The basic principle of PWM switching power supply

The basic principle of PWM switching power supply

Although pulse-width-modulated (PWM) switching power supplies have been around for a long time, they were not widely accepted and used until the mid-1970s. Switching power supplies have many advantages over linear power supplies.For the same power capacity, switching power supplies are more efficient and smaller than linear power supplies, but the design is more complex and electromagnetic interference (EMI) is more severe.

There are two approaches to the current design of switching power supplies. The design of onboard DC/DC (DC input, DC output) switching power supply can refer directly to the data manual of the semiconductor manufacturer, and can get standard components from other manufacturers, but if there are special requirements, it needs to be designed by the user, and the design is much more difficult.

This book is carefully organized to divide the vast design process of a common switching power supply into more comprehensible parts. The first part is explained in terms of "non-power engineer". For the generally accepted design method, the corresponding design formula is used here. The purpose of this section is to allow readers to choose the best design method that meets their needs, and to use their specific system parameters to make a subcircuit that can be plugged into a larger power system design. The sequence of designs is arranged according to the usual design steps of experienced power experts. These design steps proved to solve problems before they arose.

The working process of a switching power supply is fairly easy to understand. Unlike linear power supplies, which allow the power transistor to operate in linear mode, PWM switching power supplies allow the power transistor to operate in the on-off and judgment states. In both states, the volt-amperity-product added to the power transistor is always small (at on-off, the current is low and the current is high; Off, high voltage, low current). The volt-ampere product on a power device is all the losses generated on a power semiconductor device.

Compared with the linear power supply, PWM switching power supply more effective working process is through the "chopper", that is, the input DC voltage into the pulse voltage amplitude is equal to the input voltage amplitude to achieve. The duty cycle of the pulse is regulated by the controller of the switching power supply. Once the input voltage is chopped into AC square waves, the base amplitude can be raised or lowered through the transformer. The number of output voltage groups can be increased by increasing the number of secondary windings of the transformer. Finally, the AC waveforms are rectified and filtered to obtain the DC output voltage.

The main purpose of the controller is to keep the output voltage stable, and its working process is very similar to the linear form of the controller. This means that the controller's functional blocks, voltage references, and error amplifiers can be designed to be identical to linear regulators. They differ in 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 modes of operation: forward conversion and boost conversion. Although the layout of each part is very different, the working process is very different, and each has its advantages in specific application situations.