LTC3779 4 switch design problem


I have a application for LTC3779 switch. for the freewheel switch and boost rectifier, Can I use diode instead?


  • 0
    •  Analog Employees 
    on Mar 23, 2020 6:24 AM


    Did a quick simulation with LTspice, yes you can use a diode to replace the freewheel switch and boost rectifier.

    however I would not recommend doing so. The diode would greatly impact the efficiency of the DC-DC converter



  • Hi Ian1,

    >> Input voltage is 300 to 900Vdc with 400Vdc output.
    What maximum output current you need?
    How dynamic is your load, what is the regulation requirement for it?

    I dissent from - Pardon me!
    You need all 4 switches for buck & boost functionality.

    The problem is, that the LTC3779 is not intended for such extreme voltage and an additional mosfet driver-IC in between.
    -> For your application you will need a total different approach - sorry for my gut instinct...
    => Probably you are better off with SiC-mosfets. But they need special drivers and a very sophisticated board layout.
    -> the problem is that you need a half bridge driver with 100% / 0% capability AND built-in shoot-through prevention.
    The bootstrapped supplies for the drivers has to come from both half bridges
    In "real life" (on your board) you will have to design in considerably overvoltage spikes caused by extremely high dI/dT.
    I would suggest, that you search the internet thoroughly for high voltage buck-boost circuits.
    Because maybe you have to built your converter on your own with seperate building blocks.

    - Does your inductor have the neccessary voltage rating? (>900V at startup)
    - Try to get an LT(!)/AD-AFE or your distributors AFE to totally re-think your application.
    (But this could be even harder in such unprecended times)
    - I am NOT an AD-employee unfortunately