How to Apply NTC Thermistor to Suppress Surge Current in Lighting System.PMMA lens Manufacturing

　　The lighting industry continues to promote inductive loads. The trouble is that the inductive reactance generated by the inductive load is opposite to the resistance of the system, which will reduce the efficiency of the system. Power factor correction PFC can solve the above problem. However, when the PFC is initially charged, it will generate inrush current that damages other circuits in the system, and through the use of thermistors, the inrush current can be effectively suppressed to avoid circuit damage.

　　At the heart of a surge limiting circuit is a high resistance. Placing resistors in a circuit limits the capacitance that a capacitor can achieve. However, once the capacitor is charged, if the resistor is left in the circuit, it will continue to lose thermal energy and will reduce the overall efficiency. Basically, once the inrush current is limited, the switch can be used to bypass the resistor.

　　The most efficient way to handle inrush current is to use a thermistor. A thermistor is a special variable resistor whose resistance depends on temperature. Negative Temperature Coefficient (NTC) thermistors, for example, can dramatically and predictably decrease their resistance as their temperature rises.

　　To limit inrush current, place an NTC thermistor between the power supply and the PFC capacitor and inductive load capacitor (Figure 1). When turned on, the NTC thermistor temperature is low, so it can provide high resistance. In addition to limiting the current going into the capacitor, the thermal energy created by this high resistance will raise the temperature of the thermistor.

　　While the NTC automatically heats up, its resistance drops rapidly. While the inrush current levels off, the temperature of the NTC thermistor is sufficient to minimize resistance and allow current to flow without negatively impacting system operation or efficiency. In this way, the NTC thermistor effectively provides the resistance needed to limit the inrush current while eliminating the need for additional circuitry such as a bypass switch. With NTC thermistors, luminaires can protect lighting systems from inrush currents associated with PFC without the need for complex and expensive bypass circuits.

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