A DC fan is a widely known and commonly used thermal management device that can be employed individually, in series, or in parallel to provide forced-air convection cooling. It is precisely because of the DC fan’s versatility and relatively simple operation that it has remained a reliable choice for temperature control in end-use applications over the years.
According to fundamental physics principles, the airflow generated by a fan absorbs heat from the device being cooled and carries that heat away, thereby effectively cooling the device’s components. However, the effectiveness of heat dissipation is influenced by several factors. Engineers would greatly benefit from a better understanding of the existing functions and various options available for DC fans, which would in turn enhance reliability and efficiency.
Before selecting a DC fan, engineers need to perform some basic thermal analyses to calculate the minimum required airflow. A typical thermal analysis may involve modeling the heat sources, ambient conditions, and temperature rise. In addition, other factors such as fan size, rotation direction, and airflow paths within the application must also be considered to ensure an appropriate solution is chosen.
After completing the thermal analysis and selecting a fan that is appropriately sized and rated, the next step is to power on the fan and ensure it operates properly, right? While in some cases continuous operation of the fan can achieve its intended purpose, continuously forcing air cooling typically fails to deliver energy savings or provide a long-term solution. Today’s DC fans offer designers a range of control, monitoring, and maintenance options to enhance thermal management performance.
Start/Stop Cycle
As mentioned above, continuous operation of the fan will certainly keep temperature-sensitive components cool; however, it overlooks the fact that such operation increases power consumption and shortens the lifespan of the fan’s moving parts. Moreover, the fan’s operation generates audible noise, which can be undesirable in various applications and environments.
An alternative approach is to have the fan start and stop operation based on a temperature setpoint, which can eliminate some of the drawbacks associated with continuous fan operation. By employing fan on/off control technology, energy savings can be achieved by limiting the running time, reducing stress on the fan’s moving parts, and stopping the fan when the temperature drops below the setpoint, thereby lowering audible noise levels.
However, the on/off fan control oversimplifies the forced-air cooling approach in many respects and has inherent flaws of its own.
First, the start/stop control technology subjects temperature-sensitive components to thermal cycling—alternating between hot and cold states. Compared to operation at a constant high temperature, thermal cycling can pose even greater risks to critical components. This is because thermal cycling induces differences in thermal expansion coefficients, subjecting materials and solder joints to additional stress and thereby leading to premature failure.
Secondly, there’s the unavoidable factor of thermal overshoot. This refers to the time delay between the moment the fan starts and the actual onset of forced airflow that begins cooling. During this time delay, unless the “fan start” setpoint is lowered, component overheating could occur. Moreover, lowering the setpoint also increases the time it takes for the fan to power on and generate audible noise. Finally, to prevent rapid on-off cycling near the setpoint—commonly referred to as “chattering”—hysteresis is necessary.
Selection of fan control functions
Today’s DC fans offer designers a range of control and maintenance options that can be used for more sophisticated thermal management systems. These advanced designs elevate the basic on/off fan control function to entirely new levels of performance, efficiency, and reliability. In addition, maintenance options are available that can detect issues before they lead to damage of the fan’s micro-fan cooling components. Below, we’ll introduce some of the most common fan control and maintenance options:
Pulse Width Modulation
Pulse Width Modulation (PWM) is a commonly used method for controlling and adjusting fan speed based on fluctuating thermal conditions. When combined with advanced control algorithms, PWM-based variable-speed control can enhance operational efficiency and adapt to dynamic operating conditions by matching fan speed to thermal load.
Fan start/stop control can also be upgraded by adopting proportional-integral-derivative (PI and PID) closed-loop control techniques. These strategies ensure that the airflow conditions match the desired temperature setpoint, helping to prevent thermal overshoot or undershoot when the load changes.
Embedded tachometer signal
The embedded tachometer is designed for closed-loop feedback and advanced fan control. It detects and reports the fan speed by measuring the frequency of the pulse output signal. The tachometer can also serve as a locking sensor, alerting the user when the fan stops running due to power failure, blockage, or other similar issues. Promptly detecting these problems is a key advantage in ensuring system reliability, allowing for timely shutdowns to protect temperature-sensitive components.
Automatic restart maintenance
The automatic restart maintenance function can detect when the fan motor is blocked from rotating and automatically cut off the drive current. This helps protect the fan drive circuit and notifies the fan controller of any emergency issues caused by the interruption of the drive current.
Rotation Detection/Lock Sensor
The rotation detection/lock sensor is used to determine whether the fan motor is running or has stopped, thereby preventing issues during startup or operation.
Summary of this article
When an application generates excessive heat, DC fans are a common cooling solution that helps keep device components within their maximum operating temperature range and improves thermal performance. After conducting some basic thermal analysis, it’s clear that continuously running a fan is indeed a good cooling option. However, more advanced control and maintenance features can further extend the fan’s service life and enhance its efficiency.