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NTC thermistor used in EMI filter circuit

Power at work, Switching transistors will produce a lot of EMI and RFI,And this will seriously affect the normal work of other electronic equipment in the house.
So we have to ensure that the power supply is not affected by the input noise in the power grid and the impact of the peak voltage of the outgoing voltage. We need a two-way protection against this stage of the power supply.
        Noise, According to the conduction mode can be divided into two types:
Common Mode Noise (CMN) and Differential Mode Noise (DMN).

1. Common-mode noise (CMN) is in the use of AC power of electrical equipment input (transmission line and the center line) there is such noise, The two are in phase with each other. By placing an inductance in series with each of the power lines in the electromagnetic interference filter, And between the two transmission lines and the use of Y capacitor to connect, to be suppressed.

2. Differential mode noise (DMN) is from the power line and return through the center line of noise. Exist in the AC line and neutral wire.

EMI filter circuit location before the rectifier bridge, Because it is designed to filter the noise before the current passes through the rectifier bridge diodes. In the EMI filter current component, There must be two Y capacitors and two X capacitors, two solenoid coils, A MOV (varistor) and a fuse, These components are indispensable.
EMI filtering technology is an effective measure to suppress the tip pulse interference, Can filter out a variety of causes of conduction interference. Is a kind of capacitance, inductance composed of EMI filter,Connected to the input of the switching power supply. Tests show that, as long as the appropriate choice of components of the parameters, It can better inhibit the conduction power generated by switching power supply.
     Generally after the EMI filter circuit, Will be equipped with a thermistor NTC (full name: negative temperature coefficient thermistor, the higher the temperature the lower the resistance value) Usually used when the high current into the time. Protect the internal components of the power supply. Thermistor has its name, Is a resistor that controls the resistance of the resistor through the temperature level.

When the thermistor temperature is low, Resistance resistance is usually around 6-12 ohms. When the power is turned on, the resistor temperature rises, The resistance is about 0.5-1 ohms.

For high-performance power supplies, It will be equipped with a relay, In the power after the start will bypass the thermistor, Reduce the heat loss of electricity, For the power efficiency to enhance the play a certain help.
Technical guide: <What is a thermistor?>
EMI filter circuit design thermistor

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