Working Principle of the Switch Mode Power Supply
The working process of a switching power supply is quite easy to understand. In a linear power supply, the power transistor is operated in linear mode. Unlike a linear power supply, a PWM switching power supply operates the power transistor in on and off states. In these two states, the voltage ampere product applied to the power transistor is very small (low voltage and high current when conducting; high voltage and low current when turning off)/the volt ampere product on the power device is the loss generated on the power semiconductor device. Compared with linear power supplies, the more efficient working process of PWM switching power supplies is achieved through "chopping", which means chopping the input DC voltage into a pulse voltage with an amplitude equal to the input voltage amplitude. The duty cycle of the pulse is adjusted by the controller of the switching power supply. Once the input voltage is chopped into AC square waves, its amplitude can be increased or decreased through a transformer. By increasing the number of secondary windings of the transformer, the number of output voltage groups can be increased. Finally, these AC waveforms are rectified and filtered to obtain a DC output voltage. The main purpose of the controller is to maintain stable output voltage, and its working process is similar to that of a linear form controller. That is to say, the functional blocks, voltage reference, and error amplifier of the controller can be designed to be the same as those of a linear regulator. Their difference lies in that the output (error voltage) of the error amplifier needs to go through a voltage/pulse width conversion unit before driving the power transistor. Switching power supplies have two main working modes: forward conversion and boost conversion. Although there is little difference in the arrangement of their various parts, the working process varies greatly, and each has its own advantages in specific application scenarios.
