The basic working principle of switching power supply

1-1. Several basic types of switching power supplies

As the name suggests, a switching power supply uses electronic switching devices (such as transistors, field effect transistors, thyristors, etc.) to control the circuit so that the electronic switching devices are constantly "turned on" and "turned off", allowing the electronic switching devices to pulse modulate the input voltage, thereby achieving DC/AC, DC/DC voltage conversion, as well as adjustable output voltage and automatic voltage stabilization.

Switching power supplies generally have three working modes: fixed frequency and pulse width mode, fixed frequency and variable pulse width mode, and variable frequency and pulse width mode. The former working mode is mostly used for DC/AC inverter power supplies or DC/DC voltage conversion; the latter two working modes are mostly used for switching voltage-stabilized power supplies. In addition, the output voltage of the switching power supply also has three working modes: direct output voltage mode, average value output voltage mode, and amplitude output voltage mode. Similarly, the former working mode is mostly used for DC/AC inverter power supplies or DC/DC voltage conversion; the latter two working modes are mostly used for switching voltage-stabilized power supplies.

According to the way the switching devices are connected in the circuit, the widely used switching power supplies can be roughly divided into three categories: series switching power supply, parallel switching power supply, transformer switching power supply, etc. Among them, the transformer switching power supply (hereinafter referred to as transformer switching power supply) can be further divided into: push-pull type, half-bridge type, full-bridge type, etc.; according to the excitation of the transformer and the phase of the output voltage, it can be divided into: forward type, flyback type, single-excitation type and double-excitation type, etc.; if divided by purpose, it can be divided into more types.

Below we will briefly introduce the working principles of the three most basic switching power supplies: series, parallel, and transformer. Other types of switching power supplies will also be analyzed in detail step by step.

1-2. Series switching power supply

1-2-1. Working principle of series switching power supply

Figure 1-1-a is the simplest working principle diagram of the series switching power supply. In Figure 1-1-a, Ui is the working voltage of the switching power supply, that is, the DC input voltage; K is the control switch, and R is the load. When the control switch K is turned on, the switching power supply outputs a pulse voltage Up with a pulse width of Ton and an amplitude of Ui to the load R; when the control switch K is turned off, it is equivalent to the switching power supply outputting a pulse voltage with a pulse width of Toff and an amplitude of 0 to the load R. In this way, the control switch K is constantly "turned on" and "turned off", and a pulse modulated output voltage uo can be obtained at both ends of the load.

Figure 1-1-b is the waveform of the output voltage of the series switching power supply. It can be seen from the figure that the output voltage uo of the control switch K is a pulse modulated square wave. The pulse amplitude Up is equal to the input voltage Ui, and the pulse width is equal to the on-time Ton of the control switch K. From this, the average value Ua of the output voltage uo of the series switching power supply can be obtained as:

The amplitude Up of the output voltage uo of the series switching power supply is equal to the input voltage Ui, and the average value Ua of the output voltage uo is always less than the input voltage Ui. Therefore, the series switching power supply generally uses the average value Ua as the variable output voltage. Therefore, the series switching power supply belongs to the step-down switching power supply.

The series switching power supply is also called a chopper. Due to its simple working principle and high working efficiency, it is widely used in output power control. For example, electric motorcycle speed controllers and light brightness controllers are all applications of series switching power supplies. If the series switching power supply is only used for power output control, the voltage output can be directly provided to the load without connecting to the rectifier and filter circuit; but if it is used for voltage stabilization output, it must be rectified and filtered.

The disadvantage of the series switching power supply is that the input and output share the same ground, so it is easy to generate EMI interference and the bottom plate is charged. When the input voltage is the rectified output voltage of the mains, it is easy to cause electric shock, which is unsafe for people.