010
18,224
Huge.

I think you just said you are using the mosfet in the linear region where P=IE and the voltage is unknown. Power = current times voltage. That's why mosfets are usually run as variable pulse width devices. Way complicated compared to just a potentiometer, but much easier to keep cool.
 

bountyhunter

Joined Sep 7, 2009
2,512
Hi,

i am using a N channel mosfet IRF 1405 to drive a 19A fan motor. the mosfet is controlled by a 10K potentiometer & 50K NTC. my question is what type of a heat sink do i need to use for this Mosfet. thanks In advance.
You have to calculate the power dissipation, then select a heatsink based on allowable maximum temperature rise.
 

bountyhunter

Joined Sep 7, 2009
2,512
Huge.

I think you just said you are using the mosfet in the linear region where P=IE and the voltage is unknown. Power = current times voltage.
However, if a FET is used as a "saturated" switch (fully cranked ON), power is calculated as:

P = (I)squared X (FET ON resistance)
 

crutschow

Joined Mar 14, 2008
34,285
What is the fan supply voltage? From the maximum power theorem the maximum MOSFET dissipation will occur when the voltage across the MOSFET is 1/2 the supply voltage but that's for a resistive load. Since the fan is a variable load it's more difficult to accurately determine the maximum power point, but it's likely to be no more than 1/2 the supply voltage times 1/2 the maximum fan current.
 

Thread Starter

sonda

Joined Oct 30, 2012
10
What is the fan supply voltage? From the maximum power theorem the maximum MOSFET dissipation will occur when the voltage across the MOSFET is 1/2 the supply voltage but that's for a resistive load. Since the fan is a variable load it's more difficult to accurately determine the maximum power point, but it's likely to be no more than 1/2 the supply voltage times 1/2 the maximum fan current.
its supply voltage is 12 volts. thanks a lot!
 
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