Cross regulation in flyback when main output is no-load?

Hello,

We do offline flyback with vin = 90-265vac.

We have two output rails

1. 25V, 30W

2. 12V , 10W


Rail 2 is completely unregulated....it just has the correct number of turns for 12V.

Rail 1 is 100% regulated , but often goes on completely no-load........when its loaded , it is loaded by quick, sudden, full-load trasnients, then back to no-laod again.

Do i have any chance of getting any kind of decent regulation on the 12V, unregulated winding.?

It would be very difficult to achieve. You may want to try interleaved winding for both secondary, so that leakage are in same range, and keeping winding resistance of first winding (25V) to very low would help minimize the voltage swing on second winding (when first one goes from no load to full load).

I've heard that a general rule of thumb for cross regulation on flybacks is around +/-6% at steady state. During large load transients, the transient regulation will be much poorer. Of course you can try and improve the numbers with very tight secondary coupling, low ESR windings, etc, but I doubt it would improve things by more than a factor of two. If you need good steady state and transient regulation on the unregulated outputs, then you'll need some kind of postregulation circuit. It doesn't necessarily have to be a linear or switching regulator; I've seen designs that make clever use of saturable reactors to make efficient secondary regulators (though I don't understand how they work %100). For transient load conditions, crowbar circuits can suppress overshoot while undervoltage transient suppressor circuits can help smooth out dips.