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UCC256303: Problem for Vout decrease

Satoshi
Guru 19645 points
Part Number: UCC256303

Customer is designing UCC256303 for below image, condition is 24Vout / 20A (480W) and 24Vout / 28A (672W).

When output current increased for 20A to 28A, a few 28A condition occur Vout drop (about -2V) and fsw increase (about +10kHz).

Please let me know about these reason and measure by adjust external components. 

Best regards,

Satoshi

  • 0
    TI__Mastermind 27085 points

    Hi Satoshi,

    Thanks for your interest in UCC256303. Switching thresholds are determined by comparing the VCR waveform to VTH and VTL which are derived from the feedback signal. VCR has a common mode voltage of 3V with VTH=(VFBreplica/2)+3V and VTL=3V-(VFBreplica/2). As the load increases, FBreplica becomes larger and switching thresholds VTH and VTL grow further away from 3V.

    The controller is only able to push VTL to 0V, it cannot push VTL any lower than this. So if VCR peak to peak is >6V, the switching frequency is not able to be pushed any lower to get to higher gain. I would suggest to select the VCR capacitors to have <6V peak to peak at maximum load to avoid this issue. Try C1=68pF and increasing C2 to 10nF or so.

    Best Regards,
    Ben Lough