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App Note: How to Use Power Inductors

As electronic devices become more advanced, the power supply voltage of LSIs used in them is lowered, so their power consumption can be reduced and their speed increased. However, a decrease in the power supply voltage also causes the requirements regarding voltage fluctuations to become more severe, creating a need for high-performance DC-DC converters to fulfill these characteristic requirements, and power inductors are important components that greatly affect their performance. TDK has a widely varied lineup of power inductor products, and this article describes and explains effective methods for using power inductors, and key points for selecting them, according to the required characteristics of DC-DC converters.

APP NOTE HOW TO USE POWER INDUCTORS

Power inductors are important components that largely affect DC-DC converter performance

Although inductors (coils) are capable of transmitting direct current smoothly, if the current varies they will generate electromotive force to obstruct those fluctuations. This is known as self-induction, and with alternating current it has the property of obstructing transmission to the point where it can appears at higher frequencies. Therefore, if a current is passed through an inductor it will be accumulated as energy, and if the current is interrupted this energy will be discharged. Power inductors are components which effectively apply this property and are used primarily in power supply circuits for equipment such as DC-DC converters.
Figure 1 shows a basic circuit for a step-down DC-DC converter (diode rectification type). Power inductors are important components that largely affect its performance.

Parameters related to power inductor characteristics have a complex trade-off relationship with each other

The difficulty in designing a power inductor stems from the variability of its characteristics according to factors such as temperature and current magnitude. For example, inductance (L) has the property of decreasing as the current becomes larger (DC superimposition characteristics), and temperature rises caused by increase of current can cause the inductor core magnetic permeability (μ) and saturation magnetic flux density (Bs) to change as well. Even with the same inductance value, the DC resistance (Rdc) will change depending on the winding thickness and the number of windings, causing variations in the degree of heat generation, and differences in the magnetic shield structure can also affect the noise characteristics.
These parameters have a complex, mutual trade-off relationship with each other, making it critical to select the best power inductor for an application from an overall perspective, with consideration for the efficiencies, sizes, and costs of DC-DC converters.

Key PointMagnetic cores of power inductors are broadly classified into ferrite and metal types

Power inductors can be broadly classified into wire-wound, multilayer, and thin-film types, according to differences in their production methods, with ferrite or metallic magnetic materials used in their cores. Ferrite cores have a high μ value and high inductance, while metallic magnetic cores have excellent saturation magnetic flux density, making them well-suited to larger currents.

Key PointThere are two types of rated currents for power inductors: allowed current for DC superimposition, and allowed current for temperature rise

If the core of a power inductor becomes magnetically saturated, its inductance value will drop. The guideline for the maximum current that can be transmitted without reaching magnetic saturation is the allowed current for DC superimposition (example: drop of 40% from initial inductance value). The current defined by the heat generation according to the electrical resistance of the windings is the allowed current for temperature rise (example: temperature rise of 40℃ due to self-heat generation). The rated current is generally considered to be the smaller of these two types of allowed currents.

The conditions of loss will change depending on the sizes and frequencies of loads

Key PointThe main types of loss which can cause rises in temperature are copper loss due to windings, and iron loss due to core materials

Loss which occurs due to windings is known as copper loss, while that due to core materials is known as iron loss. The main copper loss is caused by the DC resistance (Rdc) of the windings (DC copper loss), and increase proportionally to the square of the current. Also, as the frequency of AC current becomes higher, there is a tendency for the current flow to become concentrated in the area near the conductor surface and for the effective resistance value to increase (skin effect). In high frequency regions, copper loss resulting from the AC current (AC copper loss) will be added as well.
The main iron loss are hysteresis loss and eddy current loss. Eddy current loss is proportional to the square of the frequency, so in high frequency regions the core loss caused by eddy current loss becomes larger. One key point for improving efficiency is selecting core materials which have low core loss even in high-frequency regions.

For More Details: App Note: How to Use Power Inductors

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