Select the buck converter's passive components

Power supply design is often the last consideration of the system. At this time, most users can choose a valid module - input a DC voltage to generate another voltage. This module can have different specifications, generate low voltages in steps, or generate high voltages in steps. At the same time, there are a number of dedicated solutions, such as step-up/step-down, flyback, and single-ended primary inductors (sepic), which can generate output voltages that are greater than, less than, or equal to the input voltage. For a system that operates on AC power, it may first be necessary to use the AC-DC module to generate the highest DC voltage required by the system. Therefore, step down converters, also known as buck converters, are the most widely used devices. Below, we first describe how to select a basic step-down voltage converter to improve light-load efficiency, and then discuss the considerations for selecting a peripheral device.

There are two main types of step-down converters, namely low-dropout regulators (LDOs) and switching regulators. The LDO gives a clean, stable voltage, and the switching regulator is suitable for more efficient operation. Efficient means that the energy lost during the conversion process is low, simplifying thermal management. Because switching regulators are highly efficient and are the most popular solution, we focus on this product. At the same time, for the sake of discussion, we will only briefly describe the buck converter. Figure 1 shows a basic type of step-down switching regulator with a synchronous buck converter. Synchronous buck represents the use of MOSFET as a buck switch (marked as Ilower in its direction in Figure 1). In contrast, standard buck regulators use Schottky diodes as buck switches. Since MOSFETs have less voltage drop than diodes, the main advantage of synchronous buck regulators is higher efficiency than standard buck regulators.

The timing information for the buck and boost MOSFETs is provided by a pulse width modulation (PWM) controller. Only one PWM input is shown in Figure 1. In many circuit diagrams, PWM has two inputs. The second input voltage to the circuit is the PWM supply voltage. Controller input and output constitute a voltage feedback loop. This loop allows the buck converter to adjust the output according to the load change. The output of the PWM module is a digital signal that switches up and down at the switching frequency. Only one MOSFET is turned on at a time. Turning on both MOSFETs at the same time will cause Vin to short to GND, which will reduce efficiency, so this method is not recommended. The duty cycle of this signal determines the proportion of time the input is directly connected to the output. Therefore, the output voltage is the result of the input voltage and this duty cycle.