How to use the inrush current limiter NTC

When starting electronic equipment, such as switching power supplies (SMPS) or inverters, transient abnormal currents with high peak values ​​pass through the equipment. This current is called magnetizing inrush current, and if there is no protection circuit, it may damage the semiconductor device or affect the service life of the smoothing capacitor.

When starting electronic equipment, such as switching power supplies (SMPS) or inverters, transient abnormal currents with high peak values ​​pass through the equipment. This current is called magnetizing inrush current, and if there is no protection circuit, it may damage the semiconductor device or affect the service life of the smoothing capacitor. When NTC thermistor is used as an ICL (excitation inrush current suppressor), it can conveniently and effectively protect the circuits of electrical and electronic devices from the influence of excitation inrush current.

How to use the inrush current limiter NTC (1)

Advantages of NTC thermistors

An NTC thermistor is a temperature-dependent resistor made from a special semiconductor ceramic with a negative temperature coefficient (NTC). They have high resistance at room temperature, generate heat when energized, and the resistance decreases as the temperature increases. Due to this characteristic, they are used as current protection devices for electrical and electronic equipment, which can conveniently and effectively limit abnormal currents, including magnetizing inrush currents when energized. NTC thermistors used as current protection devices are also called power thermistors.

Fixed resistors or NTC thermistors can be used to limit the field inrush current. However, fixed resistors always result in power loss and performance degradation. NTC thermistors limit the excitation inrush current with a high initial resistance, and then as the temperature increases due to energization, the resistance drops to a few percent of room temperature levels, thereby achieving lower power losses than using fixed resistors. In other words, the effect of limiting the excitation inrush current with an NTC thermistor is greater than using a fixed resistor with the same initial power loss.

The following is an example of the application of NTC thermistor in limiting excitation inrush current.

Application 1: Excitation inrush current limitation in switching power supplies (SMPS)

Various switching power supplies (SMPS)—which are small, lightweight, and high-performance—are commonly used as power supplies for electronic equipment. When the SMPS is energized, the excitation inrush current with a high peak value will charge the smoothing capacitor, thereby charging the device. Since this excitation inrush current will affect the service life of the capacitor and damage the contacts of the power switch or destroy the rectifier diode, it is necessary to take corresponding countermeasures.

As shown in the figure below, limiting the excitation inrush current of SMPS by inserting an NTC thermistor is widely used as a low-cost and simple circuit to limit the excitation inrush current in power supplies. Even if you connect the NTC thermistor after the rectifier circuit, you can get the same result.

Figure 1: Magnetizing inrush current limitations in switching power supplies

Application 2: Excitation inrush current limitation in AC-DC power modules

A built-in power supply that compactly integrates various power circuits and peripheral circuits is called a power module. The AC-DC power module is a power supply composed of an AC-DC rectifier circuit, a DC-DC converter and a small number of external components, which can form a space-saving optimized power supply system. Inserting an NTC thermistor (power thermistor) can effectively limit the magnetizing inrush current applied to the input and output capacitors during power-up.