Automobile Interior Fan Control (PWM + Mosfet)

[dext3r]

hello all,
i'd like to replace my stock fan switch in my car with a PWM circuit + MOSFET setup to drive the blower motor. I've been doing some research (got plenty of links and datasheets to mull over) into it and I've got a PWM circuit that should work well, but I have a couple questions.

1) MOSFET driver IC.
What is exact reasoning behind using this part? From what I've read it might have something to do with the gate capacitance (to allow faster switching)? And also to bump up the voltage from the PWM output to satisfy the gate threshold voltage requirements? So I need to match the output of the driver IC to the Vgs of the MOSFET, correct?

Since this is a blower fan, I only need one direction, so I think a single channel driver IC is the correct thing to use. Can anyone suggest a specific chip/model?

2) Current Requirements
The current circuit the fan is on has a 40A fuse, so I'm assuming that is the minimum current I should aim for. In regards to this, how should I design my PCB? What width should I make the traces connecting to the MOSFET? Is it even safe to mount it to a PCB, or would I need like 2" wide traces? (Sorry if this sounds newb-o-rific). I am aiming to switch at ~20kHz, as this seems to be the correct frequency to avoid noise from the motor (from what I've read).

thanks for lookin and i appreciate any help on my project
dex

 

[Sceadwian]

The average current is more important, the fuse rating may be excessively high or for an entire branch (like the entire AC system less the compressor at least) You really need to determine the true current requirements before you proceed but I don't see a fan needing more than 10 amps (I could be wrong) find out from the fan itself, use a current meter if you have to. You need to know the starting current as well but most FET's are tolerant of momentary surges as long as their thermal limits aren't exceeded.
I'd mount the whole thing on perf board and just use thick wire to point to point solder each component. A simple 555 timer could be used to generate the PWM signal with a pot for the duty cycle to drive the fet. Since the blower is an inductive load and you're going to be driving it directly from the FET make sure you put in a flyback diode the body diode of a FET has soft recover characteristics so it probably won't cut it. Basically all you need is a FET, a 555 timer and a few resistors and capacitors along with a POT.

 

[Hero999]

40A doesn't seem excessive for a car blower fan, 480W seems reasonable considering I can almost feel my ears pop when I open a window when the fan is on full blast.

 

[Sceadwian]

If that's true it could be pretty power hungry then. Car fuses are usually slow blow so it could probably handle 60 amp startup currents. Still power FETs are so cheap and easy to drive it shouldn't be too big an issue, just a consideration.

 

[dext3r]

Thanks for the responses.

I looked at the factory wiring diagram and the only thing the fuse is for is the blower circuit. My voltmeter can only measure up to 10A of current though, so I need to find something else to measure for sure.

I will be using the PWM circuit from here: **broken link removed**, where a NPN transistor is used to drive the MOSFET.

For the flyback diode, should it be placed closer to the MOSFET or closer to the blower?

Thanks again.