Temperature Sensing Guide | Understanding the Design Challenges[Copy link]
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.
Three major temperature design challenges
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 needs to be taken 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 up to 5°C or even 10°C performance loss. 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.
Temperature Sensing Basics
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.
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 will have an impact on 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 effective temperature design. Common temperature sensor technologies in industry include integrated circuit (IC) sensors, thermistors, RTDs, and thermocouples. Table 1 compares the key characteristics to consider when selecting the right technology for your design challenge.
For many system designs, it is necessary to monitor high-power components (processors, field-programmable gate arrays, field-effect transistors) to ensure system and user safety. Accuracy in temperature readings is important because it enables designers to increase performance while staying within safety limits, or reduce system cost by avoiding over-design elsewhere. Texas Instruments' (TI) portfolio of compact, high-precision temperature sensors can be placed closer to these critical components for the most accurate measurements.
Temperature issues in circuits can affect system performance and damage expensive components. Measuring the temperature of areas with hot spots or high-power integrated circuits (ICs) on a printed circuit board (PCB) helps identify thermal issues so preventive or corrective action can be taken.
You might want to monitor the die temperature of a power-hungry IC (such as a central processing unit, application-specific IC, field-programmable gate array, or digital signal processor) to dynamically adjust its performance, or you might want to monitor the thermal zone around a power stage to control fan speed in a system or initiate a safe system shutdown. The ultimate goal is to optimize performance and protect expensive equipment . Figure 1 shows a temperature monitoring system on a high-performance computer motherboard.
Power management in high-performance processors such as central processing units (CPUs), graphics processing units (GPUs), application-specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs) is often complex. With temperature monitoring, these systems can not only initiate safe system shutdown procedures, but also use temperature data to dynamically adjust performance.
Monitoring process temperature can improve system reliability and maximize performance . As shown in Figure 2, high-performance processors often use heat sinks to absorb excess heat from the die. Higher temperatures may activate cooling fans, modify system clocks, or quickly shut down the system if the processor exceeds its temperature threshold.
In many applications, ambient air temperature monitoring is critical to control environmental conditions or ensure safe operating conditions. Accurately and quickly measuring ambient temperature is often challenging because the sensor may not be fully exposed to the external environment and may be affected by self-heating of other components in the system . TI's high-precision, low-power single- and multi-channel temperature sensors are available in compact packages for faster thermal response.
Measuring ambient temperature using surface mount devices can be challenging because heat transfer from other power-hungry electronic components can affect the sensor’s temperature readings. Good layout techniques , such as understanding the primary thermal paths, isolating the sensor package, and placing the device away from interfering heat sources , are necessary to accurately measure ambient temperature. Figure 3 shows a simple thermostat design that uses these techniques. In Figure 3, passive airflow from the system’s self-heating draws outside air over temperature sensor A. The sensor is placed at the air intake away from the primary heat source (central processing unit) and is thermally insulated to ensure more accurate measurements.
Knowing a patient’s temperature is a critical first step in any clinical diagnosis and is a significant concern for athletes. In addition to requiring ultra-high accuracy, the industry is moving toward compact wearables to provide continuous temperature monitoring. Temperature sensors with up to 0.1°C accuracy not only meet the American Society for Testing and Materials (ASTM) E1112 requirements for medical thermometers, but are also optimized to keep battery-powered wearables compact and comfortable.
Monitoring patient vital signs in a clinical setting is typically a task performed by expensive systems that require rigorous calibration and require the patient to be tethered to a clinical monitor. Wireless patient monitoring systems can provide patient comfort and clinical convenience while still meeting strict medical standards.
When designing a wearable temperature monitor, there are many trade-offs to be made between power consumption, size, system performance (both in terms of radio frequency [RF] and accuracy), and patient comfort . For example, thinner, more flexible batteries offer greater comfort but may require more careful power management.
Smaller, lower-cost designs require trade-offs in thermal isolation and RF performance . Solutions for long-term monitoring must make the best use of board area to improve accuracy and signal integrity while minimizing current consumption. System designers must balance these requirements with patient comfort and experience.
Threshold Detection
For some applications, continuous temperature acquisition is not required, but it is critical that the system stays above or below a temperature threshold . TI's temperature switches and digital temperature sensors enable simple autonomous temperature monitoring , detecting if the temperature exceeds a limit with hysteresis. These devices allow the threshold trip point to be selected via an external resistor that can be pin-programmed, factory-programmed, or set via I2C.
In control systems, operating temperature is one of many key factors that affect system performance, reliability, and safety. Understanding the impact of temperature on control systems can help system designers predict and prevent thermal damage .
Typically, the behavior of control systems is well understood, and they operate within a limited temperature range. When operating outside of this temperature range, control systems can experience reduced efficiency, increased heat dissipation, and accelerated aging. These effects combined can result in costly failures.
The TMP303 integrated temperature switch uses a window comparator to provide design flexibility through ultra-small size (small outline transistor [SOT]-563), low power consumption (5A maximum), and low supply voltage (down to 1.4V). The device requires no additional components to operate and can operate independently of a microprocessor or microcontroller. Seven trip points are available through different device options, which can be factory programmed to any desired temperature.
The TMP390, shown in Figure 4, is a resistor-programmable dual-output temperature switch with two internal comparators and two outputs. The TMP390 features ultra-low power consumption (1μA maximum) and low supply voltage (1.62V). Both the high and low temperature trip points can be configured to any desired temperature window, allowing hysteresis options between 5°C and 30°C, all with just two resistors. Separate high and low temperature trip outputs generate independent warning signals for microprocessor interpretation.
Temperature drift must be a factor in correcting for temperature changes in any system. Temperature will affect everything from passive components (resistors and capacitors) to active components (amplifiers, data converters, references, clocks). Optical components are also affected by temperature drift, resulting in changed intensity, spectral shifts, sensitivity and noise. TI's high linearity, high accuracy temperature sensors can provide feedback to correct for temperature effects in precision systems.
Field transmitters are widely used in factory automation and control to sense process parameters such as temperature, pressure and flow rate. The sensors used in field transmitters are primarily analog sensors that must be accurately sampled using an analog front end. Due to the operating conditions caused by the way field transmitters are laid out, field transmitters may be subject to a wide temperature range and therefore require some form of temperature compensation. Temperature compensation systems have traditionally used accurate temperature sensors such as platinum resistance temperature detectors (RTDs), especially in industrial applications where high accuracy and long service life are required.
Most RTD applications use a current source to excite the RTD element and generate a voltage difference across the RTD, as shown in Figure 5. This voltage is proportional to the resistance of the RTD and the excitation current. The voltage potential is amplified, converted to a digital output by an analog-to-digital converter (ADC), and then fed into a microcontroller (MCU) where the digital output is converted to temperature using a lookup table.
The TMP117 is a digital temperature sensor designed for low-power, high-precision applications. The device provides 16-bit temperature results with a resolution of 0.0078°C; the factory-calibrated performance accuracy is ±0.1°C in the range of -25°C to +50°C; ±0.3°C in the full operating temperature range of -55°C to +150°C, which exceeds the accuracy of Class AA RTDs .
The results of an oil bath experiment on the TMP117 are shown in Figure 6. This graph shows that the TMP117 can meet the accuracy of a Class AA RTD required for CJC applications.
TI's sensor portfolio enables engineers to realize large designs with high-precision, small-size sensors. The portfolio features industry-leading packaging options and many options in the world, accurate sensors for designing reliable systems and accurately predicting intelligence.
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