When the switching power supply is turned on, the voltage of the capacitor will not change suddenly, so a large charging current will be generated. This current is often referred to as input surge current, which is generated when the filter capacitor is initially charged. Although this surge current is short in time, if it is not suppressed, it will shorten the life of the input capacitor and rectifier bridge, and may also cause the voltage of the input power supply to drop, causing other power equipment using the same input power supply to lose power instantaneously, which will interfere with the normal operation of nearby equipment. There are many ways to suppress surge current. Generally, small and medium-sized power supply uses resistance current limiting method to suppress surge current of power supply. Taking thermal resistance NTC as an example, the function of thermal resistance in circuit is described.
Role of NTC thermistor in power supply circuit
(1) temperature measurement
As a thermistor sensor for measuring temperature, its structure is generally simple and its price is low. Thermistors without external protective layer can only be used in dry places; Sealed thermistors are not afraid of moisture and can be used in harsh environments. Because of the large resistance value of thermistor sensor, the resistance and contact resistance of its connecting wire can be ignored, so thermistor sensor can be used in long-distance temperature measurement as long as several kilometers, and the measuring circuit mostly adopts bridge circuit.
(2) Temperature compensation
The thermistor sensor can compensate the temperature of some components within a certain temperature range. For example, the moving coil in the moving coil meter head is wound by copper wire, and the temperature rises and the resistance increases, which causes the temperature error. Therefore, the thermistor with negative temperature coefficient can be connected in parallel with the manganese copper wire resistor in the moving coil circuit and then connected in series with the compensated components, thus offsetting the error caused by temperature change.
(3) Overheat protection
Overheat protection is divided into direct protection and indirect protection. For small current situations, thermistor sensors can be directly connected into the load to prevent overheating damage to protect devices, and for large current situations, they can be used to protect relays and transistor circuits.
(4) Liquid level measurement
When a certain heating current is applied to the NTC thermistor sensor, its surface temperature will be higher than the ambient air temperature, and its resistance value will be smaller at this time. When the liquid level is higher than its installation height, the liquid will take away its heat, which will lower its temperature and increase its resistance. Judging its resistance change, we can know whether the liquid level is lower than the set value. The oil level alarm sensor in automobile fuel tank is made by using the above principles.
