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UC1825: Duty Cycle Skipping

H2
Prodigy 65 points
Part Number: UC1825

Hi, 

I'm working on a flyback design using a legacy part, UC1825BJ/883B, and I've encountered some technical concerns that I hope you could shed some light on.

The issue I'm facing is that OUTPUTA and OUTPUTB often miss a cycle under the same load conditions. OUTPUT_A is tied to the gate of a MOSFET, while OUTPUT_B is left open. I've already verified that ILIM stays below the Current Limit/OverCurrent threshold.

Channel#1: OUTA | Channel#2: OUTB |Channel#3: CLK

CH1: OUTA, CH3: V_DS on the MOSFET

Wondering if you have any suggestion where I should look into regarding this cycle skipping issue/feature. Which portion of the part/circuit might control this valley detection/ Somewhat Zero Voltage Switching on VDS?  

  • 0
    TI__Guru 74115 points

    Hi,

    Please provide your schematics for a review to see any clue

  • 0 in reply to Hong Huang
    Prodigy 65 points

    Hi Hong, 

    I can't share the schematic in this public space, but would love to connect via an email/a call if that works for you. If you have any recommendation on where I can take a dive, please let me know as well. 

    The schematic is similar to this TI reference design. 

  • 0 in reply to H2
    TI__Guru 74115 points

    Hi,

    Can you send the schematics to

    hong_huang@ti.com

  • 0 in reply to Hong Huang
    Prodigy 65 points

    Hi Hong, 

    Sent to your email. Let me know if you have any feedback. 

  • 0 in reply to H2
    TI__Guru 74115 points

    Hi,

    Can you remove C11 to see if pulse skipping still there?

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    As no communication for a long time, I will close this thread. You can reopen this thread if further support is needed.

  • 0 in reply to Hong Huang
    Prodigy 65 points

    Hi Hong, 

    I finally got a chance to remove the C11. OUTA and OUTB pulses still skip here and there just like before (see the capture below CH1: OUTA, and CH2: OUTB). Wonder if you have any suggestions. 

  • 0 in reply to H2
    TI__Guru 74115 points

    Hi,

    What if you remove C8 - the capacitor on ILIM pin?

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    As no communication for a long time, I am closing this thread.

  • 0 in reply to Hong Huang
    Prodigy 65 points

    Hi Huang, 

    Please bare with me on the slow response. 

    Removing C8 didn't help at all. It actually breaks the circuit. You can see the waveform as below: CH1 is OUTA, and CH2 is OUTB. It no longer functions normally as PWM output. The frequency drops to 6Hz for both OUTA and OUTB. 

    Wonder if you have any suggestion?

    Wonder if you have any suggestion. 

  • 0 in reply to H2
    TI__Guru 74115 points

    Hi,

    If remove C8 actually causes some malfunction, it is likely your current sensing signal has too much spikes which charge up C8 when it is there then causing pulse skipping. You can measure the ILIM pin and RAMP pin to find out. The pulse skipping is usually due to the current signal exceeding threshold so you also measure pin 3 to see if it exceeds 1.5V to cause pulse termination very early (no pulse). 

    Please follow the diagram on datasheet page 1 to find which signal to cause the pulse terminate too early. This is basically the basis to make debugging. 

  • 0 in reply to Hong Huang
    Prodigy 65 points

    Hi Hong,
    I appreciate your input. Removing C8 would make the circuit malfunction due to the fault condition. Your explanation and the datasheet indicate that info clearly. In this case, C8 serves as a filter, without the filter, the overshoot would come in and trigger the fault condition as expected.

    However, it doesn't explain why there's a skipping cycle while C8 is there. This skipping cycle is random. This skipping issue most likely doesn't relate to the Fault Condition due to its randomness and high frequency. Notice that the PWM waveform under fault condition should have a significant low frequency, in the Hz range, which is not true in my case.

    Do you have any strong lead to where to look next?

  • 0 in reply to H2
    TI__Guru 74115 points

    Hi,

    C8 might have voltage residual randomly charged by the current spikes so when C8 voltage becomes high enough then terminate the pulse so cause pulse skipping. The spikes are possible randomly showing so the pulse skipping randomly. Similar to RAMP  and COMP. It looks to me it is likely due to your board layout or transformer leakages which causing too much noise.

    You can capture these pins waveforms to find out and if you do see the noise you need to reduce the spikes or otherwise you would have to tolerate the pulse skipping.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    Please capture these waveforms to find out.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    As no further communications for a long time again, I am closing this thread. Please create a new thread when you need further assistance.