PWM dsPIC33F

Suggest you explain what your problem is.. that optocoupler has particularly crappy CTR so you'll be able to get (reliably) maybe

200uA out of the transistor with 4mA drive with the lowest grade device.

Best regards, Spehro Pefhany

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You're right. I have to pilot a 12V-3A motor DC using a MOSFET where the gate should be connected to the dsPIC (PWM). I want to use an optocoupler so in this case the gate will be connected at the Vout of the optocupler. In order to have the LED completely ON, I need 16mA but the dsPIC33F just can sinks/drains 4mA. My problem is to have the same PWM signal, that I generate using the dsPIC's output pin, in the Vout of the optocoupler. What do you suggest ?

Thank you, Pasquale.

What PWM frequency? MOSFET type & gate charge? You will NOT be able to drive the MOSFET gate directly with this part at any reasonable PWM frequency.

Why don't you try a logic-output optoisolator and a gate driver IC until you get things working? Or a push-pull opto designed for gate driving? You may have to put a bit more current into the input for either of those.. add something like a 2N7000 as a driver.

Best regards, Spehro Pefhany

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Do you think this won't work ?

Maybe now it's better. My problem is that I generate the PWM with the dsPIC33F which goes to the left LED. The PWM at the output of the optocoupler is negate. I'd like to have the same PWM at the input and at the output of the optoisolator. I'm try to find some solutions in driving the left LED and not to have some device after the optocoupler (such as NOT port or something like that).

Thank you, Pasquale.

Nope. If you buy the best 'bin' of that opto and drive it hard (or get lucky with CTR and/or don't care about reliability) you might be able to make it work very slowly and inefficiently.

So, use the 2N7000 I suggested to drive the input LED. High gate voltage = conducting. But the gate driver section still has problems.

Best regards, Spehro Pefhany

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And what do you suggest ? To change optocoupler ?

Can you sketch down drawing ?

Thank you, Pasquale.

Hi,

I'd like to ask, why do you need the isolation? I'd also suggest using something like this,

which operates at much higher frequencies than an opto ever can. You will pobably still need a MOSFET driver of some sort.

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DaveN

The GMR devices are really nice, stupidly fast and reasonably low power, but keep in mind that they are edge-triggered devices so their turn-on state is indeterminate (iow, could be 'on'). So you need to make sure your startup code toggles it at least once and leaves it in the correct state. There's a note to that effect in the datasheet. If one was paranoid it might be worth hitting the isolator with periodic "on" pulses (extremely low duty cycle) when it's supposed to be 100% off. Eg. a 100ns pulse every 1msec. Best regards, Spehro Pefhany

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I agree. So how you would manage that ? Any suggestion ?

Thank you, Pasquale.

You want *code* now? Sorry, posting code would be completely off-topic here and I could be strung up by my bolo tie. Hint: You can usually change the PWM to regular port pins (and back) on the fly.

Best regards, Spehro Pefhany

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I don't want any code. I agree with your discussion but you missed to ask which MOSFET I'm gonna drive (and I've also missed to write it down ;)). There are some MOSFET that can be driven directly using TTL port and 5V. I've asked you how you would manage the connection of the dsPIC and opto in order to have the same PWM at the input of the opto and the output...no *code* of course.

Sorry for the misunderstanding, I've really appreciate the consideration about the MOSFET driver,

Pasquale.

You most probably dont need voltage isolation. Just buffer the signal off of the PWM pin and got straight to the mosfet. You can use a microchip TC4427 and that would solve that.

Just remember to connect the digital electroncis to the power ground in only

1 spot that is directly on the source pin of the mosfet. You only want to reference digital ground to that point and do NOT want motor currents flowing through digital ground. You might even conect the two grounds with at 1 ohm resistor if there is a posibility of ground loops.

Thanks a lot. I think is really what I needed.

Thank a lot, Pasquale.