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UCC28951: ucc28951

Suresh Bhat1
Prodigy 90 points
Part Number: UCC28951
Other Parts Discussed in Thread: UCC28950

Our team is facing some issues regarding isolated DC DC converter designed using UCC28951. Would like suggestions 

1. Duty cycle of channel B is reducing as input voltage increases. This phenomenon is not observed in other channels.

2. MOSFET's on the primary side are turning on even before PWM is applied to their  respective gate channels. We suspected it to be false turn on and tried to solve them by reducing gate turn on resistance and introducing gate -source capacitance of upto 1nF. But no improvement

3. Audible noise is generated on the output capacitors once the regulation action occurs.  We were using MLCC initially and when we replaced them with electrolytic capacitors noise just vanished! What could be the reason behind this

  • 0
    TI__Genius 13660 points

    Hi Suresh, an expert will reply your questions tomorrow. Thanks. ——Teng

  • 0
    TI__Expert 3460 points

    Hi Suresh,

    Could you confirm where did you monitor waveform let you to find this issue of duty cycle of OUTB reducing?  on the UCC28950 OUTB PIN or on the Gate of mosFET? The UCC28950 device has outputs OUTA, OUTB driving the active leg, initiating the duty cycle leg of power MOSFETs in a phase-shifted full bridge power stage.The duty cycle of the OUTA/ OUTB pair never changes in normal status. you can check with the Figure 2 in UCC28950 datasheet and you can get know the CLK signal determine the OUTB ON or OFF. I guess there is two potential causes. one is the CLK is disturbed by Noise from the pool PCB layout. the other one is delay time between OUTA and OUTB was been changed.

    Anyway, could you describe your question detail such as what input voltage, power range, Vout and effective waveform when the issue occurred. 

    Thanks

  • 0 in reply to Bliss Zhou
    TI__Mastermind 25570 points

    Hi Suresh

    MLCC capacitors can vibrate and cause audible noise - please see the e2e post at https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/08/09/how-to-reduce-acoustic-noise-of-mlccs-in-power-applications

    Another possibility is that the original circuit did not have enough phase margin and was unstable. The ESR of the output capacitors has a stabilising effect because the ESR of the Aluminium Capacitors is higher than that of the MLCC parts. Of course, if the ESR is too high the system can be unstable too. This is especially true of systems using Voltage Mode Control.

    You can check the loop stability easily by applying a fast load transient and observing how the output voltage reacts.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Expert 3460 points

    Hi Suresh，

    Since we didn't hear from you long time. I have to close this thread. If you have more questions, you can create new thread to discuss.

    thanks