1. Basic working principle

In the most basic step-down switching power supply, the circuit structure is as shown in the figure. When the switch (corresponding to U1 in the figure) is closed, current flows in from VIN, passes through components such as resistor R1, and the VOUT voltage rises slowly. As time goes by, when the VOUT voltage reaches the voltage we expect, we quickly disconnect the switch. At this time, due to the energy storage effect of capacitor C1, the voltage can be stabilized at the ideal voltage value. Once a resistor (or other type of load) is applied to the output end, when the switch is disconnected, the load will consume the energy stored in capacitor C1, causing the voltage to gradually decrease. In order to maintain the output voltage stable near the target value, we need to continuously open and close the switch. In this way, the voltage will fluctuate slightly above and below the voltage value we want to obtain. Through this periodic switching action, the output voltage can be dynamically adjusted and the output can be approximately stable.

We can change the trend of voltage rise by changing the size of R1 and C2 (charging time = R×C). As shown, by continuously turning the switch on and off, the voltage obtained can be maintained at a certain value. However, the R1 resistor will have a great impact on the conversion efficiency of our circuit and increase energy consumption.

Therefore, we replace the R1 resistor with an inductor, because the inductor does not consume energy and the current flowing through the inductor will not change suddenly. However, there is a very big problem with the inductor. When the switch is turned off, the voltage across the inductor will change suddenly, so we need to add a freewheeling diode in front of the inductor.

As a non-isolated DC converter, the output voltage of the buck converter is less than or equal to the input voltage. The main circuit of the converter is mainly composed of a switch, a freewheeling diode, an output filter inductor, and an output filter capacitor.

The DC-DC buck chip we usually use in the circuit is equivalent to the switch of U1, which controls the closing of the switch to control the voltage. It contains one or more switching elements (such as MOSFET). These switching elements will turn on and off at a certain frequency, and adjust the output voltage by controlling the duty cycle of the switch (the ratio of the on time to the period). From this perspective, the switch is the core action component of the DCDC buck chip to achieve the buck function.

2. Synchronous Buck and Asynchronous Buck

2.1. Asynchronous Buck

When the switch is turned on, the input voltage is applied to the inductor, the inductor current rises, and the electric energy is stored in the inductor, while supplying power to the load and charging the capacitor; when the switch is turned off, the inductor generates a reverse electromotive force, the freewheeling diode is turned on, the inductor releases energy to the load through the diode, and the inductor current decreases. Since the circuit only requires one switch and one diode, the circuit components are relatively few and the cost is relatively low. The price of the diode is relatively cheap, especially the ordinary silicon diode.

The freewheeling diode has a forward conduction voltage drop. Generally, the forward voltage drop of the silicon diode is about 0.7V, and the Schottky diode is about 0.3-0.5V. When the current is large, the conduction loss of the diode will be relatively large, resulting in relatively low efficiency, especially in the application scenario of low output voltage and high current. It is suitable for application scenarios with low efficiency requirements, small output current, and cost sensitivity, such as power modules of some small electronic products.

The freewheeling diode has a forward conduction voltage drop. Generally, the forward voltage drop of the silicon diode is about 0.7V, and the Schottky diode is about 0.3-0.5V. When the current is large, the conduction loss of the diode will be relatively large, resulting in relatively low efficiency, especially in the application scenario of low output voltage and high current. It is suitable for application scenarios with low efficiency requirements, small output current, and cost sensitivity, such as power modules of some small electronic products. 

2.2. Synchronous Buck

When the high-side switch is turned on, the input voltage is applied to the inductor, and the inductor current rises; when the high-side switch is turned off, the low-side switch is turned on to provide a freewheeling path for the inductor, and the inductor current decreases. The output voltage is regulated by controlling the on and off time of the two switches. The circuit requires two switches, and a more complex drive circuit is required to control the alternating on and off of the two switches, so the cost is relatively high. However, as the price of MOSFET decreases, the cost gap between the two is gradually narrowing.

Use MOSFET to replace the freewheeling diode. The on-resistance of MOSFET is usually very small, and the conduction loss is also small. Therefore, under the same conditions, the synchronous Buck is more efficient and more suitable for low output voltage and high current applications. It is suitable for application scenarios with high efficiency requirements and large output current, such as computer motherboards, server power supplies, portable electronic devices, etc.