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TINA/Spice/LM3478: SEPIC Converter Startup Current Problem

Ignacio
Prodigy 160 points
Part Number: LM3478
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hi,

I´m designing a power supply (SEPIC Converter) for a circuit wich need an output voltage of 12V. These are the characteristics of the SEPIC:

  • Vin min = 10V
  • Vin max = 30V
  • Vo = 12V
  • Io max = 0,7A
  • Sw Freq = 1MHz

I´ve made a previous design on TINA software and it seems that the supply works fine except for one thing, I obtain a high current at the startup of the simulation. What is the reason of this current? and is it possible to avoid this behaviour?

If someone needs more information I´ll be attentive to provide it. Thank you very much!

Best Regards.

  • 0
    TI__Mastermind 21720 points

    Hi Ignacio,

    Can you please show the plot of the output voltage?

    The soft-start of the LM3478 is internal and can be increased by adding the circuit shown below.

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 160 points

    Yes, sure. Here you have.

    Thanks for your advice. How could I decide Rss and Css values?

    Thank you very much Garrett.

  • 0 in reply to Ignacio
    TI__Mastermind 21720 points
    Hi Ignacio,

    Please take a look at this application report. This should help explain on how to extend the soft-start time of the LM3478.
    www.ti.com/.../slyt669.pdf

    -Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 160 points

    Hi Garrett,

    The increase of soft start doesn´t solve the problem of inrush current at start up. 

    I´ve got 2 real LM3478 samples where I´ve applied my design, the 2 chips have broken-down. I can probe with my multimeter that Isen, Vin, FA/SD and AGND ports are now shorted. 

    I´ve only one more sample and i don´t want to continue making mistakes, so that´s the reason why I´m here.

    Thank you!

  • 0 in reply to Ignacio
    TI__Mastermind 21720 points
    Hi Ignacio,

    What coupled inductor are you using? 1MHz is a pretty high switching frequency for a SEPIC and the coupled inductor must be selected correctly. Please post the schematic.

    If the pins are shorted it implies that an absolute maximum voltage on one of the pins was exceeded.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 160 points

    Hi Garrett,

    The schematic is the one of my first question. The coupled inductor is 10uH (SRF0703-100M, 0,082Ohm, 2,1A). The mosfet is DMN6140L-7 (I = 2,3A , Vmax = 60V), and the Diode is NTS245SFT1G (Imax = 2A, Vreverse max = 45V).

    I selected this inductor appliying the following formula. I apply a 30% for the ripple current. AIL = 0,7A * (12V/10V) * 30%. L = 10V/(2*AIL*1M)*Dmax (0,55)

    So AIL = 0,25A and L = 11uH.

    Maybe the Isen pin voltage at startup was higher the maximum wich we can see in the datasheet is 500mV. The startup could be the reason of that?

    Thank you Garrett.

  • 0 in reply to Ignacio
    Prodigy 160 points

    Any idea Where im failing?

  • 0 in reply to Ignacio
    Prodigy 160 points

    Hello again Garret,

    I´ve been investigating on this.

    1. I feel my mistake was connecting the Rsen resistor without a capacitor. Thus, I obtained a peak voltage on the Isen pin higher than 500mV (witch is the maximum voltage allowed). So I decided to connect a capacitor in parallel, and seem it works.

    2. Now, I have a new problem. When the SEPIC is working without load, it works well and it produce 12V.
      But working with load (120Ohm -> 100mA), the Vout goes down to 5 or 6 Volts.
      I neither see a frequency of 1MHz on the DR output.

    Any advice?

    Vout = 12V (Without Load)

    The output without load is 12V, but I obtain some weird noise. This is created together with the DR pin pulse and is transmitted also to the GND.

    ​Vout Fall Down With Load 

    Best regards!