fan speed controller

1 Overview

IC designers strive to put more high-speed transistors into smaller and smaller packages, but this will inevitably lead to heat generation. In order to put these high-power ICs into smaller packages, thermal management issues must be effectively solved. Fans are used in many applications to cool down, but fans can cause mechanical failure, increase power consumption and noise. Therefore, fan speed should be monitored and controlled to solve these problems, making the fan work more reliably with lower power consumption and noise.

Because brushless DC fans are easy to use and reliable, they are the preferred solution for most electronic products. It is a two-terminal device that can work with DC voltage. Its DC voltages include 5V, 12V, 24V and 48V. Currently, there are many 12V fans to choose from. As the 12V power supply decreases, the use of 5V fans will increase. 48V fans are generally used in the telecommunications field. The commutation rectification of brushless DC fans is controlled electronically inside the fan. Old-fashioned DC fans use machinist brushes, which produce higher EMI and are prone to damage. The brushless fan uses electronic sensors and switches to replace mechanical brushes, thereby extending the service life. It is a two-terminal device that is simple to use and reliable in operation. Brushless DC fans have different terminal voltages, and their rotational speed and current consumption are also different (proportional).

Despite its high reliability, a brushless DC fan is still a mechanical device. When used for a long time, its fan speed and cooling efficiency may decrease or even fail, so the fan must be continuously monitored. Many fan manufacturers provide different monitoring methods, which are generally divided into two categories: alarm sensors and speed sensors. The alarm sensor can be used to give an alarm signal when the fan speed is lower than a certain threshold. Some manufacturers use a speed sensor to give an output signal of the fan speed that is proportional to the frequency, generally producing 2 pulses per revolution. Both alarm sensors and speed sensors can provide open-drain or internal pull-up outputs, which can be TTL levels or supply voltages. It is worth noting that changes in the supply voltage used to control fan speed will affect sensors and other circuits. Moreover, the usage conditions of the fan must be considered in the actual design (such as the worst temperature range, maximum power consumption, fan error and service life, etc.). Working under appropriate conditions can reduce the fan speed, and under the worst conditions , the fan speed should be increased. Properly controlling fan speed can reduce system noise and power consumption, thereby extending fan life and reducing dust.

2 Fan speed control method

2.1 Pulse width modulation method (PWM)

Pulse width modulation (PWM) is a method that sets the switching frequency of the fan power supply to a fixed value and adjusts the fan speed by changing its duty cycle. The larger the duty cycle, the faster the fan speed. The key to this control method is to choose the appropriate switching frequency. If the frequency is too low, the fan speed will oscillate with the PWM cycle. On the contrary, if the frequency is too high, the commutation rectifier circuit inside the fan will disconnect the power supply of the fan acceleration/deceleration circuit and cause the fan to stop. The general frequency range is 20Hz ~ 160Hz. In addition, the rise and fall times of the PWM must be slow enough to ensure the long-term stability of the fan. The advantages of the PWM control method are simple driving circuit, good starting characteristics, and small heat dissipation of the transistor. The disadvantage is that it increases the stress on the fan and makes it inconvenient to use the speed alarm sensor. The speed alarm sensor and the motor are powered by the same power supply. If the motor power supply is turned on or off at a frequency of 20Hz to 160Hz, the sensor will also be turned off or turned on. As a result, the proper detection function is lost.

In the PWM control method, the voltage of the fan is the rated value (such as the 12V power supply in a 12V fan) or zero voltage, but because the fan is not operating at full load, its back electromotive force will be reduced, which will cause the PWM conduction period to The current may be greater than normal. Although the fan is designed to handle larger batteries, during the fan startup phase, the current will increase by 30 times per second, which will have a large impact on the fan's reliability. Nonetheless, PWM is still ideal for low-cost applications.

2.2 Linear adjustment method

This method uses a linear regulator to adjust the DC voltage of the fan. It requires the fan to work in a wider voltage range. Compared with the PWM control method, its advantage is that it can use speed and alarm sensors; its disadvantage is that it consumes more power. , need to solve the problem of starting and stopping. When a linear regulator is used to control the DC voltage across the fan, although its power consumption will generate heat, the power consumption in the maximum cooling and minimum cooling states is approximately zero. Because at maximum cooling, the voltage difference on the regulating tube is approximately zero; at minimum cooling, the current on the regulating tube is approximately zero, so the power consumption is approximately zero. When the voltage across the fan is half of the maximum operating voltage, its power consumption is maximum. The power dissipation at this time can be estimated by the following formula:

P=1/4(Vmax×Imax)

In the formula, V and I are the rated voltage and rated current of the fan respectively. For example, the power dissipation of the adjustment tube of a 1.2W fan is only 300mW (a 12V fan works at 6V). Therefore, a certain amount of power can be saved when the fan speed is reduced. Start and stop are related, and the fan requires a certain starting voltage before starting. After the fan is running, once the fan voltage drops to the stop voltage, the fan will stop running. The starting voltage is generally equal to or slightly greater than the stopping voltage, usually 25% to 50% of the rated voltage of the fan. When there is no speed monitoring, there are two ways to judge whether the fan is running or stopped: the first is to ensure that the voltage at both ends of the fan is always greater than the starting voltage. Voltage, generally the voltage can be set to be greater than 60% of the rated voltage to reduce the fan voltage control range. The second is to use a fan with a tachometer and the tachometer is monitored by a microcontroller. Before the fan starts or stops, software can be used to understand its status, but this method will increase the design complexity and software and hardware resources.

2.3 DC-DC controller

Like the linear voltage stabilization method, the DC-DC switching method also controls the fan speed by controlling the DC voltage at both ends of the fan. The difference is the switching method used in this method. The advantages and disadvantages of these two methods are basically the same. The one difference is that the ideal efficiency of the DC-DC switching method is 100%, so no heat is generated. In fact, the efficiency of DC-DC switching controllers is usually 75% to 95%. As the efficiency increases, the cost and complexity will increase. Power is saved only when the fan operating voltage is reduced to a low level, and when the operating voltage is half of the saturation voltage, the power consumption is reduced to the greatest extent. The cost of saving power is increased cost and complexity, so the DC-DC method is generally used in portable systems and applications that require high-power fans or multiple fan controls.

2.4 High-end/low-end driver

The above three methods can be low-end drive or high-end drive. The circuit for the high-end driver is a little more complicated, but the advantage is that the negative terminal of the fan can be connected to ground, the speed and alarm sensors have the same reference ground, and it is easy to interface. Low-side drivers do not require level translation, but speed and alarm sensors require level translation circuitry. At this time, the positive terminal of the fan is generally maintained at a constant voltage, while the negative terminal voltage can be controlled and adjusted. Therefore, the reference points of the speed and alarm sensors are easy to change, so a level conversion circuit is required.

3 Fan speed control without tachometer

Figure 1 and Figure 2 are two fan speed control circuits without alarm sensors and speed sensors. The MAX1669 in Figure 1 forms a PWM control method, and Figure 2 shows the MAX1669 forming a linear adjustment control method. The MAX1669 in the picture is both a temperature sensor and a fan controller. The two parts can work independently and cooperate with the microcontroller. Its interface is SMBus. The temperature sensor is completed by a diode installed at the remote end. This diode can also be included inside some devices, such as XILINX's VIRTEX series devices, which will more accurately control the temperature of critical components and eliminate the need for external diodes.

When the temperature detected by the MAX1669 increases, software can control and increase the fan speed. The control of fan speed has nothing to do with temperature measurement, so this is an open-loop control system and there are no stability issues. However, there is no way to directly know the actual temperature of the controlled part, and it is also impossible to directly know whether the fan has failed or whether the cooling efficiency has decreased. If the temperature sensor is placed in an area to be cooled, it can be designed as a closed-loop system, where the fan speed increases and the temperature decreases. Closed-loop systems need to pay attention to stability issues and will increase the complexity of the software.

4 Fan speed control with tachometer

Figure 3 is a fan speed control circuit with a tachometer. The MAX6625 measures temperature and transmits it to a microcontroller via a two-wire I2C-like interface. The two-wire interface transmits commands to the MAX6650, which controls the fan speed. The internal interface circuit of the MAX6650 can connect to the fan's tachometer (speed sensor), and the fan speed is read through the SMB two-wire interface. Use the MAX6650 to form a fan speed controller or fan speed regulator. A fan speed controller can be used to control the voltage across the fan to achieve the purpose of controlling the fan speed. A fan speed regulator actually uses a tachometer to measure and regulate fan speed. When using the MAX6650 as a fan speed controller, a microcontroller is used to read the temperature from the MAX6625 and the speed from the MAX6650, and then control the voltage and speed across the fan through the DAC in the MAX6650. The microcontroller must constantly read the fan speed and regulate the fan speed via the DAC, which is especially important during the start and stop phases. When using the MAX6650 for speed adjustment, the microcontroller can be used to send speed instructions, and the MAX6650 can be used to automatically monitor and adjust the fan speed. Once the speed command is sent to the MAX6650, the microcontroller can no longer intervene, which can reduce the workload of the software. If the MAX6650 cannot provide the control voltage required by the command, the system will generate an alarm signal to the microcontroller to generate an interrupt.