SYSTEMS DESIGN DC FANS
in better operating efficiency and can
be implemented alongside advanced
control algorithms. These advanced
control algorithms can take into
account usage patterns, energy
costs, and more, while adapting to
operational dynamics that match fan
speed to thermal load.
This can be taken a step further
by upgrading basic on/off fan control
to the well-known proportionalintegral
derivative (PI and PID)
closed-loop control strategies. PI and
PID controls help to avoid thermal
undershoot or overshoot despite
load changes by ensuring the airflow
keeps conditions at the desired setpoint
temperature.
Embedded Tachometer signal
Sensing and reporting on a fan’s
rotational speed via a pulsed signal,
embedded tachometer is a control
used for closed-loop feedback
and more advanced fan control.
Embedded tachometer also serves
as a lock sensor if the fan seizes
operation due to loss of power,
obstructions, or any other reason.
Having these detection capabilities is
a major benefit to system operation,
as the sooner negative conditions
are detected, the quicker the system
can be shut down or put in an
inactive state to protect sensitive
components.
Auto-restart protection
Auto-restart protection detects when
the fan motor is prevented from
rotating and automatically cuts the
drive current. This protection serves
to both protect fan-drive circuitry and
indicate to the fan controller that
there is a problem due to the drive
current being cut-off.
Rotation detection/lock sensor
True to its name, rotation detection/
lock sensor detects if a fan motor
is operating or stopped in order
to safeguard against problems at
start-up or during operation.
Conclusion
DC fans are a go-to solution in many
thermal management systems,
providing forced-air cooling to
systems and keeping components
within their operating limits.
However, simply selecting a fan
and running it full time or using basic
on/off control are short-sighted
approaches to thermal management
with fans.
With a host of fan controls and
protections, engineers can better
optimise their forced-air cooling
solutions for greater efficiency and
reliability and today the selection
process of fans has been made
easier with the availability of a broad
range of DC axials fans and blowers
with multiple sizes, airflow ratings
and control options.
Below: Thermal
overshoot and lag
due to on/off fan
created to demonstrate thermal
overshoot due to unavoidable thermal
lag in on/off fan control applications.
In the graph, the light blue line
represents the desired set-point
temperature and includes a step
change, the green line is the on/off
cycling of the fan, and the dark blue
line is the actual temperature.
Protections and controls
Fans today are available with a
range of controls and protections to
help designers better optimise their
thermal management systems. These
controls can take the limitations of
basic on/off control and improve
upon performance, reliability, and
efficiency. There are also protections
available that detect fan problems
early on, which can extend fan life
and ensure systems are not put at
risk.
To better understand some of
the most common fan controls
and protections as well as their
implementations, see below:
Pulse-Width Modulation (PWM)
Pulse-width modulation (PWM) is
used to control and change the fan
speed based on varying thermal
conditions. As the first step towards
improved fan performance, PWMbased
variable-speed control results
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