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| What does this circuit do in a flyback converter? |
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| serik5000:
Hi Everyone, Could anyone tell me what the highlighted circuit with a transistor does in the feedback loop of the AC/DC flyback converter with synchronous rectification? The rest of the circuit appears to be a text book case of the compensated isolated converter with the exception of those additional components. My best guess so far that it might be some sort of a soft start. 2nd interesting thing about this converter is that Y capacitor was connected between DC rail negative/HV ground and a +12@VDC output instead of 0V/output GND. Would that be for any reason other than PCB real estate? Thanks a lot in advance for your input! |
| strawberry:
I guess to remove startup overshoot . RCD soft startup circuit would be enaugh |
| ocset:
yes its for soft start, and removal/reduction of startup overshoot..........it works by stopping the error amp on the pri side from getting railed high at startup, so it reduces the chance of overshoot occurring. |
| T3sl4co1l:
Yeah, could've been a C in parallel with the 680+2.21k resistors just as well (with some adjustment to the compensation RC). I haven't seen that before. Usually it's something like a crowbar (zener or second TL431 and SCR), or another opto for enable, etc. Y-caps can be placed pretty much anywhere, as the large bypass caps make DC+/- act as a supernode. Yes, probably placement and layout. Tim |
| serik5000:
I still don't quite get what happens on the startup. So initially transistor is closed and then voltage starts to rise charging 2nf cap via 1.5k and 221R resistors and until Vout reaches threshold of 7.5V Zener. And at that point 2nf cap is shorted to the ground via Zener and 221R. What happens to the transistor? And how does it affect the opto? Btw, this PS was designed in Germany, controller IC is SG6859A. There is also a 2nd opto with a latch on the HV side to shutdown Vcc to the controller if output voltage exceeds limit set by 13V Zener, when primary opto fails. I did see a very similar reference design from TI This PS also does not use a typical RCD snubber across the primary, just an RC one |
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