The Tech Blog

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.

Newer folks usually pick a MOSFET by chasing the lowest RDS(on) on the valid assumption that lower resistance means lower I²R loss. So it feels like a good way to keep the device cool. But that does not always give you the lowest-running design once the MOSFET is switching. Let’s discuss that today.

RDS(on) mainly tells you about conduction loss when the device is fully on. That matters a lot in high-current paths. If the current is high and switching speed is small, low RDS(on) will help. But in a converter, heat is not only coming from on-state loss. It also comes from the time the MOSFET spends moving between off and on.

This is where gate charge(Qg) comes in. Gate charge is the charge the driver must push into the gate every cycle. More charge often means slower transitions for the same driver current. Slower transitions mean more overlap of voltage and current during switching, and that means more switching loss. You also burn power in the gate driver, because charging and discharging the gate every cycle takes energy.

This is why a MOSFET with low RDS(on) can still run hotter than expected. To get that low resistance, the device silicon usually needs to use a larger die area. That typically brings higher capacitance and higher total gate charge. So you save conduction loss, but give some of it back in switching loss, especially at higher frequency.

So how do you choose then? Low RDS(on) helps when conduction loss dominates. Low gate charge helps when switching loss dominates. You can’t choose one in isolation. You try to find a balance that matches your current, voltage, switching frequency, and gate driver strength.

My rule of thumb is at lower frequency and higher current, care more about RDS(on). At higher frequency, look into Qg. A part with slightly higher RDS(on) but much lower Qg can run cooler in the real circuit.

Remember that two MOSFETs with similar input capacitance can switch very differently, so the gate charge curve often tells you more than just one capacitance number.