The power adapter is the power supply of small electrical appliances. When in use, it can obtain the electric energy required by electrical appliances from the AC power supply (ac-220v ⓐ 50Hz or ac-110v ⓐ 60Hz). The adapter is essentially a switching power supply system (AC → DC converter). Just for the convenience of carrying and small volume, as shown in the figure, it is a complete schematic diagram of switching power supply.
Due to the inevitable transient effect in the circuit, each time the power grid load changes, the surge current with violent fluctuation will be caused at the AC input. Due to the accumulation, rapid release and continuous transfer and conversion of energy, the surge will be transmitted at dot1 and dot2 in the circuit, Generally, this surge current can reach more than ten times to dozens of times of the current of the whole adapter under steady-state operation. Its existence will often cause damage to the fuse, rectifier bridge, filter capacitor, adjusting tube and high-frequency transmitter in the circuit. Generally speaking, the greater the effective output power of the adapter and the higher the value of the filter capacitor C, the greater the strength of the generated surge circuit, the longer the duration of the surge current, and the greater the impact damage to the components in the circuit.
Theoretical analysis and long-term tests show that in the initial design and thermal effect analysis of such AC → DC circuit, selecting one or more power NTC thermistors with appropriate parameters at dot1 or dot2 of the circuit can effectively suppress the surge current more than ten to tens of times higher than the working current (steady-state current), It will greatly reduce the impact damage of surge current on key components in the circuit, effectively protect and prolong the service life of components, so as to improve the service life of the whole adapter.
The principle of selecting power NTC thermistor for adapter is as follows:
(1) The steady-state working current of power NTC thermistor must be greater than the maximum steady-state working current at dot1 and dot2.
(2) Most of the surge current in the power adapter is caused by the charging of the filter capacitor, so the maximum impact energy that the power NTC thermistor can withstand should be greater than the charging energy of the filter capacitor in the circuit, that is, the maximum capacitance corresponding to the power NTC thermistor should be greater than the capacitance of the filter capacitor in the power adapter circuit.
(3) Under the conditions of (1) and (2) above, selecting the resistance value of power NTC thermistor with large R25 value is conducive to absorbing large surge current.
(4) The negative temperature characteristic of the power type NTC thermistor will naturally convert the electric energy of the surge current into Joule heat energy, so the power type NTC thermistor will heat rapidly during operation. Therefore, when the PCB board device is arranged, try to stay away from other components to prevent the thermal impact of the heating of the power type NTC thermistor on other components, And take effective heat dissipation measures (such as increasing the heat dissipation area of the pad and copper clad wire of the component pin of the power NTC thermistor).
(5) Selecting the power NTC thermistor with large B value and low residual resistance under the maximum steady-state current is conducive to reducing power consumption, reducing the heating capacity of the power NTC thermistor, improving the conversion efficiency of the adapter, and improving the service life of the power adapter, which is often ignored by users.
(6) Because the adapter is generally small, the volume of power NTC thermistor is strictly limited. Under the same electrical parameters, the volume of power NTC thermistor should be as small as possible.
