Need help choosing the right PWM controller for 72V to 12V 5A buck converter (DC-DC converter)

Discussion in 'The Projects Forum' started by nikee, Sep 19, 2014.

  1. nikee

    Thread Starter New Member

    Jan 17, 2014
    I am planning to make a buck converter able to convert from (30V - 90V) to 12V delivering 5A current.
    I am fine in calculating the inductor and capacitor and other parts requirement and selection. I am facing problem in choosing the right PWM controller.
    My converter will be
    1. Current mode controlled
    2. Non-Isolated
    3. Synchronous or asynchronous depending on complexity and part cost

    I have been looking for several IC from multiple manufacturers but each IC got some problem.
    I started with TI UC3842A but their current sense resistor is monitoring 1V voltage drop which results in 5W of power loss which is unacceptable. (specially since I am planning to increase the current in next iteration)

    Then I went for LT3800 but the maximum input voltage is 60V but I need 90 volts.
    After looking at many PWM controller IC I can say that my PWM IC should be
    1. Either not using Rsense (current detected via mosfet Rdson) or the type of Rsense which is in series to load (Rsense+:Rsense-) which consumes very low power.
    2. It should not be directly fed by incoming voltage because I don't think many PWM IC can withstand 90V at Vin
    3. Should be driving the Mosfet directly
    4. Certainly should have external switch (not the one with internal switch and limited to 2-3 Amps of current)
    5. Should not be too costly

    I think choosing the right IC should not be a big issue and I must be missing something.
    Any help choosing the right IC will be really helpful. Also if someone can point me to a similar project
    that will be really helpful (I might be missing something but I have been looking for similar project on google for past 3-4 days and believe me I cant find one)

  2. nikee

    Thread Starter New Member

    Jan 17, 2014
    I think I am wrong in interpreting the Isense and Rsense in UC3842A IC. Currently what I understand is Rsense should be such that it feeds 1V back to PWM controller which will result in 1Vx5A = 5watts of power loss. I think something is wrong here. Can someone help me understand setting Rsense value. the datasheet is confusing me.
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    Sign up for TI WEBBENCH. They have several designs with all the parts.
    nikee likes this.
  4. nikee

    Thread Starter New Member

    Jan 17, 2014
    thanx ronv, this is working really great, appreciate your help.
    Another nice link I later found out is "" which is collection of all OEMs in a single place.