Don’t Overlook Inductors in Your Designs

By Lou Frenzel, Contributing Editor
[Sponsored by Coilcraft]

Whether implemented for bypassing, common-mode chokes, or filters, inductors play important roles in electronics. Maybe it’s time to give them another look.

At one time, engineers were told to avoid inductors in their designs because they were too big, heavy, and expensive. Today that’s no longer the case. Yet there’s some evidence that engineers use fewer inductors than other passive components.

Inductors seemingly get no respect, perhaps due to a lack of understanding of inductor characteristics and benefits. Does that define your thinking about inductors? If so, you really should take another look at a component that can really improve your design if you know how to use it.

In case you forgot, inductors oppose changes in current flow through them. As the current rises or falls, the magnetic field around the inductor induces a voltage back into itself that opposes the changes. An inductor is essentially a coil of wire around a core of air, powdered iron, ferrite, or ceramic. It’s available in an enormous variety of forms.

Getting Started: An Introduction to Inductor Specifications

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There is more to selecting an inductor than the nominal inductance value. To ensure the inductor will perform as needed in a specific application, due consideration must be given to inductance tolerance, current ratings, DCR, maximum operating temperature and efficiency at specific operating conditions.

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Things We Would Not Have Without Inductors
Take a minute or so and think of some electronic products or circuits that would not exist or be practical if there were no such thing as inductors. Here’s a short list of the key applications:

  • Switch-mode power supplies (SMPS): SMPS like regulators, dc-dc converters, and inverters are good examples. How would you make a SMPS without an inductor? They have become essential to achieve power savings and efficiency.
  • LED lighting: Switch-mode pulse-width modulators (PWMs) use an SMPS driver with multiple inductors to provide the best control of LED lighting.
  • EMI filtering: As the world gets more electrical with electronic equipment, noise levels continue to rise. To ensure that wireless equipment can function with minimal interference, some form of electromagnetic interference (EMI) elimination or minimization is required. Multiple organizations have established to set standards that define some minimum EMI for different types. The most effective filters to eliminate noise use inductors.
  • Class D amplifiers: These efficient audio power amplifiers use multiple inductors to filter the switched output into a smooth, clean audio signal with minimum distortion. Without inductors, distortion rules.
  • RF filters: Filters, especially the low-pass type, are widely used in wireless equipment. How do you make an RF filter without one or more inductors? Simple passive low-pass filters are a top choice for eliminating harmonics, unwanted noise, and jitter.
  • Anything using resonant circuits: Series and parallel LC resonant circuits still play a major part in radio equipment.
  • And what would you do without inductors in impedance-matching circuits?
  • Automotive ignition systems: Inductors in the form of transformers produce the very high voltages needed in internal combustion engine.

Anyway, you get the idea. Nature created the perfect opposite of, or complement to, capacitance: inductance.

Important Inductor Specifications
One should know these critical inductor characteristics:

  • Inductors are also called coils or chokes depending on their application.
  • The unit of inductance (L) is the henry (H). Most inductors have values in the millihenry (mH), microhenry (µH), or nanohenry (nH) ranges.
  • The resistance of the inductor windings at dc and low frequencies, known as dc resistance (DCR).
  • The opposition that an inductor offers to an ac signal is called the inductive reactance (XL). It’s a function of the inductance (L) and the frequency of the ac (XL = 2πfL).
  • Q is the so-called quality of the inductor that essentially states how much energy it can store versus its losses (Q = XL/R).
  • In tuned LC circuits, the Q of an inductor defines its bandwidth, or BW (BW = fr/Q).
  • Self-resonant frequency (SRF) is the frequency where the parasitic capacitance resonates with the inductance.
  • Maximum current rating.
  • Maximum voltage rating.

Designing with Inductors
Here are some of the ways that inductors are used in new designs.

Bypassing or decoupling: Most circuits today use capacitors on their dc power rails. These capacitors help filter out noise and ripple from the power supply and prevent signals from the circuits from being propagated to other circuits using the same dc rail. A particularly effective decoupling arrangement is a combination of both an inductor (L) and a capacitor (C) (Fig. 1).

1. Decoupling the dc supply and its load with an inductor and a capacitor.

RF biasing: A common way to apply dc bias to an RF amplifier or other circuit is to use a Bias Tee as shown in Figure 2. The inductor allows the dc to be applied to the circuit of interest while isolating the ac. Broadband chokes are now available, permitting this effective circuit to be used at UHF and microwave high frequencies.

2. Bias Tees are used to apply dc supply voltages to high-frequency circuits.

Isolation: Inductors, inherently low-pass filters, are great at isolating RF signals from one another. The inductor allows dc and low-frequency signals to pass while blocking the higher frequencies. The effectiveness of an inductor in this role depends on its self-resonant frequencies. Special coil construction methods provide a way to control the resonant frequencies to ensure a high impedance over a wide range of frequencies. A unique conical wound coil is a good inductor for this application.

Suppression: When you’re trying to eliminate or suppress noise on a wire or component lead, a simple approach is to use a ferrite bead. The bead is a magnetic cylindrical material that surrounds or clips around the wire (Fig. 3). The bead makes the wire into a small inductance. It’s very effective in controlling some kinds of high-frequency noise.

3. The ferrite bead makes the wire into an inductor.

Common-mode choke: If you need to get rid of or greatly attenuate noise on the ac power line or on a differential data cable, a common-mode choke is a great solution. The regulations requiring minimization of electromagnetic interference (EMI) almost always requires a common-mode choke.

This component consists of two windings on a common core. The windings are arranged so that the combined magnetic flux blocks or cancels the noise (Fig. 4). Most ac power supplies are required to have a common-mode choke on the ac input. In addition, common-mode chokes are used on most high-speed serial data interfaces like USB, HDMI, PCIe, LVDS, and others that use a differential connection.

4. This is how common-mode chokes operate.

Filters: A passive filter with inductors and capacitors is very effective in controlling the bandwidth of a circuit, eliminating harmonics or unwanted intermodulation distortion and noise. Filter design has always been a complex nuisance, but today with design software and online tools, you can quickly create just the filter you need. The problem is, the computed values of capacitance and inductance are often odd values not available as real components. The capacitors come in more different values and are easily combined to get the desired value.

That hasn’t been so true of inductors. At RF, you can design and make your own inductors, such as air-wound coils. There’s less need to do that today; some inductor manufacturers have extensive product lines of many standard values of inductors, making filter design easier than ever.

Impedance matching: LC circuits are widely used to match impedances in equipment to maximize power transfer. Z-match networks are common in high-frequency circuits. It’s hard to do without inductors and not having access to unusual values that tend to be calculated when designing these circuits. The popular L-network of Figure 5 or some variation thereof is widely used to make RL match Rg.

5. Impedance-matching circuits like the popular L-network rely on the right combination of L and C.

Unintended inductance: Like stray capacitance, stray or unintended inductance is often a problem in some circuits. Wires are inductors; capacitor and transistor leads are small inductors. In many if not most cases, this small inductance can be ignored. But at high frequencies, even a small inductance can produce unwanted effects. You shorten wires or the copper on a PCB.

Other sources of inductance don’t come from an inductor component as such. It comes in the form of a relay or solenoid coil or a motor winding. This inherent inductance is unwanted and can cause damage to other components. Most relays, solenoids, and motors are driven by a transistor. When the transistor turns off, the voltage applied to a coil is disconnected, the magnetic field in the coil collapses rapidly, and that induces a large short duration spike in voltage that can damage the transistor. Remember:

V = −L(di/dt)

A suppression or flyback diode across a coil like that shown in Figure 6 fixes the problem.

6. When the driving transistor turns off, the resulting high induced voltage in the coil can damage the transistor if the suppression diode was not present.

Power dissipation in inductors: Because inductors use wire or other conductors to provide the winding, they will also dissipate power. The main concern then is the dc resistance of the inductor. The lower the DCR, the less power and heat produced. At higher levels of power dissipation, heat raises the effective series resistance (ESR) that’s made up of the DCR plus ac losses such as skin effect, which increases with operating frequency. This causes the Q to decrease. Be sure to include the ESR in your power calculations when designing with high-frequency chokes.

Designing Your Own Inductor
Inductor design is an EE specialty within a specialty. Some engineers may be able to design their own coils when they’re simple, but it’s a waste of time in most cases. There are just too many forms, magnetic core types, and winding configurations to know it all. It’s best to work with a well-known inductor manufacturer with a comprehensive product line to identify the inductor you need.

 

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