Electronic devices are inseparable from the power adapter. The power adapter supplies the energy required by the electronic device, which determines the importance of the power adapter in the electronic device. The quality of the power adapter directly affects the working reliability of the electronic device, so the requirements of the electronic device for the power adapter are also increasing. Existing power adapters are mainly composed of two main categories: linear regulated power adapters (referred to as linear power adapters) and switching regulated power adapters (referred to as power adapters). These two types of power adapters are widely used due to their unique characteristics.

Linear power adapter

The composition block diagram of the linear power adapter is shown in the figure.

Figure Block diagram of linear power adapter

The linear power adapter is generally composed of four parts: voltage transformation, rectification, filtering and voltage stabilization.

Transformer-turns the AC grid voltage into the desired AC voltage. The transformation process is usually completed by a transformer, and some use capacitors for voltage reduction.

Rectification-to convert AC voltage to DC voltage. Rectifier circuits usually have half-wave rectifier circuits, full-wave rectifier circuits, bridge rectifier circuits, etc. Bridge rectifier circuits are more commonly used.

Filtering—filter out the AC components in the pulsating direct current (regular change in size) obtained by rectification. Commonly used filter circuits include capacitor filter circuit, inductance filter circuit and resistance-capacitance filter circuit.

Voltage stabilization-Stabilize the DC voltage output by the filter circuit, even if the output DC voltage does not change with changes in the grid voltage and load. Voltage stabilizing function can be completed by voltage stabilizing diode, DC/DC converter, series voltage stabilizing circuit, etc.

The advantages of the linear power adapter are good stability, high reliability, high output voltage accuracy, and low output ripple voltage. Its disadvantage is that it requires the use of power frequency transformers and filters. Their weight and volume are large, and the power consumption of the adjustment tube is large, which greatly reduces the efficiency of the power adapter. Under normal circumstances, the efficiency of the power adapter will not More than 50%. However, its excellent output characteristics make it widely used in occasions where the performance requirements of the power adapter are high.

Power Adapter

The power adapter is named because its control device works in the ON and OFF state. Its essence is to change the size of the output voltage by changing the on time of the control device in the circuit to achieve the purpose of maintaining the stability of the output voltage The schematic diagram of the power adapter and the input/output waveforms are shown in the figure.

The schematic diagram of the power adapter and the input/output waveform diagram in the figure, Ui is the unstable DC voltage after rectification; Uo is the chopped output voltage; S is the switch control device; RL is the load; T is the switch on and off cycle; Ton is The switch closing time is the on-time; Toff is the switch off time, the off time.

Compared with the linear power adapter, the power adapter can meet the requirements of modern electronic equipment. Since the invention of the power adapter in the mid-20th century, it has been widely used in computers, communications, aerospace, office and electrical equipment due to its outstanding advantages It is likely to replace the linear power adapter.

The power adapter has the following characteristics.

1. efficient.

The adjustment tube of the power adapter works in the switching state, and the size of the output voltage can be changed by changing the ratio of the conduction and cut-off time of the adjustment tube. When the adjustment tube is saturated and conductive, although there is a large current flowing through, the saturation tube voltage drop is very small; when the adjustment tube is cut off, the tube will withstand a higher voltage, but the current flowing through the adjustment tube is basically equal to zero. It can be seen that the power consumption of the adjustment tube working in the switching state is very small. Therefore, the efficiency of the power adapter is relatively high, generally up to 65% to 90%.

2. Small size and light weight.

Because the power consumption of the adjusting tube is small, the radiator can also be reduced accordingly. Moreover, the power adapter can also remove the 50Hz power frequency transformer, and the switching frequency is usually tens of kilohertz, so the capacity of the filter inductor and capacitor can be greatly reduced. Therefore, the power adapter is much smaller in size and weight than the linear power adapter of the same power.

3. Low requirements on grid voltage.

Because the output voltage of the power adapter is related to the ratio of the turn-on and cut-off time of the regulating tube, and the amplitude change of the input DC voltage has little effect on it, therefore, a large fluctuation of the grid voltage is allowed. The general linear voltage stabilizing circuit allows the grid voltage to fluctuate by ±10%, while the switching voltage stabilizing circuit can still work normally when the grid voltage is 140-260V and the grid frequency changes by ±4%.

4. The control circuit of the adjusting tube is more complicated.

In order to make the adjustment tube work in the switching state, a control circuit needs to be added. The pulse waveform output by the adjustment tube needs to be filtered by LC before being sent to the output end. Therefore, compared with the linear power adapter, its structure is more complicated and debugging is more troublesome.

5. Large ripple and noise components in the output voltage.

The adjustment tube works in the switching state, which will generate spike interference and harmonic signals. After rectification and filtering, the ripple and noise components in the output voltage are still larger than the linear power adapter.

In the future, the development of switching power supply adapters, in addition to continuing to maintain the existing advantages, is mainly to adopt new technologies and new technological measures to overcome some of its own shortcomings.