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UCC28951: PWM Jitter

mike barry
Prodigy 30 points
Part Number: UCC28951

Tool/software:

What causes jitter in the PWM signal in the UCC28951, PSFB Converter?

 

At approx 20% load I am seeing the Duty Cycle vary/bounce between 2 points, approx 45% to 55% Duty

It results in audible noise. It is a negligible impact on the output ripple voltage ripple.

 

On the LHS below , there is no variation in Duty cycle. On RHS I increase the input voltage and audible noise can be heard. There is a small (~10%) variation in duty cycle and this can be seen as jitter in the oscilloscope waveforms. It  can be seen in the CS signal where there is a slight difference in the peak values, the duty cycle increase will cause CS peak to be higher as the on time has increased.

Transient plot indicates loop is stable. 

I have tried to filter the COMP pin by placing a local decoupling capacitor across it. I have also tried to improve filtering on VDD and VREF Pins.

 

What causes the Duty cycle to jitter?

Probably circuit noise but what signal/pins needs to be filtered?

 

 

  • 0
    TI__Guru* 86266 points

    Hello,

    Your inquiry has been received and is under review.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    If you are exhibiting sub harmonic oscillation most likely it is due to not having enough slope compensation.

    The other thing is that your transformer primary current does not look correct for a phase shifted full bridge.

    You should double check your schematic to see if you have setup the design correctly.

    The following link will bring you to an application note that show you how to design a UCC28951 in a phase shifted full bridge.

    There is a section that shows how to add slope compensation.  There is also a schematic of 600 W reference design that you can use to compare to your design to check for errors.

    https://www.ti.com/lit/pdf/slua560

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,