The Tech Blog

Last week we discussed high-side and low-side switching in detail. Today I wanted to extend that with three simple MOSFET cases that you will see again and again in real circuits. I have attached circuit diagrams so you can follow along.
Before that, a quick MOSFET refresher, in case you have forgotten,
For an N-channel MOSFET: Positive Vgs turns it on, Near-zero Vgs turns it off
For a P-channel MOSFET: Negative Vgs turns it on, Near-zero Vgs turns it off

Case one is the classic NMOS low-side switch. The load sits between VDD and the drain, the source goes to ground, and the gate is driven by your MCU(Lets say, 3.3V). When the gate goes high, Vgs becomes positive, the NMOS turns on, and current flows through the load to ground. This is why low-side NMOS switching is the default choice for LEDs, relays, buzzers, and many small loads. The small resistor from gate to ground is there so the gate does not float at reset.

Case two is a PMOS high-side switch when your MCU voltage and VDD are the same, like 3.3V. Here the PMOS source sits at VDD(Not like the NMOS). Pull the gate down to 0V with the MCU and the PMOS turns on. Drive the gate back up to 3.3V and it turns off. This is the simplest way to switch the positive rail when you want the load to keep a solid ground reference.

Case three is the one that trips people up. If VDD is higher than your MCU voltage, a PMOS cannot usually be driven directly from the MCU. A 3.3 V output is still lower than a 5V, 9V, or 12V source, so the PMOS may never fully turn off. That is why you add a small NMOS stage. 3.3V from the MCU turns NMOS on, so NMOS pulls PMOS’s gate to 0V and the PMOS turns on. When MCU low turns NMOS off, and R7 pulls PMOS’s gate up to VDD, turning the PMOS off.

Hope these circuits help as a refresher. One practical reminder though. In the LED examples, the series resistor sets current. If you swap the LED for a relay or coil, that resistor is replaced by the load, and you must add a flyback diode across it.

Recently, while discussing a project with a client’s junior team member, this topic came up. I thought it is worth explaining from first principles for folks starting out.

At the simplest level, both are just ways of using a MOSFET as an electronic switch to turn a load on and off. That load could be an LED, relay, solenoid, motor, or a small module. The difference is where you place the MOSFET in the current path.

In a low-side switch, the MOSFET sits between the load and ground, so it switches the return path. In a high-side switch, the MOSFET sits between the supply and the load, so it switches the positive rail. That’s why the naming is like that. You are switching the low side of the load or the high side of the load.

Low-side switching is usually the first thing you will see in practical designs because it is easier to drive, especially with an N-channel MOSFET. Your controller is already ground-referenced, so the gate drive is simple. That is why it is so common for relays, LED strips, buzzers etc.

The tradeoff is that when you switch the low side, the load is not firmly tied to ground when it is off. It can float. For simple loads this is often not a problem. For sensor boards, communication modules, or anything connected to other powered signals, that floating node can cause problems and hard-to-trace leakage paths.

High-side switching is popular when you want the load to stay referenced to ground, and you want to cut the supply instead. That often makes the overall system behave more cleanly. The con is that the drive method is usually more complicated. A PMOS can make high-side switching easier, but it is often less efficient than a similar NMOS. A high-side NMOS is better electrically, but usually needs a proper driver.

If there is interest, I can do a part 2 next week with actual circuits, drive options with NMOS and PMOS and explaining how it works + common issues to watch out for. Let me know via comments if that would be useful.