What Is a Power Inductor?
A power inductor is an element of a coil used in a power circuit among inductors.
They are also called power chokes. There are three basic types of construction: windings formed by lamination technology, windings on a bar-shaped core, and windings on a drum-shaped core.
Inductors, including power inductors, have a property that allows DC current to pass through, but prevents AC current from passing through. This property is used to eliminate high-frequency noise in circuits for voltage conversion.
Impedance is a measure of the resistance to the passage of alternating current, and the following formula for the impedance of an inductor easily explains this property.
Impedance of inductor [Ω]ZL=jωL
(L: inductance [H] ω: angular frequency [rad/s])
Uses of Power Inductors
Power inductors are used in a wide range of fields such as home appliances, communication equipment, and transportation equipment to stabilize the voltage supply of power circuits. Examples include smartphones and automobiles.
Controlling the voltage of a DC power supply is very important for any electronic device. The reason is that electronic devices have a range of supply voltages that can guarantee normal operation, and the ability to supply the proper voltage to each component affects stable circuit operation and the life of the equipment.
A DC-DC converter is a circuit that converts voltage, and the supply of stable voltage depends on the performance of the Power Inductor. There are two types of converter circuits: a step-down type that lowers the voltage, and a step-up type that raises the voltage. Both types contain switching elements, diodes, and power inductors as components.
Principle of Power Inductor
When an alternating current flows through an inductor, electromagnetic induction generates an induced electromotive force proportional to the time rate of change of the current.
Induced electromotive force [V]e = -L* (dI/dt)
(L: inductance [H] I: current [A] t: time [sec])
Since the direction of the induced electromotive force is the direction that counteracts the original magnetic flux change (Lenz’s law), it is marked with a minus sign to indicate this explicitly. An induced electromotive force is generated in the direction that cancels the current change, making it difficult for the alternating current to flow.
The proportionality constant involved in the induced electromotive force is the inductance (unit: Henry [H]). It is expressed as the ability to convert electrical energy into magnetic energy.
To increase inductance, there are several methods, such as “increasing the cross-sectional area of the coil,” “increasing the number of turns,” and “adding a core to increase the magnetic permeability.” For use in converters, the inductance is determined according to the switching frequency.
Compared to inductors for other applications, power inductors are characterized by larger external dimensions and greater inductance, and are designed to have characteristics more suitable for a stable supply of DC voltage.
How to Select a Power Inductor
When selecting a power inductor, first consider the inductance value and the allowable current flowing through the inductor. In addition to these factors, the shape and frequency characteristics of the inductance must also be taken into account to determine the optimum component. The decision is based on how much efficiency, output characteristics, and noise immunity are required for the DC-DC converter to be designed.
For power inductors, the allowable DC superimposed current and allowable temperature rise current are specified.
For inductors:
- The larger the DC current flowing through the inductor, the lower the inductance.
- As the current increases, Joule heat due to the resistance of the winding increases.
Therefore, not only the inductance value but also the allowable current is important. Manufacturers produce a wide variety of inductors for a wide range of applications. For example, a component with a large inductance has a high efficiency due to low DC loss, but there is a tradeoff in that the external dimensions of the component are larger and more heat is generated. These points also require careful attention.
Ideally, the voltage value of the DC power supply of each circuit group to be designed should be maintained at a constant value at all times, but this is rarely possible in reality. However, it is important to select devices with excellent characteristics to prevent problems before they occur.