Passives

inductance associatedwith the circuit. An inductor dissipates power both in thematerial of the core and in thewindings. Core loss is calculated by taking the difference between the energy that goes into the core during the on portion of the cycle and the energy that goes out of the core during the off cycle. The following table provides data on the core loss and related cost of four different core types, though additional calculations ormanufacturer supportmay be necessary to account for core loss in a specific design.

Core loss and cost by material type

The power

dissipation is given as the sumof the Irms current power calculated fromthe

classic equation P = 12 R and the AC ripple current for core loss. Temperature and saturation

When examining the temperature range for the inductor, a designer should be aware that the operating temperature and the ambient temperature are not one and the same, but rather the operating temperature is the difference of the ambient temperature and self-rising temperature. For example, a given component with an operating temperature in the range of -40 to 125°C with a self-rise temperature of 40°Cmeans the ambient temperature is actually -40 to 80°C where the difference is taken only on themaximal temperature. Since the operating temperature cutoff is -40°C, the range is not able to extend beneath that value. Furthermore, temperature is a function of power dissipation and surface are given by the equation:

ΔT = f(power dissipation) / f(surface area). Saturation current, denoted as Isat is defined as the amount of

DC current to cause inductance drip. This is typically 5-35% of its initial value. Isat

is a benchmark of current handling capability for

an inductor, characterising the amount of energy that can be stored based on the relationship of energy that is stored in the inductor, which is ½ LI2

and essentially infinite. The term

saturation current does not imply that the inductor core has reached the saturation stage; rather the component is at the magnetized stage. Isat

can be considered as the peak current of a

switching regulator to account for worst case saturation. Since a high saturation current does not drive the component to zero inductance, it becomes an air inductor remaining above zero. If the current goes high enough during operation then the core material and wire insulation can be damaged at high temperature. Inmost cases, the operation of an inductor is limited by its temperature rise.

Choosing the optimal inductor

Once all of the information for a part has been calculated, a search throughmanufacturers’ product catalogs will allow designers and buyers to choose the proper component for their product. Next- generation components feature inductance of 0.1 uH to 15mH, current ratings from60mA – 43 Arms

, and saturation current of up

to 65 A. Choosing the right part is simple when the following sequence is used and this process is typically applicable formost applications:

1. Choose the category that corresponds to the design’s current based on themaximal output current.

2. Find components that have the necessary inductance. 3. Choose the desired DC resistance.

4.Narrow the choices to the type of construction or coil type desired.

44 | November 2010 5. Select the specific part number.

6. If necessary, use a cross reference table to convert to the appropriate component.

Next-generation inductor designs

Applications such as cellular phones,mobile devices, notebook computers, industrial electronics, and entertainment electronics are creating new demands on inductors in terms of size, performance, and radiation. Next-generation power inductor designs include surfacemount that are non-shielded and shielded. Some non-shielded designs range in inductance from0.68 μH to 15mH. The output current capacity of these types of inductors is 0.06 to 16 A. A variety of package shapes and sizes are available including round, oval, and square offered with a profile as low as 2.2mm. Shielded surfacemount designs are offered in round or square packages ranging in inductance from0.47 to 15,000 μH. The output current capacity ranges from0.07 to 20 A. These surfacemount power inductors are ideal for car stereos, CD players, camcorders, LCD displays, portable communication devices, and at the input and output of DC-DC converters.

Additional inductor designs include shielded construction for

low radiation and packages that feature an octagonal body to eliminate part rotation in a tape carrier. This also ensures proper orientation when placed on the board. High powered shielded inductors are gaining popularity inmany applications. Other important next-gen features include the adoption of flat wire instead of round wire. This is especially beneficial in high frequency applications up to 55MHz. The advantages of using flat wire within the component are a compact package and the ability to optimize the space utilization afforded by stacking the devices on top of one another. A flat wire allows 95% of the area to be used, amarked improvement over the 70% utilization of a package containing round wire. In addition to amore compact package, more surface area is available to reduce AC resistance. At high frequencies, the skin effect dictates that the current will travel on the surface. Contrasting a round and a flat conductor with the same cross sectional area of 0.64mm2 4.64mm2

, the unit surface area of for the flat conductor is nearly double the 2.83mm2

provided by the round wire. Themolded core construction of the device provides additional benefit since the package space is fully utilized to allowmaximal performance with the corematerial molded to the winding. The space inside the package is filled with themagnetic compound and provides high inductance, shielded construction as well as high current handling capacity.

Custompower inductor designs

Somemanufacturers are now offering the ability to provide a customsolution in as little as six to eight weeks. Designers are able to work with themanufacturer to adjustmanufacturing parameters such as the core type and number of turns to deliver a custom component thatmeets their exact needs for package current, inductance, and size requirements.

Fundamental building blocks

In this consumer-drivenmarket, portable electronics are increasingly shrinking in size tomeet user expectations and demand. Tomaintain their competitive edge, designers and buyers are nowable to startwith themost fundamental building blocks such as highly reliable and compact inductors. With a grasp on the basic physics of inductors and an example to follow, the guesswork previously associatedwith selecting a power inductor has been removed. Following the guidelines set forth in this article, the appropriate inductor can be incorporated in any design to ensure reliability and longevity in even themost demanding applications.

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