# DC/DC

## Selection of Inductors and Capacitors for DC/DC Converters

### Inductor Selection

2017/04/27

When designing a step-down DC/DC converter, inductor selection is important. Inductor selection greatly influences performance and characteristics. The procedure for inductor selection and the method for calculating inductances and the like are basically indicated on the data sheets of the power supply IC that is used.

**Inductor selection procedure**

First, the procedure to use when selecting an inductor is described.

1） Calculate the required inductance L.

2） Calculate the maximum current flowing in the inductor (output current + one-half of the ripple

current).

3） Select an inductor having the calculated value L (or a value close to it) and with an inductor

saturation current at least as great as the calculated maximum current.

※In the event of a short-circuit or transient state, there are cases in which a current greater than

the calculated maximum current may flow; hence another method is to select the inductor based

on the maximum switch current.

In essence, the margin is considered and determined based on calculations. The method for determining the margin depends on the design rules of the company and empirical rules.

1）Inductance calculation

First, the following equations are used to calculate the inductance.

2）Calculation of the maximum current of the inductor

Next, the maximum current of the inductor is calculated according to the following equation.

As is clear from the equations and the current waveform, I_{LPEAK} is equal to one-half of ΔI_{L} with I_{OUT}I_{OUT} added.

From the calculated inductance and maximum current of the inductor, an inductor is selected having approximately the calculated inductance and a saturation current at least as large as the maximum current. Below an example of selection is described.

**Example of Inductor Selection**

Conditions: V_{IN} = 12V, V_{OUT} = 3.3V, I_{OUT}I_{OUT} = 2A, r = 0.3, f _{SW} = 380kHz

From the above results, the starting point will be an inductor of 10 μH, with a saturation current of at least 2.3 A. "Starting point" means that these calculations are not absolute, and modifications may be necessary when such things as short-circuits and transient states are taken into consideration.

**Inductor Current when the Inductance is Changed**

Here, in order to deepen our understanding of inductor operation, we explain the changes in the inductor current when the inductance is changed. The following chart shows I_{LPEAK} when the inductance is set to 0.4 μH, 1 μH, and 2.2 μH, under the same operating conditions.

As is also clear from the equation, when the inductance L is smaller, I_{LPEAK} increases, and a larger superimposed direct current can be accommodated. However, the increase in I_{LPEAK} necessitates tolerance of a larger superimposed direct current. If the inductance is made larger, the reverse applies, and there is the need to study phase compensation.

## Key Points:

・Inductor selection is extremely important in DC/DC converter design.

・The circuit operation, current paths, and the relation between the inductor and the output current must be understood.