What is the purpose of the three diodes in this gate drive transformer circuit?

[treez]

Hello,
What is the purpose of the three ringed diodes in the attached gate drive transformer schematic?
(Ltspice simulation also provided, including the same gate drive circuit but without the said diodes..there seems to be no difference)

(the GDT is actually for a buck converter FET, but the buck is omitted here for brevity. The circuit was given to us by a consultancy but they didn't say what the diodes were for)

 

[dick_freebird]

Looks to me like they are trying to protect the PNP from
E-B breakdown (always a good thing to avoid) and B-C
forward bias (which could perhaps leave the PNP engaged
(saturation storage time) when it should quench smartly
after "doing its thing", and as a result bother the subsequent
turnon pulse. Maybe something of a min-off-time issue
(commanded off-time less than tSaturation could get
flakey or even destructive to the power FET).

 

[FvM]

A silicon diode in place of D3 is definitely useless, I assume also the other two under regular operation conditions.

A schottky D3 could be used to prevent transistor saturation.

 

[treez]

A silicon diode in place of D3 is definitely useless, I assume also the other two under regular operation conditions.

A schottky D3 could be used to prevent transistor saturation.

Thanks, they are 1n4148 which is silicon, so you are saying this is bad, we should use three schottkys in their places instead?

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Indeed having the bottom of the three diodes as a schottky really cleans up the gate drive signal......the upper two of the ringed diodes don't seem to serve any purpose?

- - - Updated - - -

Looks to me like they are trying to protect the PNP from
E-B breakdown

..thanks, but do you think that the top gate driver circuit in the pdf of post #4 gives all the protection needed to the EB junction of the PNP turn-off transistor?......since with the diode connected across the EB junction of the pnp, the pnp can never get more than 0.7v in reverse bias....surely the two top diodes that are ringed are not necessary?