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LM3481: SEPIC converter Topology

LEVI PARSELL
Prodigy 30 points
Part Number: LM3481

Hi all,

I've been tasked to perform what my company calls a tech refresh on a power supply circuit board, essentially this involves addressing minor issues, obsolescence, small tweaks and adjustments based on operational history.  This also includes a PCB re-layout.  

This circuit board utilizes an LM3481 with a SEPIC topology, I've heard from  colleagues  who have experience with this board that the SEPIC converter performs terribly.  

I'm seeking the help of the experts to identify any glaring design issues.  I have a feeling there are fundamental schematic errors that are causing the poor performance (or none at all). 

The design Targets:

Input voltage range: 5 - 28 VDC

Output voltage: 12 VDC

Output current: 500 ma

 

A couple issues I have noticed right away: 

1) The two inductors are different values.  All the design notes that I have seen use identical values.

2)  The Compensation resistor and capacitor are swapped according to the examples shown in the design notes.(R51 and C38) in attachment.

Thanks in advance!!

  • 0
    TI__Mastermind 46905 points
    Hi LEVI

    The expert supporting this device is out of office. He will reply the question next week.
  • 0 in reply to Jasper Li
    TI__Mastermind 21720 points
    Hi Levi,

    Regarding the inductors there are two approaches, 1) a coupled inductor can be used or 2) Two Separate non-coupled magnetic can be used. The first is more common by the second allows for more flexibility for the design.

    The placement of the compensation components (R59 and C38) can be placed in any position since they are connected in series. Either way a zero is formed by these two components.

    What type of performance issue are you currently running into? Also what is the value of C47? I am assuming this is the bulk capacitance and need this to analysis the control loop.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 30 points

    Garret,

    Thank you very much for your response.

    C47 is a 150uF, 16V cap, and for whatever reason, is only installed on the 28V PCB variant (SEPIC is supposed to be designed to work with three different input voltages: 5 VDC, 12 VDC, and 28 VDC).  

    Regarding the performance issues, we have a lot of noise on the output.

    Feel free to let me know if you have any other questions

    Thanks,

    Levi

  • 0 in reply to Garrett Roecker
    Prodigy 30 points


    Can anyone  determine a ball park figure of the ACTUAL MAX output current from the SEPIC circuit in the original post?

  • 0 in reply to LEVI PARSELL
    TI__Mastermind 21720 points
    HI Levi,

    Is it just high frequency noise that you are seeing on the output? Based on calculations the loop compensation looks okay.

    Assuming the noise is high at a high frequency the first step should be to add a snubber across the output diode. I also recommend adding a snubber on the switch node as well.

    The maximum output current is just based on the peak current limit set by R46. Based on the input voltage range you should be able to get about 700mA.

    -Garrett