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UCC28950: Burst Mode with PCM vs VMC

Marc Petit
Prodigy 10 points
Part Number: UCC28950


Hi,


It appears to me that the burst mode is different for PCM and VMC. In PCM, if the voltage at VCOMP is between 800mV and 950mV, the controller is in burst mode. However, in VMC, the controller is off, i.e. OUTA-OUTF are all low, until COMP reaches 1.1V. I didn't see anything about this difference in the datasheet, so I wonder if this is expected.

With many thanks,
Marc

  • 0
    TI__Mastermind 24375 points

    Hi Marc,

    Thank you for the query on UCC28950.

    The device enters a light-load-burst mode if the feedback loop demands a conduction time less than a user settable level (TMIN). You might want to check the duty cycle in both modes. So the proper selection of the TMIN duration is dictated by the time it takes to raise sufficient magnetizing current in the power transformer to maintain ZVS which could be different in VMC and PCM modes. If Lmag is less than the value calculated by the Excel calculator then what will happen is that the system will behave more like as it is operating in Voltage Mode rather than in Peak Current mode. This make me feel there will be difference in the VCOMP voltage for the two modes. Have you designed two PSFB boards optimized for VMC and PCM and observing the above?

    Regards,

    Harish