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Original question:

LM3481: LM3481

LM3481: LM3481

Namith HL
Intellectual 570 points
Part Number: LM3481

Hi,

Please review the attached LM3481 Auxillary power supply design.

I have never worked SEPIC converter before ,so kindly check the schematic and give me review comments as earliest.

Following are my design specifications:

I/P Voltage: 21V-30V

O/P Voltage :24V

Current: 2A

Note:I have followed Ti webench power designer ,According to that design have created 

  • In current schematic from webench there is coupled inductor 150UH which is  big foot print ,kindly suggest me this inductor can be replaced by small inductance value.
  • I have gone through several design application note,based on that design calculation ,inductance value coming arround 12UH coupled inductor.
  • In webench switching frequency is 554KHZ.
  • so im considering 554KHZ for inductance calculation.(Duty cycle is 54%).
  •  Auxillary power supply design for TI review.pdf Also while considering duty cycle which duty cycle need to consider minimum or maximum duty? becoz if your considering minimum duty cycle maximum input voltage has to consider for calculation

      and suppose if your considering maximum dutycycle need to consider minimum input voltage. so give me some information on this .

  • At output side i have considered one electrolytic capacitor .need another two capacitor?

  • 0
    TI__Mastermind 21720 points
    Hi Namith,

    A coupled inductor around 10uH to 15uH should yield good results. For the application this will place the ripple ratio around 50% which is a good trade off between magnetic copper losses and AC losses. For the comments below I am assuming the inductor selected is 10uH and the operation frequency is 550kHz.

    1. Peak current will be around 7.5A
    2. Current sense resistor (R60) will need to be about 12mOhm. These should also be a resistor or ~500Ohm in between the ISEN pin and R140. this will add slope compensation, reducing sub harmonic oscillation.
    3. The AC capacitor (C272) should be around 3.3uF. The RMS rating on the capacitor should be greater than 2.3A.
    4. Output capacitor: Please increase this to 100uF. This will help with the transient response of the load and control loop stability.
    5. The loop compensation looks okay.

    Please let me know if you have any questions.

    -Garrett