Switching between two voltage sources.

Discussion in 'Digital Circuit Design' started by ashokchandra, May 16, 2018.

  1. ashokchandra

    Thread Starter New Member

    May 13, 2018
    3
    0
    Hello All,

    I would like to switch between two voltage sources, namely +28v and +20v. I have been trying to do with MOSFET. IRFR5305TRPBFand BJTMBT3904DW1T1G, I will be doing the switching using MICROCONTROLLER.Please advise on this. I also attached our circuit design please confirm it works or not


    Or, Is there any other better way to do this ?
    upload_2018-5-16_14-41-56.png


    Thanks
    Ashok
     
  2. danadak

    Active Member

    Mar 10, 2018
    662
    134
    Why the back to back mosfets for the 20V supply ? Given timing is
    controlled by UP ?

    BJTs should have a R to ground right at base, to absorb leakage and
    possible turn on when UP outputs are in tristate, like during UP power
    up. I would make UP to base R 1K, and BJ base to ground ~ 10K. In
    fact Rbasetognd <= Vbe / Ileak

    Regards, Dana.
     
  3. crutschow

    Expert

    Mar 14, 2008
    18,634
    5,201
    To block the 28V from backdriving the 20V supply through the substrate diode.

    That circuit should work okay but you should add a resistor in series with the bottom BJT as you did for the top one to limit Vgs when ON, since the maximum Vgs rating of the MOSFET is 20V.
     
  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    6,740
    1,935
    Also, I think you need back-to-back MOSFETs for both switches.

    ak
     
  5. crutschow

    Expert

    Mar 14, 2008
    18,634
    5,201
    Why?
    The top MOSFET will never be reverse biased.
     
  6. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    6,740
    1,935
    Depending on the condition of the 28 V source...

    ak
     
  7. crutschow

    Expert

    Mar 14, 2008
    18,634
    5,201
    Yes, my statement assumes it stays at least 20V (or that it's not a problem if the 28V is back driven by the 20V if it's off).
     
    Last edited: May 17, 2018
  8. Picbuster

    Active Member

    Dec 2, 2013
    689
    83
     
  9. andrewmm

    Active Member

    Feb 25, 2011
    128
    26
    Do you want to be able to switch between or just use which ever is the greatest voltage ?

    You could also look for active diodes or loss less diodes,

    ( they are really fets, but have a hunt )
     
  10. ashokchandra

    Thread Starter New Member

    May 13, 2018
    3
    0
    HI danadak,
    Thanks for the details.where should i add the resistor in series with the bottom BJT.could please show me or redraw my attachement where should i add the resistor.


    Thanks,
    Ashok
     
  11. olphart

    Member

    Sep 22, 2012
    12
    3
    For something completely different: I had to solve this issue for a test calibration jig.
    I used a LM317 (or equ adjustable regulator) with one voltage set by a resistor, then modified by a relay to another resistor.
    Just be sure to use a relay good at low current. I suppose a FET could replace the relay, but the relay is simpler.
    An upside is no glitch in transitions.
    upload_2018-5-20_8-4-26.png
     
    ebeowulf17 likes this.
  12. ArakelTheDragon

    Active Member

    Nov 18, 2016
    540
    25
    Or you could put a SPDT relay which will switch between the 2 sources. If you do not want it to be automatically switched, you can put a slide switch.
     
Loading...