Introduction to the topology, working principle and characteristics of the step-down DC/DC converter

Step-down DC/DC converter, referred to as buck converter, also known as Buck Converter in English, is one of the most commonly used DC/DC converters. A buck converter can convert a higher DC voltage into a lower DC voltage, such as converting 24V voltage into 12V or 5V voltage. The buck converter has very small losses, high efficiency, and has a wide range of applications.

The topology structure of the step-down DC/DC converter The topology structure of the step-down DC/DC converter is shown in Figure 2-1-1. In the figure, UI is the DC input voltage, VT is the power switch tube, VD is the freewheeling diode, L is the output filter inductor (also called energy storage inductor), C is the output filter capacitor, UO is the DC output voltage, RL

is the external load resistor. The pulse width modulator (PWM) is used to control the on and off of the power switch VT and is the control core of the converter.

Figure 2-1-1 Topology of step-down DC/DC converter

The working principle of the step-down DC/DC converter. The power switch tube VT of the step-down DC/DC converter is alternately turned on and off (also called cut-off) under the control of the pulse width modulation (PWM) signal. A mechanical switch closes and opens at high speed. Its working principle is shown in Figure 2-1-2. Figure 2-1-2(a) and Figure 2-1-2(b) show the current path when VT is turned on and off respectively. In order to facilitate circuit analysis, the closing and opening of switch S are used in the figure. Replaces the turn-on and turn-off of VT.

When VT is turned on (that is, S is closed), as shown in Figure 2-1-2(a), the freewheeling diode VD is turned off, and the input voltage UI is added to the left end of the energy storage inductor L, so (UI - UO

) voltage, the current IL through L increases linearly, and the energy stored in the inductor also increases. The induced electromotive force of the inductor is "+" on the left and "-" on the right. During this period, the input current (i.e., the inductor current IL

) In addition to supplying power to the load, there is also a part that charges the filter capacitor C. The inductor current IL is the sum of the capacitor charging current I 1 and the load RL current IO.

Figure 2-1-2 Working principle of step-down DC/DC converter

When VT is turned off (that is, S is disconnected), as shown in Figure 2-1-2(b), the inductor L and UI

disconnect. Since the inductor current cannot mutate, an induced voltage of "-" on the left and "+" on the right is generated on L to maintain the current IL through the inductor.

constant. At this time, the freewheeling diode VD is turned on, and the magnetic field energy stored in L is converted into electrical energy, which continues to supply power to the load through the loop formed by VD, and the inductor current IL decreases linearly. At this time, the filter capacitor C generates a discharge current I

2 is superimposed with the inductor current IL to power the load RL. The load current IO is the sum of the inductor current IL and the capacitor discharge current I 2 .

Tips

The buck converter transmits energy to the load when the power switch is turned on and is a forward converter.

The voltage and current waveforms of the step-down DC/DC converter are shown in Figure 2-1-3. PWM represents the pulse width modulation waveform, t ON is the conduction time of the power switch tube VT, t OFF

is the turn-off time of the power switch tube VT. T is the switching period, and its value is the sum of t ON and t OFF, that is, T=t ON +t OFF. where t ON

The ratio to T is called the duty cycle, represented by "D", that is, D=t ON /T.

Figure 2-1-3

UE is the emitter voltage waveform of the power switch tube VT, and IC is the collector current waveform of VT. IF is the current waveform of freewheeling diode VD. IL

is the current waveform of the filter inductor. It can be seen that when the power switch VT is turned on, its emitter voltage UE is equal to the input voltage UI; when VT is turned off, its emitter voltage UE

is zero. During the turn-on period of the power switch VT, the inductor current increases linearly; during the turn-off period of the VT, the inductor current decreases linearly. The inductor current IL is determined by the collector current IC of VT and the current IF of the freewheeling diode VD.

formed by superposition.

The output current IO of the DC/DC converter is the filter inductor current IL

average of. The difference between the peak value and the valley value in the inductor current waveform is the inductor ripple current. In order to reduce the ripple of the output current, L should be selected large enough to make the DC/DC converter work in continuous mode. Usually the ripple current should be about 20% of the rated output current.

Step-down DC/DC converters have the following characteristics.

① Output voltage UO

② The output voltage UO and the input voltage UI have the same polarity.

③ The maximum voltage that the power switch tube VT can withstand is U CE =UI.

④ The maximum current of the power switch VT collector IC =IO.

⑤ The average current IF of the freewheeling diode VD = (1-D)IO.

⑥ The reverse voltage UR that the freewheeling diode VD withstands is UR =UI.

Step-down DC/DC converters can be composed of discrete components and PWM controllers, or integrated circuit products can be selected. Typical integrated circuit products include LM2576, LM2596, L4960, etc. Among them, the peripheral circuit of LM2576 is the simplest.