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UCC28711: Poor cross regulation

Jean Eddim
Prodigy 120 points
Part Number: UCC28711
Other Parts Discussed in Thread: TIDA-00173

Dear TI E2E Team

My smps includes a UCC28711 (primary side regulation), I designed it for 4 outputs (2 of 12v and 2 of 24v).  Output current is 0,2A form each output

The input is 200 to 1000 VDC

The power supply is working correctly with an input voltage between 200 to 500v. But from 500 to 1000v i get a poor cross regulation.. The 24v is reduced gradually to 16v, and the 12v output goes up to 18v

The lose of regulation is gradual, according to increase input voltage

My question is: why do I get this out of specification voltages with high input volages? Poor cross regulation?. How can I solve this problem?

Please tell me which points to test in the ic. The schematic is similar to TIDA-00173

Is it the UCC28711 adequate to multiple outputs? better to use secondary side regulation controller?

Regards

Jean Eddim

  • 0
    TI__Guru* 86266 points

    Hello Jean,

    Primary side regulation with multilpe is not the easiest of designs but can be accomplished.   The outputs can achieve roughly +/- 10% with designs where the output load due not vary more than +/- 10%.

    There is a presenation on improving cross regulation in flyback converters that you might find useful at the following link.

    https://training.ti.com/multiple-output-flybacks-how-improve-cross-regulation?cu=1134585


    If the outputs are regulating with an input voltage of 200 to 500 V the transformer turns ratios Nps, Nsa are most likely correct.

    1. You can use a sacked winding approach similar to the tida design to improve cross regulation.

    • This is covered in the video presentation and is used in the TIDA-00173 reference design on T1. 
        • Review transformers T1 pin's 11, 12, 13 and 14 flyback converter outputs, they are stacked.

    2. You might have to adjust pre-loading to improve the flyback converter preloading.

    3. This toplogy works best with fixed loads on the output.

    4. Try adjusting/adding pre-loads to the outputs to improve cross regulation.  In the Tida design these are R11, R14, R16, R15, R8 etc...

     

    Regards,

     

    Mike