Passives optimal inductor


Given the myriad of choices for inductors currently available, choosing the right inductor for a power converter is not always a simple task By Guido Zehnder, Bourns


Bourns' Guido Zehnder


Today’s electronic devices have become increasingly power hungry and can operate at higher switching frequencies, causing themto be starved for performance as never before. Inductors are a


fundamental element in the voltage regulator topology, and virtually every circuit that regulates power in electronic applications or a DC- DC converter requires an inductor. Tomeet the high-performance requirements of next-generation systems, newinductormodels have been introducedwith rated DC current up to 60 A.


Given themyriad of choices for inductors currently available,


properly selecting an inductor for a power converter is not always a simple task for designers and purchasing professionals. Beginning with the basic physics behind inductor operations, a designermust determine the ideal inductor based on radiation, current rating, corematerial, core loss, temperature, and saturation current. A buyer thenmust choose a component that meets all these specs, is on its approved vendor list, andmeets price/performance expectations. This article will outline these considerations to illustrate the role each of these factors plays in choosing the best inductor for a circuit. It will also examine the options available for various applications with special emphasis on new cutting-edge inductor product trends that offer advantages in performance, reduced size, and ease of designmodification.


the physics behind inductor operations


An inductor can be simply described as a component that stores energy in the formofmagnetic flux. Faraday’s Law of Induction states that change inmagnetic flux induces electromotive force that opposes the driving current. The below formula describes this relationship:


e = -ndφ/dt = -l dl/dt. Current through an inductor


is transformed to produce a magnetic field. It charges and current riseswhen the circuit is on. When off, the inductor acts as a current source, discharging current that flows through the circuit as the magnetic flux collapses.


Operation of inductor in a DC-DC buck converter


In continuous currentmode, the


inductor current does not fully discharge to zero during the portions of the cycle inwhich it is off. A voltage change across the inductor is instantaneouswhile the current through an inductor varieswith time.


The voltage and current components


of the inductor are out of phase such that the voltage leads by 180°.


Inductor current over time Selecting the shielding environment


One of first considerations before selecting an inductor is its placement on the board and the sensitivity of components that will be in its immediate vicinity. A shielded or non-shielded category must be chosen and each has special advantages and disadvantages. The main advantage of a shielded inductor is its low radiation, which can be characterised, whereas the flux of its non-shielded counterpart is neither steady nor confined to a specific vicinity. Larger size, a faster inductance roll-down, greater cost, and a lower current rating are the disadvantages of a shielded inductor. Non-shielded components are smaller, rated for higher current, and cost less although these advantages may be of no value if the radiation environment is a key requirement in a design. Another option that is growing in popularity due to its unique design features is the toroid inductor. The toroid shape results in an inductor that performs like a shielded component. Distributed air gap construction provides high current handling capacity.


inductor rated currents


When current flows through an inductor the temperature of the component rises, AC ripple current causes core loss, and DC current causes inductance to drop. The amount of steady state DC current, referred to as Irms


20-40°C range and effectively provides a benchmark of power dissipation for the component. Anotherway to classify Irms output or average current of a switching regulator.


, causes the temperature to rise to typically in the is the


corematerial and loss Core loss is related to thematerial chosen and the core cross sectional area aswell as the ripple current, switching frequency, and


November 2010 | 43 Voltage across an inductor Selectingthe


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