[Basic course series on passive components (LC)] LC selection ~Part 2 Causes of inductor failure~

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Inductors and capacitors must be selected according to the application. We will explain the points that need to be considered for selection.
The contents of this time will be Part 2 "Inductor failure factors".

If you want to see other articles, there is a summary page, so please take a look there.

Click here for a list of basic courses on passive components (LC)

Inductor failure factors

Inductors are less prone to failure than ICs and have a long lifespan, but they can cause failure if used incorrectly.

There are three main causes of inductor failure:

1. Breakage of internal windings and peeling of solder joints due to overcurrent

2. Physical damage such as core cracks due to vibration, impact, etc.

3. Internal short circuit due to overvoltage application

This time, we will explain the internal short circuit due to the application of overvoltage in 3.

Overvoltage breakdown of inductor

The main points to note in selecting an inductor are described in"Selecting an LC -Part 1 Selecting a Power Inductor".

This content is important in most selection cases, but depending on the conditions, it is necessary to consider the operating voltage, and if this is not done correctly, it can be a factor of failure.

What happens when the working voltage of the inductor exceeds the allowable value? The internal insulating material is destroyed, causing a short circuit and a decrease in the inductance value. Photo 1 shows a case where the internal winding was burnt due to the application of overvoltage.

Photo 1: Internal winding damage due to overvoltage
From Wurth Elektronik application note

Inductor working voltage

When using ICs and capacitors, we always check the rated voltage and withstand voltage, but there are cases where inductors are used without much concern. The method of describing the operating voltage of an inductor differs depending on the manufacturer, and there are cases where it is not specified, and there are cases where it is specified only in catalogs.

Many inductors, even though not listed, have operating voltages of 40V or higher, and are rarely a concern if used in applications operating below that. However, it should be checked when using a case where a higher voltage is applied or when using a small inductor.

Consider two cases.
Example 1: When using with AC input line

AC input (85-265V) will be approximately 375V after rectification. When used here, general-purpose inductors cannot be used, and it is necessary to select products that support high operating voltages. Fig. 1 shows the electrical characteristics of 7687779470 of Wurth Elektronik's "WE-PD HV series". This product specifies an operating voltage of 400V, and can also be selected with this specification.

Figure 1　7687779470 electrical characteristics
From the 7687779470 datasheet

Example 2: When used in a high voltage bias circuit

In the case of a sensor that is used with a high voltage bias, there is a specification that almost no current flows when several tens of volts is applied. Since the current is small, it is thought that a small inductance can be selected, but consideration of the operating voltage is necessary here as well. Many small inductors have an operating voltage of around 20V. Fig. 2 shows the electrical characteristics of 74405020100 of Wurth Elektronik's "WE-LQSH series". This product has a specified operating voltage of 25V and cannot be used at voltages exceeding this.