DC-DC｜Design

Summary

2017.06.08

Points of this article

・In DC-DC converter design, selection of inductors and input/output capacitors is particularly important.

・Circuit operation, current pathways, and the functions of components must be understood.

・Calculations are performed in accordance with the design manual, and optimization is executed based on actual measurements.

This is a summary of ” Selection of Inductors and Capacitors for DC-DC Converters” in “DC-DC Converters: Design Edition”.

As explained in this chapter, inductors and capacitors are important components of step-down DC-DC converters, and their selection has a large influence on performance and characteristics. This selection is explained using the data sheets, application notes, design manuals, and other supplementary materials for the power supply ICs that are used in nearly all cases. We begin by using these explanations and formulae to execute design.

However, even if design is executed according to the manual, the resulting operation is unfortunately not necessarily optimal. In such cases, component values are modified after evaluations; but if changes are made haphazardly, the design process may veer out of control, and even if problems are resolved, it may not be clear why performance improved, so that as a result the designer will not have the confidence to move to mass production.

In order to avoid this kind of situation, it is important that the basic operation of step-down DC-DC converters, and the currents flowing in these devices, be understood. Moreover, it is also necessary to understand in advance what changes and how when there is a change in inductances and capacitances, not only based on equations, but also from experience drawing on actual measurements.

These points are summarized below.

Selection of Inductors and Capacitors for DC-DC Converters

① Basic operation of step-down converters

In DC-DC converter design, selection of inductors and input/output capacitors is particularly important.
The circuit operation, current pathways, and the functions of components must be understood

②Inductor selection

Calculate the required inductance L
Calculate the maximum current flowing in the inductor (output current + 1/2 the ripple current)
Select an inductor with the calculated L value (or close to it), and with an inductor saturation current greater than the calculated maximum current
In short-circuit and transient states, a current greater than the calculated maximum current may flow, and so some think the inductor should be selected based on the maximum switching current
Basically, an operating margin is considered in light of calculated values
The margin should be determined based on company design rules and experience

③Output capacitor selection

Important elements of output capacitor selection
1）Rated voltage
2）Rated ripple current
3）ESR (equivalent series resistance)
The voltage applied to a capacitor and the ripple current must be at or below the maximum rated values for the capacitor
The ESR is an important factor in determining the output ripple voltage in relation to the inductor current

④Input capacitor selection

Important elements of input capacitor selection
1）Rated voltage
2）Rated ripple current and ripple heat generation characteristic
3）In the case of ceramic capacitors, temperature characteristics and DC bias
characteristics
The rated voltage must be higher than the maximum input voltage
The rated ripple current must be greater than the maximum input ripple current occurring in the IC input
In a step-down converter, the maximum value of the instantaneous input current is the same as the output current

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