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Inductors, alongside resistors and capacitors, form the trio of fundamental components in electronic circuit design. While resistors and capacitors have their own distinct roles, inductors are critical for storing energy in magnetic fields and resisting changes in current. This guide delves into the diverse types of inductors, their construction, and their applications, offering insights to help select the right inductor for specific circuit needs.
An inductor, often described as providing "AC resistance," is a component that resists changes in electrical current while storing energy in a magnetic field. Its ability to maintain current stability makes it essential in various electronic applications. The standard unit of inductance is the henry, which quantifies the inductor's capacity to generate a magnetic field in response to current flow.
A laminated core inductor is constructed by wrapping a wire coil around a bobbin, which houses a core made from E and I-shaped steel plates with high silicon content. These plates are heat-treated to enhance permeability and minimize hysteresis and eddy current losses, forming a robust core. This design is commonly used in onboard chargers for electric vehicles, line and noise filters, and signal choke filters, offering inductance ranging from 0.12 mH to 100 mH and DC current handling from 1.0 ADC to 200 ADC. With Class B insulation rated for 130°C, these inductors are reliable in demanding environments.
Air core inductors are created by winding a wire around a cylindrical template, such as a drill bit, and stabilizing the structure with varnish or wax. The core, consisting of air, has low permeability, resulting in lower inductance, which makes these inductors ideal for high-frequency applications like RF tuning coils, filter circuits, snubber circuits, and TV or radio receivers. Their low inductance ensures minimal peak inductance and high stability, with features like a tolerance of ±2%, inductance around 0.85 mH, and a DC resistance of 0.44 ohms, capable of handling 30 watts RMS.
Ferrite core inductors are formed by winding a wire around a core made from iron oxide mixed with metal oxides like manganese, zinc, or magnesium, sintered at 1000°C to 1300°C to create a material with high permeability and low eddy current losses. These properties make them suitable for high and medium-frequency applications, such as switching circuits and pi filters. With proprietary ferrite materials and an operating temperature range from -25°C to +120°C, these inductors are flame-retardant and robust for frequencies ≥150 kHz.
Bobbin inductors are made by winding a wire around a cylindrical bobbin, typically made of ferrite, and securing it with a shrink tube. Their small size and ferrite core properties make them suitable for power adapters, SMPS circuits, input/output filters, and pi filters. These inductors offer a primary inductance standard of ±10%, dielectric strength of 0.5 kV between coil and core, and are available in vertical configurations, ensuring compact and efficient performance in power-related applications.
Toroidal core inductors feature a wire wound around a donut-shaped ferrite core, which effectively contains the magnetic field due to its closed-loop design. This results in high inductance with fewer windings, low impedance, and improved efficiency, making them ideal for medical devices, switching regulators, industrial controllers, and SMPS output filters. With inductance values like 560 µH ±15% at 10 kHz and a maximum resistance of 77 mΩ at 25°C, these inductors provide reliable performance in high-inductance applications.
Axial inductors, or color ring inductors, are constructed by wrapping a thin copper wire around a dumbbell-shaped ferrite core, with lids at both ends, followed by a molding process that encases the inductor in a green material marked with colored bands. These bands indicate the inductance value, similar to resistor color coding, making them easy to identify. They are used in line filters, filter designs, boost converters, and general applications, with a temperature rise of 35°C and an operating range from -55°C to +105°C, ensuring versatility and durability.
Shielded surface mount inductors are built by winding a wire in a cylindrical bobbin encased in a ferrite housing, designed for PCB mounting. The shielding reduces electromagnetic interference (EMI) and noise, making them suitable for high-density designs in applications like PDAs, notebooks, high-current POL converters, low-profile power supplies, battery-powered devices, and DC/DC converters for FPGAs. With a frequency range up to 5.0 MHz, low DC resistance, and ultra-low buzz noise, these inductors handle high transient current spikes without saturation.
Wireless charging coils are created by coiling multi-stranded wire around a ferrite core to reduce the skin effect, where high-frequency currents flow primarily on the conductor's surface, increasing resistance. A ferrite plate beneath the coil enhances inductance and focuses the magnetic field, minimizing emissions. These coils are used in wireless charging systems, communication products, and industrial or medical devices, with inductance values like 6.20 µH ±5% at 100 kHz and a DC resistance of 0.08 Ω, ensuring efficient power transfer.
Coupled inductors are formed by winding two wires around a common core, connected in series, parallel, or as a transformer, depending on the application. They transfer energy between windings via mutual inductance, commonly used in flyback, SEPIC, and Cuk converters with a one-to-one turns ratio. Operating in temperatures from -50°C to +155°C and frequencies up to 3 MHz, these inductors maintain a maximum temperature rise of 40°C, making them reliable for DC-DC conversion tasks.
Multilayer chip inductors are constructed by printing coil patterns with special metallic paste on thin ferrite plates, stacking these layers to form a compact coil. This design is ideal for small wearable devices, wireless LANs, Bluetooth systems, single-board computers, and motherboards. With an operating temperature range of -55°C to +125°C and robust thermal shock and humidity resistance, these inductors provide reliable performance in compact, high-frequency applications.
Shielded variable inductors are made by winding a wire around a hollow cylindrical bobbin, with a movable core of ferromagnetic material or brass that adjusts inductance. Inserting a ferrite core increases inductance, while a brass core decreases it. These inductors are highly reliable for automotive applications, meeting AEC-Q200 standards, with a frequency range of 20 to 129 MHz, inductance from 0.05 to 2.7 mH, and durability against mechanical stress, making them suitable for precision tuning in high-reliability systems.
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