Revisiting The Photocoupler
Remember those noisy-looking feedback photocoupler output waveforms I did not like earlier? Now that I have something that seems like a definitive fix, I decided to go back and have another look:
And sure enough, these look a lot more like what I was originally expecting. Here, VF is the voltage across the photocoupler's LED on the secondary side, while VCE is the voltage across the photocoupler's collector-emitter terminals on the primary side. As with previous photocoupler waveforms, nothing exciting happens before the 5VSB voltage actually reaches 5V. Until then, VF simply follows the TL431's minimum operating current. The VCE ramp merely follows the 5VSB and AUX2 voltage ramps since it is open-circuit. What changed is that we have no more negative spiking and no more 10-15V peak-to-peak noise on VCE, only a relatively smooth control signal with the error output from the 5VSB's TL431 coupled through the 817 on top. Putting some load on the 5VSB rail causes the TL431 to draw less current through the photodiode to maintain its reference voltage, which in turn reduces the phototransistor's current and allows VCE to increase, reducing the 5VSB transformer's triggering rate and power output.
On the unloaded waveform, you may have noticed how the pulse rate increases as the output voltage ramps up. The reason is that it takes longer for inductors to dump energy into low-impedance loads like discharged capacitors. You've probably heard the classic xenon flash capacitor bank charging noise: a low coil whine that ramps up to ultrasonic pitch over a few seconds. It is caused by the exact same principle.