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UCC24612: SR doesn't work in light load condition as expect.

Timmy Cheng
Prodigy 70 points
Part Number: UCC24612
Other Parts Discussed in Thread: UCC28780

Hi,

I'm using UCC24612 in an active clamp flyback converter. Although it can still maintain a 19V output as I expect. But wrong switching condition occurs in light load, it will become steady as I increase  above light load to heavy load.

What may be the main problem? I've laid the path of VD and VS as short as possible to the circuit. The Rds(on) of my secondary MOSFET is about 13m ohm from the datasheet.

Thanks!

  • 0
    TI__Expert 8210 points

    Timmy

    If you have not already done so, I recommend you review your design with our UCC28780 excel tool http://www.ti.com/lit/zip/sluc664.  We've recently updated this tool.

    The way the proportional gate drive of the UCC24612 works is that it attempts to regulate the voltage across the SR FET to 50mV.  Your current probe does not have units in amps so we're unable to determine if this is causing the issue or not.  If it is, you can resolve it by either using an SR FET with a higher RDSON or adjust the burst mode threshold to a higher power level (which will cause the current magnitude to be larger).

    If it is not due to the proportional gate drive sensing at 50mV, other issues that could be causing it are noise on the sensing VD and VS pins.  We recommend the following to reduce this noise

    • Increase the snubber across the SR FET
    • Verify that the electrical connection between VS pin and the SR FET source is as short as possible.  Section 10 (page 31) of the UCC24612 datasheet has a recommended layout

    Best Regards,

    Eric