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What does this circuit do in a flyback converter?

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T3sl4co1l:

--- Quote from: xavier60 on June 08, 2019, 03:45:53 am ---The 7.5V zenner disables the slew limiting early.

Extra: The TL431's feedback RC also causes slew limiting, but not for the first few volts of rise. Maybe the mystery circuit is just to provide slew limiting when the TL431 can't.

--- End quote ---

Another good point -- it's dead once operating.

What if... the answer is staring at us?  What if that sync rect makes a horrible mess at low voltages, tons of shoot-through (or inoperative entirely and it's hitting body diode reverse recovery to the same end)?  What if those surge currents flow impudently over the filter cap, generating many volts of high frequency ripple, activating the circuit and folding back throttle so it starts up more softly?

But again, it has to be many volts, about 30 of 'em, for this to even matter.  That's ridiculous, even for the worst transformer and commutation in the world.

Tim

serik5000:
I will try to take some measurements on it and may be try disconnecting it to see what changes.
Couple more things about this supply I don't typically see in other designs:
1. There are 2x 2.2M resistors going from the positive rail to the current sense input.
2. MOSFET drive circuit has different resistors for turn on and turn off. That would be typically beneficial in a bridge topology to avoid shot through, but in a flyback? 

xavier60:

--- Quote from: serik5000 on June 14, 2019, 03:20:15 am ---I will try to take some measurements on it and may be try disconnecting it to see what changes.
Couple more things about this supply I don't typically see in other designs:
1. There are 2x 2.2M resistors going from the positive rail to the current sense input.
2. MOSFET drive circuit has different resistors for turn on and turn off. That would be typically beneficial in a bridge topology to avoid shot through, but in a flyback?

--- End quote ---
The 2.2M resistors lower the current limit at higher mains voltage.
If a single Gate drive resistors is used and assuming 0V to 12V drive and 4V Gate Plateau, the resistor will have 8V across it  while the MOSFET is being turned on and only 4V while being turned off.
The extra diode and resistor give independent control of on and off drive current, usually to make them the same. Sometimes the on drive is purposely made lower than the off drive where lower  turn on current is expected.

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