Flyback power supply and slave output regulation and problem

core ei40

Hi everybody

I have designed a flyback power supply by uc3844 and i have 5 isolated slave outputs, one of them is for 3844 Vcc and i want to use 4 others in my system. I have isolated the master output feedback (+12) by means of optocoupler and my load regulation in master output is so good (from no load to 3 amperes). I have a little gap in my core (EI40) for preventing saturation. but i have some problems:

1- when i load my master output, my slaves get high, for example when i load my master output 3 amperes, my +8 output gets +15 volts. how i can get my cross regulation better? is this natural?

2- when i have no load on my master output, i cant load my slaves as i want, my
slaves gets so low. how i can make them beter?

3- my core gets so hot in full load, is this natural (about 50 watt output) ?

masoud

Re: Flyback power supply and slave output regulation and pro

Hi,

1.Yes,unfortunately this is a very common problem.
You might try solving it (partially) by adding RC snubbers across the secondary windings.

2.Try connecting small dummy loads to the slave outputs.This will reduce the difference between no load and full load.

3.Define 'so hot'.
Is this transformer designed properly?
Usually they don't get hotter than 50 C.

Re: Flyback power supply and slave output regulation and pro

Crossregulation is always present in multioutput supplies.
It can be improved by properly sandwiching the windings in the transformer, but you cannot expect better than 6~8%. The flyback actually is pretty good when it comes to crossregulation. Are the master and the slave windings placed so as to maximize coupling between them?
You can try to add a bleeder to the master output to improve crossregulation somewhat, but you cannot expect miracles.

The fact that the transformer has a gap is not the guarantee that it will not saturate. Your 3844 will prevent that from happening, but the flux swing can be large enough to cause overheating.

You are saying the transformer gets hot. When you calculatd the transformer, how much flux swing did you allow?
Are the windings sized properly?