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LM3481: start up circuit and Vds spike of LM3481 flyback

Frank Xiao
Genius 12055 points
Part Number: LM3481

Hi team,

my customer is using the LM3481 as the flyback controller follow the apps note SNVA761.

We want to two things below.

1. how the circuit works?

2. When we test the board, we find the SW spike is larger during start up below. Do you know the reason and  how to improve it?

Thanks.

  • 0
    TI__Mastermind 25760 points
    Hi Frank,

    This circuit is the start-up network for flyback converter. Initially, C17 is at 0V, so current should flow into C17 first through R11 and D5, preventing the startup in-rush current. Eventually, when C17 is charged, the flyback converter should be operating in steady-state. When the converter is off, C17 should discharge through D4 to ground.

    I think an improvement could be made by adding a 50kOhm pull-up resistor to U3 to the node with loop 2.

    I will try to take a look through a flyback EVM to see if this will help.

    Thanks,
    Richard