Making Sense of Thermal Sensing

Tuesday, December 12, 2023

5 minute read time.

We’ve looked at the datasheet thermal information including thermal resistance and maximum junction temperature before examining more closely thermal derating and the thermal characterization parameter. We know how important it is to keep junction temperature within the operating range. What do we do to understand the environments our devices are working in and why is a huge part of that solution the temperature sensor? Let’s figure out why we use thermal sensors and how to pick the right one for your application.

Thermal Sensing for Control, Calibration, and Protection

Industry 4.0 and the Industrial Internet of Things emphasize sensor use and data collection so we can control things—we use temperature information to control other systems like thermostats and complex fan systems. Electronics, such as real-time clocks and oscillators, are sensitive to heat and require calibration based on the ambient temperature. Lastly, protection from thermal runaway and irreparable damage requires monitoring the temperature so that we can safely shut down the device.

There are four sensors used to realize these goals: IC sensors, thermistors, resistive temperature detectors, and thermocouples.

Analog IC Sensors

The IC sensor uses a silicon bandgap reference to determine the temperature. We place matched transistors that are similarly manufactured in silicon with near-identical characteristics close together on the die so the temperatures will be the same. The base-to-emitter voltage VBE is proportional to the ratio of the collector currents and thermal voltage. This bandgap reference can be used to determine the temperature in the ICs—the sensor has a small footprint and is also low-cost! Analog Devices Fellow Paul Brokaw has patented such a bandgap reference: the Brokaw bandgap reference.
Fig. 1. Bandgap Reference Example

Fig. 1. Bandgap Reference Example

Thermistors

The common, negative temperature coefficient (NTC) thermistor has a large decrease in electrical resistance for a small increase in temperature—dependent on its semiconductor or metal-oxide material. Thermistors are accurate, stable, and easy to package, but may require lots of programming because of their nonlinearity (although behavior is highly predictable). In the voltage divider circuit shown in Fig. 2., a fixed resistor can be chosen which shifts the curve to help provide more linearization depending on application. As it is highly sensitive, the thermistor is best for detecting small temperature changes.

Thermistors are typically used for overheating protection. The MAX31740 fan-speed controller monitors an NTC thermistor much like the voltage-divider circuit in Fig. 2. The controller generates a PWM signal to control the fan from the temperature changes and range of interest from the thermistor.

Fig. 2. Characteristics of a 10 kΩ NTC Thermistor

RTDs

Resistance temperature detectors (RTDs) rely on the resistance of metal increasing with added temperature. The PT100 RTD, named for layers of platinum and 100 Ω resistance at 0°C, has high linearity and stability. RTDs need an accurate current source as well as a 2-, 3-, or 4-wire circuit configuration to compensate for cable resistance errors. The MAX31865 device converts the platinum RTD resistance to a digital value for approximating the temperature although third-order polynomials may be required to accommodate for nonlinearity.

Fig. 3. PT100 RTD Resistance vs. Temperature

Thermocouples

Thermocouples utilize a phenomenon called the Seebeck effect. Generation of a Seebeck voltage is only possible from a junction of two dissimilar conducting materials (alloys). The voltage read at the open end is proportional to the temperature at the heated junction. The K-type thermocouple (chromel - alumel) has a wide temperature range of -180 °C to +1370°C.

Fig. 4. Thermocouple Circuit

Temperature drift is common for sensors, but especially so because of thermocouple aging and changes in metallurgy properties. Thermocouples may require annual calibration, or adjustment to accommodate drift, such as the cold-junction compensation procedure. The MAX31856 is a converter that captures thermocouple data and utilizes multiple-order polynomials to help compute this calibration quite easily.

Trade-Offs for Thermal Sensors

	IC Sensor	Thermistor (NTC)	RTD	Thermocouple
Typical Temp. Range	-55°C to 150°C	-50°C to 300°C	-200°C to 800°C	-200 °C to 1800°C
Accuracy	± 1°C	± 0.1% to 0.2% of reading	±0.2% of reading	±1% of reading
Linearity	Most linear	Highly non-linear	Fairly linear	Linear (relatively)
Challenges	Can be difficult to calibrate	Narrow range, nonlinear, self-heating effect	Requires current source, self-heating effect	Drifts and recalibration

If cost is a concern and annual calibration isn’t, then thermocouples may be more appealing than the thermistor. RTDs have high accuracy and repeatability but may be expensive as they consume more power than the IC sensor.

Hopefully, this answered those questions about thermal sensing. Our next post revisits modeling with more detail.

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