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UCC28950: valley switching

kwangwoon Lee
Prodigy 230 points
Part Number: UCC28950

Hi,

I am developing a 5kW PSFB with UCC28950.

The waveform below is the waveform of main transformer (pink) and the VDS waveform (yellow) of the MOSFET QB(based on  the datasheet application)

at 15% of the total load current.

It seems that the visible waveform is valley switching, is it correct?

The load current should be about 50%(load current 100A) until the visible waveform disappears

completely.

It seems like ZVS is going in then.

But the heat of the MOSFET is too hot until then.

Is it right to take the Llk bigger to get the ZVS in earlier?

Currently DELAB is using 15kohm.

Regards, 

Kwangwoon.

  • 0
    TI__Genius 9140 points

    Hi Kwangwoon

    I have involve a PSHB controller engineer to help on this post, we hope we can feedback to you before next Monday USA time.

  • 0 in reply to David Huang33
    TI__Guru* 86216 points

    Hello,

    Your waveforms look like that turn on delay for the A, B, C and D FETs are not setup correctly.

    The following link will bring you to an application note that describes how setup up the UCC28950 timing to get ZVS and valley switching.  It goes more in depth than the data sheet. 

    At this light load if you adjust the A, B, C and D FETs to valley switch it will also ZVS when there is enough energy stored in Ls to achieve ZVS.

    The device does have adaptive delay that is used to remove body diode conduction.   However, in most applications they used a fixed delay approach with is presented in the application note.

    Regards,