Application Analysis of NTC Thermistor in Limiting Current Surge

　　At present, considering the factors such as volume and cost, most AC/DC converter input rectification filtering adopts capacitive input filtering method.

　　Since the voltage on the capacitor cannot jump, when the rectifier is powered on, the voltage of the filter capacitor is almost zero, which is equivalent to a short circuit at the rectifier output. If the power is turned on in the worst case (the instantaneous value of the voltage at power-up is the peak value of the supply voltage), an input inrush current much higher than the normal operating current of the rectifier will be generated. When the filter capacitor is 470μF and the internal resistance of the power supply is small, the first current peak value will exceed 100A, which is 10 times the normal operating current peak value.

　　The surge current will cause the power supply voltage waveform to collapse, make the power supply quality worse, and even affect the work of other electrical equipment and the action of the protection circuit; due to the surge current impacting the input fuse of the rectifier, it will be powered on several times. Under the impact of surge current rather than overload fuse. In order to avoid this kind of phenomenon, a fuse with a higher rated current has to be selected, but the fuse will not be blown when overloaded, and it will not protect the rectifier and the power circuit; Rectifiers and filter capacitors cause irreversible damage. Therefore, the input inrush current of the rectifier with capacitive filtering must be limited. Power-on inrush current limitation The most effective way to limit power-on inrush current is to add a negative temperature coefficient thermistor (NTC) between the rectifier and the filter capacitor, or on the input side of the rectifier.

　　The negative temperature coefficient thermistor has a high resistance value at room temperature to limit the power-on inrush current. After power-on, the resistance value is reduced due to the heating of the NTC flowing through the current to reduce the loss on the NTC. Although this method is simple, the problem is that the performance of limiting the power-on inrush current is affected by the ambient temperature and the initial temperature of the NTC. When the ambient temperature is high or the power-on time interval is very short, the NTC cannot limit the power-on. Therefore, this method of limiting the inrush current at power-on is only used for low-cost microcomputer power supplies or other low-cost power supplies. In color TVs and monitors, a current limiting resistor is used to limit the power-on inrush current.

　　The most common application is color TV. The advantages of this method are simplicity, high reliability, and allow operation in a wide ambient temperature range. In fact, after the rectifier is powered on and works in a steady state, the current limiting function of this current limiting resistor has been completed, and it only plays a negative role in power consumption and heat generation. Short-circuit the current limiting resistor with a mechanical contact or an electronic contact after a certain delay. This method of limiting the power-on surge current has good performance, but the circuit is complex and occupies a large volume.

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