50 January & February 2021 http://www.elektormagazine.com
High-side or Low-side Switching?
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 (Figure 1). 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 (Figure 2) 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 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 its source or emitter. To pull the gate or base of the transistor above
its source or emitter voltage, some sort of charge pump is needed or
an extra supply. This complicates the design, not only making it more
expensive but also increasing its sensitivity to noise and interference.Driving such a high-side switch with a PWM signal to control e.g.,
the speed of a motor or the brightness of an LED can be problem-
atic because of the charge pump.Developer’s Zone
Tips & Tricks, Best Practices
and Other Useful Information
By Clemens Valens (Elektor)
developer‘s zone
Figure 1: Switch a load to
the supply rail (left) or to
ground (right)?Figure 2: High-side
switching is preferred when
the risk of a short circuit
to ground is higher than a
short circuit to the supply.Figure 3: Low-side
switching is cheap and
easy. However, when a fuse
must be added because the
distance between the load
and the controller is too big
(right), this configuration
can become more
expensive than high-side
switching.