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Offline flyback with two isolated secondaries -regulation

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eem2am

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Hello,

I am doing an offline flyback:-

V(in) = 150VAC to 230VAC
C(in) = 150nF
P(out) = 14W
C(out) = 330uF

V(secondary 1) = 30V, 13W
V(secondary 2) = 9V, 1W

V(aux) = 20V

It is a fully opto-coupler isolated design.

The input DC Bus has at least 100V pk-2-pk ripple at 100Hz, due to the low C(in), which gives a reasonable power factor.

V(secondary 1) is regulated with a TL431
V(secondary 2) is un-regulated, & just relies on turns ratio to set it to 9V

V(secondary 1) & V(secondary 2) are the isolated secondaries, though are not isolated from each other.

V(aux) supplies the primary side PWM controller.



Now, I wish to get good regulation throughout all conditions of line and load.

I have read powerint.com AN18 but i did not see how to wind transformers to get good cross-regulation in such a case as my own.
-i.e. getting good cross-regulatioon for both of two isolated secondaries when one is un-regulated.

Do you think that the best way to ensure that the V(secondary 1) & V(secondary 2) and V(aux) all stay well in regulation is to wind the SMPS transformer as a “double sandwich”……………

Transformer winding as a “double sandwich”…………..->
https://i31.tinypic.com/2irrzus.jpg

if not, then how do you wind the transformer to get the best cross-regulation?
 

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