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Employing programmable SoC for fan control

Posted: 20 Nov 2014     Print Version  Bookmark and Share

Keywords:brushless DC  BLDC  pulse width modulation  fan  microcontroller 

We are now in a period of a revolution in electronics where high performance and circuit miniaturisation are major considerations. This dual increase in performance and reduction in size causes an increase in power consumption and heat dissipation. Thus, thermal management is an issue in applications ranging from personal computers to high-end servers, making cooling/thermal management a critical part of every high performance electronic system.

Thermal management is done by forced convection where heat dissipation is increased by moving the air inside and around the heat source. This is commonly achieved using brushless DC (BLDC) fans. The speed of these fans depends on the incoming RMS voltage.

Thermal management can be done by running these fans at full speed. However, running at high speed results in the following problems:
 • Increased audible noise
 • Increased power consumption
 • Decreased lifetime from mechanical wear and tear
 • Increased clogging (dust collection)

Alternatively, operating the fan below the required speed leads to inefficient cooling and potential overheating of components and component failures. In order to overcome these problems, fan speed needs to be controlled according to environmental conditions (i.e., temperature).

Fan speed can be controlled in the following ways:
 • Direct pulse width modulation (PWM): Increasing or decreasing the pulse width (that is, modifying the duty cycle) controls the fan speed.
 • Linear regulation: Tthe DC voltage across the fan controls the fan speed.
 • DC-DC regulation: Similar to linear regulation, a switching regulator is used instead of a linear regulator.

Direct PWM is commonly used because of advantages such as reduced power dissipation, low cost, and ease-of-design. Most commonly, the BLDC fans used in thermal management have four wires, although older designs may have three or two wires.

4-wire fan basics
The four wires of a BLDC fan are power, ground, tachometer output, and PWM input. A typical 4-wire brushless DC fan is shown in figure 1.

4-wire DC fan

Figure 1: Typical 4-wire DC fan.

4-wire DC fans include hall-effect sensors that sense magnetic fields generated by the rotor as it spins. The output of the hall-effect sensor is a pulse train that has a period inversely proportional to the rotational speed of the fan. The number of pulses that are produced per revolution depends on the number of poles in the fan. For the most common 4-pole brushless DC fan, the tachometer output from the hall-effect sensor will generate two pulses per fan revolution. If the fan stops rotating due to mechanical failure or other fault, the tachometer output signal remains static at either a logic low or logic high level. The speed of this fan is measured in rotations per minute (RPM). The tachometer output for one such fan is shown in figure 2.

Tachometer output of fan

Figure 2: Tachometer output of fan.

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