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Transformer/output choke problem with design using UCC28950?

Michal Sitta1
Intellectual 290 points
Other Parts Discussed in Thread: UCC28950

Hi All,

I am already solving my problem in different topic (strange H-bridge driver signal for UCC28950), but it is already solved and now i moved to different problem.

Currently I don't understand what is happening in my design. I try to do DC-DC converter about 1 kW 400 V to 12 V (regulated from 1 to 12 V) with switching frequency 200 kHz. I had a lot of troubles but with help from  I solved lot of things. 

Currently my design isn't stable and I don't know why. I changing input voltage and output voltage to try all possible conditions, but in some cases it became unstable this way of worst. screen conditions 100 V input  and 1,5 V/25 A output.

 

channel 1 is voltage over transformer

channel 4 is voltage on secondary side fet

channel 3 is feedback loop voltage (COMP) pin

Thank you in advance for any help or idea what to try.

  • 0
    Intellectual 330 points

    Hi Mike.

    Please may you try and run the converter keeping input voltage close to its final value.

    In other words, in case you have a front end PFC stage, feed the converter with an input range not wider than 250 to 400V.

    This will cover for steady state to hold-up condition.

    Please remember your wish to have an output regulated from 1 to 12V so let's reduce the overall input voltage dynamic to narrower values.

    This should help also in preliminary investigations.

    One point to investigate:

    There's a constant delay (roughly 500ns) between transformer positive rise and Fet's voltage rise. This delay is not visible during voltage fall.

    This condition is present in all pictures you posted but it should not be there.

    Last question for you: is the converter still running in voltage mode control?

    If yes, are you prepared to change control method to current mode one?

    This should help you in achieving required output voltage dynamic and will ease the stabilization network.

    All for now, enjoy it!

    Flavio