Using FETs as Switches

Discussion in 'General Electronics Chat' started by elec_mech, Jul 5, 2013.

  1. elec_mech

    Thread Starter Senior Member

    Nov 12, 2008
    Anyone know of a good site or application note on using small voltages to control larger ones?

    Found a good app note here. I unintentionally tried a common drain design with an N-CH MOSFET as a high side switch to control power going to the anode side of a common anode LED display. I then tried putting same N-CH on the cathode side in a common source design. Neither worked and after reading the app note, it appears to use an N-CH in such a way, the gate voltage must be higher than the input voltage and threshold voltage of the FET combined. I guess special driver ICs are used to accomplish this. I haven't read the app note in detail yet, so perhaps the answer is in there. but I thought I'd ask here as well. I've tried doing similar switching in the past with BJTs with limited success. I've had a heck of a time finding good solid examples on how to do this with transistors in general.

    I've used logic level MOSFETs in 5V applications. Can those be used in this one (12V to control 18V)?

    Is a P-CH the way to go?
  2. crutschow


    Mar 14, 2008
    You last sentence is the answer to your problem. A P-MOSFET is typically used as a high side-switch for positive voltages. The supply voltage is connected to the source and drain to the load. Grounding the gate turns the transistor fully ON (if the supply voltage is sufficient for an adequate gate-source voltage) and letting the gate go to the source voltage turns it OFF.

    For a 10V or greater supply voltage you don't need a logic-level FET. Any standard FET should work.
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    Here's an example.
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    I had a similar experience 2003, before intro. to AAC,with high side switching using N ch MOSFETS to switch 32V using 12V logic. As supply V was full wave rectified 24V AC all that was needed was to add a cap., diode & zener diode to give a 12V supply stacked on supply V. Gates were, or are driven with transistors. It is still making the waving action on a US flag. Thr reference that you posted is verry informatave.
  5. MrChips


    Oct 2, 2009
    So you've discovered the solution.
    On the low side, use N-type FET or NPN BJT.
    On the high side, use P-type FET or PNP BJT. In order to control higher voltages you will require additional circuitry or use a high side switch.
  6. elec_mech

    Thread Starter Senior Member

    Nov 12, 2008
    Thank you everyone. A P-channel is definitely the answer, but as MrChips and Bernard pointed out, additional circuit is needed to fully shut off the P-Ch since the input voltage is 18V and the control voltage is 12V.

    Per Bernard's comment about using transistors to drive gates, something clicked and I experimented a little. Sure enough, using an NPN transistor to drive a P-Ch MOSFET does the trick. I've attached a quick schematic for reference. I used 15V and 10V as that is what I had handy.

    Thank you again everyone!