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UCC28720: The power supply output voltage is good with no load, but drops the voltage when loaded

Part Number: UCC28720


I copied the reference design inside UCC28720 datasheet, including same part numbers for discrete. The design gives the expected output with no load connected, and when a 10 ohm load is added the voltage drops to zero with some repeatitive peaks. I don't know where is the potential problem since it is completely following the reference design

  • Hello Rafael,

    Thank you for your interest in the UCC28720 flyback controller. I'm sorry that you're experiencing difficulties with your circuit.

    I noticed that the schematic posted (apparently duplicated) matches the datasheet (DS) components in all cases except the primary snubber TVS (D3). In the DS it is an 82-V clamp, but your schematic shows it as a 15-V device. If your design does, in fact, have a 15-V TVS populated at D7, then that would be the cause of the problem. It will act as a primary-side load and divert most of the flyback energy into the TVS, with some remaining left-over energy pulsing the output each cycle. I expect that it and (the 1.62K R160) may be running quite hot.
    At no load, the peak primary current is low and the switching frequency is low, so enough residual flyback energy may be able to regulate the output voltage to appear to be working and not be fully absorbed by the 15-V TVS.

    If your schematic merely shows a typographical error and D7 really is an 82-V TVS, then some other problem exists that must be debugged.
    Please refer to this troubleshooting guide for help with that debug.

  •  Hello,

    Yes, label is wrong. The snubber mounted is: SMBJP6KE82A-TP. By the way all components are cold. 
    I'm attaching output  capture, please let me know if you need measurements at different pins

  • Hello Rafael,

    Thanks for verifying the TVS part number.
    The screen-capture shows that the controller is making re-start attempts every ~300ms when under load (assuming 10-ohm load).
    Therefore there is some fault condition that prevents sustained operation other than at no-load.

    Please follow the troubleshooting guide that I indicated above, to debug this situation.
    Any measurements or waveforms that I would request are already covered by the guide anyway. It will help you to determine at what point the fault occurs and what is the nature of the fault so that you can correct it.

    One thing to consider: although the schematic diagram may be correct, the actual pcb may have an incorrect component, something installed backwards, a missing or incorrectly routed connection or trace, or a solder-ball short-circuit or other mistake in assembly. Please check the board thoroughly to verify that all of the parts are properly installed and connected. Clean off any flux residue to avoid hygroscopic leakage current paths around high-value resistances.