The difference between switching power supply and ordinary power supply

Do you know the difference between switching power supplies and ordinary power supplies? This article mainly compares the differences between switching power supplies and ordinary power supplies, and introduces in detail the concepts, characteristics, classifications and differences between the two types of switching power supplies, so that everyone can know more about them. Have a clear understanding of switching power supply related knowledge~

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 supply is widely used in almost all electronic equipment due to its small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of today's electronic information industry. A switching power supply is a power supply that uses modern power electronics technology to control the turn-on and turn-off time ratio of the switching tube to maintain a stable output voltage. The switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET.

The switching power supply is relative to the linear power supply. Its input terminal directly rectifies the alternating current into direct current. Under the action of the high-frequency oscillation circuit, the switching tube is used to control the on-off of the current to form a high-frequency pulse current. With the help of an inductor (high-frequency transformer), a stable low-voltage direct current is output. Since the size of the transformer's magnetic core is inversely proportional to the square of the switching power supply's operating frequency, the higher the frequency, the smaller the core. This can greatly reduce the size of the transformer and reduce the weight and size of the power supply. And because it controls DC directly, this power supply is much more efficient than a linear power supply. This saves energy, so it is favored by people. But it also has disadvantages, that is, the circuit is complex, maintenance is difficult, and the circuit is seriously polluted. The power supply is noisy and is not suitable for use in some low-noise circuits.

Characteristics of switching power supply

Switching power supplies generally consist of pulse width modulation (PWM) control ICs and MOSFETs. With the development and innovation of power electronics technology, switching power supplies are currently widely used in almost all electronic equipment due to their small size, lightweight and high efficiency. Its importance is evident.

Classification of switching power supplies

According to the way switching devices are connected in the circuit, switching power supplies can be generally divided into three categories: series switching power supplies, parallel switching power supplies, and transformer switching power supplies. Among them, transformer-type switching power supply can be further divided into: push-pull type, half-bridge type, full-bridge type, etc. According to the excitation of the transformer and the phase of the output voltage, it can be divided into: forward type, flyback type, single exciting type and double exciting type.

The difference between switching power supply and ordinary power supply

Ordinary power supplies are generally linear power supplies. Linear power supplies refer to power supplies where the regulator tube works in a linear state. But in switching power supply, it is different. The switching tube (in switching power supply, we generally call the adjusting tube switching tube) works in two states: on and off: on - the resistance is very small, off - the resistance is very small. big. Switching power supply is a relatively new type of power supply. It has the advantages of high efficiency, light weight, ability to increase and decrease voltage, and high output power. However, since the circuit works in a switching state, the noise is relatively large.

Example: Buck switching power supply

Let’s briefly talk about the working principle of the step-down switching power supply: the circuit is composed of a switch (a triode or field effect transistor 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 inductor, and stores part of the electrical energy in the inductor and capacitor. Due to the self-inductance of the inductor, the current increases slowly after the switch is turned on, that is, the output cannot reach the power supply voltage value immediately.

After a certain period of time, the switch is turned off. Due to the self-inductance of the inductor (the current in the inductor can be thought of as having inertia), the current in the circuit will remain unchanged, that is, it will continue to flow from left to right. This current flows through the load, returns from the ground wire, flows to the anode of the freewheeling diode, passes through the diode, and returns to the left end of the inductor, thus forming a loop.

By controlling the time when the switch is closed and opened (ie PWM - pulse width modulation), the output voltage can be controlled. If the on and off time is controlled by detecting the output voltage to keep the output voltage constant, the purpose of voltage stabilization is achieved. Ordinary power supplies and switching power supplies have the same voltage adjustment tube, which uses the feedback principle to regulate voltage. The difference is that switching power supplies use switching tubes for adjustment, while ordinary power supplies generally use the linear amplification area of ​​triodes for adjustment. In comparison, the switching power supply has low energy consumption, a wide applicable range for AC voltage, and a better ripple coefficient of output DC. The disadvantage is switching pulse interference.

The main working principle of an ordinary half-bridge switching power supply is that the upper and lower bridge switching tubes (the switching tubes are VMOS when the frequency is high) are turned on in turn. First, the current flows through the upper bridge switching tube, and the storage function of the inductor coil is used to gather the electric energy. In the coil, the upper bridge switch tube is finally turned off and the lower bridge switch tube is turned on. The inductor coil and capacitor continue to supply power to the outside. Then the lower bridge switch tube is turned off, and then the upper bridge switch tube is turned on to let the current enter, and this is repeated. Because the two switch tubes are turned on and off in turn, it is called a switching power supply.

The linear power supply is different. Since there is no switch involved, the water supply pipe is always discharging water. If there is too much water, it will leak out. This is why we often see that the adjustment tube of some linear power supplies generates a lot of heat. The endless electrical energy is all converted into heat energy. From this perspective, the conversion efficiency of the linear power supply is very low, and when the heat is high, the life of the components will inevitably decrease, affecting the final use effect.

Main difference: How it works

The power adjustment tube of the linear power supply always works in the amplification area, and the current flowing through it is continuous. Since the adjustment tube consumes a lot of power, it requires a larger power adjustment tube and is equipped with a large radiator. The heat is serious and the efficiency is very low, generally between 40% and 60% (it must also be said to be very linear). power supply). The working method of linear power supply requires a voltage converter device to change from high voltage to low voltage. Generally, it is a transformer, but there are also others like KX power supply, which then rectifies and outputs DC voltage. As a result, it is large, bulky, low-efficiency, and generates a lot of heat; but it also has advantages: small ripple, good adjustment rate, low external interference, and suitable for use with analog circuits/various amplifiers, etc.

The power device of the switching power supply works in the switching state. During the voltage adjustment, the energy is temporarily stored through the inductor coil, so that its loss is small, the efficiency is high, and the requirements for heat dissipation are low, but it does not affect the transformer and energy storage inductor. There are also higher requirements, which need to be made of materials with low loss and high magnetic permeability. Its transformer is as small as a word. The total efficiency is 80% to 98%. The switching power supply has high efficiency but small size. However, compared with the linear power supply, its ripple and voltage and current regulation rate are at a certain discount. The above are some differences between switching power supplies and ordinary power supplies, which will be helpful to everyone when choosing.