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UCC28951: Cannot get stable duty cycle at OUTB and OUTD, but OUTA and OUTC is stable.

Poh Kok Keng
Genius 5512 points
Part Number: UCC28951

hi,

I am doing a design using UCC28951 following the recommended circuit similar to below: 

my MAX load is 42A 12V. but when the load < 6A, the signal to OutB (pin21) and OutD (Pin19) with reference to GND will have unstable duty cycle at OUTB and OUTD. But the OUTA and OUTC is stable. 

OUTA and OUTC output is having stable during cycle/ This is output CHA  , blue is output from Pin 22 , Pink is form Pin 7 IC 2

However, OUT B and OUT D is having unstabel durty cycle

you can see from this waveform, the duty cycle is unstable. 

Can you advice how can i get a stable duty cycle even at <6A load? 

  • 0
    TI__Mastermind 24375 points

    Hi,

    Thank you for the query on UCC28951.

    The OUTC and OUTD behaviour seems strange as there appears no delay between these and both are on at the same time (from your second figure above). Can you please confirm if you are having similar behaviour in waveforms after 6A? Are the delays TCDSET and TABSET set appropriately as per the app note below:

    https://www.ti.com/lit/an/slua560d/slua560d.pdf?ts=1682659733368&ref_url=https%253A%252F%252Fwww.google.com%252F

    We may want to study the COMP, CS (may be noise on this) and the voltage across the primary of the transformer as well.  To make sure your control signals are as expected.

    There are issues related to imbalance in OUTC and OUTD and ways to address them mentioned inlink below, but your issues seems to be with dead time configuration.

    https://www.ti.com/lit/an/slua275/slua275.pdf

    Are you using fixed delay or adaptive one. I would suggest us a fixed one to start with and verify the PWMs. I would also recommend disabling the SRs for now. Please share a copy of your schematic and calculator so that we can debug further.

    Regards,

    Harish