How to Choose Between High-side and Low-side Switching
August 13, 2019 | 11:05
The principles of operation of high-side and low-side load switching are easy to grasp, but when do you prefer one method over the other?
In systems employing high-side switching the switch is inserted between the positive power line and the load. Low-side switching by contrast connects the load to ground. The principles of operation of high-side and low-side switching are easy enough to grasp, but when do you prefer one method over the other? It's all application dependent for sure.
Avoid dangerous situations
High-side switching is the preferred switching technique in situations where short circuits to ground are likelier to occur than short circuits to the positive power line. Think for instance of cars or machines where most of the structure or body is connected to ground. In such cases it is safer to disconnect the load from the battery than from ground. Also, in humid environments, this usually results in less connector corrosion as the load carries no voltage in the off state.
Power switching is better with N-type devices
Because N-type transistors in general can carry more current than P-types, they are preferable for switching heavy loads. Low-side switching with N-type devices is easier than high-side switching and can often be done by microcontroller ports without the need for special drivers. Using an N-type transistor for high-side switching is possible but requires a control voltage higher than the load voltage connected to the source/emitter. Some sort of charge pump is needed to pull the gate/base above the source/emitter voltage. This complicates the design, not only making it more expensive but also increasing its sensitivity to noise and interference. Controlling such a high-side switch using PWM can be problematic because of the charge pump.
A fuse can make all the difference
Thus, low-side switching tends to be cheaper than high-side switching. However, when the load and its controller are not located right next to each other, two fuses are necessary to protect them both in such a setup; high-side switching would only need one. Saving a fuse may seem futile, but when you take into account the wiring and work required to make the fuses accessible in a fuse box, having to add a fuse may very well annihilate the cost advantage of low-side switching.
Good ground is good
Low-side switching has one ground connection for both the load and its controller, thus avoiding ground potential differences (ground shifts) between the two when currents are high. Low-side switching therefore is more robust to ground noise than high-side switching.
In cases where a (heavy) load only has to be switched on or off, high-side switching is the preferred method. When the power to a load must be controlled through (relatively) high-speed PWM, for instance in a lighting or heating system, low-side switching is recommended. And then there are half H-bridges that require both a high-side and a low-side switch... and PWM... Therefore, as always, before settling on any technique, make sure that it is a valid choice for your particular application.