Off-line SMPS with auxiliary coil

[eem2am]

Please consider Transformer isolated, Off-line SMPS's with auxiliary coils.....eg...

-when the load is removed, the auxiliary coil falls out of regulation and can't power anything...

(..this is especially so in "skip-cycle" smps controllers which skip cycles on light load.)

so why do people bother to use an auxilairy coil ?

why not instead just use say a LNK304 IC ...

http://www.powerint.com/sites/default/files/product-docs/lnk302_304-306.pdf

or TNY274 to give the auxiliary voltage? ............
http://www.powerint.com/sites/default/files/PDFFiles/tny274-280.pdf


An auxiliary coil also vastly increases the cost of an smps transformer because it will have to be hand wound (no machines can wind an off-line smps transformer properly and certainly not one with three or more coils)


So i ask why do designers bother with Auxiliary coils?

thanks in advance of any thoughts.

 

[FvM]

The shown application circuit serves a purpose. The designer apparently regarded an ausxilary winding as the most simple way to power the control circuit when the battery is shorted. Of course other options exist as well. They probably will use other part and don't fit an UC3909 datasheet.

I think, that most manufacturers would also contradict your opinion regarding costs of an auxilary winding. But if you can avoid it, it's better of course.

 

[eem2am]

thankyou,

..i gave the above picture, but would like also to talk more widely about auxiliary coils, which i find always let me down at light_load and especially no_load.