Thermistor NTC Applications for Effective Inrush Current Limiting

Thermistor NTC Applications in Inrush Current Limiting

Introduction

NTC (Negative Temperature Coefficient) thermistors are crucial components in many electronic devices, known for their ability to effectively manage inrush current. This article will explore various applications of thermistor ntc in inrush current limiting, particularly in Switch Mode Power Supplies (SMPS), AC-DC power modules, DC-DC converters, and industrial inverters.

Applications :

1. Inrush Current Limiting in Switch Mode Power Supplies (SMPS)

Switch Mode Power Supplies (SMPS) are popular due to their compact size, lightweight design, and high performance. When an SMPS is powered on, it experiences a high peak inrush current as the filter capacitor charges. This inrush current can reduce the lifespan of the capacitor, damage the power switch contacts, or destroy rectifying diodes, necessitating effective countermeasures.

NTC thermistors are widely used to limit the inrush current in SMPS, offering a low-cost and straightforward solution. Connecting an NTC thermistor even after the rectifier circuit yields similar benefits.

2. Inrush Current Limiting in AC-DC Power Modules

Power modules, integrating various power circuits and peripheral components, are termed built-in power supplies. AC-DC power modules consist of AC-DC rectifier circuits, DC-DC converters, and minimal external components, forming a space-saving optimized power system. An NTC thermistor effectively limits the inrush current applied to the input and output capacitors when powering on.

Figure : NTC thermistor for Inrush Current Limiting in AC-DC Power Modules

3. Inrush Current Limiting in DC-DC Converters

In DC power circuits like DC-DC converters, NTC thermistors serve as power thermistors to limit inrush current during the initial power-on phase when input and output capacitors are charging. The resistance of the NTC thermistor drops significantly after power-on, reducing power loss compared to using a fixed resistor.

Figure : Negative Temperature Coefficient NTC Thermistor for Inrush Current Limiting in DC-DC Converters

4. Inrush Current Limiting in Industrial Inverters

Induction motors, commonly used in factories, large facilities, and office buildings, drive fans, pumps, air conditioners, and other equipment. These motors, while simple and stable, have speeds dependent on frequency and require inverters for speed control. Motors with inverters, known as variable speed drives (VSDs), significantly reduce power consumption.

An inverter system comprises a converter section, an inverter section, and a DC link capacitor (filter capacitor) after the converter. At startup, the device charges, and an inrush current several times larger than the steady-state current fills the DC link capacitor. This inrush current can adversely affect the lifespan of the DC capacitor or damage semiconductor devices. To prevent this, an NTC thermistor (power thermistor) is connected.

Conclusion

NTC thermistors are essential in managing inrush current across various applications, including SMPS, AC-DC power modules, DC-DC converters, and industrial inverters. Their ability to effectively limit inrush current ensures the longevity and reliability of electronic components, making them indispensable in modern electronic systems. By integrating NTC thermistors, engineers can enhance the performance and safety of power systems.

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Author: Ivan Huang 

Updated on 9th June 2025