Connect with us

MOSFET controlling 5v power supply from 3.3v logic level

Discussion in 'Power Electronics' started by skinny_c, May 10, 2011.

  1. skinny_c

    skinny_c

    5
    0
    May 10, 2011
    Hi,

    I'm not an "analog" expert and only dabble in it when I have to, preferring to stick to microprocessors.

    I have a 3.3v logic level from a microprocessor that I need to use to switch on/off a 5v 750mA supply to an external circuit. It doesn't have to be "fast" as I'm not pulsing the supply, just switching it on or off.

    Using a MOSFET "seems" desirable - high current / small package.

    From what I can see I need a "high" side as the "switch" is switching the 5v supply to a circuit, not the ground.

    I've tried an Infineon BSS215P, but it's always "on" regardless of the logic level (0v or 3.3v) used to drive the gate. The logic drive to the gate is pulled high (3.3v) via a resistor and I can see that this is working correctly.

    Ideally I need the MOSFET to be a single component solution and a SOT23 device. This will enable me to fit the device easily into the footprint of the demo board I'm using. I can accept a part that will handle 500mA if necessary, but 750mA would be ideal.

    Having looked around on the net there's lots of talk about switch 12v power supplies from 5v but nothing I can find that covers what I'm trying to do. I've also seen people talking about using a transistor to drive the gate from the logic level, but I need to keep component counts low and board space is limited.

    Any thoughts? It's driving me nuts!

    Cheers,

    C
     
  2. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    The reason as to why you would use a FET instead of a transistor is that to use a transitor you would need to boost the current of the output signal from the microcontroller (extra components) a FET is voltage triggered so the 3+ volts supplied by the microcontroller is enough to switch the FET.

    Ok firstly check your connections as you mention high side and low side... The + switching supply should be connected to the DRAIN pin of the FET and the - swithing supply should be connected to the SOURCE pin of the FET. With your signal from the microcontroller connected to the GATE.

    Often people choose a FET with a source to drain resistance to high. This means that the Fet will not turn off at 0 volts. One way you could check your connections is to connect say -5 volts through a resistor to the GATE pin. that should be more than enough to swith off the fet (if the connections are correct).

    Otherwise choose a FET that will turn off at 0 volts. (check the data sheet)
     
  3. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
    ... either that or use a Darlington package.
    But I'm surprised you can't find a device to switch off at 0V. It's been a long time but I thought the TMOS transistors we played with in the '80's switched off quite nicely. I wonder if you have tried a 10k pull-down resistor at the o/p of the processor?
    You could talk to the local sales manager for Motorola or the local professional electronics supplier and ask them.
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    Using a high-side switch like that requires the use of an open-collector output and a pull-up resistor to 5V to drive it.
    Your 3.3V is actually a 5V - 3.3V = 1.7V drive to the P-channel MOSFET gate.
    That's plenty to turn on a logic-level MOSFET.
     
  5. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
    What about a cct diagram Resqueline?
     
  6. skinny_c

    skinny_c

    5
    0
    May 10, 2011
    Thanks to every one for all your replies and for replying so quickly!

    I had considered pulling the output of the micro to 5v, but as the pin is not 5v tolerant I am reluctant to do this.

    I've come across the Vishay Si2328DS - spent some time looking through data sheets at the threshold voltage having understood that the MOSFET I had tried will be on for the 1.7v in my setup.

    The Si2328DS has a threshold voltage of 2v to 4v. So if I'm reading the data sheet correctly it should be ok. I guess my only worry is that at a VGS of 5V (which if my understanding is correct will be what I'll have when the MOSFET is ON) the ON resistance is 0.35 ohms...... is this a bad thing?
     
  7. skinny_c

    skinny_c

    5
    0
    May 10, 2011
    Ignore the Si2328 - it's N-channel so can't use for hi-side. Strange as the parametric search for P-channel brought it up on the Vishay web site!
     
  8. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
    By the look of the datasheet http://www.vishay.com/docs/71796/71796.pdf, the current you plan to switch is getting pretty big for such a little fella.
    Is your load inductive?

    (didn't see skinny's post in time)
     
    Last edited: May 11, 2011
  9. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
  10. skinny_c

    skinny_c

    5
    0
    May 10, 2011
    Thanks for the suggestion....

    BUT the threshold voltage isn't going to work. From what I've been told I'm trying to switch 5v with a 3.3v logic. So, that means I need a threshold voltage greater than 1.7v....

    Or have I got my understanding wrong?
     
  11. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
    I see, finally.
    maybe you could drive a little transistor from the processor, to switch the gate of the FET.
     
  12. skinny_c

    skinny_c

    5
    0
    May 10, 2011
    Having looked around there are such things as "smart switches" which do what I want. I'm off to take a look at trying one :)

    Fairchild FPF2123 is an example. Not a "3 pin" SOT23, but it's still a SOT 23 :)
     
  13. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
    I had a look at the package. Good one!
    Still, here's my transistor driver, which has given me a chance to practise my Gimp.
    PS: It's a stronger device than the all-in-one package, too.
     

    Attached Files:

    Last edited: May 12, 2011
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-