Electronics Guide

Inductance and Energy Storage

Inductance measures a component's ability to store energy in a magnetic field per unit current. The inductance L, measured in henries (H), relates the induced voltage V to the rate of current change: V = L(di/dt). This relationship explains why inductors oppose rapid current changes while allowing DC to pass freely.

Energy stored in an inductor equals (1/2)LI^2, where I is the current. This stored energy can be released when current decreases, enabling energy transfer functions in switching power supplies. The inductor's maximum energy storage is limited by core saturation, which constrains peak current regardless of inductance value.

Magnetic Flux and Flux Linkage

Magnetic flux represents the total magnetic field passing through a surface. In an inductor coil, the flux links multiple turns, multiplying the effective flux linkage. Inductance relates directly to flux linkage per unit current: L = N * Phi / I, where N is the number of turns and Phi is the magnetic flux through each turn.

Flux density B, measured in tesla (T), indicates the intensity of the magnetic field. Core materials have maximum flux densities (saturation flux density) beyond which they cannot support additional flux. Operating near saturation reduces effective inductance and can cause circuit malfunction.

Air Core Inductors

Air core inductors use no magnetic material, relying solely on coil geometry for inductance. Without a magnetic core, they cannot saturate and exhibit perfectly linear behavior. However, achieving substantial inductance requires many turns and significant physical size. Air core inductors excel in high-frequency applications where core losses would be excessive and in precision circuits requiring linear behavior.

Typical air core applications include RF tank circuits, antenna matching networks, and precision current sensing. The inductance can be calculated reasonably accurately from coil dimensions using formulas like Wheeler's approximation, making custom design straightforward.

Ferrite Core Inductors

Ferrite cores dramatically increase inductance for a given size by providing a high-permeability path for magnetic flux. These ceramic materials combine iron oxide with zinc, manganese, or nickel to create soft magnetic materials with specific frequency characteristics. Different ferrite grades optimize for power applications, broadband transformers, or EMI suppression.

MnZn ferrites suit frequencies below about 1 MHz, offering high permeability and saturation flux density. NiZn ferrites extend to hundreds of megahertz with lower permeability but reduced losses at high frequencies. Core selection requires matching material properties to application frequency and power level.

Powdered Iron Cores

Powdered iron cores consist of iron particles insulated from each other and pressed into shape. The distributed air gaps between particles provide soft saturation characteristics and inherent gapping, making these cores ideal for DC bias applications in switching power supplies. Various powder mixes optimize for different frequencies and permeabilities.

Unlike ferrites that saturate abruptly, powdered iron cores lose inductance gradually with increasing current. This soft saturation allows continued operation under overload conditions, though with reduced filtering effectiveness. The distributed gap also makes inductance relatively insensitive to temperature and manufacturing variations.

Laminated Steel Cores

For low-frequency, high-power applications, laminated silicon steel cores provide the highest saturation flux density and permeability. Thin laminations minimize eddy current losses at power line frequencies. EI, EE, and toroidal configurations address different mounting and winding requirements.

Steel core inductors dominate in AC line filters, power factor correction circuits, and audio frequency applications. Their high saturation flux density enables smaller size for given energy storage, but losses increase rapidly with frequency, limiting use above a few kilohertz.

Core Geometries

Toroidal cores confine nearly all magnetic flux within the core, minimizing external field and EMI. However, winding toroids requires special equipment or hand labor. Pot cores and shielded drum cores also provide low external fields with easier winding. E cores and similar gapped geometries allow simple bobbin winding while providing controlled inductance and saturation characteristics through air gap adjustment.

Inductance Value and Tolerance

Nominal inductance specifies the design value, typically at a specific frequency, current, and temperature. Standard tolerances range from +/-5% to +/-20%, with tighter tolerances available at premium cost. Applications like filters may require tight tolerance, while energy storage applications often tolerate wider variation.

Inductance varies with frequency due to core permeability changes, parasitic capacitance effects, and skin effect in windings. Datasheets specify measurement conditions, and inductance at operating frequency may differ significantly from the nominal value measured at 1 kHz or 100 kHz.

Saturation Current

Saturation current indicates when inductance drops significantly due to core saturation. Manufacturers typically specify the current causing 10%, 20%, or 30% inductance reduction. Operating beyond saturation current causes ripple current to increase, potentially damaging other circuit components or causing malfunction.

Saturation current decreases with temperature as core saturation flux density falls. Designs should account for worst-case operating temperature. Soft saturation materials provide additional margin by maintaining some inductance even when heavily saturated.

Temperature Rise Current

Temperature rise current indicates the DC current causing a specified temperature increase (typically 20C or 40C) due to winding resistance losses. This rating may be higher or lower than saturation current depending on inductor design. The lower of the two ratings typically limits operating current.

Thermal design must consider ambient temperature, airflow, proximity to other heat sources, and duty cycle. Inductors in enclosed spaces or near hot components may require derating from datasheet current ratings.

DC Resistance

DC resistance (DCR) causes power loss proportional to I^2R. Lower DCR improves efficiency in power applications but typically requires more copper, increasing size and cost. High-efficiency designs minimize DCR while meeting inductance and saturation requirements. DCR increases with temperature, approximately 0.4%/C for copper windings.

Self-Resonant Frequency

Every inductor has parasitic capacitance between windings. This capacitance resonates with the inductance at the self-resonant frequency (SRF), above which the component behaves as a capacitor rather than an inductor. For proper function, operating frequency must remain well below SRF. Datasheets specify minimum SRF; applications should maintain adequate margin.

Quality Factor

Quality factor Q equals the ratio of inductive reactance to resistance at a given frequency. Higher Q indicates lower losses, important for filters and resonant circuits. Q varies with frequency, typically peaking in a range where both low-frequency winding resistance and high-frequency core losses are minimized.

Power Inductors

Switch-mode power supply inductors store energy during switch-on time and release it during switch-off time. They must handle DC bias current while maintaining adequate inductance for proper converter operation. Key parameters include saturation current, DCR, and inductance under DC bias conditions. Various core materials and constructions address frequencies from tens of kilohertz to several megahertz.

Coupled inductors contain multiple windings on a shared core, enabling efficient multi-phase converters and providing current balancing. Proper coupling coefficient design trades off leakage inductance against current sharing performance.

RF Inductors

Radio frequency inductors require high Q and stable inductance at operating frequency. Air core and specialized ferrite designs minimize losses. Tight tolerance ensures predictable filter and matching network performance. Shielded designs prevent coupling to adjacent circuits.

Chip inductors in miniature packages suit wireless applications demanding small size. Wire-wound designs offer highest Q when space permits. Multilayer construction provides compact low-inductance values for high-frequency circuits.

EMI Suppression Inductors

Ferrite beads and common-mode chokes suppress electromagnetic interference. Unlike energy-storage inductors, EMI components intentionally dissipate noise energy as heat. Impedance versus frequency characteristics, rather than inductance alone, determine effectiveness.

Common-mode chokes present high impedance to common-mode noise while passing differential signals. They are essential in power supplies, data communication lines, and motor drives to meet EMC requirements.

Filter Chokes

Power line filter chokes attenuate switching noise conducted back to the AC line. They must withstand line voltage and handle load current while providing adequate inductance at noise frequencies. Laminated steel or powdered iron cores suit these low-frequency, high-current applications.

Determine Requirements

Begin by establishing inductance value, operating frequency, DC current, AC ripple current, and acceptable losses. Consider temperature range, vibration and shock requirements, and available board space. Power supply design tools and application notes from controller IC manufacturers often specify inductor requirements for their topologies.

Calculate Ratings

Sum DC and peak AC current to determine peak current requirement. Verify this is below saturation current with margin for temperature effects. Calculate RMS current (considering AC waveform shape) and resulting power loss in DCR. Ensure temperature rise remains acceptable in the application environment.

Verify Performance

Check that inductance under actual DC bias meets requirements. Confirm SRF exceeds operating frequency by appropriate margin. For EMI-sensitive applications, consider shielded versus unshielded designs. Review available package options against PCB footprint constraints.

Prototype and Test

Actual circuit performance may differ from datasheet predictions due to layout parasitics, thermal interactions, and component variations. Prototype testing under worst-case conditions validates the design. Measure inductor temperature, ripple current, and converter efficiency to verify adequate margins.

Saturation

When current exceeds saturation rating, inductance drops and ripple current increases dramatically. In switching converters, this causes excessive switch current, EMI, and potential component damage. Symptoms include overheating, audible noise, and increased output ripple. Solutions include selecting higher saturation current inductors, reducing peak current, or using soft-saturation core materials.

Overheating

Excessive temperature indicates losses exceeding thermal dissipation capability. Causes include high RMS current, core losses at high frequency, and inadequate cooling. Verify current ratings under actual operating conditions. Consider inductors with lower DCR, better core material for operating frequency, or improved thermal management.

Audible Noise

Magnetostriction causes core dimensions to change with flux density, potentially creating audible noise at excitation frequency or its harmonics. Loose windings can also vibrate mechanically. Solutions include potting, different core materials, or shifting operating frequency outside the audible range.

EMI

Inductors with open magnetic structures radiate electromagnetic fields that couple into adjacent circuits. Symptoms include noise on sensitive signal lines and EMC test failures. Shielded inductors, toroidal cores, or improved layout and shielding address EMI problems.