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LM51561H-Q1: sepic converter

Part Number: LM51561H-Q1
Other Parts Discussed in Thread: LM5155, LM5156

I am reviewing a suitable Device for the sepic converter.
Could you recommend a suitable device and schematic for the following specifications?
Vin = 12V
Vout =+/-100V
iout = 0.04A(max)

  • Hello Lee,

    For such a high voltage rating between VIN and VOUT, I would recommend not using a SEPIC, but a flyback.

    Do you need an automotive qualified component?

    LM5156xxx devices are a good choice for this application, I think.

    If you decide to go with a flyback, this calculator is a good starting point: 

  • Thank you for quick response.
    I am a beginner in using DC/DC.
    I did not understand the answer below.
    "For such a high voltage rating between VIN and VOUT, I would recommend not using a SEPIC."
    I would like to ask you to explain why or share related document.

  • Hi Lee,

    I suggest taking a look at the following reference design. This implements the LM5155 (similar to the LM5156) as an uncoupled SEPIC controller with a positive and a negative output voltage. This design can be easily changed to meet the application requirements of your design.

    https://www.ti.com/tool/PMP40488

    Thanks,

    Garrett

  • Hi Garrett


    Thank you for good information.
    Let me refer to the circuit design.

    I have 2 questions.
    The first question is, I would like to know what Brigitte said.

    "For such a high voltage rating between VIN and VOUT, I would recommend not using a SEPIC, but a flyback."
    I would like to ask you to explain why or share related document.

    The second question is that if you select LM5155 in WEBENCH and run WEBENCH, it comes out up to 45V in SEPIC structure. please check. ( My Vout spec is 100V )

    The WEBENCH address I ran is as below.

    www.ti.com/.../LM5155

  • Hi Lee,

    Typically for large setup ratios or when isolation is needed a flyback works well. The only problem is that a flyback requires a complicated transformer and they can be very noisy. The document that Brigitte shared to calculate the components required for a flyback regulator. The reference design that I shared uses a SEPIC and has a few key benefits. First it doesn't require a complicated transformer. Second, the power level is relatively low so it fits the topology very well. Third a SEPIC is much quieter than a flyback. This is good for medical applications or sensor applications.

    Regarding WEBENCH, it will not support the dual output application that is required. This is not a very common application for a SEPIC converter so it is not supported by WEBENCH. However it will support 100V output, WEBENCH limits the output to 45V as most applications don't require output voltages larger than this value. I suggest using the reference design that I shared with you and changing it to meet you specifications. Some of the changes that will need to be made are

    • Change the feedback resistor to set the output voltage to 100V
    • Make sure the output capacitors are rate at a voltage greater than 100V
    • Ensure the MOSFET and the diodes are rated for greater than VIN+VOUT = 12V + 100V. 120V should be sufficient. 

    Hope that helps out.

    -Garrett

  • Hi Garrett

    Thank you for your kind explanation.
    I will review using the reference design you have recommended.
    In my personal opinion, WEBENCH hopes to improve.
    I think that the output from FLYBACK and the output from the SEPIC looks different from each other can be misleading to users.

    Thanks again for the kind explanation.

    Best regards
    HongGyo Lee