Question about MOSFET as a switch and a cap?

[EasyGoing1]

Talking about using a MOSFET as a switch ... like this (and in this case, the LED is actually a 12 volt incandescent bulb)



Is there any advantage to adding a cap, like this?



Does the cap do anything to help guarantee that the gate is off HARD once the voltage has been removed from it?

[eccerr0r]

Re: Question about MOSFET as a switch and a cap?

With all the posts we've written about using MOSFETS high side, I'd think you stop running high side.

Also the MOSFET is still being drawn wrong, you have a P-channel drawn with S-D swapped.

Perhaps you should just specify your requirements and ask someone to design one for you and we choose the parts.

[stj]

Re: Question about MOSFET as a switch and a cap?

replacing the cap with a 10k resistor to help it turn off would be better

[sam_sam_sam]

Re: Question about MOSFET as a switch and a cap?

This might be a little of this topic however I want to ask a question about using a mosfet to limit current using a PWM controller so it not completely of topic because I going to follow this post

Perhaps you should just specify your requirements and ask someone to design one for you and we choose the parts.

I might be interested in doing just what you are talking about here

1.5 to 24 volts Charging and discharging batteries 0.050 to 5 amps

This is what I am looking for possibly using a micro controller to control the mosfets

[eccerr0r]

Re: Question about MOSFET as a switch and a cap?

This might be a little of this topic however I want to ask a question about using a mosfet to limit current using a PWM controller so it not completely of topic because I going to follow this post



I might be interested in doing just what you are talking about here

1.5 to 24 volts Charging and discharging batteries 0.050 to 5 amps

This is what I am looking for possibly using a micro controller to control the mosfets

yes this is threadjacking, create your own post, but this is not a simple circuit unless you just buy one off from fleabay or ali...

[eccerr0r]

Re: Question about MOSFET as a switch and a cap?

replacing the cap with a 10k resistor to help it turn off would be better

Yeah if it would ever turn off...
as it's a p-channel hooked up wrong the body diode will always be on...getting that diode to turn off would be quite a challenge...

[redwire]

Re: Question about MOSFET as a switch and a cap?

How about the basic mosfet symbols... confusion is plentiful otherwise

[eccerr0r]

Re: Question about MOSFET as a switch and a cap?

Yeah whoever set up the "official" MOSFET symbol decided to buck history and made the arrow confusing to people reading schematics. They should have just stuck the arrow on the source terminal and make it follow convention: arrow points in current flow, even if MOSFETs

or they should have threw everything away and made the arrow point in electron flow.

my goodness they got vacuum tubes (diodes, triodes, ...) right, arrow/curvature of the cathode electrons are coming from the cathode. But for semiconductor diodes and bipolar junction transistors they made the arrow current flow instead of electron flow... sigh.

Perhaps the reason for MOSFET confusion was due to history from junction FETs... despite the arrow is drawn on the gate of a JFET normally you don't forward bias the gate, so technically the arrow does follow convention. Translate to MOSFETs it doesn't work...

Also back to the original topic...

What does one mean by "turning off hard" ... Usually MOSFET drivers can both source and sink current specifically to shut off the MOSFET fast. So you have a driver problem if you're just leaving the gate floating to shut it off. You can do that with BJTs but not MOSFETs.

[EasyGoing1]

Re: Question about MOSFET as a switch and a cap?

Perhaps you should just specify your requirements and ask someone to design one for you and we choose the parts.

What does one mean by "turning off hard" ... Usually MOSFET drivers can both source and sink current specifically to shut off the MOSFET fast. So you have a driver problem if you're just leaving the gate floating to shut it off. You can do that with BJTs but not MOSFETs.

Yeah if it would ever turn off...
as it's a p-channel hooked up wrong the body diode will always be on...getting that diode to turn off would be quite a challenge...

MAN ... it was just a general question ... the schematic symbol was literally one I just grabbed and screenshot just to have a visual to go with the question ... the main focal point of the schematic was about the cap at the gate ... this is not a circuit I'm actually building it was something I was thinking about and then I began to wonder if connecting something between the gate and ground would in any way help sink any potential stray current so that when the voltage at the gate goes low enough, it sinks it down to zero.

replacing the cap with a 10k resistor to help it turn off would be better

Thanks, STJ for actually answering the question... that was all I was looking for.

[eccerr0r]

Re: Question about MOSFET as a switch and a cap?

Still, be accurate in your schematics. You've been warned multiple times and drawing things wrong will lead to wrong conclusions. Also high side drive has other issues, adding resistors can cause gate failures depending on your voltages.

Note that when you shunt the gate to GND you also slow down turn on time, and you also limit your gate drive due to voltage divider issues.

As I said for fastest turn on and off time the driver needs to sink and source current. Adding resistors is a bandage.

[redwire]

Re: Question about MOSFET as a switch and a cap?

The circuit can't work - regardless of mosfet type.
P-ch mosfet will never turn off, with 5V on the gate and unknown VCC.
N-ch mosfet will never turn on, with 5V on the gate and the source moving up.

You can't answer questions about the circuit because of confusion around what kind of mosfet is being used, what VCC is etc. A 12V lamp load is mentioned.

[PeteS in CA]

Re: Question about MOSFET as a switch and a cap?

Assuming an N-channel MOSFET, for it to turn on the Gate must be several volts more positive than the source. The meaning of "several volts" will depend on the threshold voltage of the MOSFET. Assuming a logic-level threshold (the lowest threshold voltage), the MOSFET in that circuit will barely turn on, with just a volt or two across the "12 volt incandescent bulb".

OTOH, if one assumes a 12V Vcc, putting the "12 volt incandescent bulb" in the Drain circuit could work OK with a logic-level MOSFET. With a regular threshold MOSFET the MOSFET might or might not turn on, and if it does turn on it might get rather warm, due to power dissipation.

[eccerr0r]

Re: Question about MOSFET as a switch and a cap?

Yeah it's too bad we have to make assumptions... He could have actually meant putting a correctly placed p-channel device here in which there are still gate drive implications that we'd have to make additional assumptions...and we still don't exactly know what's going on here.

He's posted so many erroneous schematics so far that we've pointed out faults in and things don't change... so what can we really infer from these? Pretty much nothing other than perhaps he really should just be working with relays...

[redwire]

Re: Question about MOSFET as a switch and a cap?

When I was at that stage learning electronics, in tech school, there are two rules that I memorized:

For an N-ch mosfet, the gate must have a more positive voltage with respect to the source, for it to turn on. The drain must be more positive than the source (otherwise the body diode conducts).
For an P-ch mosfet, the gate must have a more negative voltage with respect to the source, for it to turn on. The source must be more positive than the drain (otherwise the body diode conducts).

Once you have these two basics looked after in a circuit design, then you can move on to the other issues - like level-translating, switching speed, on-resistance, heat etc.