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UCC28C55: Pulse Width Oscillation issues

Joe Brant
Prodigy 30 points
Part Number: UCC28C55

Hello,

I am working on a dc/dc flyback design.  I have the device working well with a 16V output and 600V input.  My only real issue is I am seeing some pulse width oscillation at lower power.  I am using a type 2 compensator on the comp and feedback pins.  I have the switching frequency set to 65KHz.  I have included a Vds screenshot of the oscillation I am seeing.  

Thanks,

Joe

  • +1
    TI__Mastermind 32885 points

    Joe,

    The VDS waveform looks good and shows the converter operating in DCM. Take a look at COMP and CS to really know how your converter is behaving at light load. What is the phase margin and cross over frequency for this converter? And when you say "lower power" - what does this mean with respect to full power, zero load, 5%, 10%, etc? Sometimes at no load you might expect to see what look like "Burst mode" operation visible as packets of pulses and periods of no switching but your waveform shows varying pulse width, suggesting control loop instability. Is your converter intended to operate in DCM over the entire line/load range?

    Regards,

    Steve

  • +1 in reply to Steven Mappus1
    Prodigy 30 points

    In terms of "low power" the flyback is operating at roughly 10%.  I believe the cross-over frequency is 6.5KHz.  This is my first flyback design, so I am not 100% sure on what I need.  I have read through the datasheet but the reference design covers a CCM and I am using a DCM.  The CS and COMP pin look pretty good to me.  I have included a screenshot below.  

    Thanks,

    Joe

  • 0 in reply to Joe Brant
    Prodigy 30 points

    I got it fixed.  The issue was on the CS pin causing issues with the PWM.  I appreciate the help.