1. The maximum working current of the NTC thermistor> the working current of the actual power circuit

2. Nominal resistance value of power thermistor

　　R≥1.414*E/Im

where E is the line voltage Im is the surge current

For conversion power supply, inverter power supply, switching power supply, UPS power supply, Im = 100 times working current

For filament, heater, etc. loop Im = 30 times operating current

3. The larger the B value, the smaller the residual resistance and the smaller the temperature rise during operation.

4. Generally speaking, the greater the product of the time constant and the dissipation coefficient, the greater the heat capacity of the resistor, and the stronger the resistor's ability to suppress surge current.

? Maximum rated voltage and filter capacitor value

The maximum starting current value allowed by the product and the working current loaded on the NTC thermistor for a long time

Working environment of NTC thermistor

First look at the maximum rated voltage and filter capacitor value

The size of the filter capacitor determines the size of the NTC. For an NTC thermistor of a certain size, the size of the filter capacitor allowed to be accessed is strictly required, and this value is also related to the maximum rated voltage. In power supply applications, the power-on surge is caused by capacitor charging, so the ability of NTC thermistor to withstand surge current is usually evaluated by the capacitance allowed to be accessed under a given voltage value. For a specific NTC thermistor, the maximum energy that can be withstood has been determined. According to the energy consumption formula of the resistor in the first-order circuit E = 1/2 × CV2, it can be seen that the allowable access capacitance value is inversely proportional to the square of the rated voltage. Simply put, the greater the input voltage, the smaller the maximum capacitance value allowed to be accessed, and vice versa.

Secondly, the maximum starting current value allowed by the product and the working current loaded on the NTC thermistor for a long time

The maximum starting current value allowed by the electronic product determines the resistance value of the 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, the minimum resistance value of the selected NTC in the initial state is Rmin =(220 × 1.414/60)-1=4.2(Ω). At this point, there are generally one or more NTC thermistors that meet the conditions, and then the following method is selected.

When the product works normally, the current loaded on the NTC thermistor for a long time shall not be greater than the current specified in the specification. According to this principle, a suitable resistance value can be selected from a plurality of resistors with a resistance value greater than 4.2Ω. Of course, this refers to the case at room temperature. If the working environment temperature is not normal temperature, it is necessary to carry out the derating design of NTC thermistor according to the principle mentioned below.

Finally, the working environment of NTC thermistor is greatly affected by the ambient temperature. Generally, only the resistance value at normal temperature (25 ℃) is given in the product specification. If the application condition of the product is not at normal temperature, or the ambient temperature around the NTC thermistor is not at normal temperature due to the design or structure of the product itself, the resistance value of NTC in the initial state must be calculated before the selection of the above steps can be carried out.

When the ambient temperature is too high or too low, the derating design must be carried out according to the power consumption reduction curve provided by the manufacturer.

In fact, many manufacturers have defined the ambient temperature category for their products. In practical applications, the ambient temperature of the NTC thermistor should not exceed the upper/lower limit temperature specified by the manufacturer. At the same time, care should be taken not to make it work in a humid environment, because too humid environment will accelerate the aging of the NTC thermistor.

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