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UCC3895 Phase Shift Full Bridge converter at 3kW and 100kHz.

George Oprica
Prodigy 30 points

I have a bunch of power supply designs in this configuration and they all display a large approximately 5kHz oscillation at the output  when loaded at 20  -30 % of the nominal load. Compensation on the FBK loop may lower but never cancel it and the trade offs make a slower and less performing loop. I thought it is a resonance in the power stage , but I found so far no discrete elements to resonate. Could be a continuous -discontinuous  transition mode the problem? Any known solution? Thanks. George O 

  • 0
    TI__Mastermind 31580 points
    Hi George,

    Have you measured the gain and phase of the system under this load condition?

    Alternatively you can try to drive the system open loop, very carefully, with a dc signal instead of the feedback signal to see if the system behaves ok.

    If it is a loop issue you may also see some oscillation when performing load transient steps on the output between say 10% load and 80% load.

    Regards

    Peter
  • 0
    Prodigy 30 points
    Hi Peter
    Thank you for reply. I had a demo Bode Plot Analyzer and I managed to run a couple of tests on the unit before returning it. Phase Margin was 10deg; Gain Margin was close to 0db in that particular load point, with aprox. 4.5kHz band with . The compensation network was optimized for nominal output power. I re-compensated the loop for bringing the oscillation down and still having a decent ripple and dynamic load at nominal. This time the phase margin was 20deg and gain margin was 8dB with a 1kHz bandwidth. Fact is that if you changed the load, you still find a spot where the output oscillates. There is an overall critical/conditional stability at light loads . In open loop I saw some oscillation at 800Hz but not at around 5kHz.
    Regards
    George O
  • 0 in reply to George Oprica
    TI__Mastermind 31580 points

    Hi George,

    Do you have the UC3895 configured for current mode control or voltage mode control?

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 30 points
    Hi Peter
    UCC 3895 is in voltage mode. 64 kHz. At light loads, the primary current wave forms show a current ramp surrounded by resonant waves corresponding to the freewheel intervals. Seemingly, when the ramp start approaches zero level the output tends to oscillate.
    Thank you