1. What is a switching power supply?
Switching power supply is a kind of power supply that uses modern power electronic technology to control the time ratio of switching on and off to maintain a stable output voltage. Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and MOSFETs.
The switching power supply is relative to the linear power supply. Its input terminal directly rectifies the alternating current into direct current, and then under the action of the high-frequency oscillation circuit, the switch tube is used to control the on and off of the current to form a high-frequency pulse current. With the help of inductors (high-frequency transformers), stable low-voltage direct current is output.
Since the size of the magnetic core of the transformer is inversely proportional to the square of the operating 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. And because 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 favored by people. But it also has shortcomings, that is, the circuit is complicated, the maintenance is difficult, and the pollution to the circuit is serious. The power supply is noisy and is not suitable for some low-noise circuits.
2. Characteristics of switching power supply
Switching power supplies are generally composed of pulse width modulation (PWM) control ICs and MOSFETs. With the development and innovation of power electronics technology, the current switching power supply is widely used in almost all electronic devices mainly due to its small size, light weight and high efficiency, and its importance is evident.
Three, the classification of switching power supply
According to the way the switching device is connected in the circuit, the switching power supply can be divided into three categories: series switching power supply, parallel switching power supply, and transformer switching power supply.
Among them, the transformer-type switching power supply can be further divided into: push-pull, half-bridge, full-bridge, 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 excitation type and double excitation type.
Fourth, the difference between switching power supply and ordinary power supply
Ordinary power supplies are generally linear power supplies, and linear power supplies refer to the power supply in which the regulator tube works in a linear state. However, it is different in switching power supply. The switching tube (in the switching power supply, we generally call the adjusting tube as the switching tube) works in two states: on-the resistance is very small, and off-the resistance is very high. big.
Switching power supply is a relatively new type of power supply. It has the advantages of high efficiency, light weight, voltage increase and decrease, and high output power. But because the circuit works in the switch state, the noise is relatively large.
5. Examples: step-down switching power supply
Let's briefly talk about the working principle of the step-down switching power supply: the circuit is composed of switches (triodes or field effect tubes in the actual circuit), freewheeling diodes, energy storage inductors, filter capacitors, 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 relatively 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 (it can be more vividly considered that the current in the inductor has an inertial effect), the current in the circuit will remain unchanged, that is, 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.
The output voltage can be controlled by controlling the time when the switch is closed and open (ie PWM-pulse width modulation). If the output voltage is detected to control the on and off time to keep the output voltage constant, this achieves the purpose of voltage stabilization.
Ordinary power supply and switching power supply are the same in that they all have voltage regulators, which use the feedback principle for voltage regulation. The difference is that switching power supplies use switching tubes for adjustment, while ordinary power supplies generally use the linear amplification area of a triode for adjustment. In comparison, the power consumption of the switching power supply is low, the application range of AC voltage is wide, and the ripple coefficient of the output DC is better. The disadvantage is the switching pulse interference.
The main working principle of the ordinary half-bridge switching power supply is that the switching tubes of the upper bridge and the lower bridge (the switching tube is VMOS when the frequency is high) are turned on in turn. First, the current flows in through the upper bridge switching tube, and the storage function of the inductor is used to gather the electric energy. In the coil, the switch tube of the upper bridge is finally turned off, and the switch tube of the lower bridge is turned on. The inductor coil and the capacitor continue to supply power to the outside. Then turn off the lower bridge switch tube, then turn on the upper bridge to let the current enter, and repeat this way. Because the two switch tubes are switched on and off in turn, it is called a switching power supply.
The linear power supply is different. Because there is no switch intervention, the water supply pipe has been draining water. If there is too much, it will leak out. This is what we often see that some linear power supply regulator tubes generate a lot of heat. The inexhaustible electric energy is all converted into heat energy. From this point of view, the conversion efficiency of the linear power supply is very low, and when the heat is high, the life of the component is bound to decrease, which will affect the final use effect.
Six. Main differences: working methods
The power adjustment tube of the linear power supply always works in the amplifying area, and the current flowing through is continuous. Due to the large power loss on the adjustment tube, a larger power adjustment tube is required and a large radiator is installed. The heat is serious and the efficiency is very low, generally 40% to 60% (it has to be said that it is very linear power supply).
The working method of linear power supply requires a voltage reducing device to change from high voltage to low voltage. Generally, it is a transformer, but there are also other types like KX power supplies, which are then rectified to output a DC voltage. In this way, the volume is also large, relatively heavy, low efficiency, and large heat generation; but it also has advantages: small ripple, good adjustment rate, small external interference, suitable for use with analog circuits/various amplifiers, etc.
The power device of the switching power supply works in the switching state. When the voltage is adjusted, the energy is temporarily stored through the inductance coil, so that its loss is small, the efficiency is high, and the requirement for heat dissipation is low, but it has a transformer and energy storage inductance. There are also higher requirements to use materials with low loss and high permeability. Its transformer is just 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 linear power supply, its ripple and voltage and current adjustment rate are discounted to a certain extent.