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TIDA-01505: Pulse Skipping Mode?

Prodigy 30 points

Replies: 5

Views: 68

Part Number: TIDA-01505

Hello,

does the UCC28C43QDRQ1 used in this design support light skipping or burst mode? If not, how do you ensure that the output voltage does not rise uncontrolled above 15V for very light loads or no load condition? 

As far as I can see, you have just an LED D21 and R12 connected at the output. Is the output voltage stable at 15V with no additional load?

Thanks

Utku

  • Hi Utku

    UCC28C43 is a basic general purpose legacy PWM controller. While the UCCx8C4x are industry workhorse PWMs, they lack some of the more important features needed for light load efficiency, standby power consumption etc. These controllers are fixed frequency PWMs that do not include pulse skipping or burst mode operation. TI has an extensive portfolio of flyback PWM controllers that do include these features.

    The output of a flyback will always have some minimum load to ensure regulation during no load operation. The amount of current draw during no load can be adjusted by the feedback resistor divider.

    Regards,

    Steve M

  • In reply to Steven Mappus1:

    Hello Steve,

    thanks for the reply. After I posted the question yesterday, I came across a similar discussion going on in this post and Aidan Davidson seems to contradict with your reply:

    Can you please clarify this?

    Thanks

    Utku

  • In reply to Utku Karakas:

    Hi,

    For reference I've copy/pasted what Mr Davidson wrote with regards to pulse skipping: 

    "By definition the UCC28C43 will skip pulses if the COMP pin voltage is below the cutoff threshold."

    I would refer to this behavior more like pulse missing as opposed to pulse skipping. A controller that has true pulse skipping would do something analogous to switching at a minimum duty cycle and then skipping every nth pulse. Some controllers even change to hysteretic mode to achieve pulse skipping. There are different way to accomplish this but it's done intentionally and in a controlled manner so that the output stays in regulation. Pulse skipping will also impact output voltage ripple but again, this is done in a controlled and predictable manner.

    When the UCC28C43 COMP voltage falls below the minimum threshold, pulses will stop until the COMP voltage returns. This is uncontrolled and results in erratic output operation. Pulses are missing as opposed to periodically skipping.

    Pulse skipping vs pulse missing, Mr Davidson's response is correct with regards to the UCC28C43 behavior but maybe we just needed to clarify semantics?

    Regards,

    Steve

  • In reply to Steven Mappus1:

    Helle Steve,

    thank you for pointing out the difference. In our application, both methods are fine, as long as the PWM controller doesn't put out a a constant minimum on-time pulse width.

    I think that this concludes my inquiry.

    Regards,

    Utku

  • In reply to Utku Karakas:

    Good to here

    Regards,

    Steve M