I found this circuit while looking for ways to isolate 12V; what is the purpose of D2?

 

D2 aids in ending the snub current by converting it to heat and dissipating it.

 

clipping circuit

 

Like pwm

 

 

View attachment 240351View attachment 240351
clipping circuit

That makes sense, but is clipping important for an inductive load in this case? Especially if it's just being used for isolation? Also, how does using a Zener in the flyback circuit affect the secondary voltage versus just a single diode, if at all?

 

D2 aids in ending the snub current by converting it to heat and dissipating it.

Hm, to me it seems kind of pointless to have a Zener on just for heat dissipation, especially when it's only on a 12V circuit. If that's all it's for I could get rid of it without causing harm, correct?

 

The Zener raises the voltage when the the flyback diode is conducting, which speeds the collapse of the field. This is normally done on a relay or solenoid to speed up the turn off time. Not sure why it would be used in this circuit, or why any flyback is needed at all, since the load on the other side of the transformer will absorb the energy.

Bob

 

This normally done in a flyback circuit with one Zener from the MOSFET drain (Zener cathode) to ground, to absorb the high voltage spike if the flyback output has no load.
In that case the Zener has a voltage rating of perhaps 75% of the MOSFET maximum Vds.

The circuit shown seems to be overly complex for what it apparently does.
If it's trying to provide some regulation for the output voltage, that's a poor-efficiency way to do it.