With the increasing reliability requirements of electronic products and the shrinking of energy resources, high reliability and energy-efficient electronic products will be a direction for the development of electronic products in the future. Therefore, in the power supply design of products, it must be fully considered. Reliability and power efficiency.
This paper first analyzes why electronic products have a boot surge, and then analyzes how to use a thermistor to suppress inrush current with a typical power supply circuit. Finally, how to choose the thermistor in practical applications.
Causes of starting surge current
Figure 1 shows a simplified circuit of a typical electronics power supply section. C1 is a filter capacitor in parallel with the load. At the moment of power-on, the capacitor voltage cannot be abruptly changed, thus generating a large charging current. According to the first-order linear non-homogeneous equation established by the first-order circuit zero-state response model, the current value obtained by short-circuiting the filter capacitor can be obtained. This current is what we often call the input surge current. It is generated when the filter capacitor is initially charged. The size depends on the amplitude of the input voltage when the power is turned on and the bridge rectifier and electrolytic capacitor. The total resistance of the loop.
In fact, many manufacturers have defined the ambient temperature category for their products. In practical applications, the ambient temperature at which the NTC thermistor should work should not exceed the upper/lower limit temperatures specified by the manufacturer. At the same time, care should be taken not to operate in a humid environment, as excessively humid environments can accelerate the aging of NTC thermistors.
in conclusion
It can be seen from the above analysis that the NTC thermistor type surge suppressor is used in the power supply design, and its ability to suppress the inrush current is equivalent to that of the ordinary resistor, and the power consumption on the resistor can be reduced by several tens to hundreds of times. For applications that require frequent switching, a relay bypass circuit must be added to the circuit to ensure that the NTC thermistor is fully cooled back to its original state. In product selection, the product series should be selected according to the maximum rated voltage and filter capacitor value. The resistance of the NTC thermistor should be selected according to the maximum starting current value allowed by the product and the operating current applied to the NTC thermistor for a long time. At the same time, it is necessary to consider the temperature of the working environment and appropriately derate the design.
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