Temperature problems solved for you (I) Basic principles of temperature sensing

When designing personal electronics, industrial or medical applications, engineers must address the same challenges of increasing performance, adding functionality and reducing size. In addition to these considerations, they must also carefully monitor temperature to ensure safety and protect systems and consumers from harm.

Another common trend across many industries is the need to process more data from more sensors, further illustrating the importance of temperature measurement: not only measuring system or environmental conditions, but also compensating for other temperature-sensitive components to ensure sensor and system accuracy. Another benefit is that with accurate temperature monitoring, there is no need to overdesign the system to compensate for inaccurate temperature measurements, which can improve system performance and reduce costs.

Thermal design challenges fall into three categories

Temperature Monitoring: Temperature sensors provide valuable data to continuously track temperature conditions and provide feedback to the control system. This monitoring can be system temperature monitoring or ambient temperature monitoring. In some applications, we can see that the design challenge is characterized by the need to implement both types of monitoring in the control loop. These monitoring include system temperature monitoring, ambient temperature monitoring, and body or fluid temperature monitoring.

Temperature protection: In many applications, action is required once the system exceeds or falls below a functional temperature threshold. Temperature sensors provide output alerts when a pre-defined condition is detected to prevent system damage. It is possible to increase processor throughput without compromising system reliability. Systems often initiate safety thermal shutdown prematurely, resulting in performance losses of up to 5°C or even 10°C. Engineers can autonomously initiate real-time protection measures when the system exceeds or falls below a functional temperature threshold.

Temperature compensation: Temperature sensors can maximize system performance as temperature changes during normal operation. Monitoring and correcting for temperature drift of other critical components as they heat up and cool down can reduce the risk of system failure.

This series of articles will provide some TI application briefs to illustrate the design considerations for various applications using different temperature sensing technologies. It will first introduce the main temperature challenges, then focus on the design considerations for various applications, evaluate the trade-off between temperature accuracy and application size, and discuss sensor placement methods.

Temperature sensor basic principle

In embedded systems, there is always a demand for higher performance, more functionality, and smaller form factors. Given this demand, designers must monitor overall temperature to ensure safety and protect the system. Integrating more sensors in applications further drives the need for temperature measurement, not only to measure system conditions or environmental conditions, but also to compensate for temperature-sensitive components and maintain overall system accuracy.

Temperature Design Considerations

Considerations for effective temperature monitoring and protection include:

• Accuracy. Sensor accuracy indicates how close the temperature is to the true value. When determining accuracy, all factors must be considered, including the acquisition circuitry and linearity over the entire operating temperature range.

• Size. The size of the sensor affects the design, and analyzing the entire circuit can help achieve a more optimized design. Sensor size also determines the thermal response time, which is important for applications such as body temperature monitoring. 

•Sensor placement. The packaging and placement of the sensor affects the response time and conduction path; both factors are critical to efficient temperature design. 

Common temperature sensor technologies in industry include integrated circuit (IC) sensors, thermistors, RTDs, and thermocouples. The following table compares the key characteristics to consider when selecting the right technology for your design challenge.

IC Sensors

IC temperature sensors depend on the predicted temperature dependence of the silicon bandgap. As shown in the following figure and equation, a precision current powers an internal forward biased PN junction, producing a base-emitter voltage change (ΔVBE) that corresponds to the device temperature.

Temperature dependence of silicon bandgap

Due to the predictable behavior of silicon, ICs offer high linearity and accuracy (up to ±0.1°C) over a wide temperature range. These sensors can integrate system functions such as analog-to-digital converters (ADCs) or comparators, ultimately reducing system complexity and overall footprint. These sensors are typically packaged in surface mount and through-hole packaging technologies. 

Thermistor

Thermistors are passive components whose resistance depends strongly on temperature. Thermistors are divided into two categories: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). 

While thermistors offer a variety of package options for both on-board and off-board temperature sensing, their implementation typically requires more system components than IC sensors. Silicon-based PTC thermistors have linear characteristics, while NTC thermistors have nonlinear characteristics that typically increase calibration costs and software overhead. 

Typical Thermistor Implementation

The figure above shows a typical thermistor implementation. It is often difficult to determine the true system accuracy of a thermistor. Contributors to NTC system errors include NTC tolerance, bias resistors (which are susceptible to temperature drift), ADC (which may cause quantization errors), NTC inherent linearization errors, and reference voltage.

RTD 

RTDs are temperature sensors made of pure materials such as platinum, nickel or copper that have a highly predictable resistance/temperature relationship. 

Complex Four-Wire RTD Circuit

Platinum RTDs offer high accuracy and linearity over a wide temperature range up to 600°C. As shown in the figure above, an implementation using analog sensors includes complex circuitry and design challenges. Ultimately, a complex error analysis is required to achieve an accurate system due to the large number of contributing components, which also affects the overall size of the system. RTDs also require calibration during manufacturing and annual field calibration thereafter. 

Factors that contribute to RTD system errors include RTD tolerance, self-heating, ADC quantization error, and reference voltage.

Thermocouple 

A thermocouple consists of two different electrical conductors that form an electrical junction at different temperatures. Due to the thermoelectric Seebeck effect, the thermocouple generates a temperature-dependent voltage. This voltage is converted to a temperature difference between the hot and cold junctions. 

Thermocouple with Cold Junction Compensation (CJC) Temperature Sensor

The temperature of the cold junction must be known to obtain the hot junction temperature. The accuracy here will be limited because there are two systems with separate tolerances and capabilities that affect each other. The figure above shows a typical CJC implementation where a thermocouple and an external sensor are used to determine the hot junction temperature. 

Thermocouples do not require external excitation and therefore do not suffer from self-heating issues. They also support extreme temperatures (>2,000°C). 

While thermocouples are rugged and inexpensive, they do require an additional temperature sensor to support CJC. Thermocouples tend to be nonlinear and very sensitive to parasitic junctions where the thermocouple connects to the board. Digitizing thermocouples is susceptible to the ADC errors discussed previously.

Click here to quickly locate the TI analog column and view the latest and most comprehensive information on TI sensor products. At the same time, in the next few articles, we will focus on the design considerations of various applications, evaluate the trade-off between temperature accuracy and application size, and discuss sensor placement methods.