Power thermistor (NTC) selection.Radar Microwave Induction Sensor Switch

　　Tian will explain to you the selection and application of power thermistor (NTC) in switching power supply. In the design of switching power supply, the power thermistor (NTC) is the most common. The power thermistor (NTC) is a resistor with a negative temperature coefficient, and its resistance value decreases with the increase of temperature. The function is to suppress the surge current, and it is generally connected in series with the mains input. It has a rated zero-power resistance value, and when connected in series in the power loop, it can effectively suppress power-on inrush current and consume almost negligible power. Usually, when the switching power supply is turned on, there will be a high peak inrush current to charge the filter capacitor, thereby charging the device. These inrush currents will affect the service life of the capacitors and damage the contacts of the power switch or destroy the rectifier diodes. Therefore, it is necessary to take corresponding measures. This chapter mainly summarizes the selection and application of power thermistors (NTCs).

　　In the switching power supply, the main parameters of the power thermistor (NTC):

　　1. Rated zero-power resistance (R25 ): Also known as the nominal resistance value, unless otherwise specified, it refers to the resistance value of the power NTC thermistor measured at an ambient temperature of 25°C. Commonly used resistance values are 2.5Ω, 5Ω, 10Ω, etc. The commonly used resistance errors are: ±15%, ±20%, ±30%, etc.

　　2. Maximum steady-state current (A): The maximum current that can be continuously applied to the power NTC thermistor at the nominal ambient temperature.

　　3. Maximum allowable capacitance (joule energy) (UF): the maximum allowable capacitance value of a capacitor connected to a power NTC thermistor under load.

　　4. Operating temperature range (℃): The ambient temperature range that the power NTC thermistor can work continuously under zero power state, which is determined by the upper limit category temperature and the lower limit category temperature.

　　Briefly introduce the function and selection of power thermistor (NTC) in suppressing surge current in switching power supply:

　　1. Selection of R25 resistance value of power NTC thermistor.

　　The maximum starting current value allowed by the circuit determines the resistance value of the power NTC thermistor.

　　Assuming that the rated input of the power supply is 220VAC, the internal resistance is 1Ω, and the maximum allowable starting current is 60A, then the minimum resistance value of the selected power NTC in the initial state is: Rmin=(220×1.414/60)-1=4.2( Ω)

　　For this application, we recommend that the resistance value of R25 of the power NTC thermistor is ≧4.2Ω.

　　2. Selection of the maximum steady-state current of the power NTC thermistor.

　　The selection principle of the maximum steady-state current should satisfy: the actual working current of the circuit < the maximum steady-state current of the power NTC thermistor.

　　Many power supplies are of wide-voltage design (AC 85V-264V), but the power of the product is fixed, so it should be noted that when the input voltage is low, the operating current is much higher than when the input voltage is high.

　　According to the formula: P=U*I , under the same power conditions, such as when the input voltage is 85V, the working current is 3 times that when the input voltage is 264V. Therefore, the actual working current of the circuit is calculated at the lowest voltage.

　　3. Selection of the maximum allowable capacitance (joule energy) of the power NTC thermistor.

　　For a certain type of power NTC thermistor, the size of the filter capacitor that is allowed to be connected is strictly required, and this value is also related to the maximum rated voltage.

　　The power-on surge is caused by the charging of the capacitor, so the capacity of the power NTC thermistor to withstand surge current is usually evaluated by the allowable access capacitance under a given voltage value.

　　For a specific power NTC thermistor, the maximum Joule energy that can be tolerated has been determined.

　　Joule energy calculation formula of power NTC thermistor: E=(1/2)*C*(U^2)

　　It can be seen from the above formula that the allowable access capacitance value is inversely proportional to the square of the rated voltage. Simply put, the larger the input voltage, the smaller the maximum capacitance allowed to be connected, and vice versa.

　　The specifications of power NTC thermistor products generally define the maximum capacitance value that is allowed to be connected under 220VAC.

　　Assume that the maximum rated voltage of an application condition is 420VAC, and the filter capacitor value is 200μF. According to the above energy formula, it can be calculated that the equivalent capacitance value under 220VAC should be: 200×(420)2/(220)2=729μF, so when selecting the model, it is necessary to select the power with a capacitance value greater than 729μF under 220VAC Type of NTC thermistor.

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