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DC Fan Control Need High Performance
- May 19, 2017 -

We have entered the era of electronic product revolution that requires high performance and miniaturization of the circuit. Improvements in electronic system performance and reduced size have led to increased power consumption and heat dissipation. As a result,DC Fan Control different solutions from personal computers to high-end servers are frequently hot-managed. System cooling / thermal management has become a key task for all high-performance electronic systems. Normally, forced convection is used to achieve thermal management. Forced convection by transferring heat inside and around the air to improve heat dissipation. Brushless DC (BLDC) fans can easily achieve the above purpose. The speed of such fans depends on its RMS voltage.

Thermal management can be achieved by running the fan at full speed, but the high speed of the fan can cause the following problems:Improve audible noise;Increase power consumption;Reduced service life (mechanical wear);Increase clogging (dust collection)

However, if the fan is running below the required speed, it will cause the cooling to be insufficient, causing the module to overheat. Overheating can cause component failure. In order to solve this problem, the fan speed must be controlled according to the ambient conditions (ie, temperature).

The fan speed can be controlled in the following ways:

Direct PWM achieves pulse width modulation (PWM) by increasing or decreasing the pulse width used to control the speed (ie, changing the duty cycle).

Linear adjustment Linear regulator can control the fan DC voltage, and then control the fan speed.

DC-DC regulation of this way and linear adjustment much the same, the difference is the use of switching regulator instead of linear regulator.

Direct PWM method because of its low power consumption, low cost, easy to design and other advantages, more commonly used. Most of the BLDC fans used for thermal management are 4-wire,DC Fan Control while some vintage designs are 3-wire and 2-wire.

The 4-wire DC fan contains a Hall effect sensor that senses the rotating magnetic field generated when the rotor rotates. The output of the Hall effect sensor is a pulse train whose cycle is inversely proportional to the fan speed. The number of pulses per revolution depends on the number of fans. For the most common 4-pole brushless DC fans, the Hall-effect sensor's tachometer output produces two pulses per revolution. If the fan is stopped due to mechanical or other failure, the tachometer output signal stabilizes to a logic low or high level. The fan speed is in revolutions per minute (RPM).

The fans are available in standard sizes,DC Fan Control typically 40 mm, 80 mm and 120 mm. When selecting a fan for cooling applications, the most important consideration is the amount of fan exhaust. The amount of exhaust is generally measured in cubic feet per minute (CFM) or cubic meters per minute (m3 / min). The fan blade size, shape and pitch will affect the fan's exhaust volume. Small fans eliminate the same air at a given time and need to run at a higher speed than a large fan.

Space constraints and the need for smaller fans due to physical size constraints will significantly increase the noise generated.

In order to control the generation of noise, the fan controller can be configured to drive the fan at the lowest possible speed while maintaining the operating temperature within the safety limits. This method also extends the life of the fan compared to systems that always run the fan at full speed.

Fan manufacturers specify the duty cycle and RPM relationship in their data tables with tolerances of up to ± 20%. To ensure that the fan is running at the desired speed,DC Fan Control the system designer needs to run the fan at a speed 20% higher than the rated value to ensure that all fans supplied by the manufacturer are able to provide adequate cooling. This may lead to excessive noise and increased power consumption.

The fan manufacturer specifies the relationship between the PWM duty cycle and the rated fan speed and is displayed via a data point table or diagram. Figure 3 illustrates this information, where the horizontal axis shows the PWM control duty cycle (%) and the vertical axis shows the RPM fan speed.

It is worth noting that the performance of all the fans is not consistent at the low duty cycle of the PWM pin. Some fans will stop spinning when the PWM pin duty cycle is close to 0%, and some fans continue to rotate at this time. In both cases, the duty cycle has a non-linear relationship with the RPM,DC Fan Control or it may not be specified. Likewise, two identical fans may have different speeds at the same duty cycle. When using duty cycle and RPM information, two data points that clearly define the behavior of the fan in the linear zone should be used. It can be seen from Figure 4, PWM duty cycle is 0 in the case of speed is not 0.