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UCC28950-Q1: Duty Oscillation issue in Peak current control mode

Joseph Francis S
Prodigy 130 points
Part Number: UCC28950-Q1

Tool/software:

Hi Team,

I am using UCC28950-Q1 IC for Phase shifted ZVT converter for input 380VDC output will be 28VDC/ 40A. While validating upto 28VDC 4A load in 380VDC input converter was operating fine and no abnormality found in switching node.

But in 380VDC input and 7A load at 28VDC "C" switch node was found oscillating, that is the duty cycle is not 50% it is oscillating from 40% to 60% in 22kHz. My switching frequency was 400kHz.  At this time the same oscillation is there in output voltage (28VDC) but the amplitude was less than 100mV. Refer the attached figure 1.

To verify whether there is problem in feedback loop compensation, I had validated the converter in open loop by giving constant voltage in EA+ pin through resistor divider from VREF (5V).

when giving 1.2V at EA+ the converter was operating normally (without duty oscillation in C switch node) but when I gave +1.3V at EA+ through resistor divider from VREF, the same duty oscillation in "C" switch node was observed. Can you please let me why the controller is operating like this in peak current control mode and how can i solve this duty oscillation issue in peak current control mode.

Channel 1: Switch node "C"

Channel 2: Switch node "A"

  • 0
    TI__Guru* 86016 points

    Hello,

     

    Your design has one or both of the following issues.

    1. Your design may not have enough phase margin at the heavy loads.
      1. This can be corrected with adjusting the voltage loop compensation. The following link will bring you to an application note that reviews how to compensate the voltage loop. https://www.ti.com/lit/pdf/slua560
        1. Please note you will need to check the design with a network analyzer and large signal transients to check your loop after it is compensated.
          1. https://www.ti.com/lit/pdf/slua560

     

    1. You design does not have enough slope compensation and you are seeing subharmonic oscillations.
      1. This can be removed with proper slope compensation. The following link will bring you to an application note that shows how to compensate the voltage loop.  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,