Why and How to Control Fan Fastness for Cooling Electronic Equipment
Introduction
Interest has been growing in integrated circuits for controlling the speed of cooling fans in personalised computers and other physical science equipment. Compact electric fans are tuppeny and have been used for cooling electronic equipment for more than half a century. However, in Recent epoch years, the technology of exploitation these fans has evolved significantly. This article will describe how and wherefore this evolution has taken place and will suggest much useful approaches for the designer.
Warmth Generation and Removal
The trend in electronics, particularly consumer electronics, is towards little products with increased combinations of features. Consequently, scads of electronic components are being shoehorned into precise small form factors. An obvious example is the notebook PC. Thin and "Low-cal," notebook PCs have contracted significantly, yet their processing power has been maintained or increased. Opposite examples of this trend admit projection systems and set-top boxes. What these systems complete have in common, also significantly smaller—and still detractive—size up, is that the amount of heat they must dissipate does non decrease; oftentimes it increases! In the notebook PC, a lot of the heat is generated past the processor; in the projector, most of the heat is generated by the light source. This heat of necessity to be removed quietly and with efficiency.
The quietest way to murder hot up is with passive components much as heat sinks and heat pipes. However, these stimulate proved insufficient in galore popular consumer electronics products—and they are besides reasonably expensive. A good mutually exclusive is active cooling system, introducing a fan into the system to generate airflow around the chassis and the heat-generating components, efficiently removing heating plant from the system. A fan is a source of disturbance, however. IT is also an additive source of power uptake in the system—a really important circumstance if ability is to be supplied by a battery. The fan is also one many windup component in the system, non an ideal solution from a reliability standpoint.
Speed control—one room to answer some of these objections to the use of a buff—can have these advantages:
- lengthways a buff slower reduces the noise it emits,
- running a fan slower lavatory reduce the power it consumes,
- running a fan slower increases its reliableness and lifetime.
There are many different types of fans and ways of controlling them. We will discuss here versatile winnow types and the advantages and disadvantages of control methods in use today. Nonpareil way to classify fans is as:
- 2-wire fans
- 3-wire fans
- 4-wire fans.
The methods of fan control to constitute discussed here include:
- no sports fan insure
- on/off control
- linear (continuous dc) control
- low-pitched-frequency pulse rate-breadth transition (PWM)
- high-frequency fan assure.
Fan Types
A 2-wire fan has power and earth terminals. A 3-electrify fan has index, ground, and a tachometric ("tach") output, which provides a sign with frequency proportional to velocity. A 4-wire fan has power, establish, a tach output, and a PWM-driving input. PWM, shortly, uses the relative width of pulses in a coach of happening-off pulses to align the level of king practical to the motor.
A 2-wire fan is controlled by adjusting either the dc electric potential or pulse breadth in low-oftenness PWM. However, with only two wires, a tach signal is not readily available. This substance that there is no denotation as to how fast the fan is running—or so, if information technology is lengthways at all. This constitute of speed control is open-loop.
A 3-wire fan can be priest-ridden using the same kind of drive as for 2-wire fans—variable dc or low-frequency PWM. The divergence between 2-electrify fans and 3-wire fans is the availability of feedback from the fan for squinched-loop hotfoot see to it. The tach signal indicates whether the rooter is running and its plac of speed.
The tach signal, when driven away a dc voltage, has a square-waving output intimately resembling the "ideal tachometer" in Figure 1. It is always valid, since power is continuously applied to the fan. With low- frequence PWM, however, the tach sign is unexpired only when power is applied to the fan—that is, during the along phase of the pulse. When the PWM drive is switched to the slay phase, the devotee's internal tachometer point-generation circuitry is also off. Because the tach output is typically from an open drain, information technology will float high when the PWM drive is off, equally shown in Figure 1. Thus, while the nonsuch tach is representative of the actual rush of the lover, the PWM ride effective "chops" the tach signal output and may produce erroneous readings.
In ordinate to represent careful of a correct fan speed indication under PWM control, it is required to periodically switch the fan on long sufficient to get along a complete tach cycle. This feature is implemented in a routine of Analog Devices fan controllers, such as the ADM1031 and the ADT7460.
In addition to the power, ground, and tach signal, 4-cable fans have a PWM input, which is wont to control the speed of the devotee. Instead of switching the power to the entire fan along and off, only the power to the ram down coils is switched, qualification the tach information available continuously. Switch the coils on and off generates some commutation noise. Driving the coils at rates greater than 20 k moves the dissonance outside of the hearable place, so typical PWM fan-drive signals use a rather high frequency (>20 kilocycle). Another advantage of 4-wire fans is that the fan speed can be dominated at speeds as low as 10% of the fan's full speed. Visualize 2 shows the differences betwixt 3-wire and 4-wire sports fan circuits.
Lover See
No control: The simplest method of fan control is non to employ whatever in the least; fair-minded run a buff of grade-appropriate capacity at overflowing speed 100% of the time. The of import advantages of this are guaranteed fail-safe cooling and a very reniform external circuit. Notwithstandin, because the fan is e'er switched on, its lifetime is low and information technology uses a constant quantity amount of power—even when cooling system is not needed. Also, its incessant noise is likely to be annoying.
On/off control: The next simplest method of fan control is thermostatic, or on/off control. This method is also very easy to follow out. The rooter is switched on only if cooling is necessary, and IT is switched forth for the end of the time. The user needs to dress the conditions low-level which cooling is needed—typically when the temperature exceeds a preset threshold.
The Analog Devices ADM1032 is an ideal sensor for on/soured winnow control using a temperature setpoint. Information technology has a comparator that produces a THERM output—one that is normally high only switches low when the temperature exceeds a programmable threshold. IT mechanically switches back to high when the temperature drops a preset number below the THERM Limit. The advantage of this programmable hysteresis is that the winnow does not continually substitution on/off when the temperature is close to the brink. Figure 3 is an example of a circuit victimization the ADM1032.
The disfavor of connected/off control is that it is selfsame limited. When a devotee is switched along, it immediately spins high to its loaded speed in an audible and annoying style. Because humans before long get moderately wonted to the sound of the fan, its shift off is too same noticeable. (IT can be compared to the refrigerator in your kitchen. You didn't notice the noise it was making until information technology switched off.) So from an natural philosophy perspective, on/off control is distant from optimal.
Unsubdivided control condition: At the next even of lover control, simple control, the emf applied to the fan is variable. For lower speed (less cooling and quieter operation) the potential is decreased, and for high pep pill information technology is magnified. The relationship has limitations. Consider, for example, a 12-V fan (rated maximum voltage). Such a fan may require at least 7 V to start spinning. When it does start spinning, it will probably spin at about incomplete its full speed with 7 V applied. Because of the call for to overcome inactiveness, the voltage required to kickoff a fan is higher than the electric potential mandatory to keep it spinning. Thusly as the voltage applied to the fan is reduced, it may spin at slower speeds until, sound out, 4 V, at which point it will stall. These values will disagree, from manufacturer to manufacturer, from worthy to model, and even from fan to fan.
The Analog Devices ADM1028 linear fan-control IC has a programmable output and just about every feature that might be needed in devotee control, including the ability to interface accurately to the temperature-detection junction rectifier provided on chips, such as microprocessors, that news report for most of the looseness in a system. (The purpose of the diode is to provide a fast indication of critical junction temperatures, avoiding all the thermal lags inherent in a system. Information technology permits quick initiation of cooling, supported a rise in chip temperature.) In rules of order to donjon the major power used by the ADM1028 at a stripped, it operates on supply voltages from 3.0 V to 5.5 V, with +2.5-V full exfoliation outturn.
5-V fans grant lonesome a limited range of speed hold in, since their lead off-up voltage is close to their 5-V replete speed level. But the ADM1028 can be used with 12-V fans by employing a simpleton step-finished booster amplifier with a circuit much as that shown in Figure 4.
The principal advantage of additive control is that it is silence. However, as we take up noted, the speed-hold in range is limited. For example, a 12-V fan with a control voltage range from 7 V to 12 V could exist running at half swiftness at 7 V. The spot is even worse with a 5-V fan. Typically, 5-V fans will want that 3.5 V or 4 V be applied to get them started, only at that voltage they will make up running at close-fitting to full speed, with a very limited range of speed control. Simply running at 12 V, using circuits such as that shown in Figure 4, is ALIR from optimal from an efficiency view. That is because the boost junction transistor dissipates a relatively enormous amount of power (when the lover is operating at 8 V, the 4-V drop crossways the transistor is not very economical). The external circuit required is also comparatively expensive.
PWM Control: The current method presently used for dominant fan speed in PCs is low-frequency PWM control. In this draw close, the voltage applied to the fan is always either zip or brimming-scale—avoiding the problems experienced in lengthways control at lower voltages. Figure 5 shows a typical drive circuit used with PWM output from the ADT7460 thermal voltage controller.
The principal advantage of this ram method is that information technology is orbicular, inexpensive, and very efficient, since the fan is either fully happening or fully off.
A disfavour is that the tach information is chopped by the PWM drive signal, since power is not always applied to the fan. The tach entropy can Be retrieved using a technique called pulse stretching—shift the fan along long enough to gather the tachometer information (with a realizable step-up of audible noise). Figure 6 shows a causa of pulse stretch.
Another disadvantage of low-frequency PWM is substitution noise. With the fan coils continuously switched on and off, sounding noise may exist present. To deal with this noise, the newest Analog Devices fan controllers are designed to push on the lover at a frequency of 22.5 kilocycle per second, which is outside the audible mountain range. The outer negative feedback circuit is simpler with luxuriously-frequency PWM, but it commode entirely equal used with 4-electrify fans. Although these fans are relatively new to the grocery store, they are rapidly comme il faut more favorite. Figure 7 depicts the circuit used for high-frequency PWM.
The PWM signal drives the devotee directly; the drive FET is integrated inside the fan. Reducing the external component count, this border on makes the external circuit much simpler. Since the PWM drive signal is applied directly to the coils of the fan, the fan's electronics are always powered on, and the tach signaling is always available. This eliminates the need for heart rate stretch—and the noise it tin can produce. The commutation disturbance is also eliminated, or reduced significantly, since the coils are beingness switched with a frequency extraneous the audible orbit.
Summary
From the standpoints of acoustic noise, reliability, and power efficiency, the most preferable method of sports fan control is the use of tall-frequency (>20 kHz) PWM drive off.
Besides eliminating the need for noisy pulse stretching and the exchange noise connected with forward-frequency PWM, it has a much wider assure range than linear control. With commanding- frequency PWM, the fan can be run at speeds as low every bit 10% of full speed, while the unvaried fan may only run at a minimum of 50% of fraught speed victimisation linear check. It is many energy effective, because the devotee is always either fully happening or fully off. (With the FET either disconnected or in saturation, its dissipation is very low, eliminating the significant losses in the transistor in the linear case.) Information technology is quieter than always-on or along/off control, since the fan can run at lower speeds—that can be varied gradually. Finally, gushing the fan slower too improves its lifespan, increasing system reliability.
| Control Method | Advantages | Disadvantages |
| On/Off | Low-cost | Whip physical science performance—rooter is always running. |
| Linear | Most quiet | Valuable circuit Inefficient—loss of power in the amplifier circuit |
| Low-Frequency PWM | Efficient Wide accelerate-control range when measuring stop number | Fan commutation noise Heart rate stretching required |
| High-Frequency PWM | Efficient Good acoustics, almost as good Eastern Samoa linear. Affordable external circuit Wide-eyed fastness-control range | Must use 4-wire fans |
Author
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Source: https://www.analog.com/en/analog-dialogue/articles/how-to-control-fan-speed.html