NMOSFET input protection circuit

[Teun]

I found this circuit being used on a 150W DC-DC converter:


Is D1 nessesary? And if so, why?

[mvs]

MOSFET gate-source voltage is usually limited to 20V, so D1 is probably 12V to 15V zener diode.

[Teun]

Ok that would make sense. But the max input of the converter is 90V. I only have a 60V power supply, and at 60V the voltage at the gate is 21V. It seems to be rising linear to that point. So could it be a MOSFET with a higher Vgs then?
If so it would have been easier to just adjust the voltage divider right?

[fourtytwo42]

This looks like a fragment of a larger circuit and as such it doesn't seem to do anything by itself, what's the load ?

[Teun]

It's the input protection of a 150W DC-DC converter. So for this the load would be a CM inductor and then the transformer etc.

Here is the typical application for the controller used:

[mvs]

Ok that would make sense. But the max input of the converter is 90V. I only have a 60V power supply, and at 60V the voltage at the gate is 21V. It seems to be rising linear to that point. So could it be a MOSFET with a higher Vgs then?

There is no reason to drive most MOSFETs with such high voltage, even if they do have 30V Vgs max.
Perhaps it was a mistake at production and 1N4148 was used instead of zener... Who knows?
Do you know the part number of used MOSFET?
 

If so it would have been easier to just adjust the voltage divider right?

Divider should be chosen so that MOSFET is fully open (e.g. Vgs=10V) at min input voltage.

[S. Petrukhin]

This diode probably limits the locking voltage at reverse polarity.