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Discharging a P-Channel MOSFET quickly

  1. Jul 14, 2017 #1
    I am using trying to familiarize myself with P-Channel mosfets since I don't have too much experience with them. I am trying to use the FET as a switch, being driven in the typical highside FET manner using an NPN transistor.

    Is there some standard way of quickly discharging the FET during turn off? I am trying to use maybe another BJT but can't seem to bias it correctly during turn off.

    Pretty much just like this.


    thanks in advance for the help.
  2. jcsd
  3. Jul 15, 2017 #2
  4. Jul 15, 2017 #3
    Thanks a lot Tom. I sometimes struggle finding information due to not knowing the terminology, I'll be sure to remember this though. Thanks again.
  5. Jul 15, 2017 #4
    You are welcome. Have Fun!
    Oh, and I almost forgot:
    Be careful you don't exceed the maximum Gate-to-Source voltage in the ON condition. The circuit you show puts the full supply voltage on the Gate. Unless it is a really low supply voltage, that generally leads to a very short life of the FET.
  6. Jul 15, 2017 #5
    I made it work with an extra NPN transistor. Apparently just adding the totem pole will invert the output. Thanks again though, is there some document that lists common circuits in analog and power along with their name?
  7. Jul 15, 2017 #6
    About 7 780 000 hits: Just be aware that not all of the circuits actually work.

    About 4 390 000 hits: Much more reliable results than the above search.

    After you get tired of those, with the search field still showing the search string, click in the field again. You will receive more search suggestions.

    (Should keep you busy for a little while, at least until your hair turns gray.)
  8. Jul 15, 2017 #7
    Wow, thanks! This definitely will keep me busy for quite a while. I had been looking up "List of analog circuits" with much poorer results.
  9. Jul 19, 2017 #8
    Quick question Tom, is there some circuit that can take a pulse that goes from, say 0 to +V and makes it go from. -V to V? to ? I'll be looking through the analog circuit indexes but thought you might know the name of such a circuit off hand.

    Thanks in advance
  10. Jul 19, 2017 #9
    PNP transistor
    Emitter to +V
    input to Base
    Collector to resistor to -V

    (Get the biasing right.)
    (Note that it does invert the signal.)
  11. Jul 19, 2017 #10
    Thanks again for the input Tom. I had another question more from a theoretical vs. applied point of view:

    Is it possible to have an NPN transistor be in the linear region, never saturated, if the emitter is connected to ground, the base to a resistor (which goes to your signal or something), and the collector to another resistor that goes to some high rail?

    In other words, can the following circuit, in real life, ever NOT be saturated?

  12. Jul 19, 2017 #11
    Sure. Just restrict VB so the transistor is always operating in its linear range.
    It's a simple enough circuit you can build it even without a breadboard. Use a potentiometer (pot) across VCC and take VB from the wiper. Measure the voltage on the Collector while you adjust the pot.
  13. Jul 20, 2017 #12
    IS this NPN Transistor question completely unrelated to the MOSFET gate driver situation? Is there a case when you would want to drive the MOSFET gate using the NPN transistor in linear?
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