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Original question:

UCC256403: Support required for Primary Side Mosfet selection in LLC half bridge converter

UCC28950-q1: Open loop oscillation

Joseph Francis S
Prodigy 140 points

Part Number: UCC256403
Other Parts Discussed in Thread: UCC28950-Q1, , UCC28950

Tool/software:

Hi team,

Continuing the previous discussion.

I found that it is loop oscillation. So, I tried to validate the converter in "open Loop" (EA+ voltage is given from VREF through resistor divider). Even in open loop also the same oscillation exists. That is what i mentioned in the previous query.

Can you able to help me to figure out why this oscillation occurs in open loop itself. (when EA+ voltage is 1.3V)

But I tried to change the peak current mode control to Voltage mode control.

In Voltage control mode the sub harmonic oscillations is not present.

It seems like the controller itself oscillates in peak current control mode when the EA+ voltage is 1.3V.

  • 0
    Prodigy 140 points

    The above observation was wrongly mentioned as UCC256403 instead of UCC28950-q1

  • 0 in reply to Joseph Francis S
    TI__Guru* 86286 points

    Hello,

    Could you repost in the e2e with the correct thread title?

    Regards and thanks,

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

    Hello,

    I was able to rename the thread so please disregard the request to rename the thread.

    Regards,

  • 0
    TI__Guru* 86286 points

    Hello,

     

    It sounds like your design might be hitting hiccup mode over current protection.  This occurs when the cycle by cycle current limit hits 2 V and cause the soft start to cycle.

    By you putting the design into voltage mode control you are disabling the over current hiccup mode protection.  The following information from the data sheet explains how this works.

    I am thinking that your current sense transformer sensing circuitry maynot be setup correcly and/or current sense is too large.  I would recommend that you double check this to make sure it is setup correctly.  The following link will bring you to an application note that goes through the step by step design process of the UCC28950 in a phase shifted full bridge.  There is a section on how to select the CS resistor and setup the current sense transformer (CT); as well as, showing the proper location of the CT .  There is also a link to an excel design tool in the file in the application note.  Not only can you use this to check your current sense transformer configuration but you can use this to check the entire design.

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

    The other thing that it could possibley is current sense noise.  If this is the case you can add a low pass filter to the CS pin from the current sense resistor.  A 1 k ohm and 220 pf will do the trick if this is the case.

     

    Regards,