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ホーム > MEMS / sensing technology > Temperature measurement and circuit protection application of NTC

Temperature measurement and circuit protection application of NTC

更新時間: 2022-07-20 16:01:11

NTC sensor element is the basis for the function of NTC temperature sensor in various applications, in fact, not only in the temperature sensor but also very common in various other electronic products applications, such as reversible fuses and self-regulating heaters, etc.. As a semiconductor sensing element, the application of NTC can be broadly divided into two categories, one is for temperature measurement, and the other is for circuit protection.


Achieving high sensitivity and high precision temperature measurement


NTC thermistors have been well used for temperature measurement, where durability, reliability, and stability are important due to their high sensitivity and high accuracy. Although there are many materials for thermistors, semiconductor resistors such as NTCs are easier to process, smaller, and lighter than conductors such as metals. In addition, its fast response time is also suitable for small diameter precision devices.


In low-cost temperature measurement solutions, a pull-up resistor is typically connected between the NTC thermistor and the power supply's positive side. The ambient temperature change is determined by collecting the change in voltage division across the NTC thermistor. In most cases, a resistor of the same value as the NTC's room temperature resistance is connected in series. The current flow is ensured to be small enough to avoid self-heating to ensure the accuracy of the measurement process.


To avoid self-heating to ensure the accuracy of the measurement process, on the other hand, because the NTC thermistor is by nature a non-linear resistor, it must be linear before the temperature measurement and to ensure that the measurement is in the appropriate temperature range; otherwise, the results will be very different. Currently, it can generally cover the measurement from -50℃ to 150℃, and some industry-leading NTC manufacturers can cover -55℃ to 175℃. With a special coating, NTC thermistors have no problem covering up to 300°C.


The resistance temperature coefficient of NTC thermistors is reduced by 3 to 5% per 1°C. Compared to other metal resistance values that change by only a few percentage points or so per 1°C, it can be seen that NTC thermistors exhibit a large change in resistance even with small temperature changes. From these characteristics, it is easy to see that NTC thermistors can achieve sensitive detection of small temperature differences with high accuracy within a suitable temperature range.


NTC sensing elements.png


Chip type embeddable NTC thermistors can be integrated directly into IGBT modules and IPM modules, such as TDK's new chip type L860 NTC thermistors, which can be embedded directly into power supply modules without leads by sintering and heavy aluminum wire welding connections, completely different from conventional SMD NTC elements. We all know that power modules are usually most efficient when operating near their power limit, but the temperature must be precisely controlled to maintain the limit state operation. Compared to traditional NTC elements, the thermal coupling between the chip NTC thermal element and the power module is more stable. This means the element can achieve ultra-fast response and is more conducive to high-precision temperature measurement and control.


Whether in industrial, automotive, or consumer applications, monitoring the temperature of all power components is a critical and fundamental requirement to ensure the safety, reliability, and longevity of the entire system, in which NTC thermistors are widely used.


NTC suppresses surges and improves system reliability


In circuit protection control, NTC is also used in many applications. Generally, when the power is turned on, there will be a large current flow through the diode. If the current is too large diode may be damaged. Inrush current suppression methods are many, generally small and medium power supplies in the resistor current limiting approach to suppress the power-on inrush current.


Traditional fixed resistance current limiting resistor wasted power consumption in the resistor is no way to change. And in series with an NTC thermistor on top of the line, the resistance is relatively large before the power is on due to low thermistor temperature. It can be very good at limiting the inrush current at power on. After the power is on, the thermistor temperature rises, the resistance value decreases significantly, and it does not produce excessive losses.


ntc.png


This process happens very quickly in practical applications. The NTC thermistor heats up quickly as the temperature rises. Its resistance value drops rapidly to a very small level in milliseconds, typically only a fraction of an ohm to a few ohms in size. In contrast, the power lost in the resistor is reduced by tens or even hundreds of times.


Based on the NTC thermistor suppression approach power, high inrush current suppression capability, high reliability, long life, and small residual resistance. In general small power supply NTC without adding a relay, high-power need to add a relay can be. However, it should be noted that the use of NTC to suppress power-on surge power supply equipment, not to be able to switch on and off frequently, NTC cooling needs dozens of seconds to a few minutes, ranging from recovery to its cold state resistance before it can be turned on again.


Suppressing power-on surges to prevent electronic equipment damage to the NTC can be the simplest and most effective. Of course, the NTC only plays a role in power-on protection. If the circuit is working normally or short-circuits, NTC is no way to deal with the surge.


Final words


NTC thermistor is a typical class of temperature-sensitive semiconductor resistors. Its resistance value will decrease with the temperature rise. With the temperature change and change in the resistance of the characteristics of one hand, NTC thermistor element also has the advantages of small losses, almost no hysteresis phenomenon, but also can play a very good protection role.


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