AEI by Dempa Publications, Inc., May 2020
Fifth-generation (5G) wireless standard is beginning to spread in earnest. In 5G networks, where transmission speeds will increase dramatically, the load on related components will also increase. This is because the amount of information to be processed by each individual component per unit time will also increase massively.
Moreover, images and videos, which will probably account for much of the data traffic, will have higher definition, and components surrounding the camera will handle information in larger amounts and at higher speeds. Power supplies that sustain such information processing also require high-speed charging of high-capacity batteries.
These factors mean that there are many sources of heat generation in electronic devices. Furthermore, in electronic devices where multiple sources of heat generation function complicatedly, these sources receive each other's heat. Simply taking a measure for a single source of heat generation cannot handle situations where multiple functions are used simultaneously.
Monitoring Circuit Board Temperatures
Under the circumstances described above, it is becoming increasingly important to monitor the temperatures of multiple locations on the circuit board of an electronic device to control the performance of components that are sources of heat generation, according to the complex functions of the electronic device.
For example, when an app, which puts a large load on the processor, is executed, the processor operates at its full capacity at the initial stage where its temperature remains low, and as the temperature increases, its performance is limited so that the temperature does not exceed the threshold level. At this time, if the power supply to the processor generates high heat and the processor receives it, then the temperature of the processor may surge. It is necessary to consider the temperatures surrounding the processor and the power IC and control their performance in a more detailed manner.
If heat generation persists despite control being implemented, ultimate overheat protection is performed, such as display of a warning and transition into a shutdown sequence.
It is necessary to consider not only the internal temperatures of ICs and modules—individual sources of heat generation—but also their mutual reception of heat and changes in the temperatures of the environment surrounding the electronic device. It is impossible to control temperatures in the manner described above without monitoring the temperatures surrounding the sources of heat generation.
Chip NTC Thermistors Suit Temperature Monitoring
To that end, surface-mount chip negative temperature coefficient (NTC) thermistors are selected as temperature sensors [Fig. 1].
They are sized according to EIA standards and are easy-to-mount like chip resistors and capacitors that conform to the same standards. They can be mounted on a location where thermistors can be wired, simply by preparing surface-mount lands. They can be placed very freely as temperature sensors at a location where temperature measurement is needed.
In addition, for chip components like chip NTC thermistors, various mass production technologies, fabrication techniques, and management methods have already been established to mass produce many product models with different properties. If production volume increases, appropriate mass production
facilities and fabrication techniques can be used according to the increase, resulting in reduced costs. Miniaturization has also been vigorously pursued by chip components manufacturers. The 0402mm size has already become common for thermistors.
Not only are thermistors cost effective and small at this point, their cost and size hold the potential to be reduced even further compared with other temperature sensors.
Additional Advantages of Thermistors
Fig. 2 shows an example of a temperature sensing circuit using a thermistor. The thermistor is connected in series with a resistor, and a constant voltage is applied. Fig. 3 shows the relationship between the divided voltage potential and the temperature of the thermistor.
A very large voltage change can be achieved in a wide temperature range. This voltage change is treated as temperature information. More specifically, if the circuit is directly connected to the analog-digital (A/D) port of a microcontroller to carry out A/D conversion, then the A/D values can be treated as temperature information by the logic of the microcontroller. For example, a warning can be output at a certain temperature, simply by programing the microcontroller so that it outputs a warning when detecting the A/D value corresponding to the temperature.
What should be noted is the large voltage change. It can be noticed that there is no amplifier in the stage preceding the A/D converter (ADC) in the circuit diagram in Fig. 2. Not only temperature sensors, but also other sensors used in electronic devices generally output a very weak signal, requiring an amplifier (signal amplification circuit). A thermistor is one of the few types of sensors that do not need any amplifier.
Meanwhile, looking into the resolution of the ADC, as shown in Fig. 2, if it is assumed that the voltage applied to the thermistor is the same as the voltage supplied to the ADC in the microcontroller and that the input range of the ADC is from 0 to 3V, then the quantization unit (LSB: least significant bit) is about 3mV with the resolution of the ADC at 10 bits.
Fig. 4 shows voltage change (gain) per unit temperature that can be obtained in the same temperature range as in Fig. 3 (-20 to +85°C). A gain of about 10mV/°C can be obtained even at the upper and lower limits of the temperature range, where the gain becomes minimal. At this time, one LSB is equivalent to about 0.3°C. It means a 10-bit ADC incorporated in a microcontroller can offer a temperature resolution of about 0.3°C. Of course, at room temperatures, a gain of 30mV/°C or more can be obtained, so one LSB is 0.1°C or less.
The use of a thermistor makes it possible to easily build a temperature sensing circuit out of simple circuits by using a standard ADC incorporated in a microcontroller. This is why thermistors are widely used to sense temperatures in electronic devices.
High-Precision Temperature Measurement
So, how accurately can general thermistors or resistors measure temperatures? The graph in Fig. 3 shows voltage and temperature characteristics obtained when a thermistor and a resistor that both have a resistance tolerance of ±1 percent are used. The central value of obtained voltages and the upper and lower limits of voltages calculated from the maximum tolerances of the components, etc. as shown with the thin lines are plotted. As differences are almost invisible, the results of temperature conversion of the upper and lower limits obtained with the central value at zero are graphed in Fig. 5.
This shows that about ±1°C and ±1.5°C of error is produced at +60°C and +85°C, respectively. It means that sufficient temperature measurement accuracy is achieved to monitor temperatures in electronic devices such as circuit board temperatures.
Components and circuits used are simple. At this stage it can be understood how high the cost performance is.
Murata Manufacturing's Design Support Tool
Murata Manufacturing's design support tool, SimSurfing, was used for the calculations and graphs mentioned above [Fig. 6].
When designing a temperature sensing circuit, it is difficult to think about what kinds of voltage changes can be obtained from temperatures.
SimSurfing allows the user to select the constants of thermistors and resistors, and circuits that use them, by intuitive operation, and to check obtained voltage changes and expected temperature error levels on graphs. Also, it can save all results of calculations as text data in steps of 1ºC, allowing the designer to continuously review these results on his or her own circuit simulator or spreadsheet software. Moreover, it has the function of calculating an approximation formula for temperature and voltage characteristics to obtain temperatures from obtained voltage and temperature characteristics, or reversely from voltages. It should be used to carry out voltage-to-temperature conversion by programmed calculation.
Why are Thermistors Selected?
The sections above describe flexibility in the location of thermistors, their potential of being further reduced in cost and size, their simple circuits, and accuracy expected from them.
In fact, proper efforts are needed to utilize a thermistor, including verification of the coherency between temperature information from the thermistor and the condition of the electronic device, and optimization of the thermistor along with the surroundings of the ADC. However, once these are introduced, the advantages described above can be used for a long time in the future.
Murata Manufacturing will help electronic device designers with temperature sensing in the coming 5G era by providing high-performance thermistors and the design support tool mentioned above, sensor-related thermal design support, and others.
About this Article:
This article is provided by Murata Manufacturing Co., Ltd.