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Switching Mode Power Supply中文版
Switching power supply principle
2018-11-09      Browse times: 5386

        The switching power supply is a voltage conversion circuit. The main work is boost and buck, which is widely used in modern 

electronic products. Because the switching transistor always works in the "on" and "off" states, it is called the switching power 

supply. The switching power supply is essentially an oscillating circuit. This way of converting electrical energy is not only applied 

to the power supply circuit, but also widely used in other circuit applications, such as backlight circuits of liquid crystal displays, 

fluorescent lamps, and the like. Compared with the transformer, the switching source has the advantages of high efficiency, 

good stability, small volume, etc. The disadvantage is that the power is relatively small, and high frequency interference is 

generated to the circuit, and the circuit is complicated and difficult to repair.

         Before talking about the switching power supply, familiarize yourself with the transformer feedback oscillation circuit. The 

circuit that can generate regular pulse current or voltage is called the oscillation circuit. The transformer feedback oscillation circuit 

is the circuit that can satisfy this condition. It is used in the basic amplification circuit. A feedback loop is composed, wherein C2 

and L1 form a parallel resonant frequency selective circuit, and VT is turned on at the moment when the circuit is energized. 

At this time, a very rich harmonic is generated on the parallel resonant circuit composed of C2 and L1, when the applied frequency 

and the parallel are applied. When the natural frequencies of the resonant circuit are equal, the circuit enters an oscillating state 

and is further amplified by L3 feedback to the base of the VT, eventually forming a regular pulse current or voltage output to the 

load RL. The switching power supply is designed around the feedback oscillating circuit of the transformer. However, some 

protection and control circuits are added on the basis of the original. We can analyze the oscillating circuit to analyze the switching 

power supply.

          The switching power supply is divided into two types: self-excited and exciter. The self-excited type does not require an 

external signal source to oscillate by itself. The self-excited type can be regarded as a transformer feedback type oscillating circuit. 

However, it is completely dependent on the externally maintained oscillation, and the self-excited application is widely used in 

practical applications. According to the structure of the excitation signal; it can be divided into pulse width adjustment and pulse 

amplitude modulation. The pulsewidth adjustment is he width of the control signal, that is, the frequency, the amplitude of the pulse 

amplitude modulation control signal, and the same effect of both is to maintain the oscillation frequency at Within a certain range, 

the effect of stabilizing the voltage is achieved. The windings of a transformer can generally be divided into three types, one is a 

primary winding that participates in oscillation, one is a feedback winding that maintains oscillation, and one is a load winding. 

In the switching power supply used in household appliances, the 220V AC power is converted into a DC current of about 300V 

by bridge rectification, filtered, and then input into the transformer and then applied to the collector of the switching tube for 

high-frequency oscillation, and the feedback winding is fed back to the base maintaining circuit. Oscillation, the electrical signal 

induced by the load winding, the DC voltage obtained by rectification, filtering, and voltage regulation supplies power to the load. 

While providing power, the load winding also shoulders the ability to stabilize the voltage. The principle is to connect a voltage 

sampling device to the voltage output circuit to monitor the change of the output voltage, and feedback the oscillation circuit to 

adjust the oscillation frequency in time to achieve a stable voltage. Purpose, in order to avoid circuit interference, the voltage 

fed back to the oscillating circuit is isolated by a photocoupler. Most switching power supplies have a standby circuit, and the 

switching power supply is still oscillating in standby mode, but the frequency is lower than during normal operation.

          Some switching power supplies are complex, components are dense, and many protection and control circuits are a 

headache when they are not supported by technical support. In the face of this situation, I will first find the switch and its peripheral 

circuit that participates in the oscillation, and separate it from the circuit to see if it meets the oscillation conditions, such as whether 

the detection bias is normal, whether the positive feedback is present or not. The fault, as well as the switch tube itself, has a very 

powerful protection function for the switching power supply, and the inspection control and protection and load circuits are eliminated 

after the elimination.