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LM20333 in Flybuck

Robert Oglesbee
Prodigy 40 points
Other Parts Discussed in Thread: LM20333, LM5022

Hello,

I was wondering if anyone has used the LM20333 in a flybuck configuration.  I need the following:

- synchronize to a wide range of switching frequencies (500kHz or lower up to 1MHz)

- 6.5-7V @ greater than 600mA

- Negative 6.5-7V @ greater than 300mA

- Isolated

If a flybuck configuration won't work, is there another fairly simple topology that might?

Thanks,

Rob

  • 0
    Prodigy 40 points

    I should clarify, the two output rails I need are on the isolated side.  The primary side would not draw much power.

    Rob

  • 0
    TI__Intellectual 1135 points

    Hi Rob,

    Generally, the low side sync FET will see higher negative current  in flybuck configuration than that in Buck configuration.  In your application,  there is greater than 900mA DC current in total on secondary side but almost no power on primary side, the low side sync FET will see certain amount of negative current depends on your transformer design (what is your input voltage range, and transfomer turn ratio you intended to use?).  LM20333 might not be the best solution to be used in the flybuck configuration for your application, since the LM20333 features  a typical negative 2.8A peak current limit while the positive peak current limit is 5.2A. If the negative peak current limit is triggered, the low side sync FET will be turned off, and consequentialy, the secondary side will be disconnected from the primary side.

    Flybuck configuration will work for your application, but the part with high enough current capability  and proper negative current limit should be choosed .  In addition to the flybuck configuration, flyback can also do the job. I would suggest you take a look at LM5022 if the input voltage range fits your application need.  

    Haifeng Fan