• State Resolved
  • +1 person also asked this people also asked this
  • Locked Locked
  • Replies 1 reply
  • Subscribers 64 subscribers
  • Views 51 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

UCC28782: Excessive current flows when transitioning from LPM to ABM

Kaji@PAN
Mastermind 7775 points
Part Number: UCC28782

Tool/software:

Hello,

When the UCC28782 is operating, an excessive current flows when transitioning from LPM to ABM.
The waveform is attached.
The excessive current is the winding current on the secondary side of the transformer, shown in light blue in the waveform.
It seems to occur when PWMH is output.
What is the cause of this excessive current?
Also, please tell me what measures can be taken to reduce this current.

Best regards,

  • +1
    TI__Mastermind 40120 points

    Hello Kaji-san, 

    The LPM-to-ABM transition current an expected behavior during the first PWMH pulse as ABM resumes after LPM ends.  
    This behavior is described in the UCC28782 datasheet in Section 8.4.12 starting on page 51.  

    This current comes from resonating the excess charge on the clamp capacitor to the output as Vclamp drops from ~95V down to ~70V in your waveform.  
    This 25V drop in the zoomed image is the first of four drops to eventually settle at ~25V(?) across the clamp cap.
     
    Actually, I expect this resonant discharge current to resemble a half-sine wave, but your current (blue trace) seems to be distorted for some reason. 

    The peak current can be reduced by limiting how high the Cclamp voltage can rise during LPM. 
    This can be done by adding a TVS clamp across Cclamp to absorb leakage energy and limit the voltage and/or by reducing the resistance across Cclamp to discharge it faster. 
    However, lower Rclamp will increase standby losses.  

    Regards,
    Ulrich