• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 210 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5023: LM5023 RCD snubber

john Pucci
Prodigy 30 points
Part Number: LM5023

I need help with the following question.
How to calculate RCD snubber for Quasi-Resonant controller, the application note AN4147 explained very well but fixed frequency. Quasi-Resonant with no load, the frequency is around 60KHz, but with maximum load, the frequency increases from 60 to 130KHz.

  • 0
    TI__Mastermind 31180 points

    Hello John, 

    Thank you for your interest in the LM5023 QR flyback controller. 

    You pose a good question, and I'll try to answer that.  But first, per Figure 13 (page 17 of LM5023 datasheet) max load generally switches at a lower frequency than 130kHz.
    In your case it may be the 60kHz that you mention, while lighter loads increase in frequency until the controller's 130kHz limit is reached.  

    High load switching operates in QR mode, but once load reduces to where the 130kHz limit is reached, lower loads operate in DCM, until at very light loads, the controller begins to skip cycles.  In skip-cycle mode the apparent switching frequency can drop very low. 

    Anyway, the main issue is that at maximum load, peak primary current is at its maximum while the fSW is relatively low (say... 60kHz).   At middle loads, fSW has gone up to 130kHz but the peak current is lower.  Using simple numbers to make the math convenient: if Pmax has Ipp(max) at 60kHz and Pmax/2 doubles fSW to 120kHz, Ipp(half) = Ipp(max)/2.  In this simple example, the half-load peak current is 1/2 of the max peak current.  

    From the snubber point of view, leakage power loss is 0.5*Llk*Ipp^2*fSW (ignoring the peak to reflected voltage factor).  Power loss at 60kHz, Ipp(max) is twice as much as loss at 120kHz, Ipp(max)/2. 

    Therefore, it is appropriate to design the snubber at maximum load, low-line where Ipp is maximum, and snubber losses will be lower at lower load conditions. 
    Note: they will also be lower at high-line max load, because the higher voltage makes Ipp reach peak sooner, so fSW is higher but Ipp is lower than Ipp(max) at low-line. 

    Regards,
    Ulrich