One way to regulate the cooling fan/blower speed according to the temperature inside the enclosure is with pulse width modulation. Find out if this method will work for your cooling process.

Figure 1. In this example, the fan speed signal (tach) is fed back to the controller for closed-loop control.
One trend in electronic enclosure cooling is to regulate the cooling fan/blower speed according to the temperature inside the enclosure. One method is pulse-width modulation (PWM), which does not generate additional heat when used to regulate motor speed. Also, when compared to linear regulating (voltage control) of the motor, it is energy efficient. The PWM duty cycle is defined as T on/Toff(percent) in one period, and the range is 0 percent to 100 percent.

PWM controllers for brushless DC motors are offered with application notes that typically suggest controlling the fan ground line by an N-MOSFET. The MOSFET is switched on and off by the PWM signal that is sent by the controller, hence controlling the fan speed. While this is the method to control a brush-type DC motor, it may cause a brushless DC motor to malfunction. Because there is electronic commutation circuit inside a brushless DC motor, some models use a microcontroller to perform the commutation function. Obviously, the microcontroller and its associated electronic components such as capacitors will not work normally when the ground line is switched at a frequency of 30 Hz or more. In figure 1, the fan speed signal (tachometer) is fed back to the controller for closed-loop control.

Figure 2. The speed signal is generated by sensing the motor current. The current and the signal are zero when the MOSFET is off, even when the motor is running at a certain speed. Therefore, the speed signal is only accurate when the PWM is at 100 percent duty cycle.
In most cases, the tachometer signal is invalid when the circuit generating the signal is turned off by the MOSFET. The speed signal is generated by sensing the motor current (figure 2).

Figure 3. A more elaborate way to control the fan's power occurs when the motor commutation circuit is switched on and off by the PWM signal. The tachometer signal is likely to be invalid during the off cycle.
Even when the motor is running at a certain speed, the current and the signal are zero when the MOSFET is off. Therefore, the speed signal is only accurate when the PWM is at 100 percent duty cycle.

Figure 3 is a more elaborate way to control the power line of the fan. The motor commutation circuit still is being switched on and off by the PWM signal, and the tachometer signal is still likely to be invalid during the off cycle.



Figure 4. Using a four-wire PWM fan allows you to connect the PWM and the tach line, leaving the fan power and ground lines uninterrupted. As a result, a simple automatic closed-loop speed control system is formed.
A better approach to using a PWM controller is to use a four-wire PWM fan. Connect the PWM and the tachometer line, leaving the fan power and ground lines uninterrupted (figure 4).

Figure 5. When using only one PWM fan, a customer can program the desired temperature/RPM curve simply by changing the PWM controller software. This figure shows temperature/RPM curves.
In this way, the circuit inside the fan is working normally, sending a valid speed signal and accepting PWM control to change motor speed accordingly. As a result, a simple automatic closed-loop speed control system is formed.

With closed-loop speed control, the tolerance of the fan PWM duty cycle vs. the speed curve can be wide. The controller can command the fan to achieve a desired speed (rpm) goal by adjusting the PWM duty cycle. If the speed is below the goal, the PWM duty cycle will be increased, and vice versa. The speed goal also will be maintained when there is voltage variation or load variation on the fan, working in the same manner that an automobile's cruise control system works. In figure 5, fan A and fan B can achieve the same temperature/rpm curve if the controller software is designed properly.



Figure 6. Fan A and Fan B can achieve the same temperature/RPM curve if the controller software is properly designed. When comparing PWM controlled fans to industry-standard thermistor controlled fans, PWM fans exhibit several advantages. One PWM fan model can replace many models of the same top speed/cfm thermistor fan.
When comparing PWM-controlled fans to industry-standard, thermistor-controlled fans, PWM fans exhibit several advantages. One PWM fan model can replace many models of the same top speed/cfm thermistor fan. Currently, customers require many different temperature/rpm curves, which will generate many different fan models though the top speeds are the same. Thus, a fan supplier's manufacturing and logistical processes can become elaborate. When using only one PWM fan, however, a user can program the desired temperature/rpm curve simply by changing the PWM controller software (figure 6).



Figure 7. This type of PWM fan uses only the duty cycle information out of the PWM signal, which results in a certain percentage of its full speed at 0 percent duty cycle. PWM frequency is not important.
It takes a certain effort by the enclosure designer to precisely define a temperature/rpm curve specification because the temperature sensed by the thermistor, which is at the fan exhaust side, is not the same as the critical component-case temperature. The enclosure may have several critical components at different locations with different thermal resistance to the thermistor location. Fan manufacturers implement the curve specification by modifying the hardware or firmware of the fan circuit. Due to different test equipment and testing methods, it may take several iterations between the supplier and user to finalize on a fan design to meet the specification. Sometimes, a minor test thermocouple location difference, or a fan test orientation inside the thermal chamber, can shift the curve outside of the specification. Once the designer receives the approved fan, he or she cannot adjust the curve. Even a slight change on the curve specification will require a new fan design. With a PWM fan, however, the user can fine-tune the temperature/rpm curve spec when designing the enclosure.

After a thermistor fan is installed in an enclosure, its speed is not controlled by the system. Usually, the fan sends a locked rotor signal to the system: Low indicates the fan is running and high indicates the fan has stopped. The fan can run out of rpm specification and the system will not know. Other types of thermistor fans can send tachometer signals but for monitoring purposes only. With PWM control loop, the fan self-adjusts to the required rpm.

One type of PWM fan uses the PWM signal to directly drive a MOSFET inside the fan. This results in 0 rpm when the duty cycle is below 5 percent to 8 percent, initial rotation when the duty cycle is more than 10 percent, and full speed when the duty cycle is at 100 percent. The PWM signal frequency must match the motor characteristics and normally is within the 30 to 80 Hz range.

Another type of PWM fan uses only the duty cycle information out of the PWM signal, which results in a certain percentage of its full speed at 0 percent duty cycle (figure 7).

PWM frequency is not important. PCE

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