Flyback transformer output current

[zdrvk]

Hello,
My question is:
What is the maximum current drawn from a SMPS flyback converter at 7V if the transformer is rated for 10VA, 12V at the single output?
Is this the same as with the regular transformers - 0.83A?

The converter is going to be used for supplying 7V from 85-275V AC at the input. The switching device is TNY268. The transformer is SPW717014 from HARTU.

Regards
Zdravko

 

[logbook]

Unfortunately life is not that easy. You have suggested that an ordinary mains transformer rated at 12V, 10VA can supply 0.83A. That is true, but only when the load is resistive. Manufacturers want to put on the best number they can and so quote the VA rating for a resistive load. In real life you are more likely to use a bridge rectifier and capacitor to get a stable DC voltage. In that case the RMS current will increase, making the copper losses higher. I have seen all sorts of de-rating factors according to what sort of load you applied to the transformer, but can’t put my finger on them at the moment.

Now to your question:
"What is the maximum current drawn from a SMPS flyback converter at 7V if the transformer is rated for 10VA, 12V at the single output?"

I assume the correct wording should have been:
"What is the maximum current that can be drawn from a SMPS flyback converter … ?"
Well certainly you would not want to be drawing more than 830mA RMS from the winding, but that may not be the limiting factor. The way you are driving the transformer must affect how much power can be delivered. Things like flux in the core, flux waveshape, frequency of operation, will be significant contributors to evaluating how much power can be pushed through the transformer.

 

[zdrvk]

Yes, this is the picture. But let me try to concretize my question. The regular transformers have some room for overloading. How is the situation with the flyback transformers as they are not typical transformers? As we know due to the air gap, they are storage inductors. In the TNY268 flyback design there is a current limited operation of the switch and a feedback control circuit. So if the transformer is rated for 10VA, 12V and if I am going to operate it at 7V, do I have some room to draw 1A continuously from the secondary winding. Or the current is limited from the manufacturer's choise of the diameter of the wire to at about 0.83A.

Thank you for responding.

 

[logbook]

Yes, I understand your query better now. You are wondering whether you can get more current out of the transformer because you are running it at a lower voltage, thus lowering the output VA. The simple answer is no.

Consider a mains transformer rated for 230V. It has a VA rating which tells you how much current can be drawn from it. If you now run the transformer at 180V, which can be done, the VA rating will be lower. It is true that the iron losses (core losses) will be lower at 180V but the copper losses at the same current will be the same. I suppose you might increase the current rating a little because the core is not getting quite so hot, but that is not a good way to go.

At the end of the day the VA ratings of a transformer is based on temperature. If it gets too hot, the self-fluxing wire on the transformer will break down, and the transformer will fail. If the transformer does not overheat then that power level is acceptable. The only warning is that the temperature has to be measured inside the winding, as the hottest point will be somewhere in the middle of the winding. This hot spot will govern the limiting power level. Only the manufacturer will really be able to measure this.

Why not just ask the manufacturer’s applications engineers? Failing that I would avoid overrunning the transformer in this way.

 

[zdrvk]

Thank you for the time spend to answer.
I am concerning to change the transformer.

Regards
Zdravko

 

[hacksaw]

you I2R losses in the secondary increase, at the same time you may need to check for saturation.