Re: [TCML] flyback driver sucess.

[Scott Bogard <sdbogard@xxxxxxxxx>]

It was an optocoupled FET driver, 2.5A peak output, and it still works
surprisingly...  I'll work on a schematic, it was pretty plane Jane
however.  After thinking about it, I'm suspecting it overheated, the
heatsink was very undersized (a 2*3 inch * 1/8 aluminum plate) and the
transistor was not fastened tightly.  It might have been voltage killed, it
was not running zero current or zero voltage switching at all, and I had no
snubbing of any kind, but it was a 400V FET if memory serves, and it was
only running 25V...  I'll try again using a proper heat sink with thermal
compound and see how it goes...  Thanks for the quick reply!

Scott Bogard.



> Do you mean an optocoupled FET driver, or just a plain phototransistor
> optocoupler?  If possible give us a schematic.
>
> >  Also I believe it was pumping out 18V on the output, is that too much
> > voltage to drive a gate, I believe they are 5V gates...
>
> It's possible, but most power FETs can handle gate voltages of 20V at
> least,
> even if their threshold is a lot lower.  You'd have to check your datasheet
> to be sure.
>
> Another possibility is that you exceed the drain voltage the FET can
> handle.  You'll probably want a clamp on the primary side, and maybe some
> kind of snubber as well.  And you could have just given it more current
> than
> it could handle.  Anythings feasible, but it's hard to be sure without some
> hard numbers to go on.
>
> -Mike
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