lektor January & February 2021 51shifts) between the two when currents are high, and the controller
and load are not close to each other. Low-side switching therefore
is more robust to ground noise than high-side switching.
On the other hand, a switch, when closed, introduces a small resistance
in series with the load. This results in a small voltage drop, meaning
that the load’s ground is slightly above the controller’s ground.So, What Do I Do?
In cases where a (heavy) load only must 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.A Fuse Can Make the Difference
Thus, low-side switching tends to be cheaper than high-side switch-
ing. However, when the load and its controller are not located
closely to each other, efficient system wiring may impose separate
fuses for the load and its controller instead of just one that protects
both (Figure 3). High-side switching would only need one. This may
seem futile, but when you take into account the wiring and work
required to make a fuse accessible in a fuse box, having to add one
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. This avoids ground potential differences (ground
LOW-SIDE OR HIGH-SIDE CURRENT SENSING?
A problem similar to high-side and low-side switching is current
sensing. Several techniques for sensing currents in a circuit
exist, but probably the most common one is to place a small
resistor of a known value in series with the load. Since the
current through the load and the series resistor is the same,
Ohm’s law lets you calculate the current by first measuring
the voltage over the series resistor and then dividing it by
the resistor’s value. The question that now arises is: where to
connect that resistor? Before or after the load? In other words,
high-side or low-side current sensing?Low-Side Current Sensing Is Cheap
In the case of low-side current sensing, the typically small
voltage over the sense resistor is referenced to ground and can
be amplified with low-cost, low-voltage op-amps before being
digitised and processed further by for instance a microcontroller.
However, like the switch in low-side load switching, the sense
resistor introduces a small voltage drop that lifts the load from
system ground, possibly resulting in noise and ground bounce.Also, low-side current sensing cannot detect if the load is
shorted to ground, either internally or at one of its terminals.Handle High Common Mode Voltages
High-side current sensing does not suffer from these
shortcomings, but it comes with its own issues. For instance,
the voltage difference over the sense resistor is not referenced
to ground. Re-referencing it to ground requires extra circuitry,
making the system more expensive and complex.Furthermore, the amplifier must be capable of coping with a
common-mode voltage all the way up to the positive supply
(and even higher in case of transients), which can be hundreds
of volts in certain applications. Therefore, the amplifier must
either support a high supply voltage or have inputs that can
handle such high common-mode voltages. Also, keeping
common-mode voltage errors low implies the use of precision
parts, which is more expensive.To make the circuit designer’s life a bit easier, several
semiconductor manufacturers have added dedicated high-side
current sensing ICs to their product catalogue.Conclusion?
The current sense method to choose depends on the application
(again). High-side current sensing can detect if the load is
shorted or open, and the load remains referenced to ground.
However, due to the common mode requirements, it is more
complex and more costly than low-side sensing. Opting for a
dedicated current sensing IC may be the best way forward.High-side versus low-side
current sensing.