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Original question:

UCC256304: UCC25630x resonant capacitor

UCC256304: UCC25630x swithcing frequence during light load

Lin Zhao
Intellectual 1230 points
Part Number: UCC256304


Hi,

My customer intend to use UCC25630x in their LED driver application, one concern is that during light load, device would enter burst mode, then Iout ripple would go large, result in LED blinking, this is not acceptable for customer.

We could turn down the burst mode threshold to avoid enter burst mode prematurely, however, the frequence would go up and increasing switching loss would damage MOS, the target fsw: <200kHz @ 100W, could we meet this target? or any other workaround solution could help to solving this blinking issue?

Thanks.

  • 0
    TI__Expert 6080 points
    Hello Leo,

    Thank you for your interest in UCC256304.
    In general, the burst mode level should be set low enough to avoid acoustic noise and output ripple.
    One way of setting the burst mode to the lowest level(0.4V) is to disconnect the resistor connecting the RVCC and LL/SS pins.
    Another way is to make sure that the output capcitors are solid alluminum caps and have high ripple current ratting and they are multiple capacitors in parallel to the output.
    For better uderstanding, please shar your design spec and your schematic.

    Regards,
    Sonal
  • 0 in reply to Sonal Singh
    TI__Intellectual 1230 points
    Hi Sonal,

    The burst mode threshold is OK, the concern is that when operate in light load, fsw would go up which would bring risk to MOS, so what's the fsw range during light load, as I mentioned above, assuming it's universal AC input.
    Thanks.
  • 0 in reply to Lin Zhao
    TI__Expert 6080 points
    Hi,

    I think the reason why the MOSFETs are getting damaged at lighter burst mode threshold is that the controller is losing ZVS. ZVS is obtained when there is current left in the resonant inductor during the turn-off edge of the gate pulse to discharge the switch node. It is important that the time required to discharge the switch node at maximum switching frequency does not exceed the dead-time slew rate (1V/ns to 50V/ns) provided by UCC25630 or ZVS operation is lost. The datasheet assumes that the max switching frequency occurs at 5% load and the controller starts to burst at 5% load as well.
    To investigate it further please send me the scope shots of both the gate drives and the switch node both before and after changing the burst mode threshold. Please share your design spec and schematic so I would be able to help you better.

    Regards,
    Sonal