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Application of PWM controlled fan Application of PWM Fan Control

Michael Huang, Senior Engineer

JMC Products, Austin, Texas


The trend in electronic enclosure cooling is to regulate the cooling fan/blower speed
according to the temperature inside the enclosure. Pulse Width Modulation (PWM) is the
preferred method to regulate motor speed because no additional heat is generated, and it
is energy efficient when compared to linear regulating (voltage control) the motor. The
PWM duty cycle is defined as Ton/Toff (%) in one period and the range is 0% - 100%.

PWM controllers for brushless DC motors are available from companies
like Analog
Devices (ADT7460), Microchip Technology (TC664), and National Semiconductor
(LM63).

In these devices application notes it is typically suggested to control the fan ground line
by a 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 (MCU) to perform the commutation function. Obviously, the MCU and
its associated electronic components, such as capacitors, will not work normally when the
ground line is switched at a frequency of 30HZ or above. In fig. 2, the fan speed signal
(tach) is fed back to the controller for closed-loop control. In most cases, the tach signal
is invalid when the circuit generating the signal is turned off by the MOSFET. In fig. 1,
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% duty cycle.

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

A better approach to using a PWM controller is to use a 4-wire PWM fan. Connect the
PWM and the tach line, leaving the fan power and ground lines uninterrupted, as shown
in fig. 4. 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.

All company and product names that appear in this paper are the trademarks or service marks of their respective
owners, including: Analog Devices (ADT7460), Microchip Technology (TC664), and National Semiconductor
(LM63).


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Fig 1. Basic connection of PWM fan control



Fig 2. Using fan tach signal for feedback control







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Fig 3. Using P-MOSFET to switch the fan power line



Fig 4. PWM fan control

With closed-loop speed control, the tolerance of the fan PWM duty cycle vs. the speed
curve can be very 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 will also be maintained when
there is voltage variation or load variation on the fan, working in the same manner that an
automobiles cruise control system works. In fig 6, Fan A and Fan B can achieve the
same temperature RPM curve if the controller software is properly designed.

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When comparing PWM controlled fans to industry-standard thermistor controlled fans,
PWM fans exhibit the following advantages:

One PWM fan model can replace many models of the same top speed/CFM
thermistor fan. Currently, customers require a multitude of different temperature -
RPM curves, which will generate many different fan models, though the top speed
are the same. Thus, a fan suppliers manufacturing and logistical processes can
become very complicated. However, when using only one PWM fan, a customer
can program their desired temperature - RPM curve by simply changing the PWM
controller software. Fig. 5 shows temperature - RPM curves.

It takes certain effort by the enclosure designer to precisely define a temperature
RPM curve spec 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 spec 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 customer to finalize on a fan design to meet
the spec. Sometimes, a minor test thermocouple location difference, or a fan test
orientation inside the thermal chamber, can shift the curve outside of the spec.
Once the designer receives the approved fan, he or she cannot adjust the curve.
Even a slight change on the curve spec 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 RPM is not controlled by the
system. Usually the fan sends a locked rotor signal to the system - low indicating
the fan is running and high indicating the fan is stopped. The fan can run out of
RPM spec and the system will not know. Other types of thermistor fans can send
tach signals, but for monitoring purposes only. With PWM control loop, the fan is
always self-adjusting to the required RPM.

JMC Products currently offers two types of PWM fans in all frame sizes: a PWM-D and a
PWM-S.

A PWM-D fan uses the PWM signal to directly drive a MOSFET inside the fan. This
results in 0 RPM when the duty cycle is below 5~8%, initial rotation when the duty cycle
is more than 10%, and full speed when the duty cycle is at 100%. The frequency of the
PWM signal needs to match the motor characteristics and is normally within the 30~80
HZ range.

PWM-S fans use only the duty cycle information out of the PWM signal which results in
a certain percentage of its full speed at 0% duty cycle, as shown in fig 6. PWM frequency
is not important.

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For more info, please visit
www.JMCProduts.com
or email
MHuang@JMCProducts.com




Fig. 5 One PWM fan can replace model A, B, C



Fig. 6 PWM fan RPM curves.


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Fig 7. PWM waveform and duty cycle

_____________________

Mike Huang
Senior Engineer
JMC Products
10315A Metropolitan Drive
Austin, TX 78758
512.834.8866
MHuang@JMCProducts.com



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