The working process of the switching power supply is quite easy to understand. In the linear power supply, the power transistor is operated in linear mode. Unlike the linear power supply, the PWM switching power supply allows the power transistor to work in the on and off state. In the state, the volt-ampere product added to the power transistor is very small (when it is on, the voltage is low and the current is large; when it is off, the voltage is high, and the current is small) / the volt-ampere product on the power device is the power semiconductor The loss generated on the device.
Compared with the linear power supply, the more effective working process of the PWM switching power supply is achieved by "chopping", that is, the input DC voltage is chopped into a pulse voltage whose amplitude is equal to the amplitude 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 increased or decreased by the transformer. The output voltage value can be increased by increasing the number of secondary windings of the transformer. After these AC waveforms are rectified and filtered, the DC output voltage is obtained.
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. That is to say, the functional blocks, voltage reference and error amplifier of the controller can be designed to be the same as the linear regulator. The difference between them is that the output of the error amplifier (error voltage) passes through a voltage/pulse width conversion unit before driving the power tube.
Switching power supplies have two main operating modes: forward conversion and boost conversion. Although the layout of their parts is very different, the working process is very different, and each has its own advantages in specific applications.