I. Switching Power Supply
Unlike linear power supplies, switching power supplies utilize transistors that primarily switch between fully on (saturation region) and fully off (cutoff region) modes. Both modes are characterized by low dissipation. While the transition between modes involves higher dissipation, the time is very short, thus saving energy and generating less waste heat. Ideally, the switching power supply itself does not consume electrical energy. Voltage regulation is achieved by adjusting the on and off times of the transistors. Conversely, in a linear power supply, the transistors operate in the amplification region during the output voltage generation process, consuming electrical energy. The high conversion efficiency of switching power supplies is a major advantage. Furthermore, because switching power supplies operate at high frequencies, they can use smaller, lighter transformers, resulting in smaller size and lighter weight compared to linear power supplies.
If efficiency, size, and weight are key considerations, switching power supplies are better than linear power supplies. However, switching power supplies are more complex, as their internal transistors switch frequently. If the switching current is not properly managed, it can generate noise and electromagnetic interference that could affect other devices. Furthermore, without special design, the power factor of a switching power supply may not be high.
The predecessor of switching power supplies was the linear regulated power supply. Before the advent of switching power supplies, many control devices used linear regulated power supplies. As the integration of electronic devices such as computers continues to increase and their functions become more powerful, while their size becomes smaller, there is an urgent need for new power supplies that are small in size, light in weight, highly efficient, and have good performance. This has become a powerful driving force for the development of switching power supply technology.
The development of new power electronic devices provided the material conditions for the development of switching power supplies. In the late 1960s, the advent of the giant transistor (GTR) enabled the development of switching power supplies operating at high frequencies; the basic structure of switching power supplies established at that time is still in use today. Later, with the application of power MOSFETs, the frequency of switching power supplies was further increased, resulting in smaller size, lighter weight, and higher power density. In the 1980s, the emergence of IGBTs allowed switching power supplies, previously only suitable for low-power applications, to be used in medium to high-power DC power supplies. In the latter two decades of the 1980s, soft-switching technology was developed to address electromagnetic interference problems caused by increased switching frequencies. Subsequently, in the 1990s, power factor correction (PFC) technology emerged to improve the power factor of switching power supplies.
Currently, except in applications with extremely high requirements for DC output voltage ripple, switching power supplies have completely replaced linear regulated power supplies, primarily used in low-power applications. Examples include power supplies for computers, televisions, and various electronic instruments. In many medium-capacity ranges, switching power supplies are gradually replacing phase-controlled power supplies, such as in the fields of communication power supplies, welding machines, and electroplating equipment.
II. Composition of Switching Power Supply
A switching power supply generally consists of four main parts: the main circuit, the control circuit, the detection circuit, and the auxiliary power supply.
1. Main circuit
Inrush current limiting: Limits the inrush current on the input side at the moment of power-on. Input filter: Its function is to filter out noise present in the power grid and prevent noise generated by the unit from feeding back to the power grid. Rectification and filtering: Directly rectifies the AC power from the power grid into smoother DC power. Inverter: Converts the rectified DC power into high-frequency AC power; this is the core component of a high-frequency switching power supply. Output rectification and filtering: Provides a stable and reliable DC power supply according to the load requirements.
2. Control circuit
On the one hand, the output is sampled and compared with the set value, and then the inverter is controlled to change its pulse width or pulse frequency to stabilize the output. On the other hand, based on the data provided by the test circuit and identified by the protection circuit, the control circuit provides various protection measures for the power supply.
3. The detection circuit provides various parameters and instrument data that are being used in the protection circuit during operation.
4. The auxiliary power supply enables the software (remote) startup of the power supply, providing power for the protection circuit and control circuit (PWM chips, etc.).
III. Basic Principles of Switching Power Supplies
Simply put, the working principle of a switching power supply is:
1. The AC power input is rectified and filtered into DC;
2. By controlling the switching transistor with a high-frequency PWM (Pulse Width Modulation) signal, that DC current is applied to the primary winding of the switching transformer;
3. A high-frequency voltage is induced on the secondary side of the switching transformer, which is then rectified and filtered to supply the load.
4. The output section feeds back to the control circuit through a certain circuit to control the PWM duty cycle in order to achieve stable output.
