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UC3842/3 Compensation Feedback

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VanitarNordic:
Hi,

I want to design a Flyback, no problem with the transformer, Clamp .. etc. Now I want to design the feedback circuit using a TL431 and an Optocoupler.

1) How to calculate the Feedback component values for the UC3842/3?
2) How to regulate several outputs?

Yansi:
1) Not that easy, can be only estimated. See: https://www.ti.com/seclit/ml/slup340/slup340.pdf
Proper verification methods will either way be needid with real hardware. Additionals tweaks of the compensation will be likely needed.

2) You need a transformer with low leakage inductance (at least split the primare in two halfes: and put all other windinggs in between these layers). Too high leakage inductance an the output tracking will get mad.
Place the regulation loop on the most critical (or most loaded) output rail. 


T3sl4co1l:
Also, if the output rails are common ground, consider jointly regulating them, that is, taking a weighted sum from several (or all) outputs.  This doesn't fix the cross-regulation introduced by leakage inductance, but it helps the outputs track an overall average, which improves regulation on average (~equally weighted) loads.

Note that negative outputs have to be handled differently, because the TL431 does not have a noninverting input (or it does if you look at it the right way, but therein lies the difference of handling).

Tim

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