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Multi output flyback Powersupply -control loop

Other Parts Discussed in Thread: UCC2813-1


I have designed Multi-output flyback power supply by using UCC2813-1 PWM IC,

Input and output specifications are as follows

Vinput- 16V-40V


Switching frequency -200kHz

The converter is working fine in no load and full load condition as per design specification ,when 5 V output is not loaded the  15 V &-15 V output voltage drops to 8.5 V since the control loop is sensing the feedback voltage from +5 V output.if +5 V output loaded to 3 A the output of  +15 V &-15 V are as per specification .

How to regulate the 15 V &-15 V output without loading +5 V output (or at least loading +5 V output lightly)

  • HI bupathi,
    This is problem with Multi output Flyback system, we should take feedback from the heavy power output rail and other rails like 15V will see huge variation, becuase they are jsut follower, the only way is to change 15V winding to make 15V at 5V/5A loaded condition which will make 15V output actually higher like 25V ~and then used LDO to make 15V.
    Or you can used standard ATX power supplu fundamental where a coupled inductor ouput method is used for unequal output regulation but with Forward converter.
  • Dear Shri Narendra,

    I have designed the dual output DC-DC Forward Converter, but come across the similar problem of cross regulation, Kindly guide me or send the link how to use ATX power supply fundamental where a coupled an Inductor output method is used for unequal output regulation but with Forward converter.

    Many many thank you.

    Atul Mishra
  • Bupathi,
    I would suggest lightly pre-loading with a resistor the main regulated output as well as the other outputs. this will improve the cross regulation. alternatively you could clamp the rails with Zener diodes. There are also transformer winding methods to improve the regulation for the main rail you care about so that it is most closely coupled with the regulated rail.
    Would you like some suggestions of magnetics vendor contacts to discuss these details?