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HOME >> What is the difference between switching power supply and normal power supply?
What is the difference between switching power supply and normal power supply?
Source: | Author:pmo89cafb | Published time: 2020-11-02 | 4158 Views | Share:
What is switching power supply?

With the development and innovation of power electronic technology, switching power supply technology is also constantly innovative. At present, switching power supply is widely used in almost all electronic equipment with the characteristics of small size, light weight and high efficiency. It is an indispensable power supply mode for the rapid development of electronic information industry.
Switching power supply is a kind of power supply which uses modern power electronic technology to control the time ratio of switching on and off to maintain stable output voltage. The switching power supply is generally composed of pulse width modulation (PWM) control IC and MOSFET.
Switching power supply is a relatively linear power supply. Its input end directly rectifies alternating current into direct current, and then under the action of high-frequency oscillation circuit, the switch tube is used to control the on-off of the current to form high-frequency pulse current. With the help of inductor (high frequency transformer), stable low voltage direct current is output.
Because the size of the magnetic core of the transformer is inversely proportional to the square of the working frequency of the switching power supply, the higher the frequency, the smaller the core. In this way, the transformer can be greatly reduced and the weight and volume of the power supply can be reduced. Moreover, the efficiency of this power supply is much higher than that of linear power supply because it directly controls DC. This saves energy, so it is favored by people. But it also has its disadvantages, that is, the circuit is complex, difficult to maintain, and seriously pollutes the circuit. The power supply is noisy, so it is not suitable for some low noise circuits.


Characteristics of switching power supply
Switching power supply is generally composed of pulse width modulation (PWM) control IC and MOSFET. With the development and innovation of power electronic technology, switching power supply is widely used in almost all electronic equipment with the characteristics of small size, light weight and high efficiency.

Classification of switching power supply
According to the connection mode of switching devices in the circuit, switching power supply can be divided into three categories: series switching power supply, parallel switching power supply and transformer switching power supply.
Among them, transformer switching power supply can be further divided into push-pull type, half bridge type, full bridge type and so on. According to the phase of excitation and output voltage of transformer, it can be divided into forward excitation type, flyback type, single excitation type and double excitation type.


The difference between switching power supply and common power supply
The general power supply is linear power supply, linear power supply, which refers to the power supply of regulating tube working in linear state. But in the switch power supply, the switch tube (in the switch power supply, we generally call the regulator switch tube) is working in two states: On - resistance is very small, off - resistance is very large.
Switching power supply is a new type of power supply. It has the advantages of high efficiency, light weight, high lifting, voltage reduction and high output power. But because the circuit is in the switch state, the noise is relatively large.


Examples: buck switching power supply
Let's briefly talk about the working principle of buck switching power supply: the circuit is composed of switch (transistor or field effect transistor in actual circuit), freewheeling diode, energy storage inductor, filter capacitor, etc.
When the switch is closed, the power supply supplies power to the load through the switch and inductance, and part of the energy is stored in the inductor and capacitor. Due to the inductance's self inductance, 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 disconnected, because of the self inductance of the inductor (it can be more vividly considered that the current in the inductor has inertial effect), it will keep the current in the circuit unchanged, that is, continue to flow from left to right. This current flows through the load, returns from the ground wire, flows to the positive pole of the freewheeling diode, passes through the diode and returns to the left end of the inductance, forming a loop.
The output voltage can be controlled by controlling the closing and opening time of the switch (i.e. PWM pulse width modulation). If the time of opening and closing is controlled by detecting the output voltage to keep the output voltage unchanged, the purpose of voltage stabilization is realized.
Common power supply and switching power supply have voltage regulator, which uses feedback principle to stabilize voltage. The difference is that switching power supply uses switch tube to adjust, while common power supply uses linear amplification area of triode to adjust. In comparison, the switching power supply has low energy consumption, wide application range for AC voltage, better ripple coefficient of output DC, and the disadvantage is switching pulse interference.
The main working principle of the common half bridge switching power supply is that the switches of the upper bridge and the lower bridge (the switch tube is VMOS when the frequency is high) turn on in turn. First, the current flows through the upper bridge switch tube. Using the storage function of the inductance coil, the electric energy is gathered in the coil. Finally, the upper bridge switch tube is closed, and the lower bridge switch tube is opened. The inductance coil and capacitance continuously supply external power. Then turn off the lower bridge switch tube, and then turn on the upper bridge to let the current enter. This is repeated. Because the two switch tubes need to be switched on and off in turn, it is called switching power supply.
The linear power supply is not the same. Because there is no switch intervention, the water supply pipe has been discharging water. If there is too much, it will leak out. This is what we often see in some linear power supply. The heat generated by the regulating pipe of some linear power supply is very large, and the unused electric energy is converted into heat energy. From this point of view, the conversion efficiency of linear power supply is very low, and when the heat is high, the life of the components must be reduced, which will affect the final use effect.

Main difference: working style


The power regulator of linear power supply always works in the amplification region, and the current flowing through it is continuous. Due to the large power loss on the regulating tube, a large power regulating tube is required and equipped with a large radiator. The heating is serious and the efficiency is very low, which is generally 40% ~ 60% (it is also a good linear power supply).
Because of the working mode of linear power supply, there must be a voltage device for changing from high voltage to low voltage. Generally, it is a transformer, and there are other power sources like KX, which output DC voltage after rectification. In this way, it has a large volume, bulky, low efficiency and high calorific value, but it also has advantages: small ripple, good adjustment rate, small external interference, and suitable for analog circuits / amplifiers.
The power device of switching power supply works in the on-off state. When the voltage is adjusted, the energy is stored through the inductance coil. In this way, the loss is small, the efficiency is high, and the requirement for heat dissipation is low. However, it also has higher requirements for the transformer and energy storage inductor, which should be made of materials with low loss and high permeability. Its transformer is a word small. The total efficiency is 80% ～ 98%. The switching power supply has high efficiency but small volume. Compared with linear power supply, the voltage and current regulation rate has a certain discount.