High current PWM controller ?

Areal Person

Hi,

I’ve built a constant current PWM. It will drive about 110 amps at 12-35 vdc
I currently use three POT’s to adjust the current, duty cycle and frequency
which ranges from 1KHz-10KHz. I use an LM324N quad op-amp with a few
resistors a couple of caps and a 7810 10v regulator in a to-92 pak.
It also uses a power MOSFET (Obviously)

QUESTION:

What would be the simplest way to substitute the POTS for my Propeller
So I can have Dynamic MCU control over these three adjustments.

I want to use this circuit with my Propeller. I know I’ll need to create the correct
voltage regulator for the Propeller. My problem is I don’t know how to substitute the POT’s for the propeller.

Can someone help me ?

It may be simple, I'm just not sure how to do it.

Here’s my PWM circuit.


Thanks,
-Areal

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I have a tree growing out of my head, but

what do you expect ? I'm a programmer.

Post Edited (Areal Person) : 9/18/2008 9:53:07 PM GMT

Ale

There are several options that I'd separate into:

a) The propeller generates PWM,

This way two of your potis will be gone due to software control.
The third that I assume controls the gate current can be replace with a digitally controlled potentiometer like the ones MAXIM has.

b) The propeller does not generate PWM

This way you replace all potis with digitally controlled versions.

Areal Person

I would like to use option B. and replace all the pots, and only use the Propeller to adjust them through a spin program.

Are your saying that I can purchase digitally controlled potentiometers and replace the manual ones then interface them
directly to the Propeller and control them with a spin program ?

If I can do that, That is what I want, a simple solution to use the existing circuit.

Is this correct ?

Thanks

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I have a tree growing out of my head, but

what do you expect ? I'm a programmer.

Ale

yes, is what i said. Have a look here:

http://www.maxim-ic.com/products/digi_pot/

Ther are no 20k models here, but maybe you can do with a 10 or 50k: (just as examples)

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2480

Have fun

Philldapill

Areal,
As always with any response, you'll get a little more than you asked for... I've made this same circuit using the LM324 and I've found that the slew rate of the opamp is just too slow. I believe the LM324 is something like 1V/us or something. You'd be much better off using another opamp with a higher slew rate. The slew rate is how fast the output changes. If it is too slow, your mosfet will spend alot of time in it's transition phase and will dissipate alot of heat. Ideally, you would use a mosfet gate driving chip which can charge the gate in a snap. You mentioned wanting to drive 110A - You'd definately need a gate driver for that! I know many mosfet datasheets state that it can handle 110A, but that's really an overstatement. In a TO-220 package, the legs of the mosfet would melt under that much current. Basically, the 110A max current is under totally ideal conditions, and only applies to the junction itself, not the packaging.

Anyway, as for the topic at heand, I'm working on something similiar(or at least have though about it!). I would suggest using the propeller itself to generate the PWM, and feed that signal to a mosfet gate driver. Controlling it would be done through buttons to increase/decrease a parameter. I've though about making a unit with simplicity in mind, and what I've come up with is a 3 button control. Button A would toggle between adjusting current/duty cycle/frequency with each push. The other two buttons, B and C, would increase and decrease the selected parameter. Pretty simple interface and only uses 3 buttons.

Ale

Phill is right, I assumed you have some gate driver in between. Thick mosfets need quite a bit of current to switch. There was a thread at mikrocontroller.net (in german) some time ago about a kart driver with mosfets. They where basically recommending very low Ron, high current (real continuous) mosfets, and a gate driver. The PWM was going to be made with an AVR.

http://www.mikrocontroller.net/topic/101929#new