How does this flyback feedback circuit work? - Page 1
Forum section: electronics / projects

author: hkBattousai
author_rank: Regular Contributor
timestamp: October 25, 2014, 08:15:28 pm
content: I was readingdatasheet of SG6859A. I saw the example circuit at page 10, which is also added at the bottom. There is an opto-coupler which sends signals to the control circuit when the output voltage level exceeds a certain limit, and an IC called "voltage reference" which I don't understand what purpose it serves for.Can you please explain me how this feedback circuit detects output voltage level? As far as I know, SMPS circuits like this has very low ripple voltage at the output, and this small ripple wouldn't make any change in the brightness of the LED inside the opto-coupler (PC817 -datasheet). I feel that the IC TL431 (datasheet) has a critical job in the circuit function, but I don't understand it.Feed-Back CircuitEntire Circuit

author: qwaarjet
author_rank: Regular Contributor
timestamp: October 25, 2014, 09:49:22 pm
content: U3 acts like a programmable zener diode. the "zener voltage" it provides is set by resistor R13 and R12 (see figure 17 in the tl431 datasheet) the ratio should apply 2.5V to pin1 on U3 at the designed output voltage VO. Effectively this device will shunt current to ground to maintain the voltage of the cathode (pin 3) and VO at the same level. this shunt current will cause the LED in U2 to emit light that will sink current in the phototransistor that will pull the FB pin on U1 to ground. FB on U1 has an internal pull up to make this possible. The reason that this will work is that U2 is notandan on/off device but can be partially on at low LED currents. R14 and C10 are just for noise suppression.

author: Neilm
author_rank: Super Contributor
timestamp: October 25, 2014, 09:54:52 pm
content: This circuit is not only feeding back the voltage, it is also providing the loop compensation required to give a stable controlSimply put - the TL431 is programmed to the required voltage by R12 and R13. When the voltage on pin 1 approaches 2.5V, it will start to conduct. This will cause the PC817 optocoupler to start to turn on. R11 provides a current limit to stop it burning out.R11, R14 and C10 provide the loop compensation. The latter provide the shaping, while R11 provides gain.Hope this helps[EDIT: rats - beaten to it]

author: hkBattousai
author_rank: Regular Contributor
timestamp: October 25, 2014, 10:39:07 pm
content: Quote from: Neilm on October 25, 2014, 09:54:52 pmThe point that I don't understand it, will such a small voltage change on U3 really turn on and off the optocoupler?Let us assume that the output ripple voltage is +/-100mV (a reasonable assumption, right?). That will make the voltage change on the pin-1 of U3 +/-50V; which makes a 100mV full swing. That LED inside the optocoupler is an ordinary LED, isn't it? When I light a LED on my breadboard (by connecting a series resistor and applying a supply voltage), I don't see a critical supply voltage level which immediately turns off the LED as soon as I apply less than 50mV of it. What I observe is, as I decrese the supply voltage, the LED slowly gets dimmer and dimmer until the last tiny light hardly visible on the junction point disappears slowly. While the LED light is weak, the transistor (inside the optocoupler) will still be in conduction (but not saturated anymore). However both your messages explain it as the optocoupler is either on or off, and there is no other state between them.Can you please explain this point.

author: qwaarjet
author_rank: Regular Contributor
timestamp: October 26, 2014, 12:04:33 am
content: Sorry Typo on my first post. the LED is NOT just on or off. LEDs are very non linear devices so it best not to think in terms of voltage but current instead. A few tens of uA is enough for the LED to be making light, normally you won't see this in a liteup room and since you normally are using a voltage controlled supply a very small change in voltage will make a large change in current so it goes from a visible amount of light to none very rapidly get a constant current supply that has uA precision and you can control the light amount much better so as to make gradually fade away. The phototransistor is going to be sensitive to very small amount a light. When the system is in steady state operation there may only be a few hundred uA going through the LED this will not be enough to put the transistor into saturation but it will be in the active region which will provide the necessary feedback control.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2014, 04:02:09 am
content: DON'T think of TL431 as a "variable zener". It's not! It's an op-amp with a rather large, but rather stable, input offset (2.50V), grounded noninverting input, and an output stage that's missing the pull-up transistor (so it only pulls down as a current sink).So, the voltage divider from the output (R13-R12) senses output, and the difference from 2.50V is gradually amplified (the amp itself has a dominant pole characteristic; the R14-C10 compensation network augments this to suit the SMPS' frequency response) until, through the feedback loop, the output voltage stabilizes accordingly.Ripple at line frequency is attenuated by way of the op-amp turning slightly more or less on (certainly not fully on and off -- such language is very sloppy indeed, and often leads to self-confusion rather than understanding). Near line frequencies, the loop gain is very high, so this is pretty good. At higher frequencies, the loop gain is lower, until somewhere near the switching frequency, it goes away completely; at these frequencies, the output filter caps (C9, and maybe C8 as well -- the schematic doesn't give enough information to tell what operating mode is used) must provide a low impedance to filter ripple instead.Tim