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Buck circuit working principle and analysis of three working modes
The **Buck circuit** is a type of DC-DC converter that steps down the input voltage to a lower output voltage. It is widely used in power supply designs due to its efficiency and simplicity. The circuit consists of a switch (typically a transistor), an inductor, a diode, and a capacitor. The key feature of a Buck circuit is that it provides a continuous output current while the input current is pulsating.
When the switch Q1 is turned on, the inductor L1 begins to store energy, and the current through it increases linearly. During this time, the capacitor C1 charges up and supplies power to the load R1. This phase is known as the **on-state** or **conduction mode**. The equivalent circuit during this phase is shown in Figure 2.
When the switch Q1 is turned off, the inductor discharges through the freewheeling diode, causing the inductor current to decrease linearly. At this point, the capacitor maintains the output voltage, ensuring a stable power supply to the load. This phase is referred to as the **off-state** or **non-conduction mode**, and the corresponding equivalent circuit is illustrated in Figure 3.
The Buck circuit can operate in three different modes: **Continuous Conduction Mode (CCM)**, **Boundary Conduction Mode (BCM)**, and **Discontinuous Conduction Mode (DCM)**. Each mode affects the behavior of the inductor current and the overall performance of the circuit.
In **CCM mode**, the inductor current never drops to zero during the switching cycle. This results in smoother output voltage and lower ripple. The waveform for CCM is shown in Figure 4.
In **BCM mode**, the inductor current reaches zero at the end of each cycle, but just before the next cycle starts. This is a transitional state between CCM and DCM, and the waveform is depicted in Figure 5.
In **DCM mode**, the inductor current drops to zero and remains there for part of the cycle. This mode is often used in low-power applications and results in higher ripple but simpler control. The waveform for DCM is shown in Figure 6.
External parameters such as input voltage, output load, and inductance value significantly influence which conduction mode the circuit operates in. For example, increasing the inductance tends to promote CCM operation, while decreasing it may lead to DCM. A diagram illustrating these effects is provided in Figure 7.
To verify the operation of the Buck circuit, simulations are conducted under different conditions. In **CCM mode**, with an inductance of 120 µH, the simulation results confirm that the inductor current remains continuous. The theoretical calculation and simulation waveforms are shown in Figures 8 and 9.
For **DCM mode**, using an inductance of 40 µH, the simulation confirms that the inductor current becomes discontinuous. The relationship between input and output voltage, as well as the average output current, is also analyzed. These results are presented in Figures 10 to 13.
Overall, understanding the working principles and operating modes of the Buck circuit is essential for designing efficient and reliable power conversion systems.