What is a switching power supply?

With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating. At present, switching power supplies are widely used in almost all electronic devices because of their small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of the electronic information industry.

A switching power supply is a power supply that uses modern power electronics technology to control the switching time ratio of the switch tube and maintain a stable output voltage. A switching power supply is generally composed of a pulse width modulation control integrated circuit and a field effect tube.

A switching power supply is relative to a linear power supply. Its input directly rectifies the AC into DC. Under the action of a high-frequency oscillation circuit, the switch tube controls the on and off of the current to form a high-frequency pulse current. With the help of an inductor (high-frequency transformer), a stable low-voltage DC is output.

Since the size of the transformer core is inversely proportional to the square of the operating frequency of the switching power supply, the higher the frequency, the smaller the core. This can greatly reduce the transformer and reduce the weight and volume of the power supply. Since it directly controls DC, the efficiency of this power supply is much higher than that of a linear power supply. This saves energy, so it is very popular. However, it also has disadvantages, namely, the circuit is complex, maintenance is difficult, and the circuit pollution is serious. This power supply has high noise and is not suitable for some low-noise circuits.

Characteristics of switching power supplies

The switching power supply is generally composed of a pulse width modulation control integrated circuit and a field effect tube. With the development and innovation of power electronics technology, the switching power supply is widely used in almost all electronic devices with its small size, light weight and high efficiency, showing its importance.

Classification of switching power supplies

According to the connection method of the switching device in the circuit, the switching power supply can generally be divided into a series switching power supply, a parallel switching power supply and a transformer switching power supply.

Among them, the transformer switching power supply can be divided into a push-pull type, a half-bridge type and a full-bridge type. According to the excitation of the transformer and the phase of the output voltage, it can be divided into a forward excitation type, a flyback type, a single excitation type and a dual excitation type.

The difference between switching power supply and ordinary power supply

Ordinary power supplies are generally linear power supplies. A linear power supply refers to a power supply in which the regulating tube works in a linear state. However, it is different in terms of switching power supplies. The switching tube (in the switching power supply, we usually call it the regulating tube switching tube) works in two states: on-resistance is very small, off-resistance is very large.

The switching power supply is a relatively new type of power supply. It has the advantages of high efficiency, light weight, buck-boost, and high output power. However, since the circuit works in a switching state, the noise is relatively large.

Example: Step-Down Switching Power Supply

Let's briefly talk about the working principle of the step-down switching power supply: the circuit consists of a switch (a transistor or field effect tube in the actual circuit), a freewheeling diode, an energy storage inductor, a filter capacitor, etc.

When the switch is closed, the power supply supplies power to the load through the switch and the inductor, and stores part of the electrical energy in the inductor and the capacitor. Due to the self-inductance of the inductor, the current increases slowly after the switch is turned on, that is, the output cannot immediately reach the power supply voltage value.

After a period of time, the switch is closed, and due to the self-inductance of the inductor (the current in the inductor is visually considered to have an inertia effect), the current in the circuit will remain unchanged, that is, it continues to flow from left to right. This current flows through the load, returns from the ground, flows to the positive pole of the freewheeling diode, flows through the diode, and returns to the left end of the inductor, forming a loop.

The output voltage can be controlled by controlling the time when the switch is closed and opened (that is, pulse width modulation). If the on/off time is controlled by detecting the output voltage to keep the output voltage unchanged, the purpose of voltage regulation is achieved.

Ordinary power supplies and switching power supplies have the same voltage regulator tube

The main working principle of the ordinary half-bridge switching power supply is that the switch tubes of the upper bridge and the lower bridge (VMOS at high frequency) are turned on alternately. First, the current flows through the upper bridge switch tube, and the electrical energy is concentrated in the coil by utilizing the storage function of the inductor coil. Finally, the upper bridge switch tube is closed, and the lower bridge inductor coil switch tube is opened, and the capacitor will continue to supply power to the outside. Then the lower bridge switch tube is closed, and the upper bridge is opened to allow current to enter. This is repeated. It is called a switching power supply because the two switch tubes need to be turned on and off in turn.

The linear power supply is different, because there is no intervention of the switch, so that the water supply pipe is always draining, and if there is a lot, it will leak out. This is what we often see. The heat of some linear power supply regulation pipes is very large, and the electricity is used up and all converted into heat energy. From this point of view, the conversion efficiency of the linear power supply is very low. When the heat is very high, the service life of the components will inevitably decrease, affecting the final use effect.

Main difference: How they work

The power regulation tube of the linear power supply always works in the amplification area, and the current flowing through it is continuous. Due to the large power loss of the regulating tube, a high-power regulating tube is required, and it is equipped with a large-volume heat sink. It has serious heat generation and low efficiency, generally between 40% and 60% (it is also a good linear power supply).

The working method of the linear power supply is that there must be a voltage-reducing device from high voltage to low voltage. Generally speaking, it is a transformer, and there are other power supplies like KX, and then the DC voltage is output through rectification. In this way, the volume is also large, relatively heavy, inefficient, and the heat generation is also large; however, it also has the advantages of small ripple, good adjustment rate, and small external interference, which is suitable for analog circuits/various amplifiers, etc.

The power device of this switching power supply works in a switching state, and the energy is temporarily stored through the inductor coil during voltage regulation, with small loss, high efficiency, and low heat dissipation requirements, but it also has high requirements for transformers and energy storage inductors. The switching power supply is made of low-loss and high-permeability materials. Its transformers are a small word. The total efficiency is between 80% and 98%. The switching power supply has high efficiency but small size, but compared with the ripple of the linear power supply, the voltage and current adjustment rate has a certain discount.