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I am designing a circuit with a load that will consume up to 2A @ 5V. I decided to use a slide switch rated at 100mAh to drive an N-channel MOSFET for the (relatively) high current, since a slide switch rated for 2A is significantly bulkier.

I understand that when using N-channel as a switch it is connected on the low side between the load and GND.

I'm just wondering about best practices for separating the circuit GND from the load GND that needs to go through the MOSFET. I'm guessing I should use different net names for those two GNDs.

  1. Which one of the grounds should I call GND? The external "raw" one? Or the one that goes through the MOSFET? I already have a dozen GND nets/symbols in my circuit that should be going through the MOSFET
  2. Should I use different symbols for the two grounds? Or same symbol with different net names?
  3. Does anyone have an example schematic with a similar circuit I can look at for reference?

I am mainly asking about best practices and ensuring maximum readability. Thanks!

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    \$\begingroup\$ Could you provide a schematic of your circuit? Edit it into the question body. As a quick note, unless you're going for full isolation you don't separate grounds. It's an old practice. It's still done, but in very specific applications. Generally you want your ground reference to be well connected to everything. \$\endgroup\$ – alphasierra Dec 24 '15 at 21:13
  • \$\begingroup\$ If you give us a schematic it my be easier for us to explain why you probably don't really need to separate all those ground nets. \$\endgroup\$ – The Photon Dec 24 '15 at 21:16
  • \$\begingroup\$ @alphasierra The grounds are connected via the MOSFET. The MOSFET is acting as a switch and only isolates the ground when turning off the circuit. \$\endgroup\$ – Shlomo Zippel Dec 25 '15 at 0:40
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If I understand correctly, you are asking what to call the net on the drain of your low-side N FET and what to call the net on the source of that transistor, correct? That's very fine to do, but check my warning further down about the danger of accidentally bypassing the transistor.

I would call the GND that is connected to the source of your transistor as "the" power GND. I would assign a net-name to the node on the drain as "LOW_SIDE_LOAD" or similar. I would not give it its own GND symbol unless it is truly spanning multiple sheets. If it is, I would have one schematic sheet with a large square drawn around the circuit element that connects it to GND through your FET. As an engineer I generally follow GND on a schematic to its source, which is usually the power supply page that ends in a connector of some type. If I saw some other GND symbol, I would follow it through the schematic until I find the tie / relationship to GND.

You should try to make it as clear as possible that any other connection between LOW_SIDE_LOAD and GND will effectively bypass your switch. This is one of the dangers of low-side switching.

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  • \$\begingroup\$ Your warning is what prompted this question. Since every single GND net in the circuit (except for the battery / usb connector) should actually be the drain of the N FET I figured it would be safer to name it GND and call the power GND something else. That way the "default" of just using GND wouldn't bypass the switch \$\endgroup\$ – Shlomo Zippel Dec 25 '15 at 0:43
  • \$\begingroup\$ I see -- is a high-side switch an option at all? It sounds like you are switching quite a bit of stuff on the low-side, not a single load. \$\endgroup\$ – Krunal Desai Dec 25 '15 at 0:44
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    \$\begingroup\$ I have used GND and BATT_DIRECT nomenclature in the past for this type of thing as well. Sounds like this is a low-side switch for a battery? In that case, make sure the body diode of the MOSFET does not allow charge or discharge when it is not supposed too. Might need two back to back. \$\endgroup\$ – Krunal Desai Dec 25 '15 at 0:46
  • \$\begingroup\$ I'm switching: MCU, a few LEDs, LDO buck converter, logic level converter. I can have them all together on the schematic but currently have them broken out by functionality so I have multiple GND symbols. Yes, high side switch is an option but I understood that N FET is "better" than P FET for this scenario (I don't know enough EE to articulate why though). \$\endgroup\$ – Shlomo Zippel Dec 25 '15 at 19:17
  • \$\begingroup\$ The circuit can be powered from a USB connector or from a LiPo battery. Can you tell me a bit more about the body diode not allowing charge / discharge, or what I should google / look for in the datasheet? The MOSFET I'm looking at is FDN327N \$\endgroup\$ – Shlomo Zippel Dec 25 '15 at 19:18
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You can't separate the grounds when using an NMOS as a low side switch. At least not without introducing some other component (like an optocoupler) between the control logic and the switch.

The control signal has to set the FET's gate-source voltage, and since the source is also connected to the load's ground, the control voltage has to be set relative to that also.

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The low side of your load will not be GND. It will be a switched line. Depending on the characteristics of the Mosfet it may actually sit at a voltage slightly above GND potential. Either allow the system to assign the node name, or create a new name, (eg. Switched_XX), if you need to link it to another point.

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Here:

schematic

simulate this circuit – Schematic created using CircuitLab

R1 prevents possible damage to your MOSFET (keeps G-S current low, just incase). R2 is a pull-down for your MOSFET's gate.

The voltage at the drain is opposite to your gate (gate goes high, drain goes low and load turns on).

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