Does this MOSFET implementation look correct?

Hi all,

I’ve been working on an arduino compatible macro focus stacking controller, dubbed Stackduino, for a little while now. The last revision worked fine, and I’ve since added a few new features to the latest board which I was hoping to double check the logic of before I get it made. Mostly I’m hoping to confirm my choice of logic level MOSFET is appopriate, and that the resistor network for each looks correct.

Here’s the schematic:
The schematic is attached.

And an OSH Park render of the board layout:

The MOSFETS I’m interested in are MP1, MP2 and MP3. I’m looking at using a NTR4501N (http://www.onsemi.com/pub_link/Collateral/NTR4501N-D.PDF) for all of these. All will be performing a basic occasional switching function, there are no high frequency requirements.

1/ MP1 switches a bluetooth add-on board on and off as required by toggling 5v VCC supply.
It is controlled by a pin on the MCP23017, through a 150 Ohm current limiting resistor (R4). A 10K pull-down resistor(R3) makes sure the switch stays off when it should be.

2/ MP2 intermittently connects the battery to an analogue pin on the ATMega via a voltage divider to read the current battery voltage. It is controlled by a pin on the ATMega, through a current limiting resistor (R10). A 10K pull-down resistor(R7) makes sure the switch stays off when it should be.

3/ MP3 is switched on by VCC. It allows unregulated input voltage (9-12v) to flow to the VMOT pin of the A4988 stepper driver board - essentially switching the a4988 on and off with the main board. There’s a current limiting resistor (R15) between VCC and the gate - but as this one isn’t being driven by a micro controller pin, is it actually required? My thinking to include it anyway was that the speed of switch on isn’t critical and the resistor might help smooth the current surge on startup slightly. If it’ll make no difference though I’ll happily remove it. In theory VMOT could pull up to 2A but in reality it’s likely to be half that or less.

If anything looks horribly wrong with the above I’d be very grateful to be put right on my mistakes :slight_smile:

Cheers
Richard

The only thing you need to check is that 70 milliohms is low enough in each case, so the power dissipation in the FET is OK and the load gets enough current.

The device has only a few 100pF input capacitance because its so small, current limiting resistors aren't really needed. Big MOSFETs can be 20,000pF input capacitance and that current limiting will be needed to protect Arduino outputs.

The biggest picture is 1600 wide, but is still super small, so I can't read all the text. You can attach a picture to a post, click on the additional options on the left under the text field.

Q:Does this MOSFET implementation look correct?
A: Not to me!

But I might be wrong. Let’s have some more people take a look at it.

The schematic is unreadable as its been shrunk too much.

Sorry, I've attached a larger version to the first post which should be better.

AREF should not be connected to Vcc. Connect AREF to a capacitor of 100nF and the capacitor to ground. I lack the knowledge to see if all the mosfets are correct, sorry.

Peter_n: AREF should not be connected to Vcc. Connect AREF to a capacitor of 100nF and the capacitor to ground. I lack the knowledge to see if all the mosfets are correct, sorry.

Great spot - thank you. I think I followed a schematic with them connected a while ago and didn't question it :blush: Didn't occur they are internally connected anyway. Schematic updated following your suggestion.

AREF can be switched internally to the internal reference, or to Vcc, or left floating. Just give it its decoupling, that's why its pinned out. See the docs for

analogReference()

Ok brilliant thanks, that makes sense :)

Returning to the MOSFET issue, I'm wondering if the FDC855N might be a better choice, particularly for switching the stepper driver: http://docs-europe.electrocomponents.com/webdocs/12cf/0900766b812cf73c.pdf

Compared with the NTR4501N it's got:

  • a higher Drain to Source Max Voltage rating (30v compared to 20v)
  • a higher continuous Drain Current rating (5.3A compared to 3.6A at VGS = 4.5V)
  • a lower Gate resistance ( 36m? compared to 70m? at VGS = 4.5V)

So if I understand that right, that's a higher ceiling in terms of safety margins, and a bit more efficient too. Is there any reason I'm missing not to go for it?

Thank you all for helping with this! Richard

No, I think you are not missing anything. But don't call it the Gate resistance ]:)

The Rds(on) is important. It is the resistance between drain and source when switched on. The lower it is, the less heat is produced by the mosfet, and 36m? is a mighty fine low value.

I would not worry so much about which N-channel mosfet you use. Because it will never turn on.

Cryptic :)

Which one? All of them? Could you elaborate a little please...do I have my load on the wrong side maybe?

iamrichard: Sorry, I've attached a larger version to the first post which should be better.

Now I can see the diagram you seem confused.

At the bottom left there is a p-channel MOSFETsymbol labelled "NTR4501N" which is an n-channel MOSFET. You also draw the drain to Vcc and source to the load which is wrong.

You need p-channel device with source to Vcc, drain to the load, to do high-side switching.

A simple hand-drawn sketch would have sorted this all out in the first instance...

This is high-side p-chan switch:

Note the diode is part of the device in all power MOSFETs, and note the direction of the arrow on the channel which indicates its a p-channel device.

+1 to MarkT!

Here’s a nice explanation, with pictures:

Ok, thanks both! I think the fog is beginning to lift and I'm starting to understand how backwards I've got it. 'Confused' was a kind way to put it.

Ideally I think I do want to use an N-Channel, at least for MP3 which switches the A4988 board - because I'm using VCC to do the switching.

So in that case I want a low-side switch, and the MOSFET needs to go between the A4988 ground and the board ground like this?:

VCC | | GATE

A4988 GND ---> DRAIN SOURCE ---> BOARD GND

The problem with low-side switching is that grounds are no longer common, which can have various issues... Or it might be OK.

Either way you also have to be aware of accidental powering via signal lines in CMOS, which is why its great to have a shutdown pin on a chip so you can put it in low power quiescent mode without having to worry about such things.

Those are very interesting points.

Would I be right in thinking though that for the particular application of MP3 it should be ok, due to the fact that the MOSFET is controlled directly by VCC?

I presume that avoids the problem with accidental powering as the board being off is what will actually switch off the A4988. The separate grounds presumably become a shared ground as soon as the board is powered up and MP3 switches on?

I'll have to bear that in mind for the other two MOSFETS though, they are used in a more normal way which I think could be affected by this.

Thanks again for your help, I'm learning a lot.