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UCC28065: Negative current spikes at Switch node of Primary side

Prebin Babu B
Prodigy 10 points
Part Number: UCC28065
Other Parts Discussed in Thread: UCC28064

Hi, 

We used UCC28065 for our AC-DC board from 360Vac to 15Vdc, 20A. We are seeing negative current spikes (6A) at primary side of the flyback transformer. This is happening at irregular intervals. Attaching the wave from below. The switch node voltage is different before this happening. Could you please help and suggest us how to rectify this?

 

Yellow :- Gate signals GDA

Green :- O/p voltage

Blue :- Switch node current at primary

Pink : Switch node voltage. 

Regards,

Prebin

  • 0
    TI__Guru 74695 points

    Hello,

    This is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 74695 points

    Hello,

    Could you share a schematic and what do you mean by switch node current?

    The switch node is the connection between the boost inductor, FET drain and boost diode.

    I am guessing this is the current in the FET but I am not sure.  Could you clarify?

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    By switch node current I mean the current flowing from the transformer primary to the FET drain. During switching we are seeing negative spikes as shown in the previous image shared (BLUE WAVEFORM). 
    We were able to solve this by removing the Syn Rectifier FETs on secondary for now. Please take a look at the above schematics. 

  • 0 in reply to Prebin Babu B
    TI__Guru 74695 points

    Hello,

    Your inquiry is under review and I will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 74695 points

    Hello,

    The current that shows is going negative through FET A is when the FET gate drive is off.  This should not be possible in an interleave CrM boost converter.

    For the current to go negative the voltage across the inductor would have to reverse direction.

    You might want to check your design vs the 300W reference design to make sure your connections are correct.  The following link will bring you to the UCC28064 reference design which can also be used by the UCC28065. 

    https://www.ti.com/lit/pdf/snvu587

    The following link describes what needs to be changed on the reference design to get to work with the UCC28065.

    https://www.ti.com/lit/pdf/sluaa28

    Regards,

    Mike

  • 0 in reply to Mike O'
    Prodigy 10 points

    Hi Mike, we did the comparison with the link you suggested above. On careful review we found the issue. The sync rectifier at secondary which is controlled by UCC24612-1DBVR is not getting gate signals. Apart from this rest of the circuit look good now. 

  • 0 in reply to Prebin Babu B
    TI__Guru 74695 points

    Hello,

    This is under review and I will get back to your shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 74695 points

    Hello,

    The links were to a PFC boosts which do not use an sync rectifier.  I could not open your schematic and it seems like you have designed flyback converters.  Is that correct?  Could you post your schematic again please?

    In regards to the negative current, if the SR does not have a gate signal, I would not think there would be any opportunity for reverse current to occur in a flyback.

    Regards,