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Original question:

LM34966-Q1: soft start function

TPS40210-HT: Primary side soft- start for isolated flyback converter

N Sarangn
Prodigy 10 points
Part Number: TPS40210-HT
Other Parts Discussed in Thread: LM5155, TPS40210

Tool/software:

Hello team, 

Please can you let me know if the primary side soft- start circuit using a PNP transistor between the COMP and SS pins (as stated in the datasheets for LM34966 and LM5155) would work for this controller?

This is in regard to a primary side regulated flyback converter. 

Kind regards.

N

  • 0
    TI__Mastermind 30120 points

    Hi N,

    Thanks for using the e2e forum.
    A main difference between TPS40210 and LM5155 / LM34966 is the transconductance error amplifier. Compensation at COMP connected to ground instead of to FB.
    Also the softstart voltage has an offset with TPS40210.

    Allow some time to look deeper into this topic and check if there are any problems with using the PNP transistor approach for TPS40210.

    I will get back to you by tomorrow.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 10 points

    Hi Niklas,

    Appreciate the quick response! Will wait for your update re: use of this circuit for the TPS40210. 

    Depending on the value of the capacitance placed between the SS pin and GND, I can see a corresponding change in the SS pin voltage ramp. However, this does not reflect in the output voltage rise time. As this is a primary regulated isolated flyback converter, I am keen on a simple approach to allow soft start on the output voltage. Would appreciate any pointers you might have on this.

    Best wishes.

  • 0 in reply to N Sarangn
    TI__Mastermind 30120 points

    Hi N,

    After discussing this approach within our team, there might be a problem with the error amplifier.
    A transconductance error amplifier as used in LM5155 is current limited. This may not be the case for a regular amplifier as in TPS40210, so when pulling COMP to GND via the PNP, it might get overloaded. Hence, the PNP transistor approach does not work with TPS40210.

    The regular SS ramp does still work in PSR flyback just like in normal boost application, but I assume you intent to further reduce possible overshoots with the additional circuit, correct?

    I can look if we have any other reference designs for a TPS40210 PSR Flyback.
    Again, I will need some additional time for this and get back to you within 1-2 days.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 10 points

    Hi Niklas,

    Thank you for confirming that the PNP transistor is not the correct solution to achieve soft start in this instance. 

    I have noticed that the SS capacitance value only seems to affect the ramp of the SS pin voltage. I was unable to see a corresponding change in the output voltage, which continues to increase from 0V to about 90% of the regulated value in a few 100 microseconds, regardless of the SS capacitance value. My target soft start time is in the tens of milliseconds.

    Would be incredibly helpful if you have any leads wrt reference designs (PSR flyback+ soft start) for this controller. Appreciate your support with my query!

    Kind regards.

  • +1 in reply to N Sarangn
    TI__Mastermind 30120 points

    Hi N,

    I was checking for designs, but all references I found for TPS40210 PSR Flyback do not use any additional startup circuit and all rely on the internal softstart.

    I am confused that the SS cap does not seem to have an influence at all in your design.
    May I ask which values you have tried for the SS cap so far?

    If you are interested in a design review, you can also attach the schematic and the Vin/Vout/Iout specs here.

    Best regards,
    Niklas

  • +1 in reply to Niklas Schwarz
    Prodigy 10 points

    Hi Niklas,

    Thank you for getting back to me on this. I figured out that there was a pre-charged condition on the rail connected to the FB pin which was affecting the start-up. I have now resolved this from my end by correcting this connection in the circuit and can see the soft start working correctly.

    Appreciate the prompt support you have provided with my query. I will mark my question as resolved.

    Kind regards.