Automatic Fan Control Techniques: Trends in Cooling High-Speed Chips
Abstract:Cooling fans are an important part of thermal management for high-power chips (such as CPUs, FPGAs, and GPUs) and systems. Unfortunately, their use can sometimes raise a system's acoustic noise level to the point where it is objectionable to the user. By measuring temperature and adjusting fan speed accordingly, the fan's speed (and noise level) can be minimized when temperature is low, but increased under worst-case conditions to prevent damage. This article describes two techniques for automatically controlling a cooling fan's speed.
High-speed chips tend to run hot. As they get faster, they
get hotter. New generation high-speed digital chips use
smaller processes that allow the supply voltage to be
reduced, which helps somewhat, but the number of transistors
increases faster than the supply voltage decreases.
Power levels, therefore, continue to rise.
As chip temperature increases, performance can suffer.
Parameters shift, maximum operating frequencies
decrease, and timing can fall out of specification. From
the user's point of view, the product is no longer
operating properly when this occurs. The first reason for
cooling high-speed chips, therefore, is to maintain good
performance for the longest possible operating time and
over the widest possible range of environmental conditions.
The maximum allowable temperature for a highspeed
chip to meet its parametric specifications depends
on the process and how the chip is designed (how "close
to the edge" the chip is operating), among other factors.
Typical maximum die temperature values range from
+90°C to +130°C.
Beyond the point where performance degradation
begins, excessive die temperature causes catastrophic
damage to chips. The maximum die temperature limit is
usually well over +120°C and depends on such factors as
process, package, and duration of high-temperature
conditions. High-speed chips are, therefore, cooled to
avoid reaching a temperature that could both degrade
performance and cause irreparable damage.
A single cooling technique is rarely used with high-speed
chips. Instead, combinations of techniques are
generally necessary to ensure high performance and
continued reliability. Heat sinks, heat pipes, fans, and
clock throttling are commonly employed to cool highspeed
chips. The last two, fans and clock throttling, can
help solve the heat problem, but introduce problems of
their own.
Fans can dramatically reduce the temperature of a highspeed
chip, but they also generate a great deal of
acoustic noise. The noise from a full-speed cooling fan is
annoying to some consumers and is also becoming a
target of government agencies concerned about the longterm
effects of noise in the workplace. Fan noise can be
reduced significantly by varying the fan's speed based
on temperature; the fan can turn slowly (and very
quietly) when temperature is low, and can speed up as
temperature increases.
Clock throttling—reducing clock speed to reduce power
dissipation—works by reducing system performance.
When throttling the clock, the system continues to
function, but at a reduced speed. Clearly, in high-performance
systems, throttling should be done only when it is
absolutely necessary—that is, when the temperature
reaches the point where functionality is about to be lost.
Controlling fan speed or clock throttling based on
temperature requires that the temperature of the highspeed
chip is measured first. This can be done by placing
a temperature sensor close to the target chip—either
directly next to it or, in some cases, under it or on the
heat sink. The temperature measured this way corresponds
to that of the high-speed chip, but can be significantly
lower (up to around 30°C), and the difference
between measured temperature and die temperature
increases as the power dissipation increases. Therefore,
the temperature of the circuit board or heat sink must be
correlated to the die temperature of the high-speed chip.
A better alternative is possible with a number of highspeed
chips. Many CPUs, graphics chips, FPGAs, and
other high-speed ICs include a "thermal diode", which is
actually a diode-connected bipolar transistor, on the die.
Using a remote-diode temperature sensor connected to
this thermal diode, the temperature of the high-speed
IC's die can be measured directly with excellent
accuracy. This not only eliminates the large temperature
gradients involved in measuring temperature outside the
target IC's package, but it also eliminates the long
thermal time constants, from several seconds to minutes,
that cause delays in responding to die temperature
changes.
The need for fan control forces the designer to make
several key choices. The first choice is the method of
adjusting the fan's speed. A common method of
adjusting the speed of a brushless DC fan is to regulate
the power-supply voltage of the fan. This approach
works well for power-supply voltages as low as about
40% of the nominal value. There is a drawback. If the
power-supply voltage is varied using a linear pass
device, the efficiency is poor. Better efficiency can be
obtained using a switch-mode power supply for the fan,
but this increases cost and component count.
Another popular fan-speed control technique is to power
the fan with a low-frequency PWM signal, usually in the
range of about 30Hz, whose duty cycle is varied to adjust
the fan's speed. This is inexpensive because a single,
small pass transistor can be used. It is efficient because the
pass transistor is used as a switch. A disadvantage of this
approach, however, is that it can make the fan somewhat
noisier because of the pulsed nature of the power supply.
The PWM waveform's fast edges cause the fan's mechanical
structure to move (somewhat like a badly designed
loudspeaker), which can easily be audible.
Another fan-control design choice is whether the fan's
speed is measured as part of the control scheme. In
addition to power and ground, many fans are available
with a third wire that provides a "tachometer" signal to
the fan-control circuitry. The tachometer output produces
a specified number of pulses (two pulses, for example)
for each revolution of the fan. Some fan-control circuits
use this tachometer waveform as a feedback signal that
allows the fan's voltage or PWM duty cycle to be
adjusted to give a desired RPM. A simpler approach
ignores any tachometer signal and simply adjusts the
fan's drive to speed up or slow down with no speed
feedback. Speed control using this method is less precise,
but cost is lower and at least one feedback loop is
removed, simplifying the control system.
In some systems, it is important to limit the change rate
of the fan speed. This is most critical when the system is
in close proximity to users. Simply switching a fan on
and off or changing speed immediately as temperature
changes is acceptable in some environments. When users
are nearby, however, sudden changes in fan noise are
apparent and annoying. Limiting the rate of change of
the fan's drive signal to an acceptable value (e.g., 1% per
second) ensures that the acoustic effects of fan control
are minimized. The fan speed still changes, but it does so
without attracting attention.
The fan-control profile is another important design
variable. Typically, the fan is off below a specific
threshold temperature and then begins to spin at a slow
rate (for example, 40% of full speed) once the threshold
is exceeded. As temperature increases, the fan's drive
increases linearly with temperature until it reaches 100%
drive. The best slope depends on system requirements. A
more rapid slope results in somewhat more consistent
chip temperature, but fan speed has more variation as
power dissipation changes from one moment to the next.
If highest performance is the goal, the starting temperature
and the slope should be chosen so that the fan
reaches full speed before the die temperature is high
enough to initiate clock throttling.
Implementing fan-control circuitry can be done in
several ways. A variety of remote temperature sensors
with up to five sensing channels is available that can
detect the die temperature of the high-speed chip and
transmit temperature data to a microcontroller. Fan-speed
regulators with multiple channels of fantachometer
monitoring can provide reliable control of
fan RPM or supply voltage based on commands from an
external microcontroller. For low cost and simple implementation,
ICs are available with temperature sensing
and automatic fan control included in a single package.
Sensor/controllers also normally include overtemperature
detection for clock throttling and system shutdown,
thereby protecting the high-speed chips from catastrophic
failure due to overheating.
Examples of two such ICs, one with DC drive and one
with PWM drive, are shown in Figures 1 and 2. The IC
in Figure 1 senses remote temperature and controls fan
speed based on that temperature. It produces a DC
supply voltage for the fan through an internal power
transistor. Figure 2 shows an IC that performs a similar
function, but drives the fan with a PWM waveform
through an external pass transistor. Both include
complete thermal fault monitoring with overtemperature
outputs, which can be used to shut down the system if
the high-speed chip gets too hot.
Figure 1. Linear (DC-output) temperature sensor and automatic fan-speed controller. Fan speed is controlled automatically based on the temperature of the high-speed chip. Tachometer feedback from the fan allows the fan controller to regulate fan speed directly. System shutdown output prevents the high-speed chip from reaching destructive temperatures.
Figure 2. PWM-output temperature sensor and automatic fan-speed controller. Fan speed is controlled automatically based on temperature. Clock throttle and system shutdown outputs prevent a high-speed chip from reaching destructive temperatures. CRIT0 and CRIT1 pins can be strapped to supply or ground to select default shutdown-temperature thresholds, ensuring protection even when system
software hangs.
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