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How Does a Switching Power Supply Work?

Views:79     Author:Site Editor     Publish Time: 2019-09-24      Origin:Site

Switching power supply, also called switching power converter, is a high-frequency power conversion device and a kind of power supply. Its function is to convert the voltage of one level into the voltage or current required by the user through different forms of architecture. The input of switching power supply is mostly AC power supply or DC power supply, while the output is mostly equipment requiring DC power supply, such as personal computer, and switching power supply converts voltage and current between the two. We always use it in our life, but we don't know much about it. Next, let's learn some basic knowledge about switching power supply.

The Main Components of Switching Power Supply

Switching power supply consists of main circuit, control circuit, detection circuit and auxiliary power supply.

(1) Main Circuit

Impulse current limiting: limit the impulse current on the input side when the power supply is switched on.

Input filter: Its function is to filter the clutter existing in the power grid and prevent the clutter generated by this machine from being fed back to the power grid.

Rectification and filtering: directly rectifies the AC power supply of the power grid into smoother DC power.

Inversion: Converts rectified direct current into high-frequency alternating current, which is the core part of high-frequency switching power supply.

Output rectification and filtering: Provide stable and reliable DC power supply according to load requirements.

(2) Control Circuit

On the one hand, samples are taken from the output end and compared with the set value, and then the inverter is controlled to change its pulse width or frequency so as to stabilize the output; on the other hand, according to the data provided by the test circuit, the control circuit is provided to carry out various protection measures on the power supply through identification by the protection circuit.

(3) Detection Circuit

Provide various parameters and various meter data in the protection circuit during operation.

(4) Auxiliary Power Supply

Software (remote) startup of power supply is realized to supply power for protection circuit and control circuit (PWM and other chips).

Working Principle of Switching Power Supply

The working process of switching power supply is quite easy to understand. In linear power supply, the power transistor works in linear mode. Unlike linear power supply, PWM switching power supply makes the power transistor work in on and off states. In these two states, the V-A product added to the power transistor is very small (when on, the voltage is low and the current is large, When turned off, the volt-ampere product of high voltage and low current/power device is the loss generated on the power semiconductor device.

Compared with linear power supply, PWM switching power supply works more effectively through "chopping", that is, chopping the input DC voltage into pulse voltage whose amplitude is equal to that of the input voltage.  The duty cycle of the pulse is adjusted by the controller of the switching power supply.  Once the input voltage is chopped into an AC square wave, its amplitude can be raised or lowered by the transformer.  The output voltage can be increased by increasing the number of secondary windings of the transformer.  Finally, these AC waveforms are rectified and filtered to obtain DC output voltage.  

The main purpose of the controller is to keep the output voltage stable, and its working process is very similar to that of a linear controller.  In other words, the controller's functional block, voltage reference and error amplifier can be designed to be the same as the linear regulator.  The difference between them is that the output of the error amplifier goes through a voltage/pulse width conversion unit before driving the power tube.

Switching power supply has two main working modes: forward conversion and boost conversion.  Although there is little difference in the arrangement of their parts, the working process is quite different and each has its own advantages in specific application situations.

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