开关电源原理与设计——开关电源的基本工作原理室外一体化电源机柜,高频开关电源，-48V开关电源

1.1 几种基本类型的开关电源

    顾名思义，开关电源就是利用电子开关器件（如晶体管、场效应管、可控硅闸流管等），通过控制电路，使电子开关器件不停地“接通”和“关断”，让电子开关器件对输入电压进行脉冲调制，从而实现DC/AC、DC/DC电压变换，以及输出电压可调和自动稳压。

    开关电源一般有三种工作模式：频率、脉冲宽度固定模式，频率固定、脉冲宽度可变模式，频率、脉冲宽度可变模式。前一种工作模式多用于DC/AC逆变电源，或DC/DC电压变换；后两种工作模式多用于开关稳压电源。另外，开关电源输出电压也有三种工作方式：直接输出电压方式、平均值输出电压方式、幅值输出电压方式。同样，前一种工作方式多用于DC/AC逆变电源，或DC/DC电压变换；后两种工作方式多用于开关稳压电源。

    根据开关器件在电路中连接的方式，目前比较广泛使用的开关电源，大体上可分为：串联式开关电源、并联式开关电源、变压器式开关电源等三大类。其中，变压器式 开关电源（后面简称变压器开关电源）还可以进一步分成：单管式、推挽式、半桥式、全桥式等多种；根据变压器的激励和输出电压的相位，又可以分成：正激式、 反激式、单激式和双激式等多种；如果从用途上来分，还可以分成更多种类。

   下面我们先对串联式、并联式、变压器式等三种最基本的开关电源工作原理进行简单介绍，其它种类的开关电源也将逐步进行详细分析。

　串联式开关电源的工作原理

图1-1-a是串联式开关电源的最简单工作原理图，图1-1-a中Ui是开关电源的工作电压，即：直流输入电压；K是控制开关，R是负载。当控制开关K接通的时候，开关电源就向负载R输出一个脉冲宽度为Ton，幅度为Ui的脉冲电压Up；当控制开关K关断的时候，又相当于开关电源向负载R输出一个脉冲宽度为Toff，幅度为0的脉冲电压。这样，控制开关K不停地“接通”和“关断”，在负载两端就可以得到一个脉冲调制的输出电压uo 。

图1-1-b是串联式开关电源输出电压的波形，由图中看出，控制开关K输出电压uo是一个脉冲调制方波，脉冲幅度Up等于输入电压Ui，脉冲宽度等于控制开关K的接通时间Ton，由此可求得串联式开关电源输出电压uo的平均值Ua为：

式中Ton为控制开关K接通的时间，T为控制开关K的工作周期。改变控制开关K接通时间Ton与关断时间Toff的比例，就可以改变输出电压uo的平均值Ua 。一般人们都把称为占空比（Duty），用D来表示，即：

串联式开关电源输出电压uo的幅值Up等于输入电压Ui，其输出电压uo的平均值Ua总是小于输入电压Ui，因此，串联式开关电源一般都是以平均值Ua为变量输出电压。所以，串联式开关电源属于降压型开关电源。

串 联式开关电源也有人称它为斩波器，由于它工作原理简单，工作效率很高，因此其在输出功率控制方面应用很广。例如，电动摩托车速度控制器以及灯光亮度控制器 等，都是属于串联式开关电源的应用。如果串联式开关电源只单纯用于功率输出控制，电压输出可以不用接整流滤波电路，而直接给负载提供功率输出；但如果用于 稳压输出，则必须要经过整流滤波。

串联式开关电源的缺点是输入与输出共用一个地，因此，容易产生EMI干扰和底板带电，当输入电压为市电整流输出电压的时候，容易引起触电，对人身不安全。

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1.1 basic types of switching power supply

As the name implies, the switch power is to use electronic switching devices (such as transistors, field effect tubes, thyristors, etc.) to make electronic switch devices "turn on" and "turn off" by controlling the circuit, so that the electronic switch devices can modulate the input voltage to realize the voltage transformation of DC/AC and DC/DC. And the output voltage can be reconciled with the automatic voltage regulator.

There are three modes of switching power supply: frequency, pulse width fixed mode, fixed frequency, variable pulse width mode, variable mode of frequency and pulse width. The former work mode is mostly used in DC/AC inverter or DC/DC voltage conversion, and the latter two modes are mostly used in switching regulated power supply. In addition, switching power supply output voltage also has three ways of operation: direct output voltage mode, average output voltage mode and amplitude output voltage mode. Similarly, the former work is mostly used in DC/AC inverters or DC/DC voltage conversion, and the latter two methods are mostly used in switching regulated power supply.

According to the connection of switch devices in the circuit, the widely used switching power supply can be divided into three categories: series switching power supply, parallel switching power supply, transformer type switching power supply and so on. The transformer type switching power supply can be further divided into single tube type, push-pull type, half bridge type and full bridge type. According to the phase of the excitation and output voltage of the transformer, it can be divided into several kinds, such as forward excitation, flyback, Dan Ji and double excitation. It can be divided into more types.

The following three basic principles of switching power supply, such as series, parallel and transformer, are briefly introduced. The other kinds of switching power supply will be analyzed in detail.

Working principle of series switching power supply

Figure 1-1-a is the simplest working principle of a 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; the K is the control switch, and the R is the load. When the control switch K is connected, the switch power output a pulse width of Ton to the load R and a pulse voltage of Up with a amplitude of Ui; when the control switch K is turned off, it is equivalent to the switching power supply to the load R with a pulse width of Toff and a pulse voltage of 0. In this way, the control switch K keeps switching on and 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 a series switching power supply. From the diagram, the control switch K output voltage uo is a pulse modulated square wave, the pulse amplitude Up is equal to the input voltage Ui, the pulse width is equal to the switching time Ton of the control switch K, thus the average Ua of the output voltage uo of the series switch power source can be obtained as follows:

The Ton in the form is to control the time when the switch K is connected, and T is the working cycle of the control switch K. Changing the ratio of control switch K switching time Ton to turn off time Toff can change the average value of UO of output voltage Ua. In general, people call it the duty cycle (Duty), which is expressed by D.

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 usually outputs the voltage with the average value Ua as the variable. Therefore, the series switch power supply belongs to the buck switching power supply.

The series switching power supply is also called the chopper. Because of its simple working principle and high working efficiency, it is widely used in the output power control. For example, electric motorcycle speed controller and lighting brightness controller belong to the application of series switch power supply. If the series switching power supply is only used for power output control, the voltage output can not be connected with the rectifier filter circuit, and directly provides the power output to the load, but if it is used for the voltage regulator output, it must pass through the rectification filter.

The shortcoming of the series switching power supply is that the input and output share one place. Therefore, it is easy to produce EMI interference and floor electrification. When the input voltage is the output voltage of the city electricity, it is easy to cause electric shock and unsafe for the person.