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UCC28730-Q1: Abnormal sw frequency on VGS and VDS

Frank QIN
Expert 8920 points
Part Number: UCC28730-Q1
Other Parts Discussed in Thread: UCC28730

Tool/software:

Hi Team,

We are facing an odd problem. We found the frequency of the UCC28730 output is not as expected. According to the datasheet, the max switching frequency should be 83kHz. While we measured the frequency is around 140khz~150kHz. We can get this frequency on VGS/VDS and VS pin.

Another problem I'm not sure it is relevant or not. We found the output is not stable, changing following the input voltage.

Could you please kindly help us to advise what reason might lead the sw frequency like this?

Thank you!

Regards,

Frank

  • 0
    TI__Guru* 86246 points

    Hello,

     

    The magnetizing inductance and peak current limit is what controls the switching frequency.  If your design is running at 140 to 150 kHz the magnetizing inductance is too small and could make the design unstable.

    The following link will bring you to the UCC28730 excel design tool.  You can use it to calculate the magnetizing inductance and the turns ratio of the transformer your design needs.  You need to use a transformer that within +/- 5 % of the calculated values.

    https://www.ti.com/tool/download/SLUC579

     

    Regards,

  • 0 in reply to Mike O'
    TI__Mastermind 40520 points

    Hello Frank, 

    The 150kHz operation is due to an undocumented operating mode called "No Sample Response" (NSR). 
    The No Sample response happens when the internal sampler at the VS input cannot get a clean valid reading of the reflected AUX voltage knee at the end of demagnetization.  Loss of valid sample information makes the controller blind to what the actual output voltage is, and the NSR is intended to repeat the sampling as quickly as possible to reacquire valid samplings of the output voltage.   

    It is expected that occasional transient disturbances might distort the VS signal and invoke a NSR but it should finished within a few switching cycles. 

    From the datasheet: 

    Looking at your waveform, it is consistently distorted by excess ringing at the sampling knee point. 
    The controller is constantly in NSR mode. 

    If you can damp out the ringing from the leakage energy clamp sooner, it should help to avoid this problem. 
    I suggest to increase the value of your TVS clamp so the the leakage reset time is shorter (tLK_RESET)>
    Double the reset voltage, for example, to cut the reset time in half.  

    Also, if you reduce your Nps ratio for lower reflected voltage, the demag time will be extended and allow the ringing to dampen further.  

    Regards,
    Ulrich