I tried to use this RDR242 design which uses TOP266VG offline switcher.
Right now my issue is with instability of the circuit. There's ~12VDC at the output, but it drops when I connect a load, and doesn't recover back to 12VDC fast enough when I remove load.
Also, not enough voltage at C7, which is near aux/bias winding, it's around 5.9V, instead of 9V.
This is the original RDR242 schematic. This is my schematic in Altium Designer. I changed a few components as not everything that's on the original old schematic is available nowadays. For instance, Shunt regulator U3, T1, C3 input capacitor etc.
For U3, shunt regulator ATL431 I used such values of resistor dividers R21, R23 as to yield 2.54V at its reference pin as this is the minimum ref voltage that it will work on, compared to LMV431 shunt regulator on the original RDR242 version, which was set to 1.24V at its ref. pin.
I put TOP266VG into 66Khz switching frequency between its drain and source pins, by connecting F pin to C pin, as suggested on p.3 in datasheet.
For T1 I used an ER25 transformer with the following parameters. Originally it was intended for Viper22A circuit, and it worked fine at 60Khz MOSFET switching frequency at primary winding. There was 12V at the output, and about 24V at auxiliary winding.
Yet when I try to use this 25W ER25 transformer in RDR242 circuit, I get voltage that's about 11.7-12VDC at the output (X1, X2), and drops when I connect a light load. C7 near aux winding shows about 5.9V at no load. I also noticed that the frequency at TOP266VG's drain and source pins doesn't go above ~2Khz (I monitor those pins continuously with an oscilloscope).
Here's my PCB design. I connected transformer to PCB with wires. As you can see from ER25 parameters, its aux winding would yield about 24V when its secondary is at 12V (since aux has twice the turns that secondary) and it appears that original rdr242 design was intended for aux/bias winding that would yield 14V, as VR3 zener diode would start reverse conducting at above 15V and the V pin of TOP266VG would receive too much current, prompting shutdown. I tried to use an LDO that would output 12VDC, but the circuit didn't work. However, I read that TOP266VG uses power from the opto-coupler's feedback path, and as long as C7 is above 9V, the U2 opto-coupler would be properly biased. Yet for some reason C7 doesn't go above 5.9V at no load. I also noticed that the 15V 200mA diode D7 would fry up quickly, so it appears that too much current is drawn from the aux winding, I tried lowering its theoretical 24V with an LDO or resistor divider network which would have also limited current, but ended up simply putting a resistor in series with D7.
Through experiments I realized that an SMD 0805 1/8W resistor with a value of less than ~30Ohm, can't handle whatever current is passing. So I tried replacing it with like 120Ohm to limit current and D7 was able to handle whatever current was passing (<200mA for sure), and the circuit kinda worked fine, except the voltage at the output would drop when I try to connect a load, such as Nanopc T4 single-board computer.
Why isn't this circuit working? Feels like I messed up transformer polarities, but I checked it and it seems fine. Perhaps I should fix the opto-coupler feedback network? That could be the reason my circuit doesn't work properly/has stability issues.
Over here a guy used TOP259EN controller with 12.2VDC, 9.4A output. If you look at his his circuit diagram you'll see that he used different resistors for biasing opto-coupler feedback network. There's such thing as a gain for opto-couplers, perhaps I should lower it for my circuit? I don't want to mess up feedback network though, so is there a tool that would allow me to calculate proper values of resistors and where to put them at the feedback network? I don't think just copying his opto-coupler resistor feedback part of the circuit is a good idea as my current limit is not 9.40A, right?