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UCC28C43: Premature On-Cycle Termination

Prasad Radhakrishnan
Prodigy 10 points
Other Parts Discussed in Thread: UCC28C43

Hello,

I am working on improving an existing flyback convertor design for EMC and while I was under the hood I noticed a potential issue. In particular, I am noticing the current mode controller PWM IC, UCC28C43, terminate it's ON cycle prematurely under certain load conditions. The output voltages are fine and the product on the whole does not report any glitches or errors from any of the voltage supervisors. I captured the CS (current sense, blue) and the OUT (gate drive, green) at the controller pins on the scope, shown below in fig1. Vcs is not crossing the 1V threshold (thanks to RC filtering), which rules out the current-limiting condition for terminating the ON cycle, so I am trying to grasp the reason why this may be happening. Maybe some of you may have seen such a scenario in your designs. I am attaching a plot of how a good ON cycle should look like in fig 2. The OUT (gate drive) was captured using infinite persistence in fig 3, to show good and bad ON cycles. 

Thanks,

Prasad

fig 1:

fig 2:

fig 3:

  • 0
    TI__Genius 15910 points
    Check your feedback to ensure stability over operating range (all 4 corners: low line/light load, low line/max load, high line/light load, high line/max load). Make sure all your decoupoling caps (VDD and VREF) are adequate (good quality ceramic) and placed as close to the pins of the device as possible to prevent high frequency noise from creating havoc (note the decoupling caps are in additio to bulk hold up caps). You also may need a small RC filter on CS to smooth out the leading edge spike on the CS pin for noise immunity.