Murata’s solution “Thermistor”
“Thermistor” for safe electronic circuit
Unique advantages from superb material technology
Mr. Kingo Ohmura
Chief of thermistor products section, Sensor business division, Murata Manufacturing Co., Ltd.
When excess current flows in electronic circuit and device, temperature can surge to an extreme level and may lead to serious problems. A thermistor is one of the important devices that can prevent such problems. Murata Manufacturing Co., Ltd. carries a variety of in-vehicle use thermistors to respond the demands from auto and in-vehicle equipment manufacturers. Murata is actively developing unique and effective products such as the industry’s smallest parts and the ones that have an easy-to-use features for circuit protection.
The thermistor is one of thermal sensitive resistors in which a resistor changes depending on the ambient temperature. In general, there are 2 types of thermistors. One is a PTC thermistor (Positive Temperature Coefficient thermistor) in which the resistance rises with temperature. Another is a NTC thermistor (Negative Temperature Coefficient thermistor) in which resistance drops with temperature. (See Fig-1) For PTC thermistor, its resistance increases not only when ambient temperature increased but also when large current flows leads to heat generation in the resistor itself. For this characteristic, PTC thermistor is utilized in an over current protection circuit or a temperature detection circuit. “Resistance of PTC thermistor returns to original low value when temperature drops. It is a useful device for a reversible fuse.” (Mr. Kingo Ohmura, Chief of thermistor products section, Sensor business division, Murata Manufacturing Co., Ltd.) Similarly, a NTC thermistor is also utilized in a temperature detection circuit, temperature compensation circuit, in rush current protection circuit, and etc. Both types of thermistors have a broad application field ranging from consumer electronic equipments, AV equipments, laptop PCs, to mobile phones. Also in the automobile field, with the advancement of digitalization, the need for thermistors is on the rise (See Fig-2).
Fig-1. Basic characteristics of PTC thermistor (POSISTOR®) and NTC thermistor(click to enlarge)
Murata develops both PTC and NTC thermistors as in-vehicle use and is preparing products with optimized characteristics in mainly 3 applications: over current protection, over heat detection and temperature detection. Furthermore, the company carries both chip type and lead type depending on its application. Precisely, Murata has the PRG21 series (0805(EIA) size, 2.0 x 1.25mm) a chip type and the PTGL series a lead type for over current protection. For over heat detection, the PRF18 series (0603(EIA)size, 1.6x0.8mm) a chip type is recommended. For temperature detection , the NCP18 series (0603(EIA) size, 1.6 x 0.8mm), NCP15 series (0402(EIA) size, 1.0 x 0.5mm) both chip type and the NTS series a lead type are available. Among them, the over current protection and over heat detection series are PTC thermistors, and the temperature detection series is a NTC thermistor. “Each product guarantees the function under the required operation temperature of -40°C to +125°C as in-vehicle application and is compatible with the reliability standard for in-vehicle use components, “AEC-Q200”” (Mr. Ohmura)
Fig-2. Main applications of thermistor in vehicle (click to enlarge)
PTC thermistor (POSISTOR®) with many unique advantages
Murata offers a PTC thermistor with a registered trademark of “POSISTOR®”. This thermistor has many unique features. One of these features is stable operation. “POSISTOR® uses ceramic material, Murata’s competitive edge. Other PTC thermistor uses organic material. In general PTC thermistor using ceramic material is superior to the organic ones in characteristic reproducibility. That means since resistance to temperature characteristics is stable, the protection circuit can operate as designed without major changes over time.” (Mr. Ohmura)
Fig-3. Resistances at room temperature and at detection temperature match one another
Murata has added unique features to each series of POSISTOR®. For example, PRF series for overheat detection has 2 major noteworthy features. One is that relationship between resistances at room temperature and at detection temperature is constant and independent of detection temperature. (See Fig-3) PRF series’ feature is rapid resistance increase when device temperature reaches a certain detection value. Products with various detection temperatures are available. Designer can select any product with their own detection temperature choice. For PRF series, all products have a fixed resistance at normal temperature (about 470Ω). Moreover, resistance at respective detection temperature is also fixed (4.7kΩ). “That is, even if designers changed the POSISTOR® used in order to change the detection temperature, they don’t need to change the design of peripheral circuit.” (Mr. Ohmura)
Another notable feature of the PRF series is that temperature characteristics of resistance are very steep and large. Once device temperature reaches detection temperature, resistance increases abruptly at around that temperature. “If we construct a voltage dividing circuit using a fixed resistor and a POSISTOR® with a desired detection temperature, we can realize a circuit that determines the overheating by setting a threshold of output voltage of this circuit. Unlike a circuit using a conventional temperature sensor, it is not required to process the output signal using an A/D converter circuit.” (Mr. Ohmura) With this feature it is possible to rationalize an overheating detection circuit that detects status at multiple points. (See Fig-4) As above described, we can construct a voltage dividing circuit using a fixed resistor and POSISTOR®, and connect multiple POSISTOR® in series by number of detection points. By doing so, if any one of these series-connected POSISTOR® detected temperature rise, output voltage will increase.
Fig-4. Since resistance to temperature characteristics is steep around the detection temperature, it is possible to use multiple POSISTOR® in series.(click to enlarge)
In this case, although output voltage at normal temperature becomes somewhat higher as the result of that multiple POSISTOR® were connected in series, the temperature detecting function can be realized without any problem. Since resistance of the POSISTOR® increases sharply at detection temperature, it does not need to add a detection circuit to each temperature sensor like the overheat detection circuit that is conventionally used in PCs, or to use a complex detection circuit that processes outputs from multiple temperature sensors simultaneously. (See Fig-5)
In lead type POSISTOR®, “PTGL_SAR series” for overcurrent protection, Murata precedes miniaturization and resistance reduction at normal temperature level. For example, the company offers a product of as small as size of 6.5 (width) x 10.0 (height) x 3.5mm (thickness) and resistance as low as 0.8Ω at normal temperature (+25°C). A POSISTOR® used for over current protection is inserted to circuits in series. So, it is desirable to reduce the resistance at normal temperature as low as possible to avoid voltage drop. But, in general, if we simply reduce the device size, resistance becomes higher. “We have developed new material with low resistance utilizing our own ceramic technology.” (Mr. Ohmura) Especially, since growing number of electronic equipments is installed in vehicles, for effective utilization of the restricted in vehicle space, the demands for smaller and thinner in-vehicle equipments are high. POSISTOR® that is featured with small size and excellent characteristics is an advantageous device for designers who are facing such problems. “POSISTOR® is getting high reputations from designers of in-vehicle equipments.” (Mr. Ohmura)
Murata has already prepared a product using a conductive adhesive agent
Fig-5. A circuit that can monitor multiple points with a single path(click to enlarge)
As for NTC thermistor, Murata has been providing designers with various advantages by developing unique features. For example, the company is preparing a product type matching its resistance to temperature characteristics between NCP15 series and NCP18 series that have the same chip type but differ in package size. By utilizing such products, it is possible to replace a component with a smaller one without changing the design of the peripheral circuit. That means it becomes possible to reduce the PCB size by replacing a component with a smaller size product leaving the existing circuit design unchanged
In an early stage, Murata has offered a product using a conductive adhesive agent in chip type NTC thermistor that is critical for automobile and in-vehicle equipment manufacturers. In mounting a chip type component using solder under high temperature or thermal shock test, solder can be brittle or electrode part of component can be damaged because of thermal expansion or thermal shrinkage of solder.
Fig-6. NTC thermistor enables to control in continuous temperature change (click to enlarge)
Conductive adhesive agent can prevent component or bonding part from being damaged by reducing the stress caused by thermal expansion or shrinkage. To optimize bonding strength in using a conductive adhesive agent, Murata has developed a new product with a noble metal external electrode instead of Ni/Sn plated one. “This technology is deployed horizontally at Murata and has already been adapted to in-vehicle use monolithic ceramic capacitor or ferrite beads.” (Mr. Ohmura)
Murata will continue to lead the development of in-vehicle use thermistors in accordance with the foreseeable market demands. “For the PTC thermistor (POSISTOR®), we will improve the accuracy of temperature detection and advance miniaturization. As for the NTC thermistor that is often utilized in temperature control circuit, we will minimize deviation of resistance to detection temperature, one of our major challenges. In addition we are planning to prepare products that expanded the maximum operation temperature to 150°C in all types.” (Mr. Ohmura)
*: POSISTOR® is the trademark of Murata Manufacturing Co., Ltd.
