Principle of BUCK Step-Down Power Supply

In electronic circuits, power supplies are generally divided into two types: linear power supplies and switch-mode power supplies. Linear power supplies have the advantages of simple circuitry, small size, and low noise. Switch-mode power supplies, although they generate more noise and have a larger footprint, are widely used due to their high efficiency and low heat dissipation.

Figure 2-1The schematic diagram and internal block diagram of a buck converter.

Switch-mode power supplies can be further classified into buck, boost, and buck-boost types. They can also be subdivided based on whether they are isolated or non-isolated, as well as synchronous or non-synchronous. In the field of consumer electronics such as mobile phones and computers, the buck type is very popular and is considered an entry-level course for power engineers. Figure 2-1(a) shows the schematic diagram of a DC-DC buck converter. If one only designs based on the official reference schematic without understanding the principles of the buck converter, it may not result in an excellent and reliable power supply. Therefore, we must approach the problem from a more in-depth perspective to improve our personal abilities. Figure 2-1(b) shows the functional block diagram inside the chip. We can see that there are two MOS transistors, Q1 and Q2. This type of buck converter with two control MOS transistors is called a synchronous buck converter. In asynchronous buck converters, a diode is used instead of Q2. The switching part and the external inductor in Figure 2-1 can be extracted to form the classic asynchronous buck topology shown in Figure 2-2. In Figure 2-2, S corresponds to Q1 in Figure 2-1, and D corresponds to Q2 in Figure 2-1.

Below, we will introduce the basic operating principle of a buck converter and conduct a simulation based on this principle. To focus our main efforts on understanding the BUCK principle, we will choose an asynchronous BUCK for open-loop analysis, where the circuit only has one switch transistor and the diode is used for inductor discharge. This can be seen in Figure 2-2. Simple components, when combined, can often produce incredible results, and that's exactly what the BUCK circuit does.

Figure 2-2 The topology of a buck converter.

In Figure 2-2, when the switch S is turned on, the SW voltage is high and equal to Vin. Vin charges the inductor L, and the current flowing through the inductor gradually increases. The charging current path of the inductor is shown by the dotted arrow in the figure, and the inductor current charging waveform can be seen in Figure 2-3. When the switch S is turned off, SW goes low, and the inductor L discharges through the load and diode D. The current flowing through the inductor gradually decreases, and the discharge current path can be seen in the solid arrow part of Figure 2-2. The inductor discharge waveform can be seen in Figure 2-3. The basic operation of the BUCK converter is the process of charging and discharging the inductor.

Here's a note: in synchronous BUCK converters, the diode D is replaced by a switch transistor to improve efficiency. When S is turned off, the voltage at the SW position is 0, but in this chapter, a freewheeling diode is used, so there is actually a negative voltage component of about -0.7V at SW when S is turned off.

Figure 2-3 The waveforms of the BUCK switch node voltage and inductor current.

Now let's derive the calculation relationship between the input and output voltage of a BUCK converter. Let's ignore the volt-second characteristics for now and focus on the most basic, essential formula related to the inductor:

V is the voltage across the inductor, L is the inductance, △I is the change in current across the inductor, and △t is the elapsed time. By transforming equation (2-1), we obtain equation (2-2):

After the BUCK converter reaches steady state, the current flowing through the inductor during one switching cycle is equal for charging and discharging. TD is the charging time and T(1-D) is the discharging time (T is the switching period; D is the duty cycle, which is the percentage of time that the switch (upper transistor) is conducting during the whole switching period). At steady state, the charge and discharge currents are equal, and we can obtain equation (2-3):

During charging, we can obtain equation (2-4), where the voltage across the inductor during charging is equal to Vin-Vout.

Similarly, we can calculate the change in inductor current during the discharging process as follows:

By combining equations (2-3), (2-4), and (2-5), we can obtain:

Next, let's take a look at the BUCK simulation based on Multisim software. The simulated circuit provided in this book is only for communication and learning purposes. In actual engineering, there are too many parameters and factors to consider. This book only focuses on the knowledge content involved. The simulation schematic can be seen in Figure 2-4, with an input voltage of Vin=10V, a switching frequency of 2kHz, a 10Vpp square wave, a duty cycle of 50%, and an inductance of 2.2mH.

Figure 2-4 The simulation of a BUCK power supply.

Figure 2-5 shows the results after adding a 47uF capacitor in parallel to the output of the BUCK power supply. The red square wave is the voltage waveform at the switch node SW (the -0.7V voltage of the square wave is explained in detail in section 2.5.1), and the black smooth curve is the current waveform of the output, which is the waveform of the inductor charging and discharging current. By controlling the inductor charging and discharging through the switch transistor, we can clearly see the smooth triangular waveform of the current during charging and discharging (the capacitor has a filtering effect). The input voltage is 10V, the duty cycle is 50%, and the measured output voltage DC value is 4.3V, close to the 10*50%=5V calculated from formula (2-6). Why is there a difference of 0.7V between the calculation and simulation?

Figure 2-5 The simulation waveform when a 47uF output capacitor is connected in parallel.

Figure 2-6 shows the simulation result with the C1 47uF capacitor disconnected, and the black current curve is closer to a triangular waveform (without the filtering effect of the capacitor), and the ripple of the output voltage becomes steeper. Changing the output capacitor helps to alleviate the output voltage ripple. However, due to the inherent characteristics of the BUCK switching architecture, this ripple cannot be eliminated but only suppressed.

 

Figure 2-6 The simulation waveform without an output capacitor.

At the same time, we can also see that due to the presence of the freewheeling diode, there is a negative voltage segment in the red waveform when the switch is turned off, which is approximately -0.7V and related to the diode's conduction voltage. Meanwhile, due to the presence of this diode, the input-output relationship is slightly changed, resulting in a slight difference from formula (2-6). This diode consumes energy, so a synchronous BUCK circuit was developed to further improve the efficiency of the BUCK power supply by replacing the freewheeling diode with a switch transistor.

This concludes the introduction to the principles of the BUCK step-down power supply.