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UCC28910: Reason of Oscillation in the Voltage across MOSFET and Diode and Voltage dip at starting

Arun Thawait
Prodigy 155 points
Part Number: UCC28910

Dear sir ,

While designing the flyback converter (Specs-Vin 380V ,Vout =15V,0.5A) using UCC28910 we are facing the following problems -

(1) When both MOSFET and diode are off after demagnetization time we are getting the oscillation in the MOSFET Vds and voltage across diode as shown in the figure 1 and 2 .What is the reason of this oscillation since when demagnetization time is over there is no current remain in the circuit ? For some cycles this voltage oscillating  are very close to zero results in false turn on of device .How to reduce this voltage oscillation so that it does not results in false turn on of device ?

(2) At starting with 30 mA load at Aux out 1 and 150mA load at Aux out 2 we are getting dip in the output voltage for some times . After couple of seconds the output voltage gets stable .What could be the possible cause of dip in the output voltage at starting ?

Sir I have shared the schematic file as well .

Regards,

                                                                       Fig.1 MOSFET  Vds

                                                                   Figure 2 Diode Voltage 

                                                                                 

  • 0
    TI__Guru 72325 points

    Hello,

    The UCC28910 is a DCM flyback section 9.3 in the data sheet will give a description of how this works.

    Below is a snippet from the data sheet. This is the same information from the data sheet if you need a clearer picture.

    There is an evaluation module you can find at the following link that shows what the typical waveforms look like for this design.

    https://www.ti.com/lit/ug/sluuai4a/sluuai4a.pdf  The following waveforms came from the User's guide.  For some reason I can't make the pictures clearer on the e2e.  Please go to the link above to get a clearer picture.

    The device will work near critical conduction at the minimum input voltage at maximum load.  As the input voltage goes up or the load goes down there will be an extended dead time.  This is where you observing ringing on the drain.  The converter seem to be operating as designed.

    Regards,

  • 0 in reply to Mike O'
    Prodigy 155 points

    Dear sir ,

    Thank you for your reply .

    Is there any way so that I can able to reduce the magnitude of the oscillating voltage that appear across Vds after diode is off ?

    Also what could be the possible cause of voltage dip at starting ?

    Regards

  • 0 in reply to Arun Thawait
    TI__Guru 72325 points

    Hello,

    When it comes to the resonant ring on Vds after the dmag time in DCM is based on the transformers primary magnetizing inductance and switch node capacitance.  Generally designers do not reduce the magnitude of it.

    I am not sure why you output voltage is dipping during turn on.  You would have to study Vout, Vaux, Ripk to figure this out.

    Please note that the aux winding signal needs to be extremally clean with less than 100 mV at the end of the dmag time.

    If it is noisey this could cause regulation issues.

    The following is a link to an application note that describes the most common issues with primary side regulated sensing and how to resolve them.  I believe this information will be helpful in resolving your issues.

      https://www.ti.com/lit/pdf/sluaac5

    Regards,

  • 0 in reply to Mike O'
    Prodigy 155 points

    Thank you sir