Homebrew PCBs

> BTW the discussion of external suppression diodes for
> mosfets is likely moot for devices where an internal
> parasitic diode is present.  The main reason to add
> an external schottky in SMPS designs is that of
> efficiency as the voltage drop across the internal
> diode is typically greater than available via the
> external schottky.


   The main reason for schottky is to make sure it turns on and takes the heat, 
if it is not schottky then the internal diode may turn on instead and the extra 
dissipation will go back into the transistor.  Very important it goes through 
the right one in a high powered circuit that's being run near the transistor's 
limit to start with.


   Oversize the FETs, make sure they have the intrinsic reverse diode, and 
forget about extra reverse diodes.  Big FETs will easily deal with 5 or 8 V, 1 
or 2 A steppers if everything else is correct, without any extra parts helping out.


   99% of general failure blown FETs come from blowing the gates either through 
poor handling or poor drive, and then get blamed on every other possible failure 
idea under the sun.  Difficult to even blow them on purpose after they're in a 
proper circuit.  Always install about everything else first, then the 
transistors.  You'll easily damage the gates in handling the board if they're 
installed first and the rest of the gate circuit isn't in place.

Alan