Flyback Optocoupler

[VolkerR]

Hi,

I need to design a simple flyback circuit, but I am having problems understanding the voltage regulation principle, mainly concerning the feedback optocoupler.

I was googling and reading datasheets for hours, but I was not able to find an answer to my question.

In most example flyback circuits there is a shunt regulator connected to an optocoupler on the secondary side. The regulator is set to the desired output voltage, and supposedly the optocoupler will signal when the voltage gets too high.

Why isn't the optocoupler LED forward voltage drop of importance to the output accuracy? The LED voltage drop can change depending on the current or optocoupler type. What kind of impact does it have on the output voltage?

This is such an obvious question, yet it is being completely ignored in datasheets or topology handbooks, why?

 

[Skogsgurra]

If your regulator consists of an opamp with PI feedback, the only thing determining output voltage is the reference voltage and the usually small offset error in the opamp.
If your "controller" is a zener diode plus optocoupler, then optocoupler fwd drop is important. But it is usually quite constant at some 1.3 V and the 0.1 V variation you see usually doesn't matter.

Gunnar Englund

http://www.gke.org

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