Ideas on debugging TxFer issues on Push Pull circuits

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phaedrus

Member
Hi ,

For a bit of background , this is the same project here :
http://www.diysmps.com/forums/showthread.php?473-0-500V-SMPS-based-on-SG3525
In a nutshell , i am building a converter
12V ---> 400V(40mA) output.

I have one transformer which works as required and give the required performance with and without load.Under a load of 10k , it drops by about 10V,which is fairly acceptable.
Subsequently I had 3 more made of the same type.These give very poor performance.First the open voltage does not go higher beyond 380V.On the same load of 10k,the voltage drops by 100V.Except for the transformer , everything else is identical.
I am not able to get any meaningful inputs from the guy who wound the transformers.Are there any checks I could do to find out why this is happening ?

TIA.
 
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KX36

New member
Sorry for the late reply.

Should make clear for anyone else reading that the input winding drawn above is actually the secondary and the outputs are the primaries, so we expect it to step down voltage through this as the design is a step up transformer. The output voltages seem to be about an order of magnitude out from what we'd expect from your design though, but that must just be a transcription error.

The polarities look correct, but obviously the turns ratio is not the same between transformers. The secondary voltage on the bad one would be 73% of that on the good one. If you don't have much headroom in your duty cycle, the maximum voltage that is well regulated will be lower.

I can't say if this mistake is yours or whoever wound the transformer as I don't know what specs you gave them. I'd just wind your own transformers so you know what you're getting.

You reported in the other thread that your max output voltage on the good transformer under 10k load is 380V. (set to 400V). 380V*73%=277V, a figure suspiciously close to what you reported the max voltage under 10k load to be. (380V-100V=280V).

My guess is that you're hitting the ceiling of achievable duty cycle there, at the same point in both circuits, but at a lower output voltage in the transformer with lower turns ratio. My quick design made the assumption that you could achieve 90% duty cycle, which might not be completely practical, especially as you're doing a push-pull on voltage mode control, something I continue to discourage because of the risk of flux-walking. (And flux walking might be part of the problem. I don't remember if you've got any primary current limiter to avoid flux walking into saturation but if you do, that could be prematurely turning off the transistor and so decreasing the output voltage.)

What you might have to do is measure the max duty cycle you can get out of your "good" circuit under load and recalculate a turns ratio so that this is not exceeded. Remember that duty cycle is highest when the input voltage is at its lowest and the output voltage is at its highest. You may then have to recheck voltage tolerances on secondary rectifier, whether the compensation network is still valid etc.

If you do have a flux walking problem, you might find that successive cycles do not have the same duty cycle, extrapolated to the extreme you might see 0%, then 100%, then 0% as the transformer saturates one way, which if you're not careful you might think is a perfectly healthly 50% duty cycle except that it'd be at the wrong frequency. I don't think it will be this extreme though as you'd probably already know about it if it was.
 
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phaedrus

Member
Hi Matt,
Thanks for the reply.
As you correctly deduced,I had made an error in noting down the parameters of the "good" transformer and gave the wrong details to the winder.But as a result of this mishap,I have wound some more turns on the HV side,with the improvement that with the 10k load the voltage is now drooping by only 3V as compared to before.
I have not yet put the current limiter as yet,"currently" I am working on stepping up the 50~400V to 5~30kV pulsating V. Designing the IGBT snubber for this part is a bit of a challenge.

"I'd just wind your own transformers so you know what you're getting."
Thanks , but already done ;-).
 
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