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UCC28950: UCC28950 leading leg driving duty cycle Asymmetric.

Ernest Yong
Expert 4710 points
Part Number: UCC28950


Hi Team,

In my customer's project, found PSFB leading leg mosfet driving voltage duty cycle variation first few switching cycle in dynamic test from no load to full load, and transformer saturate due to leading leg driving duty cycle asymmetric. refer to below picture.

What is the reason and how to solve it, thanks.

  • 0
    TI__Guru* 86286 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86286 points

    Hello,

    The UCC28950/1 devices output A and B are 180 degrees out of phase and synchronized to a master clock and output C and D are phase shifted to control duty cycle.  The C and D pulses will phase shifted 180 degrees from A and B when 100% duty cycle is required.  When 0 % duty cycle is needed C and D are phase shifted to 0 degrees.  This will make the C and D Gate drive pulses look like they are double and/or asymmetrical.

    To prevent transformer saturation, in a phase shifted full bridge, I recommend using peak current mode control instead of voltage mode control.  If you have a voltage mode control to prevent transformer saturation you would require a DC blocking capacitor in your design.

    Regards,