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UCC28710: Snubber Calculation Question

Cameron Perl
Expert 3990 points
Part Number: UCC28710
Other Parts Discussed in Thread: UCC28711

Hey team, 

I'm working with a customer to help fix a ringing issue they are seeing on the UCC28710 and we are working to design a snubber. Using the TI RCD Snubber Calculator we received the following, but wanted to double check how some values were supposed to be calculated. From measurements on the board they made, they see the UCC running at around 50kHz, does that make sense? They also are using the power rating as Vsnub^2/Rsnub which comes out to 8 Watts. Is this the correct equation for calculating power over the snub?

My thoughts are that based on https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/05/05/calculate-an-r-c-snubber-in-seven-steps 

  • 0
    TI__Genius 9790 points

    Hi, Cameron 

    I've assign your post to Mike, he need sometime to finish the analysis.

    Yunsheng

  • 0 in reply to Yunsheng Qu
    TI__Guru 66135 points

    Hello,

    The snubber in the Webench tool is actually an RCD clamp and will protect the switch node from an over voltage and will dampen your high frequency ringing after turnoff.

    You can use a snubber across the output rectifier to help dampen this ringing.  I have attached and application note that discusses the most common issues is PSR fly back control and how to resolve them.  Please note this application note had detailed instruction no how to setup the RCD clamp and snubber across the output rectifier.

    Regards,

    3122.Clean PSR Sensing App Note 10 5 20B.pdf

  • 0 in reply to Mike O'
    TI__Expert 3990 points

    Hey Mike,

    Their specific question is with regards to the RCD snubber on the transformer primary and the values shown above are for this specific RCD. The app note goes over an RC snubber across the secondary diode. 

    Another question we have is what is the recommendation for the CBC input? Can this be left open or does it need to be tied to ground with a large resistor (ie can this input pick up noise if left open since it is unneeded) 

    Thanks,

    Cameron

  • 0 in reply to Cameron Perl
    TI__Guru 66135 points

    Hello,

    The RCD clamp really is not a snubber. This is a clamp to protect the switch node.  In most cases a 511 k and 1 nF capacitor will do the trick and the application note does cover that.

    If they are interested in calculating the snubber resistor power dissipation it should be ((Vout*Np/Ns)^2)/R

    In regards to CBC this is mostly for battery charging to improve efficiency.  This pin can be left open.  There is a version of this device I believe that does not have cable compensation but uses this pin for an NTC.  I believe this is the UCC28711, you just need to double check the data sheet.

    Regards,