Controlling a Ducted Fan (12V 10A) with a PWM

I have four ducted fans on a platform that require 12V, 10A. I'm using a car battery as the power source for the fans, but I need a way to control the fans with a PWM wave. ive tried using a hardcore Mosfet in a simple power switch (Load on Drain, Ground on Source, PWM on Gate) but MOSFETS are so fragile, I keep blowing them, plus the heatsinks are weighing down my project. Is there a better way to do this?


More Info( The PWM is +-5V, but I can ramp it up to +-20V)

Bret

 

[don01]

hi newkleer
Some assumptions on my part :-
* each fan is 12v 10a
* you are using 1 output per fan

If you are using the mosfet as a true switch I would have thought you wouldn't need a very big heat sink. If the fet is "off" then the voltage drop across it times the current (0) or near as should give a very low power disapation If it is "on" the power should still only be Vds *10amp = say 10 to 20 watts (times the duty cycle say 50% at half speed) This is the beauty of pwm circuits (eg switch mode power supplys)

Ok having said that consider the cct again just in case the fets aren't turning fully off or are draining current through the cct some how.

Now IF you are worried about fragile fets then consider say MJ15003 npn? or mj15004 pnp? transistors. You may have to use mje 340 & or mje350 as a darlingtonpair arrangement but these transistors are 20 amp Ic 250 watts = should be able to power a london bus.

But honestly I'm concerned about the need for big heat sinks and blowing the fets I think the units are not turning off during the cycle or the ramp rate is too slow Giving you power losses.

Let us know how it goes
Don

 

Don,

Further information:

1.) Your assumptions are right.
2.) We're right the +-5v PWMs through a 7407 Buffer to get -+20V. (I'm not sure this is the right way to do this)

3.) After the buffer the +-20V (Not a perfect square wave) is fed into the FET.
4.) The PWM period is 2ms (~500Hz)

Bret

 

[jbartos]

Suggestions:
1. Depending on the circuits, some filtering of harmonics may be considered.
2. Snubbers
3. Higher-current rated switchers