TDK technical 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 Point: Magnetic 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 Point: There 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 Point: The 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.

Key Point: Copper loss becomes dominant during moderate to heavy loading, while iron loss becomes dominant during light loading

Power inductor loss also changes depending on the size of loads. For currents flowing through inductors during moderate to heavy loading, the DC bias current is large so the copper loss occurring due to the DC resistance (Rdc) of the windings becomes dominant. On the other hand, during light loading there is almost no DC bias current flow so the copper loss is reduced, but since constant-frequency switching operation is performed even during standby status, the iron loss due to the core materials becomes dominant and the efficiency decreases significantly (Figure 3).

It is essential to specify appropriate inductance values by considering factors such as ripple current

Key Point: Use in discontinuous mode will affect power supply stability

In power inductors for step-down DC-DC converters, there will be a flow of ripple current (ΔIL) with a waveform of continuous triangular waves, in association with the ON/OFF operation of its switching elements (Figure 4). During moderate to heavy loading, the ripple current will be superimposed on the DC bias current, so there will be no interruption in the inductor current, a state known as continuous mode(Iout＞1/2ΔIL). However, in diode rectification DC-DC converters, during light loading where Iout＜1/2ΔIL, there will be intervals where the inductor current will become zero. In this state, known as discontinuous mode, the inductor current will be interrupted intermittently and the stability of the power supply will therefore be affected. If the inductor operates in discontinuous mode, acoustic noise will also occur and ringing will be generated in the pulsed voltage waveform as a result of switching, which will contribute to the generation of noise.

Key Point: Choose the inductance value such that the ripple current becomes 20-30% of the rated current

The magnitude of the ripple current is related to the inductance of the power inductor. Diode rectification DC-DC converters should therefore be designed to avoid problems associated with operation in discontinuous mode, by restricting the ripple current. The inductance value L required for a power inductor can be determined by the following formula.

It can be seen from this formula that there is a trade-off relationship between the magnitudes of the inductance and the ripple current. If it is desired to use a power inductor with small inductance due to issues of size or cost, the ripple current will become larger. Conversely, if it is desired to reduce the ripple current, a large inductance will be required, which could lead to drawbacks from the viewpoint of size or cost, or could cause the transient response characteristics to worsen during sudden load changes, as will be explained later. Therefore, it is normally considered best to specify the inductance value so that the ripple current becomes 20-30% of the rated (it will be discontinuous at roughly 10% of the rated current) (Figure 5).

Key Point: Properly lowering the inductance value can improve the load response characteristics

At times such as when the load has increased suddenly, there will be a drop in output voltage, and to recover from it, an abnormally large peak current may flow through the power inductor over a short time to charge the output capacitor, along with the load current. If the ripple current has been set to a small value, however, it will not be possible to obtain the transient response characteristics required to recover quickly from a sudden drop in output voltage. One method of addressing this situation would be to reduce the inductance value and thereby increase the ripple current. As shown in Figure 6, the output voltage drops substantially if the load response characteristics are poor, but if the inductance value is lowered appropriately and the ripple current is increased, the change in the inductor current will become larger, causing the drop in voltage to be reduced and allowing faster recovery. However, when lowering the inductance value it will be important to use a setting that takes sufficient consideration for overall balance.

Key Point: To respond to the peak current generated during sudden load changes, set the current peak value to be 110-130% of the overcurrent setting.

Power supply ICs with modular switching elements, control circuits, or other such components are equipped with internal overcurrent protection circuits. Although the overcurrent setting values and detection methods may vary widely, overcurrent protection circuits must also be considered when selecting an externally-mounted power inductor. If there is insufficient leeway in the allowed current of a power inductor with respect to peak current, it may cause the overcurrent protection circuit to operate and stop the output. In general, the peak value of current flowing in a power inductor should be set to be roughly 110-130% of the overcurrent setting value. Also, as indicated by the DC superimposition characteristics graph in Figure 7, in situations where a flow of excessive peak current could occur, it would be suitable to use a metallic inductor whose core has gradual magnetic saturation characteristics (soft saturation) and which will not cause sudden drops in inductance.

Care must also be taken regarding leakage flux and acoustic noise

Key Point: Leakage flux from power inductors can affect their surroundings and cause noise

If the leakage flux from power inductor is large, it could affect their surroundings and cause noise. Power inductors with magnetic shield structures are available as a measure to reduce leakage flux, so it is important to select the most appropriate product type for application. For details, refer to the following article.
Application Guide: “Selection Guide for Power Inductors in Consideration of Leakage Flux”

Key Point: Use of the PFM method during light loading could create problems with “acoustic noise” in power inductors

One method used to improve the efficiency of DC-DC converters is switching from PWM mode, which performs switching at constant frequency and controls the pulse width under light-load conditions, to PFM (pulse frequency modulation) mode, which keeps the pulse width constant and controls the frequency. However, if the switching frequency becomes 20kHz or lower, caution will be required since a problem may occur in which the vibrations resulting from the core’s magnetostrictive effects and magnetic attraction will become audible as what is known as “acoustic noise“. This problem may also be caused by excessive fluctuations in load current.
Solution Guide: “Measures against Acoustic Noise in Power Inductors”

Required characteristics of DC-DC converters and power inductors

Table 1 shows the required characteristics of DC-DC converters and the related characteristics of power inductors, and Table 2 summarizes the main types of power inductors offered by TDK. For details on their characteristics and specifications, please use the Product Information pages and Selection Guides from the TDK Product Center, or our tools such as functions to search by characteristic values.