Flyback Voltage Feedback Quality

Question

I wonder about the "quality difference" between using an secondary side reference + optocoupler to provide voltage feedback in an isolated flyback design and using the third winding of the transformer.

As far es I understand things, "better" regulation is possible with first variant because the tolerances of voltage reference and optocoupler can be much tighter than the tolerance of the transformer winding ratio from secondary side to the third winding.

Now assume that i am happy with an output voltage regulation of +/- 10% , are there other benefits than better output voltage regulation when using the optocoupler variant? I heard that load regulation should be better but don't know why this should be the case.

I also saw that in the variant with third winding, the voltage is more or less (rectification) fed back to the controller FB pin whereas with optocoupler feedback you can directly "bypass the controller's error amplifier" because the optocoupler's feedback is a current signal that can be fed into the output of the controller's error amplifier(i.e. the compensation network). I don' t know if this is somehow a benefit.

NOTE: All the text abobe assumes an isolated flyback design with current mode regulation and a constant frequency controller. Also, I assume to only have one output, so please no explanations concerning cross regulation topics.

Answer 1

Load regulation will tend to be much worse with transformer feedback because the windings are not perfectly coupled - there is leakage inductance in a real transformer. This can be minimized by keeping the feedback and secondary windings as close as possible but there are limits to that since galvanic isolation depends on the insulation between the two windings. To put it bluntly, if the wires short, people could die.

The voltage across the output rectifier diode(s) will also increase with increasing load current and the average voltage across the filter caps will decrease a bit, and there will be drop due to non-zero output winding resistance. None of these changes occur in the feedback winding so they will not be automatically compensated.

You can see these effects in DC-DC converters where only one output is regulated, an loading the regulated output causes the other outputs to increase in voltage. With minimum loads applied this can be tolerable, but it is very noticeable. It really is a cross regulation thing- the feedback winding is another (lightly loaded) output that happens to be tied to the primary side.

It may be possible to (partially) compensate for these factors by fiddling the feedback a bit with increasing primary current, and I believe some controllers adopt this strategy to avoid the opto. Another approach is to follow the poorly regulated output with an LDO linear regulator.