Photo Transistor Optocouplers

[DIYMark]

On page 14 of this (http://www.onsemi.com/pub/Collateral/MC33035-D.PDF) data catalogue, in figure 25 is shows a way to use a high voltage h-bridge via a lower voltage controller if you use an optocoupler and few resistors and a diode.

Now i was wondering couldnt i just hook up the ics output straight to the gate of the mosfet and still use a high voltage supply to the mosfets drain pin...as long as the fet and the ic share the same ground connection it should work shouldnt it?

if not and i have to use an optocoupler, does it need to be able to withstand the voltage i intend using for the motor? becuase the diagram specifies a 400 volt optocoupler but the Vm isnt going in that part of the circuit is it?

Any comments welcome

[AndyKunz]

You need to get the signal from the controller 5+V above the FET drain. It's kinda hard to do that w/o some powering circuitry.

You don't need (slow) optos any more, though. Look at iCoupler technology. It's a little more expensive, but FAST and provides it's own power (if you pick the right chip). Several companies sell inductive coupling chips.

Andy

[DIYMark]

5 volts above the drain - so if i have a 120 volt motor i need 125 volts on gate?

[AndyKunz]

Relative to the ESC V- input, yes. But if you float the gate, then only 5V above the source. 5V is a lot easier to make than 125V (and you really want more like 10V). The other reason is that you can't make it go too low to turn it off, either. If you go to too far below the source, then you damage the FET that way. Look at the Vgs ratings for your FET.

Remember also that the voltage on the gate is ONLY relative to the source. The source changes on the high side FET because it will vary between V+/2 and V+ as the low FET is turned on and off by PWM.

It's MUCH MUCH easier to just float the FET. I wouldn't know where to begin trying to do it any other way.

Andy

[DIYMark]

"float the FET" thats a new term to me (but then again im not that "Skilled" at electronics) could you please explain it a bit more?

Also, i take it from the above its possible to get away without an optocoupler?

[AndyKunz]

The high side FET needs to be "floating" relative to ground. It will be rising and falling with the PWM on the low side.

You can get away w/o an opto. Use one of these. (http://www.analog.com/en/prod/0,2877,ADUM5241,00.html) You still need a FET driver and an isolated supply to feed the FET driver.

The expense is avoided if you use a simpler circuit like that shown in the online examples.

Andy

[DIYMark]

ive also seen some where on the net an optocoupler that had somthing to do with mosfets ("IGBT/Mosfet Driver" or somthing like that) are these mosfet drivers or do they use a mosfet driver in the ic's circuit?

secondly, in that datasheet (page 14 figure 25) they mention Vboost (Voltage Boost) as part of a supply to the circuit - any ideas as to how much volts this actually is?

[DIYMark]

Got it - page 15 figure 30 on this (http://www.onsemi.com/pub/Collateral/MC33035-D.PDF) data sheet shows what Vboost is [as shown, but not explained in figure 25 (page 14)].

The mc1555 is just a tripple five timer and it seems that you input your Vm (motors voltage) plus 12v for Vc and the output of fig 30 is Vm + 8ish volts (so this is like you mentioned eariler of how you need to have a couple more volts on gate than their is on drain to power the fet).

So if i go this way all i need to order is some HV optocouplers and those fets - ill use this method because if the manufactuer chose it im sure it would work with their bldc ic.

Thanks for helping Andy.