Discharging a P-Channel MOSFET quickly

[Dextrine]

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.

 

[Tom.G]

Yes, use a Totem Pole stage drive the Gate. Such an approach replaces R1 in your circuit with another transistor, supplying a low resistance path to discharge the Gate capacitance.

More than you wanted to know in this 48 page App Note:
http://www.ti.com/lit/ml/slua618/slua618.pdf

This search has lots of circuits and less technical data:
https://www.google.com/search?q=totem+pole+mosfet+driver

 

[Dextrine]

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.

 

[Tom.G]

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.

 

[Dextrine]

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?

 

[Tom.G]

About 7 780 000 hits: Just be aware that not all of the circuits actually work.
https://www.google.com/search?site=...rcuit+collection&oq=analog+circuit+collection

About 4 390 000 hits: Much more reliable results than the above search.
https://www.google.com/search?q=analog+circuit+design+design+note+collection

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.)

 

[Dextrine]

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.

 

[Dextrine]

About 7 780 000 hits: Just be aware that not all of the circuits actually work.
https://www.google.com/search?site=...rcuit+collection&oq=analog+circuit+collection

About 4 390 000 hits: Much more reliable results than the above search.
https://www.google.com/search?q=analog+circuit+design+design+note+collection

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.)

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

 

[Tom.G]

PNP transistor
Emitter to +V
input to Base
Collector to resistor to -V

(Get the biasing right.)
(Note that it does invert the signal.)

 

[Dextrine]

PNP transistor
Emitter to +V
input to Base
Collector to resistor to -V

(Get the biasing right.)
(Note that it does invert the signal.)

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?

 

[Tom.G]

...ever NOT be saturated?

Sure. Just restrict V_B 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 V_CC and take V_B from the wiper. Measure the voltage on the Collector while you adjust the pot.

 

[Windadct]

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?