Temperature-based fan-speed control: Combining accurate monitoring of the die temperature with a cooling fan provides the basis for intelligent fan control. Reducing the fan speed as the temperature drops will minimize the system acoustic noise. This can be accomplished with an integrated temperature-monitor and fan-controller IC. The best solution is to use an automatic-fan-speed control loop or closed-loop fan control. This offers the big advantage of operating completely independent of software once the control loop is configured. In the un-fortunate event of some system failure (either hardware or software), thermal management will still work.
Two distinct techniques of controlling fan speed exist: linear control and pulse-width modulation (PWM). Each has its advantages and disadvantages.
Linear, or voltage control, varies the voltage applied to the fan to adjust its speed. A typical 12-V fan will have an effective operating-voltage range of about 7 to 12 V. The lowest voltage at which the fan will operate is 7 V, but this may be the lowest voltage at which the fan will run once already spinning. For a stalled fan, the actual starting voltage re-quired to overcome inertia may be higher. It can only be found through experimentation.
For a typical 5-V fan, the operating-voltage range shrinks to about 4 to 5 V. Some fans won't spin at all below 4 V, while they spin at almost full speed above 4 V. Therefore, there's little or no adjustment range when using linear control to drive a 5-V fan. Another distinct disadvantage of linear control is the physical space necessary for the gain-setting resistors and op amp required to drive the fan.
PWM control drives the fan by adjusting the duty cycle of an applied square wave. Thus, the full voltage swing is always applied to the fan—0 to 5 V for a 5-V fan, or 0 to 12 V for a 12-V fan. Most fans will run reliably with a 33% to 100% duty cycle, so the control range can be wider than with linear control. With PWM control, the fan drive circuitry is much simpler, requiring only a single FET. Additionally, switching the fan on and off reduces the average power consumption and improves the efficiency.
Automatic-fan-speed control: The automatic-fan-speed control function turns the fan on at a predefined temperature point and automatically adjusts its speed. Operation of the automatic-fan-speed control loop is illustrated in Figure 4. The loop is programmed by defining the temperature parameters TMIN and TRANGE.
TMIN is the temperature at which the fan switches on and runs at minimum speed with a 33% duty cycle. When the measured temperature rises above TMIN, the fan will switch on and run at full speed for 2 seconds before dropping back to minimum speed. This ensures the reliability of fan startup every time.
TRANGE defines the temperature range over which the fan automatically varies in speed. The temperature at which the fan will reach full speed is TMAX:
TMAX = TMIN + TRANGE
Built into the loop, 5°C hysteresis prevents the fan from cycling on and off when the measured temperature is near TMIN.
The most tangible benefit of the control loop is that the fan operates at its optimum speed for any given temperature, greatly reducing current consumption and system acoustic noise. The acoustic behavior of the system im-proves with automatic-fan-speed control (Fig. 5). Here, with TMIN = 40°C and TRANGE = 40°C, the fan runs full-speed at 80°C. Notably, fan noise is well below the 36-dB target value for most of the temperature profile.
Filtered automatic-fan-speed control: The automatic-fan-speed control loop is easy to understand and configure. Yet, additional filtering is sometimes needed to enhance system acoustic behavior. For example, large temperature transients may occur when a program runs on a computer. The CPU temperature rises sharply and peaks while the program loads. Afterwards, it drops back to its stable state.
Clearly, the fan shouldn't speed up for a short time and then slow down again. That would create very unsettling dynamic acoustics for the user. Studies of the human ear reveal that the brain adapts to a constant noise or tone far more effectively than to a sound that's shifting in frequency or intensity. For instance, automobile alarms warble to catch your attention. Unfortunately, a cycling fan will seize your attention too.
Filtered automatic-fan-speed control mode effectively smooths the fan's response to changes in temperature. With this approach, the fan ramps up to its new speed instead of trying to jump there instantaneously. This filtering effect is much more pleasing acoustically, because the fan changes speed gracefully, eliminating cycling due to fast temperature transients.
Filtering is achieved by programming the ramp rate and the number of temperature updates per second. Both factors influence how fast the fan responds to changes in system temperature (Fig. 6).
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