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UCC28782: Startup delay issue

Ashok Kumar Challa
Prodigy 140 points
Part Number: UCC28782

Tool/software:

Hello,

My Name Kumar, We have designed a flyback 40W power supply Vin:105Vac to 132Vac, Vout:20.8Vdc@ 2Amp load. I have seen a start up delay issue in my design as below. till 13V it was increasing linearly, after that it was taking more time to reach 20.6V as below. it was taking 1.43mSec to reach 13Vdc and then ~38mSec to reach 20V. In first pic you can see the full wave forms, in the second we can see the linear rising till 13V and then slowdown to reach 20V.

  

As per TI excel the transformer winding turns P:S:A = 25:5:5 turns, but we have used 25:4:4 and we set the V feedback for 20V. so we thought system will increment the voltage. is this reduced turns ration causing the delay? or any other thing that i am missing. I also attached the ref design.

 

  • 0
    Prodigy 140 points

    missed the attachment 

  • 0 in reply to Ashok Kumar Challa
    TI__Mastermind 24375 points

    Hi Ashok,

    Thank you for reaching out.

    I reviewed the schematic and it looks fine to me.

    As you have pointed out, the turns ratio is a major factor which decides this increase to final output value. It seems the aux winding takes a lot of time to take control of the power supply of Vcc pin. You can probe the Vcc pin to see the charging time proflle along with output voltage. But increasing the turns ratio will be needed.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 140 points

    Hi Harish,

    Thank you very much for the reply. I compared the output voltage with IC Vcc (After boost converter) and observed as below.

    1. First waveform observed at Node 'B' (After Boost circuit) and Vout. Observed IC Vcc taking nearly 120msec to reach 19.7Vcc, where Vout=14V.

    2. The second one at Node 'A' and Vout.

    I couldn't fount any information related to that in datasheet. is this 120mSec causing this issue? 

  • 0 in reply to Ashok Kumar Challa
    TI__Mastermind 24375 points

    Hi Ashok,

    I will have to check on the 120ms, I will check internally, but it looks like more related to softstart implemented internally based on charging the Vdd capacitor and FB pin. The first plot looks like charging the Vdd cap and it looks fine. The following is the waveform for startup observed on our EVM at 50 ms/div

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    Prodigy 140 points

    Hello Harish,

    Good day,

    I just replaced the existing low turns ratio transformer with High turns ratio transformer (P:Aux:Sec = 1:0.14:0.19 ). but i couldn't see much improvement. as I mention in my first comments, After changing the transformer the Vout is rising linear till 14V after that rising slowly to 20V. 

    Can you please tell me is the any other thing effecting in the circuit.

    Thank you!

  • 0 in reply to Ashok Kumar Challa
    TI__Mastermind 24375 points

    Hi Ashok,

    Thank you for the feedback and trying out the higher turns ratio transformer.

    I have check with the ACF EVM, the start up time is consistent with what I see on your test setup. Looks like it is more related to soft startup process inside ACF.

    Regards,

    Harish