Mixing output of two DC/DC converters??

Discussion in 'General Electronics Chat' started by supermankid, Dec 7, 2013.

  1. supermankid

    Thread Starter Member

    May 26, 2013
    There must be some clever solution for this problem but I am finding it even
    difficult to ask in proper order. Please read it all an help me find a clever solution.
    I have two DC/DC converter
    One is
    Boost Vin(3-12V) and Vout(15V)
    other is
    Buck Vin(20-24V) and Vout(15V)
    Now I want to combine both of these voltage(15V)
    to provide more current for my load.
    Now, I noticed one thing that if the Vin of boost is
    higher than 15V it gives Vout of 16 V...etc...not 15v
    also if Vin is less than 3V vout is about 5-6 volts.
    Now what would be the clever way to give only 15V on the output
    so that I can combine with the output of both buck and boost.
    The main problem is as you may have already noticed by now that
    at times i could be mixing two different voltage to cause
    I have attached a block schematic for further understanding.
    adder circuit.jpg
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    A two-rectifer OR gate. Anode of first diode to first converter, cathode to output. Anode of second diode to second converter, cathode to output. Use Schottky rectifiers to minimize the forward drop.
    supermankid likes this.
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
    To keep the output of the boost converter at 15V, keep the input voltage in specification (3V - 12V).
  4. ian field

    Distinguished Member

    Oct 27, 2012
    Are you describing a buck/boost converter that can take a range of Vin both above and below the regulated Vo?

    They go by various names like; CUK or SEPIC that I can think of off the cuff - once you start googling, some other names should surface.
  5. supermankid

    Thread Starter Member

    May 26, 2013
    How can I limit the input within the specs 3-12v. I thought if Zener was not a help??
  6. ian field

    Distinguished Member

    Oct 27, 2012
    If you're struggling to get the current you need, a zener dropper that can handle the power might work out big and expensive - not to mention the heat sink! A better way is a chain of forward biased dioes so the sum of all the Vf adds up to the voltage you need to lose - but it still dissipates as much heat as the giant zener.

    To get any decent current draw, what you really need is a 3rd step down switcher to provide an in-spec voltage for your existing PSU - by the time you've paid to buy that, you might just as well have bought a PSU that does the job you want done.