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LM51561H: Schematic Review

Aditya Singh
Intellectual 280 points
Part Number: LM51561H

Hi E2E Team,

We are using LM51561H as a flyback boost converter in our design. Below are operating conditions-

1) Input voltage: 18V to 32V

2) Output power: 65W (1.25A@54V)

3) Flyback Transformer: Attaching the transformer specification sheet. Please review and let us know if any modification is required.

I am attaching the schematic and BOM for the design. Please review and share your review comments.

Below are our queries:

1) Please let us know how we can estimate the input and output snubber circuit components value (R1, R2, C1 and C6) during design time.

2) As per my calculation output reflected voltage will be approximately 14V so MOSFET VDS > 32V +14V. Is our assumption correct? Also, is the calculated output reflected voltage correct? If not, please share the formula for the same.

3) Below are the specifications of selected diode (SK320A R3G):

     a) Repetitive peak reverse voltage: 200V

     b) Reverse voltage (RMS value): 140V

As per our understanding diode reverse voltage > 185V. Is SK320A R3G suitable for our design or we need to change it? If yes please let us know the reverse votlage of the diode.

Best Regards,

Aditya Kumar Singh

LM51561H_Boost_BOM.xlsxBoost Regulator.pdfLM51561H_Flyback_Transformer_Spec.xlsx

  • 0
    TI__Guru 58225 points

    Hello Aditya,

    1. Snubbers are extremely dependent on parasitics, so it is hard to impossible to calculate the values before you receive the real board. Then there is a relatively easy to define these, see here: https://e2e.ti.com/blogs_/b/powerhouse/posts/calculate-an-r-c-snubber-in-seven-steps

    2. Calculation gives 13.5V, so yes, minimum for VDS is 32V+14V, but dependent on how high you allow parasitic ringing, 60V or 80V transistor should be used.

    3. The voltage rating of the diode seems to have enough margin. How did you get the 185V?

    In general, I would recommend to use the quickstart calculator to review the schematic: https://www.ti.com/lit/zip/snvc240

    Best regards,
    Brigitte

  • 0
    Intellectual 280 points

    Hi Brigitte,

    Thanks for your quick response.

    I am referring to SNVA866 for calculations. Below is the link of the document:

    How to design an isolated flyback using LM5155 (ti.com)

    In section 3.4, selection of MOSFET is described. When calculating the min. Vds from equation 18, I am getting 182V since Ns/Np is 4 for our use case. I am missing something here since as per your experience we have enough margin and as per calculation min. Vds is 182V.

    Please let me know while selecting the output diode do we need to check the peak repetitive reverse voltage or RMS reverse voltage of the diode.

    I am checking the quickstart calculator now. It will be great if can you share your comments for the schematic.

    Best Regard,

    Aditya Kumar Singh

  • 0 in reply to Aditya Singh
    TI__Guru 58225 points

    Hello Aditya,

    Your calculation is absolutely right, in my head I used 1/4 instead of 4, please excuse me.

    To be honest, I would use a higher voltage diode as you need to keep ringing in mind as well, but the decision on how much margin you need is dependent on the rest of the system. I would always use the RMS reverse voltage to be on the safe side.

    Schematic comments (please share as well the outcome of the calculator):
    - Do not use a 1k resistor for the gate. This will prevent the converter from operating at all. 1 ohm is a better value there.
    - I would add a capacitor from COMP to GND as well (only as placeholder in case it is needed).
    - In case it is needed, I would add the slope resistor on CS pin.
    - For testing purposes, I would add the capability to turn dithering on or off. Especially for EMI purposes, it might be nice to be able to enable this feature.

    Best regards,
    Brigitte