Tag Archives: Inductors

Is Metal Better than Ferrite for Inductors?

Many power and signal conditioning applications use power inductors as a basic component to store, block, filter, or attenuate energy. Today’s power circuits use increasingly higher switching frequencies and high powers that impose challenges in packaging and material levels for component manufacturers. Consequently, power inductors, while shrinking their form factors, are pushing to provide higher-rated currents.

The above presents a dual challenge to component manufacturers and designers alike. For instance, component designers must use materials other than the traditional ferrite core materials to miniaturize these devices, while maintaining other parameters such as DCR and inductance without change. Taiyo Yuden is meeting the dynamic challenges of these applications by using metal for power inductors.

Engineers typically select power inductors primarily by their inductance value, then by their current rating and DCR or DC resistance value, followed by their operating temperature range. They may also consider whether the inductor will require to have shielding or none. The application circuit that will use the inductor requires optimization of the above parameters.

Applications of power inductors can range from filtering EMI at the AC inputs of a power supply to filtering ripples at the output of a DC power supply. Inductors are indispensable for reducing the ripple in voltage and current in switching power supply outputs. DC-DC converters use inductors for their self-inductance property of storing power—as the switching circuit turns off, the inductor discharges its stored current. Almost all types of voltage regulation circuits, for instance, power supplies, DC-DC converters, switching circuits, and others, take advantage of the characteristics of power inductors.

Semiconductor power supplies are transitioning from the higher 3.3 V rails and lower currents to lower voltages of 1-1.2 V rails and higher currents for catering to advances in chip design technology. This entails the need for a high-current handling power inductor. Furthermore, smaller form factors of enclosures following the development of smaller-sized electronic components are increasing the demand for miniaturization of all associated electronic components, including the power inductor.

However, the size of power inductors and their higher current capability present a tradeoff. Withstanding higher currents typically requires a bigger case size, resulting in a change in land patterns on PCBs. On the other hand, a small size translates into saturation current due to insufficient inductance. Taiyo Yuden uses the patented construction of a wire-wound multilayer power inductor with a unique metal alloy. This construction allows the designer to achieve both the required inductance in a small case size and a high saturation current.

Taiyo Yuden create their multilayer inductor by printing a pattern on a ceramic sheet that contains ferrite. They laminate these sheets before firing them. Then they assemble the final piece, pressure bond them and fire them. At the last stage, they form external electrodes at both ends. The use of material with a high magnetic permeability results in an inductor with a high inductance value.

The construction of wire-wound inductors follows the traditional method. The coil is either on the inside or on the outside surface of a magnetic material, such as ferrite. A high number of turns results in a higher inductance and a higher DC resistance.

Why Does An Inductor Need A Fly-Back Diode?

An inductor usually stores energy when current flows through it, and releases it once the current flow stops. When the power supply to an inductor is suddenly reduced or removed, the inductor generates a voltage spike, which is also referred to as an inductive fly-back. Any current flowing through the inductor cannot change instantly and is limited by the time constant of the inductor. This is similar to the time constant of a capacitor, which limits the rate of change of voltage across its terminals.

The time constant of an inductor is the product of its inductance in Henries and the resistance present in the circuit. Usually, all current can be considered to have been dissipated within five time constants once the inductor has been disconnected. The process of inductive fly-back is best explained with an example – a 10H inductor in series with a 10Ω resistor, is charged long enough through a closed switch so that maximum amount of current is now flowing through the circuit.
When the switch is suddenly opened, the current flow has to come to zero within five seconds (five time constants). However, the switch opens far faster than five seconds, which implies current flow through an open switch – an impossible situation.

However, this can be explained by considering the switch to be bridged by air resistance of an extremely high value – 40,000,000 MΩ. Therefore, the inductor, in trying to keep the current flowing through the circuit will send a minute amount of current through this big air-resistor. According to Ohm’s law, every resistor will have a voltage drop commensurate with the current flowing through it. To maintain the current flow in the same direction, the inductor will have to change the polarity of the voltage across itself.

At the instant the switch opened, the current through the circuit would have been about 99% of the maximum current. Such a current multiplied by the extremely high resistance of the air gap will result in a huge voltage. Such a large voltage drop is possible because the inductor has stored energy, which it will use to create a very large negative potential on one side of the gap. That ensures the current flow will match the dissipation curve of the inductor. This is the origin of the huge fly-back voltage spike associated with the sudden disruption of current through an inductor.

The fly-back voltage generated by an inductor can be potentially damaging. Not only can the arc generated damage the insulation of the inductor, it can damage the switch or component being used to open or close the circuit. The arcing effect has been dramatically captured in this short video.
The use of a fly-back diode precludes the possibility of damage from an inductive fly-back. The diode provides a path for the inductor to drive the current flow once the circuit has been opened. As long as the circuit is closed, the diode is reverse biased and does not contribute to the functioning of the circuit.

When the switch opens, the inductor has a path to maintain the current flow through the diode. As the inductor reverses its polarity, it forward biases the diode, which then conducts current for the five time constants, until the current reduces to zero. That prevents the voltage spike.

What Are Inductors and How Do They Work

An inductor or an induction coil is a tightly woven coil of wire. Now, you would not expect an ordinary piece of wire to show any special property on passage of current through it. A coil with several loops or turns however, exhibits a remarkable property when current passes through it. The current through the coil creates a magnetic field in the immediate space surrounding the coil. The field stores electrical energy during the passage of current and for a very short while, even if you cut off the current.

Another amazing fact of an inductor coil is that if you place the coil in a varying magnetic field, a current starts to flow through it. The amount of current depends upon the rate at which you change the field.

Bulb and Coil Experiment

You can make out this amazing property of an inductor coil from a simple experiment. Consider a simple circuit with a battery, bulb and a switch. The bulb glows when you close the switch while it stops glowing the moment you open or release the switch.

If you now include a coil of wire wound around an iron bar across the bulb, the bulb will light up as you close the switch. However, instead of glowing at a constant brightness, the intensity of the light changes from bright to dim. If you now open the switch, the bulb does not turn off immediately as you would expect. Instead, the brightness gradually decreases before turning off completely.

Explaining the Observations

You can attribute this curious behaviour to the inductor coil placed across the bulb. When you close the switch, current flows from the battery through the bulb, causing it to glow. At the same time, current flows through the inductor coil too. This generates a magnetic field in the space surrounding the coil. The magnetic field varies in the short time the current builds up. The changing magnetic field induces a current to flow through the coil. However, according to the rules of electricity, this current is opposite to the original current sent by the battery. Hence, the effective current through the coil increases with time, while decreasing that passing through the bulb. This causes the bulb to reduce its glow from bright to dim.

When you open the switch, the magnetic field falls. During the fall of the field, the induced current causes the voltage across the inductor to rise for a moment. This causes the bulb to brighten up briefly. When the current reduces to zero, the bulb turns off.

Inductance

The physical quantity associated with this property is called inductance. The value of this quantity is measured in Henrys. Inductance depends upon four features, which include the number of turns in the coil, the degree of overlap, area of the cross section of the wire and the material of the core inside the coil.

You can increase the inductance by increasing the number of turns and the cross section area of the coil. You may also increase the value by increasing the degree of overlap i.e. by using a tightly wound coil.

Uses of Inductors

You must have wondered how traffic signalling works. Traffic light sensors make use of inductors, which form filter circuits along with capacitors. Inductors are essential components in electronic circuits and devices like receivers, transmitters, oscillators and voltage regulators, as well.