What is an NTC Thermistor?
Negative Temperature Coefficient Thermistor (NTC) is a semiconductor device made of metal oxides such as manganese, cobalt, and nickel through a ceramic process. Its core characteristic is an exponential decrease in resistance with increasing temperature, which is determined by the energy band structure and carrier behavior within the material.
Advantages: high sensitivity, fast response, small size, and low cost.
Disadvantages: non-linear characteristics require calibration, and long-term stability may be affected by the environment.
What are NTC thermistors made of?
Material composition: manganese, cobalt, nickel oxide-based, part of the use of silicon carbide or tantalum nitride, and other non-oxide materials.
NTC thermistors are mainly made of metal oxides such as manganese (Mn), nickel (Ni), cobalt (Co), and copper (Cu), and are manufactured using a ceramic process. These metal oxides are combined through a specific process to form thermistors with negative temperature coefficient characteristics.
Process: Made by ceramic processes such as mixing, molding, and sintering, the resistance characteristics can be adjusted by the composition ratio and sintering conditions.
What is the role of an NTC thermistor?
NTC (Negative Temperature Coefficient) thermistor is a semiconductor device whose resistance value decreases as the temperature increases, and its core roles can be summarized in the following three categories.
- Temperature Measurement
NTC thermistors are widely used as temperature sensors due to their high sensitivity and wide temperature measurement range (usually -10°C to +300°C, some models can be higher). Example:
Internal temperature monitoring of smartphones to prevent overheating damage to precision components.
Temperature sensing and control of household appliances (e.g. air conditioners, kettles);
Industrial temperature measurement field, compared with platinum RTDs or thermocouples, has the advantages of low cost and a simple circuit. - Temperature compensation
Utilizing its negative temperature characteristics, an NTC thermistor can compensate for the parameter drift in the circuit due to temperature change. Typical applications include:
Temperature compensation circuits for precision instrumentation;
Temperature management of lithium battery packs to ensure the safety of charging and discharging. - Inrush current suppression
In power supply circuits, NTC thermistors limit the inrush current at the moment of power-on through a high initial resistance to protect components such as capacitors and rectifier diodes. Subsequently, the resistance drops due to self-heating and the power consumption is negligible. Commonly used:
Input circuits of switching power supplies and UPS power supplies;
Electronic energy-saving lamps and ballasts.
Other characteristics support: NTC thermistor life and stability is the key performance indicators, especially in harsh environments (such as high temperature, high humidity) needs to work reliably for a long time. Its materials are mostly metal oxide ceramics, such as manganese, cobalt, nickel, etc. The semiconductor characteristics are realized through the spinel structure.
Principle of operation of NTC thermistors
Semiconductor Mechanism: When the temperature rises, the concentration of free electrons and holes in the semiconductor increases, and the change in carrier mobility results in a decrease in resistance.
Comparison with metals: Metal resistance increases with temperature (due to increased lattice vibration), while NTC thermistors exhibit the opposite characteristic.
NTC Thermistor Specifications
The main parameters of a power type thermistor (NTC) in a switching power supply:
1, rated zero power resistance (R25 ): also called the nominal resistance value, in the absence of special instructions, refers to the power type NTC thermistor in the 25 ℃ ambient temperature, measured resistance value. Commonly used resistance values are 2.5Ω, 5Ω, 10Ω, etc., commonly used resistance error is: ±15%, ±20%, ±30%, etc. .
2, the maximum steady state current (A): in the nominal ambient temperature, can be continuously applied to the power type NTC thermistor at the maximum current value.
3、Maximum Allowable Capacitance (Joule Energy) (UF): The maximum allowable capacitance value of a capacitor connected with a power type NTC thermistor under load conditions.
4、Operating Temperature Range (℃): the ambient temperature range in which the power type NTC thermistor can continuously operate under zero power state, which is determined by the upper limit category temperature and the lower limit category temperature.
The role and selection of power type thermistor (NTC) to suppress inrush current in switching power supply:
- Power type NTC thermistor R25 resistance value selection.
The maximum allowable starting current value of the circuit determines the resistance value of the power type NTC thermistor.
Assuming that the power supply rated input is 220VAC, internal resistance of 1Ω, the maximum allowable starting current of 60A, then select the power type NTC in the initial state of the minimum resistance value is: Rmin = (220 × 1.414/60) - 1 = 4.2 (Ω)
For this application, we suggest choosing the power type NTC thermistor with R25 resistance value ≧4.2Ω. - Power type NTC thermistor's maximum steady state current selection.
The principle of maximum steady state current selection should be satisfied: the actual operating current of the circuit < the maximum steady state current of the power type NTC thermistor.
Many power supplies are wide voltage design (AC 85V-264V), but the power of the product is fixed, so be careful in the low voltage input; the operating current is much higher than the high voltage input.
According to the formula: P=UI, under the same power condition, for example, at the input voltage of 85V, the working current is 3 times higher than that at the input voltage of 264V. Therefore, the actual operating current of the circuit to the lowest voltage when the calculation shall prevail. - power type NTC thermistor maximum allowable capacitance (Joule energy) selection. For a certain type of power NTC thermistor, the size of the allowed access to the filter capacitance is strictly required; this value is also related to the maximum rated voltage. Power-on surges are generated by capacitor charging, so the allowable capacitance at a given voltage value is usually used to evaluate the ability of a power NTC thermistor to withstand surge currents. The maximum Joule energy that can be withstood has been determined for a specific power type NTC thermistor. Joule energy calculation formula for power type NTC thermistor: E =(1/2)C*(U^2)
From the above formula, it can be seen that the capacitance value of its allowed access is inversely proportional to the square of the rated voltage. Simply put, the larger the input voltage, the smaller the maximum capacitance value allowed to be accessed, and vice versa.
The specifications for power NTC thermistors generally define the maximum capacitance allowed under 220VAC.
Assuming that the maximum rated voltage of an application is 420VAC, the filter capacitance value is 200μF. According to the above energy equation, the equivalent capacitance value under 220VAC can be converted to: 200×(420)2/(220)2=729μF, so that when selecting a model, you must select the power NTC thermistor with the permissible capacitance value of more than 729μF under 220VAC. This means that the power type NTC thermistor with capacitance greater than 729μF under 220VAC must be selected.
Precautions for the application of power-type thermistor
- From the analysis of the circuit operating principle, we can see that in the normal operating condition, there is a certain current through the power type NTC thermistor, the current tends to make the surface temperature of the power type NTC reach more than 100 ℃.
When the product is turned off, the power type NTC thermistor must be completely restored from the high-temperature low low-resistance state to the normal-temperature, high-resistance state to achieve the same surge suppression effect as the last time.
The recovery time is related to the dissipation coefficient and heat capacity of the power type NTC thermistor, and the cooling thermal time constant is generally used as a reference. The cooling thermal time constant is not the time required for the power type NTC thermistor to return to normal, but the larger the cooling time constant, the longer the recovery time required, and vice versa, the shorter it is. So, the power type NTC thermistor can not provide a good protection effect in the case of frequent switching. - Power type NTC thermistor is always connected in series in the protection circuit. If a power type NTC thermistor can not suppress the inrush current alone, it can be connected in series with two or more power type NTC thermistors in the circuit. Connecting two or more power NTC thermistors in parallel is not desirable because the load is not evenly distributed. If one of the power NTC thermistors passes a higher current than the other power NTC thermistors connected in parallel, it will become hotter until it finally passes almost all of the current, and this current may end up damaging the power NTC thermistor while the other power NTC thermistors connected in parallel remain cool. Therefore, the power type NTC thermistor used for inrush current suppression can only be used in series in the protection circuit.
- In the actual application, it is recommended to try to make the power type NTC thermistor work in the rated operating temperature range, as exceeding the specified upper and lower temperature limits may cause power type NTC product failure or damage. Since the power type NTC thermistor is greatly affected by the ambient temperature, the maximum steady state current at room temperature (0～25℃) is generally given in the product specification. Under the highest or lowest operating temperature conditions, the rated current will be linearly derated to zero. Power type NTC thermistor products are not applied at room temperature (0～25℃), or due to the design or structure of the product itself, such as the power supply has some devices with large heat generation. When the ambient temperature is too high or too low, it must be derated according to the derating current curve.
Calculation formula: ITa=[1-(Ta-25)/(Tu-25)]×Imax
Where: ITa: current value A at ambient temperature; Ta: ambient temperature ℃, TU: maximum working temperature ℃
If the maximum ambient temperature is 60℃, the maximum operating temperature of the thermistor is 200℃.
ITa=[1-(60-25)/(200-25)]×Imax=80%Imax
According to the above calculation, when the ambient temperature is 60℃, the maximum operating current can only be selected as 80% of the nominal operating current. The maximum current derating curve of the power type NTC thermistor is shown below.
What is the difference between PTC and NTC thermistors?
The main differences between PTC thermistors (Positive Temperature Coefficient Thermistors) and NTC thermistors (Negative Temperature Coefficient Thermistors) lie in the way they respond to temperature changes, their material composition, application scenarios, and performance characteristics.
Response to Temperature Change
NTC thermistor: As the temperature rises, its resistance decreases, i.e., the resistance is inversely proportional to the temperature. This characteristic makes NTC thermistors perform well in temperature measurement and control, and they can respond quickly to temperature changes.
PTC thermistor: As the temperature rises, its resistance value increases, i.e., the resistance is directly proportional to the temperature. When the temperature exceeds its Curie temperature, the resistance value will increase dramatically, showing the self-recovery characteristics.
Material Composition
NTC thermistor: usually made of semiconductor materials, such as manganese, nickel, cobalt and other metal oxides. The conductivity of these materials increases at high temperatures, resulting in a decrease in resistance value.
PTC thermistors: Usually made of ceramic materials such as barium titanate (BaTiO₃), these materials show a sharp increase in resistance value above a specific temperature (Curie temperature).
Application Scenarios
NTC Thermistors: Commonly used for temperature measurement, temperature compensation, inrush current limitation, and overheat protection. Due to its fast response speed, it is suitable for applications that require a fast response to temperature changes.
PTC Thermistors: Commonly used for overcurrent protection, self-recovery fuses, heating elements and temperature control. When the temperature exceeds the set value, the resistance of the PTC thermistor increases dramatically, limiting the current and protecting electrical equipment.
Performance Characteristics
NTC thermistors: usually lower cost, suitable for mass production and cost-sensitive applications. Sensitive to humidity and chemicals, and may experience slight resistance drift with long-term use.
PTC Thermistors: Higher cost, but worth the investment in applications requiring high reliability and safety. Offers better long-term stability and longer life, and can maintain stable performance under harsh conditions.
NTC thermistors are used in which places？
NTC thermistors (Negative Temperature Coefficient thermistor) are widely used in a variety of electronic equipment, mainly for temperature detection, temperature compensation and overheating protection.
Temperature Detection and Compensation
NTC thermistors are commonly used in various electronic devices for temperature detection and compensation. For example:
Smart phones and tablet PCs: used to detect and compensate the temperature of CPU and power module to ensure stable operation of the device.
Mobile device battery charging: to monitor the battery temperature and prevent overheating.
Microcontrollers: to monitor the temperature of microcontrollers to ensure their stable operation.
LED Lighting System: Monitor the temperature of LED lights to prevent overheating.
Hard Disk Drive (HDD): Monitor the temperature of the HDD to ensure its stable operation.
Crystal Oscillator and Semiconductor Pressure Sensor: Maintains its operational stability through temperature compensation.
Overheat Protection
NTC thermistors are also commonly used for overheating protection to prevent equipment from being damaged due to overheating. Example:
Batteries for mobile devices: Prevent safety issues caused by overheating of the battery by monitoring the battery temperature.
Thermal printers: Monitor the temperature of the print head to prevent damage from overheating.
Specific application examples
Examples of specific applications for NTC thermistors include:
Smartphones and tablets: Built-in multiple NTC thermistors are used for temperature detection and compensation to ensure stable operation of the device in high-temperature environments.
Battery charging of mobile devices: monitoring the battery temperature to prevent overheating during charging.
Microcontrollers: Monitor the temperature of microcontrollers to prevent them from failing due to overheating.
LED Lighting System: Monitor the temperature of the LED lights to prevent flickering or damage to the lights caused by overheating.
Hard Disk Drive (HDD): monitors the temperature of the HDD to prevent it from damaging data due to overheating。