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UCC28740: Circuit Loading Problem

Enes Aytekin
Prodigy 10 points
Part Number: UCC28740
Other Parts Discussed in Thread: TL431

Hi,

I designed a flyback converter with UCC28740 using WEBENCH tool and datasheet. My converter operated well, when i connect 200V from input, i can get 5V from output. But when i load the converter, while I should be able to load the circuit up to 2A, I can load it up to a maximum of 200mA and then the circuit turns itself off. Moreover, from the moment I connect the load, even the slightest increase in the load causes the output voltage to decrease rapidly. What could be the reason for this? My circuit design image is below. 

Thank you.

Transformer spec- Np;Naux;Ns = 152;20;6 , Lp=2.56mH

I also tried to change Rcs and using respectively 2.1 - 5.1 - 10ohm but it didnt change anything.

  • 0
    TI__Mastermind 33115 points

    Enes,

    Control variable for this topology/controller are CS, FB and VS. The Webench schematic is not really a complete schematic suitable for review but based on what you mentioned that changing the CS has no effect so I would recommend to look closely at FB and the optocoupler/TL431. Is the optocoupler transferring the error signal as you expect? Monitor the optocoupler voltage/current while you vary the load.

    Also, for additional support, please refer to the TI Reference Design Library and search UCC2870 designs here.

    Regards,

    Steve