What does this circuit do in a flyback converter? - Page 1
Forum section: electronics / projects

author: serik5000
author_rank: Contributor
timestamp: June 07, 2019, 02:29:18 am
content: 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!

author: strawberry
author_rank: Super Contributor
timestamp: June 07, 2019, 06:04:53 am
content: I guess to remove startup overshoot . RCD soft startup circuit would be enaugh

author: ocset
author_rank: Super Contributor
timestamp: June 07, 2019, 08:04:19 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2019, 08:12:57 pm
content: 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

author: serik5000
author_rank: Contributor
timestamp: June 07, 2019, 08:58:23 pm
content: 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 TIThis PS also does not use a typical RCD snubber across the primary, just an RC one

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2019, 09:23:04 pm
content: Hmm, it's also a C divider, and loaded with a nice low resistance.I wonder if it's actually protection against capacitors drying out. Really unusual.See, if it were dV/dt, it should be one series cap, and a modest size parallel resistor. In quasi-steady-state ramping, Vbe = 0.6V and I = Vbe/R = C * dV/dt. If the 100nF were absent, this would give 1.35V/us, an unimaginably fast rate for a DC power supply with, I assume, electrolytic capacitors on the output.But with the C divider, we get a pole-zero transfer function, where at low frequencies, the response is asymptotically a differentiator, but at high frequencies it shelves to a constant 2/102 ratio. For high frequencies to activate the base, it would need Vbe * 102/2 = 30V. This would be on the order of the 100n * 221R time constant, or shorter than 22us (frequencies above 7.2kHz).This would basically only be plausible if the output filter capacitors went completely open circuit.I'd think a more sensitive circuit would be desirable if the concern is detection of capacitor aging. For example if the output ripple is nominally, say, less than a volt, but we want to detect and disable output if it's more than, say, several volts: then the series resistor and capacitor would still be used, but:- Instead of a zener as shown, a 1N4148 from GND to base would absorb reverse charge- Instead of 100nF B-E, a smaller value (or the 2nF being larger, and probably the 1.5k being smaller), say 4.7 to 10nF- Instead of 221R B-E, a larger value, say 10k, just to provide some ground reference without much loading the signal (this will give some dV/dt control as well)- An LED could be connected in series with the collector, giving visual indication of the protection circuit activating.But this even assumes anyone will look at the power supply ever, and not simply replace it whole, as is typical for maintenance (and that's in turn assuming the equipment itself is worth servicing at all). I suppose it might've been appropriate back in the day, but it's very hard to justify something like that today.Today, a "health monitor" sort of function might be useful; you could sample the error amp level, output ripple, input ripple (through an opto -- optos have poor gain consistency, so this would be a very rough figure, but that's okay), output current and voltage, and maybe some temperatures (ambient / on-board, heatsink, etc.), and just put limits on them and forget about them for the most part. Maybe have them viewable with a service or debug mode. But when a limit is exceeded, log an error, or report to the user or cloud or whatever, so that appropriate action can be taken.Which, as you can see, is quite a bit more complicated, but it's potentially the same sort of idea, just on a different scale. Most computers, test equipment, automobiles, etc. do this, at least in a broad way (POST and such). Expensive things -- expensive enough that it's worth spending the extra time to implement these features, to save on service time later.Tim

author: serik5000
author_rank: Contributor
timestamp: June 07, 2019, 09:33:05 pm
content: And here is the entire output section. Wire at the top goes to the Y cap and the one at the right bottom corner goes to the additional protection opto/13V Zener

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2019, 03:30:42 am
content: It looks to me like it just limits dV/dT during startup which can be important to limit current into a capacitve load.After startup, the base-emitter shunt resistor lowers the base-emitter voltage to zero and Vbe prevents the transistor from detecting ripple or noise on the output.

author: xavier60
author_rank: Super Contributor
timestamp: June 08, 2019, 03:45:53 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2019, 12:54:36 pm
content: Quote from: David Hess on June 08, 2019, 03:30:42 amYeah but at 1.35V/us? Show me a power supply with that rate, with these ratings, and I'll show you a unicorn!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 08, 2019, 12:59:45 pm
content: Quote from: xavier60 on June 08, 2019, 03:45:53 amAnother 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 bemanyvolts, about 30 of 'em, for this to even matter. That's ridiculous, even for the worst transformer and commutation in the world.Tim

author: serik5000
author_rank: Contributor
timestamp: June 14, 2019, 03:20:15 am
content: 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?

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2019, 04:04:27 am
content: Quote from: serik5000 on June 14, 2019, 03:20:15 amThe 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.