SG3525A power suplly

[maher471]

Hi, Today I have an old switching power supply to be repaired based on the circuit SG3525A, which the bridge rectifier and all the transistors are exploded (2 MOSFET IRF740 and 2 BJT BD135). The problem is that is repaired by another repairer and following that I'm not sure of this repair, I ask you if you can confirm the type of the 2 command transistors: Q6 and Q7 if they are NPNs like schema or PNPs, and thank you for 'advance. in the same case, and basing with the value of the mosfet IRF740 and the output diode BYV72, what is the best value for the bridge rectifier BR.
The schematic was traced by me, so I suppose I have traced it correctly.
and what is the exact role of Q6 and Q7 in this configuration

[xavier60]

I have never seen transistors used in the Gate circuit as Q6 and Q7 are shown.
If they are supposed to be PNP, then the presence of C8 and C9 makes no sense.
If it really is meant to be as shown, the only reason that I can think of is to reduce the On drive current which is commonly done by other means.
Also from my experience, the SG3525A suffers a lot of stress trying to supply drive current directly.

[xavier60]

Actually I was misinterpreting the Gate drive circuitry. Now I think its purpose is  to prevent small voltage fluctuations during the dead time from being applied to the Gates. The  SG3525A will be doing a poor job of clamping the dead time drive at near to zero volts.

[maher471]

hi, thanks for the answer, but can you be more detailed on how it works I can't understand you.

[xavier60]

When the driver transformer outputs a small  positive voltage, little of it gets applied to the Gate  because Q7 is off. When the drive voltage gets high enough, Q7 turns on and all of the drive voltage gets applied to the Gate.
Because this seems to happen at about 0.6V, it doesn't seem to be worth while. It would make more sense if there was another resistor between Base and Emitter of Q7 to raise the drive voltage at which it's turned on.
The whole control circuit can be powered from just a bench supply so that the drive waveform to Q2 can be safely observed.