AC / DC adapter converts AC to DC, and its power and current flow can be bidirectional. The transformation of power current to load is called "rectification", and the transformation of power current from load to power supply is called "active inverter". The input of AC / DC adapter is 50 / 60Hz AC, which must be rectified and filtered. Relatively speaking, the filter electrolytic capacitor with large volume is essential. At the same time, due to the security problems, such as ulccee and other standards and the restrictions of EMC directives (such as IEC, FCC, CSA), the AC input must add EMC filter and use components that meet the security standards, which limits the further miniaturization of AC / DC adapter. In addition, due to the internal high-frequency, high-voltage and high current switch action, it is more difficult to solve the EMC problem, which puts forward high requirements for the design of internal high-density installation circuit. Due to the same reason, high voltage. The high current switch increases the power loss and limits the modularization process of AC / DC adapter. Therefore, the power system optimization design method must be used to make its work efficiency to a certain degree of satisfaction. AC / DC adapter can be divided into half wave circuit and full wave circuit according to the wiring mode of the circuit; it can be divided into uncontrollable, half controlled and full controlled according to the control characteristics of the circuit; it can be divided into single-phase, three-phase and multi-phase according to the number of power supply phases; it can be divided into one quadrant, two quadrants, three quadrants and four quadrants according to the working quadrant of the circuit.
AC / DC adapter has the following advantages
1.Small size and light weight. AC / DC adapter does not use bulky power frequency transformer. Because the power dissipation on the switch MOS transistor is greatly reduced, the larger heat sink is saved. Because of these two reasons, the AC / DC adapter is small in size and light in weight.
2.Low power consumption and high efficiency. In the AC / DC adapter circuit, the switching MOS transistor works alternately in the on-off and off-on switching state under the excitation of the excitation signal. The conversion speed is very fast, and the frequency is generally above 50 kHz. In some advanced switching power supply circuits, it can achieve several hundred or near MHz. This makes the power consumption of the switch MOS transistor very small, and the efficiency of the power supply can be greatly improved, and its efficiency can reach 90%.
3.The efficiency of the filter is greatly improved, and the capacity and volume of the filter capacitor are greatly reduced. At present, the working frequency of switching AC / DC adapter is basically above 50 kHz, which is more than 1000 times of that of linear regulated power adapter, so that the filtering efficiency after rectification is almost increased by 1000 times; even if half wave rectification is adopted and capacitor filtering is added, the efficiency is also increased by 500 times. Under the same ripple output voltage, when AC / DC adapter is used, the capacity of filter capacitor is only 1 / 500 ~ 1 / 1000 of that in linear regulated power supply.
4.Wide voltage range. The output voltage of AC / DC adapter is regulated by the duty cycle of excitation signal, and the change of input signal voltage can be compensated by frequency modulation or width modulation. In this way, when the power frequency network voltage changes greatly, it can still ensure a more stable output voltage. Therefore, the AC / DC adapter has a wide voltage range and good voltage stabilizing effect. In addition, there are two ways to change the duty cycle: pulse width modulation and frequency modulation. AC / DC adapter not only has the advantages of wide voltage range, but also has many ways to achieve voltage stabilization. Designers can flexibly choose various types of switching power supply IC chip schemes according to the requirements of practical application.