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LM3481-FLYBACKEVM: Reason for diodes on FLYBACKEVM

RYO
Intellectual 2000 points
Part Number: LM3481-FLYBACKEVM
Other Parts Discussed in Thread: LM3481

Hello all,

I have a question about the diodes D4 and D5 on the secondary side of the EVM. Can I ask why you put these diodes? What does these diodes do?

Thank you,
RYO

  • 0
    TI__Mastermind 21720 points

    Hi RYO,

    The diodes are used in combination with C17 and R11 to form a soft-start circuit on the secondary side of the transformer. this allows for the controlled start-up of the output voltage. The soft-start time can be increased by increasing R11 or C17. D4 is there to discharge C17 if the output voltage is shut off allowing for the soft-start to happen again.

    -Garrett

  • 0 in reply to Garrett Roecker
    TI__Intellectual 2000 points

    Hello Garrett,

    Thank you for your reply.I could understand how softstart works by the combination of D5 and C17. I think the current path at start should be red line in the picture below. But what R11 do? The green path below is the current path when the LM3481 is disabled, right?

    Best regards,
    RYO

  • 0 in reply to RYO
    TI__Mastermind 21720 points
    Hi RYO,

    R11 is there to ensure that the CATHODE of D5 is high than the reference voltage when soft-start is done. This will reverse bias D5 and keep the control loop from being effected by the soft-start circuit in steady state operation.

    -Garrett