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UCC28C44: UCC28C44 burst switching at light load

Part Number: UCC28C44
Other Parts Discussed in Thread: UCC38C42

 Hello guys,

 One of our customer is evaluating UCC28C44 on their own AC/DC converter which generates 24V/6A DC from 115V AC input. When their AC/DC converter operates with light loard, burst switching of "OUT" is observed like the attached waveform (Ch2 waveform (Color Pink) is the voltage of photo transistor emitter. Ch3 waveform (Color Blue) is the voltage of UCC28C44 COMP pin. The spike noize timing timing is "OUT" switching timing.) . They can't disclose their board schematic for us but they are using a similar schematic and parts parameter to the application example, Figure 25 on page 22 in UCC28C44 English datasheet.

 The customer wants to know whether the burst operation is normal or not. (If we have UCC38C42 EVM, we would check the OUT swiching signal with light load. But we don't have the EVM at the moment.)

 If you have the EVM board released by TI, could you check the OUT switching waveform at light loard?

 Also if any burst switching is not observed, could you please give me the switching waveform for showing it to the customer and please advice me which part parameter should be changed to reduce the bursting if you have any idea?

 Your reply would be appreciated.

 Best regards,

 Kazuya Nakai.        

  • Hi Kazuya-san,

    I have asked one of our applications engineers to respond to your mail, you should see a response later today.

    Regards

    Peter
  • Nakai-san,

    Thank you for your interest here. While this device does not have burst mode as part of its control, it will skip cycles if it's output is greater than the regulation set point. This situation can arise during light loads so it's an expected behavior.

    There is not an easy way to get around this. Anything that will cause the output to dip lower faster cycle by cycle perhaps will help. Like heavier loading, smaller output cap, running at a lower frequency, etc.  

    Is this causing too high of ripple or some other issue that concerns the customer?

    Regards

    John