# Logic Level Gate FET

#### DC_Kid

Joined Feb 25, 2008
796
i am using the Maxim 6643 for a cooling project. the maxim PWM output drives a FET, but its not clear to me if the FET needs to be one that is rated as having logic level FET gate. my project uses fans that are 30A at full power.

IR has a FET that has logic level gate (http://www.irf.com/product-info/datasheets/data/irlz44n.pdf). does "logic level" mean full rated current @ 5v on gate?

do you see a issue driving two of these FET gates directly from the Maxim? when is a FET gate driver needed?

#### balisong

Joined Feb 26, 2008
27
Yes, "logic level" FETs should be fully on with 5V at the gate.
I don't see any problem using those FETs with the MAX6643.

A gate driver would be needed if you were switching at a high frequency.
The MAX6643 is switching at 32Hz, which is quite slow.

Remember to heat-sink well. Those FETs could be dissipating over 20W each.

#### SgtWookie

Joined Jul 17, 2007
22,221
Remember to heat-sink well. Those FETs could be dissipating over 20W each.
How do you figure that?

IRLZ44's have a Rds(on) of around 0.028 Ohms.
You'd need to have around 26.7v across the MOSFET itself while it's conducting to get that kind of power dissipation.

#### DC_Kid

Joined Feb 25, 2008
796
does I$$^{2}$$R still hold true? if so then at 30A & 100% duty cycle on the gate the dissipation would be around 25watt (30*30*0.028) ?

for this project i might switch back to the IRF1404's which have Rds = 0.0035 typ. looks like 1404 can get about 40A with Vgs of 5v, although they are not logic gate rated i could pop in a gate driver (another fet) to drive the 1404 gates, etc.

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#### balisong

Joined Feb 26, 2008
27
We don't call it Ohm's law for nothin'.

What's the voltage driving the fans? When the FET turns off, it's going to see the supply voltage plus the voltage generated by the spinning fans. You'll need FETs with Vds rated twice the fan voltage. Or you'll need to put a diode across the fan.

#### AchMED

Joined Aug 5, 2008
41
You have to use the correct current the correct current is RMS .You are switching the FET at 32Hz and at a duty cycle and its unlikely the case will remain at 25C unless you are using liquid nitrogen for cooling. Then apply OHMS law for conduction losses P=Irms*Irms*rDSON @100C. Rdson at a realistic case temp is usually about 1.8 *RDSON at 25C.

The simple application of OHMS law is only valid at DC and 25C.You are PWMing the FAN?

and i did note ohms law @ 100% duty cycle. we could just add PWM % as a factor to get W = I$$^{2}$$*R*PWM%. it's ideal. the T$$_{J}$$ vs Rds$$_{on}$$ is characterized and given in the datasheets.