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UCC28911: RCD circuit parameters

Hailiang Zhang
Expert 7445 points
Part Number: UCC28911

Tool/software:

Hi Expert,

In the datasheet, section 10.2.1.2.12 recommends that the diode recovery time is 0.3us < trr < 1us. However, the diode recovery time in the reference schematic is 200nS, which seems to conflict. Do you have any comments?

在手册中，10.2.1.2.12章节建议二极管恢复时间为0.3us<trr<1us。但是在参考原理图中的二极管的恢复时间为200nS，这个似乎冲突。你有什么comments？

Regards,
Hailiang

  • +1
    TI__Mastermind 34790 points

    Hello Hailiang, 

    I cannot say for sure how the primary clamp diode (D3, in this case) was selected to be 1N4937 (200ns typical recovery time).
    However, I believe that more than one diode was tested in this application and the chosen part (1N4937) had sufficiently good performance to be selected over the other possibilities. 

    Although this seems to contradict the literal recommendation of  0.3us < trr < 1us, I think the spirit of the recommendation is being followed. 
    D3 in not an ultrafast diode with trr in the 25~50ns range.
    D3 is not a general-purpose diode with trr > 2~3us. 

    I think when diodes heat up, their reverse-recovery time starts to extend, and peak Irr starts to increase. 
    So when hot, this diode 1N4937 may have performance similar to diodes with 0.3us < trr < 1us.  

    In any case, selection of the best snubber/clamp diode is an empirical process, not a calculated one. 
    The recommended range of 0.3us < trr < 1us is referenced for starting the selection process during prototype evaluation. 
    It is not a guarantee that any one particular part rated within that range will be the optimal part for a particular application. 
    Please see this paper on the topic: TI_SNVA744_Ap-note_on_Snubbers.pdf

    Regards,
    Ulrich