Basic Operation of Step-Down Converters

In this chapter, we explain the methods for selecting the inductors and capacitors that are important components of step-down DC/DC converters in particular, the selection of which can have a major impact on performance and characteristics. In order to understand the components, it is necessary to know about the basic operation of a step-down DC/DC converter and the flow of currents in its operation. Hence by way of a review, we begin by explaining the basic operation and current paths.

Basic operation of step-down converters

Below, the basic circuit and operation of a step-down converter, as well as the flow of current, are illustrated.

The basic circuit diagram shown is of a diode-rectifier type (non-synchronous rectification) step-down circuit. In the case of synchronous rectification, D₁ is replaced with a switching element (transistor), which is turned on and off in operation opposite that of Q₁, but the basics are the same.

The diagrams below summarize Fig. 1 and Fig. 2 above, showing the voltage or current waveform at each major node.

It is easy to see how, as Q₁ is turned on and off, the drain current I_D, inductor current I_L, as well as the output capacitor current I_CO and input capacitor current I_CIN change.

With a firm understanding of the circuit operation, one can understand the jobs performed by the various components and the characteristics these components should have.

Hereafter, with this operation and current flows in mind, we will proceed to explain inductor selection.