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Original question:

TPS23756: want to check what's the purpose of the D17、R18、C26 of the TPS23756

TPS23756: overshoot while power cycle

CF Hsu
Prodigy 30 points
Part Number: TPS23756
Other Parts Discussed in Thread: TPS23734

In our design, we use TPS23756 and Flyback topology to accomplish DC12V to DC5V.
We observed that if we remove the DC12 and then reconnect the DC12 immediately, the +5V overshoot up to 5.7V
We think that is because the C383 is not fully discharge, and the soft start circuit is malfunciton.
Is there any way to prevent the overshoot in this scenario?

▲ +5V overshoot while DC12  reconnect immediately

Regards,

CF

  • 0
    TI__Mastermind 21320 points

    Hello, 

    Thank you for the question. Your reasons could be correct. Can you try increasing the soft start capacitor to see if that affects the circuit?

    I will need to see a full schematic to provide further comments.

    Additionally, the TPS23734. the new version of this part, has two new features that can resolve this issue. First is soft stop, which will quickly drain the output capacitance. The second is soft start, which works in conjunction with the soft-start circuit on the secondary side to deliver a safe and effective turn on. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 30 points

    Dear Michael,

    Increase the soft start capacitor doesn't fix the overshoot phenomenon,

    I add a 30KΩ resistor parallel to the soft start capacitor to discharge the cap, it looks good. 

    Is there any concern about my solution?

    Regards, 

    CF

  • +1 in reply to CF Hsu
    TI__Mastermind 21320 points

    Hey CF, 

    No this is perfectly acceptable. Of course it will decrease efficiency, but it will drain the caps faster so that overshoot does not happen. Again, our new part the TPS23734 has a feature called soft-stop that eliminates this phenomenon. Please consider this part for any new projects. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments