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UCC28C56H-Q1: Poor Load regulation

Michael Cain
Prodigy 30 points
Part Number: UCC28C56H-Q1

Tool/software:

Hello All,

I am using the UCC28C56H-Q1 controller in a flyback design to take 120V nominal and output 5V at up to 25W. If the output load is 10 ohm, the output voltage is 5V and power is ~2.5W. When the output load is decreased to 1 ohm, the output voltage should be 5V, but instead it droops to 4.3V and ~19W. I would expect a lower voltage due to resistance in secondary windings and output inductor, but this is worse than expected.

Any suggestions to output load regulation or improve any other part of the circuit?

  • 0
    TI__Guru 74115 points

    Hi,

    It depends on the coupling from Vout to AUX. The regulation is on your AUX - you should see good voltage regulation on AUX voltage. How good it is on Vout depends on how much coupling from Vout to AUX.

  • 0 in reply to Hong Huang
    Prodigy 30 points

    Hello, 

    Thanks for the quick reply. Currently the inductor coupling is 0.98 between secondary and aux so I would expect the voltage ratios between AUX and Vout to be consistent. When the output load is 1 ohm, Vaux is 17.57v, Vout is 4.3v, the ratio is 4.086. When the output load is 10 ohms, Vaux is 17.6v, Vout is 5v, and the ratio is 3.52. 

    Why do the ratios change when the coupling is fixed and is there anything I can change to improve it?

  • 0 in reply to Michael Cain
    TI__Guru 74115 points

    Hi,

    If the AUX is in regulation then the Vout regulation is the transformer effect or transformer model effect. On the IC side, nothing can help. Your transformer model is rough and whatever you can get you need is the model you need to adjust. As there is no good model for transformer - you may need to talk to PSpice vendor or do some research, as no good transformer model existing.