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Understanding primary side regulation in this Flyback converter

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Red_Micro:
I'm going through this Reference Design and there is something I don't get. The controller LM5020MM-2 was not designed for primary side regulation, how does this power supply regulate through the primary side? How precise it is? Does it require certain minimum load to regulate?

ahbushnell:

--- Quote from: Red_Micro on August 10, 2020, 03:14:33 am ---I'm going through this Reference Design and there is something I don't get. The controller LM5020MM-2 was not designed for primary side regulation, how does this power supply regulate through the primary side? How precise it is? Does it require certain minimum load to regulate?

--- End quote ---
It is regulating on the  5/6 secondary of the transformer.   This feeds the voltage divider made of R17 and R30. I'm sure it's not tight regulation.   It's simple and provides isolation. 

eblc1388:
I also has trouble understand how two MOSFET Q1 & Q2 rated at only 800V can share the high voltage when the supply voltage is 1260V.

Would OP or others shed some lights on the mechanism of this voltage sharing?

magic:
Q2 on: Q1 source is at 0V and gate is at 15V, Q1 is on
Q2 off: Q1 gate is pulled to 600V and source rises up until it reaches 600V, Q1 turns off

I couldn't promise that everything here works out alright, but it's a TI reference design so they hopefully tested it.

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