Selecting Temperature Sensors for System Measurement and Protection
Abstract: A number of technologies are available to designers who need to measure temperature within a system. Thermistors, thermocouples, RTDs, and temperature-sensor ICs each have advantages and disadvantages in any given situation. This article compares the most popular temperature sensing technologies. It discusses the suitability of each technology for monitoring common targets such as PC boards, ambient air, and high-power circuits like as CPUs and FPGAs.
Temperature-Sensing Technologies
Sensors are often used within electronic systems to
monitor temperature and provide protection from
excessive temperature excursions. The most common
technologies for use within systems are listed below.
Thermocouples are made by joining two wires of dissimilar
metals. The point of contact between the wires
generates a voltage that is approximately proportional to
temperature. Characteristics include wide temperature
range (up to 1250°C), low-cost, very low output voltage
(on the order of 40µV per °C for type K), reasonable
linearity, and moderately complex signal conditioning
(cold-junction compensation and amplification). There are
several thermocouple types, which are designated by
letters. The most popular is type K. Maxim manufactures
ICs (MAX6674 and MAX6675) that perform the signal
conditioning functions for type-K thermocouples, simplifying
the design task and significantly reducing the number
of components required to amplify, cold-junction compensate,
and digitize the thermocouple's output.
Thermocouples are available in probes and with bare leads.
RTDs are essentially resistors (often made from platinum
wire) whose resistance varies with temperature.
Characteristics include wide temperature range (up to
750°C), excellent accuracy and repeatability, reasonable
linearity, and the need for signal conditioning. Signal
conditioning for an RTD usually consists of a precision
current source and a high-resolution ADC. Cost can be
high. RTDs are available in probes, in surface-mount
packages, and with bare leads.
Thermistors are temperature-dependent resistors, usually
molded from conductive materials. The most common
thermistors have a negative temperature coefficient
(NTC) of resistance. Characteristics include moderate
temperature range (up to 150°C), low-to-moderate cost
(depending on accuracy), poor but predictable linearity,
and the need for some signal conditioning. Thermistors
are available in probes, in surface-mount packages, with
bare leads, and in a variety of specialized packages.
Maxim manufactures ICs that convert thermistor resistance
to digital form.
IC temperature sensors are complete, silicon-based
sensing circuits with either analog or digital outputs.
Characteristics include moderate temperature range (up to
about 150°C), low cost, excellent linearity, and additional
features like signal conditioning, comparators, and digital
interfaces. Digital formats are numerous and include 3-
wire and 4-wire (such as SPI™), 2-wire (I2C™ and
SMBus™), and single-wire (1-Wire®, PWM, frequency,
and period). Note that signal conditioning, analog-todigital
conversion, and thermostatic functions all add
costs to the other sensing technologies, but are normally
included within sensor ICs. IC temperature sensors are
available primarily in surface-mount packages.
Choosing the Proper Temperature Sensor for
System-Measurement Targets
Picking the right sensor technology begins with understanding
the characteristics and requirements of the target
whose temperature needs to be measured. Some common
temperature-measurement targets are listed below and are
summarized in Table 1.
PC board
Surface-mount sensors are best for PC board measurement.
RTDs, thermistors, and IC sensors are available in
surface-mount packages and temperature ranges that are
compatible with sensing the temperature of a PC board.
RTDs are quite accurate and produce highly repeatable
measurements, but can be costly compared to thermistors
and ICs. Thermistors are very nonlinear, but the nonlinearity
is predictable. When used over a narrow temperature range, they can often be linearized reasonably well
with just an external resistor or two. If accuracy is not
critical, thermistors can be inexpensive; but, precision
thermistors can be moderately expensive. The system cost
and complexity can increase significantly if linearization
calculations or lookup tables must be used. ICs have
excellent linearity and additional features, such as digital
interface or thermostat functions. These features usually
give them the edge over other sensor technologies in
terms of system cost, design complexity, and performance
when measuring PC board temperature.
One of the keys to measuring PC board temperature accurately
is positioning the sensor in the right place. It is
common to measure the temperature of a specific
component or group of components, either to ensure that
the temperature does not exceed the safe operating range,
or to compensate for temperature-induced changes in a
component's performance. When location of the sensor is
critical, look for temperature sensors in small packages,
such as SOT23s, that can be easily placed in the appropriate
location without disturbing the layout. Digital
outputs are useful when sensors need to be located in sites
that may be electrically noisy or far from the other
temperature-related circuitry.
Table 1. Optimum sensor types for system-temperature monitoring
Measurement Target
Best Sensor Types
Advantages
Disadvantages
PC board
IC (analog)
Cost, linearity
IC (digital)
Cost, digital output, linearity
Thermistor
Cost
Nonlinearity
RTD
Repeatability
Cost
Air
Thermistor
Cost, low thermal mass
Nonlinearity
Thermocouple
Cost, low thermal mass
Signal conditioning (increases cost)
RTD
Repeatability
Cost
IC (analog or digital)
Cost, linearity
Difficult to isolate from PC board temperature
CPU, FPGA, Power
Device, Module, etc.
(measured under or
near device)
IC (analog)
Cost, linearity
IC (digital)
Cost, digital output, linearity
Thermistor
Cost
Nonlinearity
RTD
Repeatability
Cost
CPU, FPGA, Power
Device, Module, etc.
(contact)
Thermistor
Cost, low thermal mass
Nonlinearity
Thermocouple
Cost, low thermal mass
Signal conditioning (increases cost)
RTD
Repeatability
Cost
CPU, FPGA, Power
Device, Module, etc.
(with thermal diode)
IC (remote digital temperature sensor)
Linearity, digital output, response time, accuracy
Ambient air
Ambient-air temperature is difficult to measure because
the sensor's temperature must be influenced by the air,
but isolated from other components (PC board, power
supply, CPU) that might be at a different temperature.
Thermistors, thermocouples, and RTDs are available on
long leads that isolate the sensing elements from the PC
board temperature. If the leads are long enough, the
sensing element will be at the ambient temperature, while
the leads are connected to the PC board, which is
probably at a different temperature. ICs are usually
difficult to use for measuring ambient temperature
because the best thermal path for an IC sensor is through
its leads, which are at the same temperature as the PC
board. If the PC board is not at ambient temperature (for
example, if it contains components that dissipate enough
power to raise its temperature), the IC will not measure
ambient temperature. Note that even conventional IC
packages, such as TO92s, that raise the IC sensor above
the PC board conduct heat so well through their leads that
the measured temperature is effectively equal to the PC
board temperature. However, because they have additional
system features, such as digital outputs or thermostat
functions, IC temperature sensors are sometimes used
for ambient-air temperature sensing. This is usually done
by placing them on small "satellite" PC boards that are at
ambient temperature. ICs are also available that help with
signal conditioning of other types of sensors. These ICs
include ADCs and amplifiers for RTDs, thermistor-todigital
converters such as the MAX6691, and thermocouple-
to-digital converters such as the MAX6675
(Figure 1).
Figure 1. Using a thermocouple to sense ambient temperature, the
MAX6675 provides cold-junction compensation and converts
the output of the thermocouple directly to digital form.
CPU, graphics processor, FPGA, power device,
module, etc.
The temperature of a high-power component can often be
measured with a surface-mount sensor (thermistor, IC, or
RTD) near or under the device. If this is impractical, or if
the device has a heat sink or some other surface that must
be measured, sensors with long leads (thermocouples,
RTDs, and thermistors) can be placed in contact with the
surface to be measured. If the temperature to be measured
is more than approximately 150°C, a thermocouple or
RTD is the best choice. Near or above 750°C, thermocouples
become the only choice.
CPU, graphics processor, FPGA, power device,
module, etc. (with on-board thermal diode)
Some components, especially high-performance ICs such
as CPUs, graphics processors (GPUs), and FPGAs,
include a diode-connected bipolar transistor for sensing
temperature. Because the thermal-sensing transistor is on
the IC die, measurement accuracy is far better than with
other sensing technique and thermal time constants are
quite small.
Maxim manufactures several ICs that are specifically
designed to accurately measure the temperature of a
thermal diode and convert it directly to digital form.
Some of these ICs measure a single thermal diode, while
others measure as many as four. The signal levels are
small (on the order of 200µV per °C), but still larger than
those of thermocouples. Internal and external filtering,
combined with reasonable care in layout, allow remote
diode sensors to be widely used in electrically noisy
equipment such a computers, servers, and workstations.
Most of these ICs provide additional functions to protect
the target IC, such as overtemperature alarm pins that can
be used to shut the system down if temperature exceeds
the safe operating limits of the target. An example of a
remote diode sensor (MAX6642) is shown in Figure 2.
This IC measures the thermal diode temperature and its
own temperature up to 150°C, and also provides an
overtemperature alarm output with a trip temperature that
is programmable over the SMBus.
Figure 2. The MAX6642 is the world's smallest remote temperature
sensor. It has an ALERT pin that may be used as an interrupt
or as a system shutdown signal to protect the target IC from
damage due to overheating.
Conclusion
There are several different temperature-sensing technologies
available for the system designer. The right technology
depends on the target temperature to be measured,
and also on other system requirements such as cost, circuit
size, and design time. Maxim's comprehensive selection of
temperature-sensing ICs can help the designer solve
common temperature-measurement problems with
excellent performance and low overall cost.
1-Wire is a registered trademark of Maxim Integrated Products, Inc.
SMBus is a trademark of Intel Corp.
SPI is a trademark of Motorola, Inc.
ダウンロード、PDFフォーマット
