Working Principle of PWM DC/DC Converter

Taking the Buck PWM DC/DC converter as an example, the working principle of the PWM DC/DC converter is introduced.

The working principle of the Buck PWM DC/DC converter in continuous conduction mode (COM) can be illustrated as shown in Figure 2 (a): In one switching cycle, the switching process of the switch tube chops the DC input voltage u to form a square wave with a pulse width of TON (Ton is the switch tube conduction time). When the switch tube is turned on, the diode is turned off, and the input DC power supply Ui transfers power to the negative

The load is connected to the inductor, and the inductor stores energy (the inductor current increases); when the switch is turned off, the diode conducts and the energy stored in the inductor is released to the load (the inductor current decreases). In one switching cycle, the average value of the inductor current is equal to the load current r. (The ESR of the filter capacitor C is ignored). The same method can be used to analyze the working principle of Boost, Buck-BccGt PWM BC/DC converters.

The output/input voltage conversion ratio of the PWM DC/DC converter operating in CCM mode is derived below. The starting point is that in steady-state operation, the volt-second area of ​​the switch tube during the on and off time is balanced within one switching cycle of the PWM DC/DC converter, that is, the integral of the voltage borne by the inductor over time within one switching cycle is zero:

Where uL is the voltage that the inductor bears;

Ts——switching period;

Where fa is the switching frequency;

Ton, Toff - the on-time and off-time of the switch tube in one switching cycle.

Duty Cycle Ratio or conduction ratio:

Taking the CCM Buck PWM DC/DC converter as an example, as shown in Figure 2 (a), we can get

Ui and Uo are the input voltage and output voltage of the Buck PWM DC/DC converter respectively.

From formula (2-1), we get

or (Ui-Uo)Ton=UoToff
(Ui-Uo)DuTs=Uo(1-Du)Ts

Therefore, the conversion ratio of the output/input voltage of the Buck PWM DC/DC converter working in COM mode is

Uo/Ui=Du

The duty cycle Du is always less than 1, so the Buck converter is a step-down converter. The efficiency of the ideal Buch converter is 1, so it can be considered

Ui/Ii=UoIo

From formula (2-2) and formula (2-3), we can get

Io/Ii=1/DU

It can be seen that the Buck converter is a current-boosting converter, and Io and Ii represent the average output current and input current of the PWM converter respectively.

Using the same method, it can be deduced that the output/input voltage conversion ratios of the Boost and Buck-Boost PWM DC/DC converters operating in COM mode are

Uo/Ui=1/(1-Du)

Buck-Boost PWM DC/DC Converter

Uo/Ui=Du/(1-Du)

From equations (2-5) and (2-6), we can see that the Boost converter is a step-up converter, which proves that the ideal Boost converter is a step-down converter. The Buck-Boost converter is a step-up/step-down converter. Whether it is step-up or step-down depends on the duty cycle Du. When Du＞0.5, Du/(1-Du)＞1, the Buck Boost converter is a step-up converter. When Du＜0.5, Du/(1-Du)＜1, it is a step-down converter.

In DCM mode, the output/input voltage conversion ratio of Buck, Boost, and Buck-Boost converters is different from that of Equation (2-2), Equation (2-5), and Equation (2-6). They can be obtained using the same method, but the calculation process is more complicated.

Buck, Boost, and Buck Boost PWM DC/DC converters are the most basic circuits (Basic topological configuration) of PWM DC/DC converters. Other forms of PWM DC/DC converter circuits can be derived from these three basic circuits.