P-channel mosfet practical question: If I pull to Gnd, how do I avoid exceeding?

Using Arduino General Electronics
1

Hi!

I have a circuit where I use an NPN controlled by a teensy 3.1, that pulls the gate of a P-channel mosfet from 26V to Gnd.
I'm a little confused about Vgss, and I've searched the forums already for this specific question.

I'm not switching quickly (once on, once off.)

The NPN deals with making this 'logic level' and also so that my teensy doesn't see 26V (I hope!)

But I have been looking for P-channel mosfets, and the maxinum the seem to tolerate for Gate-to-source voltage difference is 20ish volts. So I worried I am over-overdriving my mosfet.

Is there a solution to this?

Do I keep searching, and find a mosfet with a higher Vgss tolerance? Or do I use a resistor ladder to reduce the Vgss applied [at the cost of majorly slowing down the ON-switch, or causing a lot of current waste through the ladder. The slow-on in this application is tolerable, and I can avoid applying much load while it turns on.]

For reference I'm attaching an image:

Thanks!

I've never used a P-channel, only logic-level N-channels. Perhaps I should just use a logic-level P-channel, but with a driving voltage of 3v3 on the teensy, I don't have great options. I understand if I had a logic-level p-channel i could just pull-down with the pin, and have a weak pull-up to my MCU Vcc for this application.

circuit_Excerpt.jpg

2

Your MOSFET is "upside down" (D & S swapped)
I haven't come across any parts with Vgs >25V either, so a voltage divider might be the needed solution.

3

o a voltage divider might be the needed solution.

That means putting a resistor between the collector and the gate / pull up junction.

4

CrossRoads:
Your MOSFET is "upside down" (D & S swapped)
I haven't come across any parts with Vgs >25V either, so a voltage divider might be the needed solution.

Can to clarify for me what you mean by upside down? Thanks! I'm a bit confused. If I were to swap D and S in my circuit, then that would say that the drain is the load, and the source is the 26Vdc. I always thought source refers to source of electrons, which is always the side closer to ground.

Do you mean I need to swap S and D, but keep the electrical configurations as the are? I think I see the problem in this case: The symbol as drawn shows the intrinsic diode in a way that will be active! I'm surprised as I thought this was a default part in the KiCad library, but perhaps I made it and messed up. Would it be proper to just mirror the diode so that it is properly shown?

Grumpy_Mike:

o a voltage divider might be the needed solution.

That means putting a resistor between the collector and the gate / pull up junction.

Grumpy Mike! I have learned a lot from your posts over my last two years foray into electronics. Thank you.

When you say a resistor between the collector and the gate/pull up junction, I think that makes sense to me. I went to draw it out and realized that a single resistor between collector and the gate/pullup junction makes a divider between my pullup and the extra resistor.

Sooo...with 26V, a 1k pullup, 20V VgssMax and 10V Vgs, I guess I'll aim for -15V applied to the gate.

So I want 9V left on the gate to make "-15V". So if 1k drops 15V, 600ohms will do for the 9V.

26VDc ----->[1k resistor]-----GATE----[600ohms]----thruNPN"0hm"---Gnd

In this case, if my source voltage was <26V, it would dip below -10V differential at 16.25V, and above -19V at 30.4V. That's O-K for me, though I might want to fudge my numbers to allow for above 30.4V.

5

Do you mean I need to swap S and D, but keep the electrical configurations as the are?

Yes.

Sooo...with 26V, a 1k pullup, .....

Yes, but I would make the resistors ten times the size.

6

yeah, I'm sorry, I still don't quite get the schematic change on the P-channel mosfet.

Grumpy_Mike:

Do you mean I need to swap S and D, but keep the electrical configurations as the are?

Yes.

I probably shouldn't have been so vague, when I said "swap S and D, but keep the electrical configurations as the are?" It was an ambiguous statement of mine, so I'm not sure what your "yes" response means.

Are you saying that, yes, 26V is the source?

Or rather just that I f'ed up my diode on that image and need to mirror it?

I realize I should know what's wrong, I'm still working on getting my transistor conceptual understanding down.

As for the resistors, wooh! I'll go ahead and do that, with 5-10x the size resistors.

7

erp, I "understand" now the schematic change. Not entirely, but I see what I have to do.

I found the attached image in the datasheet to a P-channel i settled on. The diode lets me see that the Source is indeed 26V.

My assumption is that this runs counter to my understanding of how mosfets go, because I am used to N-channel mosfets. I say that I know "understand" the problem, because I know how to fix it in my schematic now but I'm not 100% sure why the source is 26V, when I thought source means source-of-electrons.

derp.png

8

Finally: Here's the new (sub)circuit, fixed by knowledge from ye good folks.

I settled on somewhat lower value resistors than you (grump_mike) suggested, on account of the fact that those values are used elsewhere in the circuit and it's easier to keep track/order/payfor larger quantities of fewer components.

For anyone curious, I decided on FQD17P06TM. Maybe if i wasn't just using a couple there would be a better/cheaper option out there, but this looked good to me. Also, 25V+/- gate rating, so it was slightly better than other mosfets I saw (which were typically 20V).

finalCircuitExcerpt.png

9

Source does NOT mean source of electrons, it means source of charge carriers (electrons in the case of N-channel, holes in the case of P-channel).

I remember that source goes to the supply (positive side for P-ch, negative side, ie, ground, for N-ch), which is easy for me to remember, because the supply is the "source" of the power :stuck_out_tongue:

And yeah...

Say you have 10k pullup on the gate to keep the fet normally off - put another 10k resistor between that and the transistor you're using to control the gate. Then, when you turn on the NPN transistor, the gate will be at half the supply voltage, which should give you plenty of Vgs to turn it on, without exceeding it's specs.

10

DrAzzy:
Source does NOT mean source of electrons, it means source of charge carriers (electrons in the case of N-channel, holes in the case of P-channel).

I remember that source goes to the supply (positive side for P-ch, negative side, ie, ground, for N-ch), which is easy for me to remember, because the supply is the "source" of the power :stuck_out_tongue:

Super awesome! Thank you! That explains it. It's nice to have a better understanding.

11

You can use a 10V zener and 10k between gate and source, then a 2k2 or something
about that value between driver transistor's collector and the p-FET's gate. The zener
allows the circuit to tolerate much more supply variation than a simple resistive
divider would. The zener importantly protects the gate from capacitive coupled
spikes from the drain - more important as the supply voltage gets larger.

Because of the high impedances on the gate, don't try to use high speed PWM on such
a setup, 1kHz is probably OK though.

12

Kinnishian:
Finally: Here's the new (sub)circuit, fixed by knowledge from ye good folks.

I hope you do not connect Drain to GND

Pelle

13

When you switch to complementary devices (ie NPN <-> PNP or p-channel <-> n-channel)
you simply flip the circuit upside down as the voltages and currents are all reversed. Simple.

14

Pelleplutt:

Kinnishian:
Finally: Here's the new (sub)circuit, fixed by knowledge from ye good folks.

I hope you do not connect Drain to GND

Pelle

Hello Pelle,

You are right, that is not correctly drawn. It says Drain goes to GND, when it really goes Drain to Load. Thank you for pointing out that error, it was a product of me just print-screen'ing the subsection of my schematic and labeling "GND" where I should have said "load"

Anyway...I hope that's true. I'm going to double check the schematic in a moment ;).

Also, to the other two commentators: thanks! I made note of both helpful comments.