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Original question:

UCC28C43: PCN# 20221216002.1

UCC28C43: Pulse narrowing in UCC28C43DR

Shritesh
Prodigy 235 points
Part Number: UCC28C43

Dear TI team,

I am observing pulse narrowing in the output gate signal of UCC28C43DR IC.

For which I have probed following signals

1. Gate pulse and Sense pin voltage

2. Gate pulse and COMP pin voltage

I found no issues with COMP pin voltage and sense pin voltage which is used to terminate the output pulse.

Please let me know what can be possible cause of this narrow pulse.

  • 0
    TI__Guru 74115 points

    Hi,

    The narrow pulse is due to the CS noise based on your waveforms. The CS reaches its threshold very fast which I believe is due to the noise. You need to fix this to resolve the narrow pulses.

  • +1 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    Another check is to find if your converter actually in CCM operation. You need to capture the secondary side current waveform along with the primary side current. If the secondary side current not reach zero when a new switching cycle begins then it is in CCM. Then the narrow pulses may be due to from CCM operation - so one cycle in CCM and follow cycle in DCM, and repeat. If this is confirmed, one possible solution is to add the current slope compensation so to get each switching cycle the current start at the same level. Your waveforms show similar behavior of insufficient slopt compensation.

  • 0 in reply to Hong Huang
    Prodigy 235 points

    Thanks a lot Hong for very detailed explanation of the issue which I am facing.

    As you pointed out correctly, I have removed slope compensation which might be causing the narrow pulse issue. I will add slope compensation and check behavior of my circuit. 

    In my circuit value of C_csf is 10 pF and R_csf is 39 kOhm for 100 kHz switching frequency. Are these acceptable values?

  • 0 in reply to Shritesh
    TI__Guru 74115 points

    Hi,

    You would need to test to find if the values are ok to achieve your expectation. If your expectation is ok to achieve then they are acceptable. But if you want to know if these are good as starting point for debugging, I think typically 100pF and 1k should be the values then test to make adjustment until ok to get what expected.