1.3 Types of power inductors
Starting from here, we will introduce the different types of power inductors. The Murata lineup includes power inductors with winding Metal Alloy, winding ferrite, and multilayer ferrite structures. We provide the optimal inductors for a wide range of applications from mobile devices such as wearables and smartphones to medical, industrial electronics, and automotive equipment.
Figure 1-8 Power inductors structures
Let’s take a look at the advantages and technologies behind each structure.
＜Winding Metal Alloy＞
Winding Metal Alloy is an inductor which bonds the winding and resin-coated metal magnetic powder with thermocompression. It can be applied to high current areas in large-sized to small-sized products. While metal magnetic materials have a lower magnetic permeability when compared to the ferrite materials described below, they have superior DC superposition characteristics, so they are materials which are well-suited to high currents. As DC-DC converters have shifted toward high-speed switching in recent years, low inductance is required, so winding Metal Alloy is becoming a leading product in the majority of the market.
Moreover, the superior temperature characteristics compared to ferrite materials are also a major advantage. Because the fluctuations in magnetic permeability are small with regard to the ambient temperature, it can maintain stable DC superposition characteristics even at high temperatures. The wide-ranging target markets include automobiles, smartphones, HDDs, etc.
Figure 1-9 Structure and appearance of winding Metal Alloy
Winding Metal Alloy technologies include metal magnetic materials and their processing techniques as well as winding techniques using copper wire (Figure 1-10). At Murata, we have established material technologies which realize our own winding structure, high magnetic permeability, and insulation. By combining these technologies, we have improved the efficiency of achieving inductance, enabled low DC resistance, and created a product lineup which supports high currents.
Figure 1-10 Cross-sectional view of winding Metal Alloy
Winding ferrite： winds copper wire in a spiral shape around a ferrite core. Many of Murata’s winding ferrite products coat the copper wire, wound around the ferrite core, with a magnetic resin. The purpose of the resin coating is to reduce the leakage flux, improve the achievement of inductance, and increase the intensity. Because the magnetic permeability of ferrite material is high, there are advantages to selecting winding ferrite when using it in a high inductance area. The wide-ranging target markets include smartphones, TVs, HDDs, etc.
Figure 1-11 Structure and appearance of winding ferrite
Figure 1-12 Cross-sectional view of winding ferrite
Multilayer ferrite is an inductor which alternately laminates and sinters the magnetic material and inner electrodes. Compared to the winding structure, it enables compact and low-profile form factors. While the cases where winding Metal Alloy is used to satisfy the demand for small size and low inductance have been increasing, the characteristics of multilayer ferrite will be needed in areas requiring a small size with a high inductance and high voltage.
Figure 1-13 Structure and appearance of multilayer ferrite
Multilayer ferrite technologies include ferrite material technology and technology for forming inner electrodes with a high aspect ratio, circuit design technology, and lamination technology (Figure 1-14). Murata has obtained high aspect technology for inner electrodes, which was not achievable with previous sheet lamination that enables us to provide even lower resistance. In addition, the magnetic path gap forming technology with a high degree of freedom suppresses magnetic saturation to achieve a vastly superior DC superposition characteristic. These technologies realize a product lineup which supports areas that require a compact and low-profile form factor.
Figure 1-14 Cross-sectional view of multilayer ferrite
Next, let’s compare the performance of these power inductor structures.
＜Performance comparison of power inductors＞
The primary factors for comparing power inductor performance are the 1) inductance value, 2) DC superposition characteristic, 3) temperature characteristic, 4) voltage endurance, and 5) leakage flux. Knowing these factors will enable you to select the power inductor structure which is suited to the required level of performance.
1) Inductance value
The range of obtainable inductance values is determined by the structure of the inductor. Winding ferrite can obtain a wide range starting from ferrite materials with a high magnetic permeability up to a high inductance of 10 uH or more. Due to the fact that multilayer ferrite is more compact when compared to winding ferrite, it achieves a low inductance of 10 uH or less. Winding Metal Alloy is noted for its low inductance of 10 uH or less due to its material characteristics.
Figure 1-15 Range of inductance by structure
2) DC superposition characteristic
A circuit which conducts a high current, such as a digital circuit, requires a power inductor with an inductance that does not decrease due to a high current, that is to say, a power inductor with a good DC superposition characteristic. This means that because the inductance does not decrease, the ripple currents are kept constant, and stable circuit operation can be maintained. In the case of winding Metal Alloy, because it is difficult for magnetic saturation to occur in comparison to ferrite, it possesses a superior DC superposition characteristic.
Figure 1-16 Superposition characteristic by structure
3) Temperature characteristic
When power inductors are used under high temperatures, such as in the power supply circuit of an automobile, the temperature characteristic becomes extremely important. Within magnetic materials, there is a temperature characteristic whereby the magnetic permeability changes according to the temperature, but metal magnetic materials have smaller changes in magnetic permeability due to temperature than ferrite materials and can be said to have superior characteristics.
In the case of winding Metal Alloy, there are no significant changes in the inductance value or the DC superposition characteristic. Figure 1-17 shows the DC superposition characteristics of winding Metal Alloy and ferrite products across a range of ambient temperatures from 25°C to 125°C. We can see that characteristic of winding Metal Alloy is unchanged between 25°C and 125°C.
Figure 1-17 Temperature characteristic by structure
4) Voltage endurance
It is important to pay attention to the power inductor voltage endurance in an LED or other voltage boosting circuit and power supply circuits with a high step-down voltage ratio. In the case of winding Metal Alloy, it ensures the insulation by covering the metal magnetic powder with an insulating resin, but the insulation tends to be lower when compared to winding ferrite. For this reason, while winding Metal Alloy possesses many superior characteristics, confirmation is required when using it in a high voltage endurance situation.
5) Leakage flux
The leakage flux from an inductor affects other circuits as noise which can lead to problems such as signal degradation and malfunctions particularly in power supply circuits with restrictions on the distance between components. The magnitude of the leakage flux is highly dependent on the structure of the inductor, and the closed magnetic circuit structures of the winding Metal Alloy and multilayer ferrite are helpful in this regard. This is because the winding Metal Alloy and multilayer ferrite enable you to reduce the leakage flux to the outside when trying to obtain the same inductance (Figure 1-18). The comparison results for leakage flux by structure are shown in Figure 1-19. From the results, we can see that the winding Metal Alloy and multilayer ferrite are able to suppress the leakage flux to low levels compared to the winding ferrite.
Figure 1-18 Cross-sectional views of winding Metal Alloy and multilayer ferrite
Figure 1-19 Leakage flux by structure
Table 1 summarizes the performance comparison. When selecting a power inductor, please refer to this table to select the inductor which is best suited to your application.
Table 1 Performance comparison by structure
Finally, we will introduce our lineup of recommended power inductors. Power inductor applications can be largely classified into general use and automotive use, but in this example we looked at general use inductors in terms of the inductance value and size. Winding Metal Alloy can take on a wide range of inductance values and sizes. Winding ferrite possesses an advantage as a high inductance product while the strength of multilayer ferrite lies in its compact size.
Figure 1-20 Power inductor (for general use) lineup
Contents：Chapter 1: What is a Power Inductor?