Flyback - Isolated feedback with aux. winding

[berni80]

flyback with feedback winding

Hi,

has anyone some experience on this kind of flyback circuit?

I having problem to minimize error between the feedback (from the aux. winding) and Vout.
This mean an excessive variation of Vout under load.

The voltages across the transformer (secondary and aux winding) are very similar but their rectified version are different.. under load condition the output in -20% form the fb.

Do you have any idea to investigate?



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[FvM]

islated feedback controller + flyback

The effect is by design, due to the transformer leakage inductance. Some reference designs with primary respectively auxilary windings voltage sense use a series resistor with D2 and tune this resistor and C2 to minimize the load dependance.

You may want to consult respective reference designs or figure out your circuits behaviour either empirically or by simulation with realistic transformer parameters.

 

[berni80]

feedback winding flyback

Thanks, I'll try with this resistor.

Do you have any docs/AN to advise me about this?

 

[FvM]

No, I just remember a discussion in a datasheet, maybe some Powerint device. And I'm not sure about the achievable quality, thus I would always prefer secondary side sense with an optocoupler.

 

[eem2am]

auxiliary coil feedback control is always going to be rough and ready especially with offline flybacks

...but you only do low voltage.

so you need to increase coupling between aux and secondary by winding them with each other, say.

or you wind secondary in two sections and "sandwich" the aux coil between the two sec halves this will also increase coupling.

you may find that near maximum load the agreement is better.

continuous mode is better coupling given than discontinuos mode.

BUT....i do not understand,.......because at such low voltage , howcome you dont just directly use the secondary to do feedabck???...
..this is your biggest mystery to solve b4 anything else.

 

[FvM]

...but you only do low voltage.

doesn't necessarily mean no or only low isolation voltage!

 

[berni80]

doesn't necessarily mean no or only low isolation voltage!

You're right, I need 1KV of insulation.

 

[eem2am]

thankyou FvM & Berni80 for reminding me.

That isolation requirement will certain stop you getting good coupling between the aux and the secondary coil.

but now yuo can fiddle it to work if the load is always constant.

but i dont know if this is your case?

have you tight requirements for secondary voltage transient response and static error?

so yes i can say just use opto.

tho' there is a way of doing it where you put small inductors just before the output diodes on aux and sec coils.....the size of the inductor is some ratio involving the number of turns on the other winding.

but yes, I cannot think of better solution than FvM's idea with opto isolation.

 

[yus]

you might be interested in reading the paper/patent done by Systems General Corp.:
Control circuit for controlling output current at the primary side of a power converter
I believe, they already have the IC (8-pin PWM controller IC) for this application.. (i forgot the part #)
basically, it eliminates the use of an optocoupler then getting the feedback from the aux winding, and still achieve a good regulation at secondary..