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PMP40555: Inductance, shunt resistor

Other Parts Discussed in Thread: PMP40555, LM5155

Hi,

I'm checking the PMP40555 schematic by the following calculator.

PMP40555_LM5155_56_Excel_Quickstart_Calculator_for_Flyback_Regulator_Design.xlsx

I think that the inductance is too small compared with the result calculator.

Is the inductance of PMP40555(14uH) appropriate?

And, I think also that the actual inductor peak current limit value is too much.

Is this selected current sense resistor appropriate?

Or, is this calculator wrong?

Best Regards,

Kuramochi

  • Hello Kuramochi-san,

    for a 24W flyback beside the spreadsheet some more considerations should be analyzed;
    24W power range allows DCM operation, CCM operation or both of them, so transition mode
    (CCM at low input voltage and full power, DCM at high input voltage).
    DCM helps at high voltages to get less EMI trouble by reverse recovery, DCM also helps
    for dynamic requirements - the RHPZ is in the high kilohertz range and is not limiting loop bw.
    Last but not least DCM results in a smaller transformer.

    Attached two examples for your power stage, CCM by magnetizing inductance 14uH as-it-is,
    DCM by magnetizing inductance 2uH. So 2uH up to 25uH are fine for this application, depends
    on mode of operation, maximum peak current and needed slope to meet internal slope compensation.

    The sense resistor depends on your maximum peak current by magnetizing inductance
    (calculate at full load plus some margin and lowest input voltage).

    For your convenience added two calculations of your power stage DCM/CCM, hope this helps,

    Best regards, Bernd

    CCM Flyback Kuramochi-san.pdf

    DCM Flyback Kuramochi-san.pdf