An inductor stores energy in a magnetic field and resists changes in electrical current. If a capacitor is the electronic equivalent of a water tank (storing voltage/pressure), an inductor is a heavy mechanical flywheel (storing current/flow). When current tries to increase, the inductor's magnetic field absorbs the excess energy; when current tries to drop, the collapsing field pushes energy back into the circuit to keep the flow moving.
In practical power electronics—like buck converters, boost regulators, and EMI filters—the inductor's primary job is to smooth out pulsed current from switching MOSFETs into a steady DC stream for the load. Understanding what an inductor does requires looking past the abstract physics and examining how core materials, saturation limits, and parasitic resistance dictate real-world circuit behavior.
The Core Function: Energy Storage and Current Smoothing
The fundamental governing equation for an inductor is:
Voltage equals Inductance multiplied by the rate of change of current over time.
This formula reveals why inductors are indispensable in switching power supplies. If you abruptly interrupt a 2A current flowing through a 10µH inductor in 5 microseconds, the inductor will generate a voltage spike to keep that current moving:
V = 10µH × (2A / 5µs) = 4 Volts.
In a DC-DC buck converter, the switching transistor turns on and off at high frequencies (e.g., 500 kHz). When the switch is ON, the inductor resists the sudden rush of current, storing energy in its magnetic field and passing a ramping current to the load. When the switch turns OFF, the magnetic field collapses, maintaining current flow through the freewheeling diode. According to Analog Devices, selecting the correct inductance value ensures the current ripple remains within 20% to 40% of the maximum DC load current, preventing excessive output voltage ripple and core saturation.
Inductor Types: Which Core for Which Job?
Not all inductors are created equal. The core material dictates the component's saturation current, frequency response, and thermal stability. Here is a breakdown of the most common types you will encounter on the bench.
| Type / Core Material | Construction | Typical Tolerance | Tempco (ppm/°C) | Best Application |
|---|---|---|---|---|
| Molded Ferrite (SMD) | Ferrite powder mixed with binder, molded under high pressure | ±20% | +100 to +300 | High-density DC-DC converters, point-of-load (POL) regulators |
| Shielded Drum Core | Copper wire wound on a ferrite drum, enclosed in a ferrite shield | ±20% | +50 to +200 | General-purpose buck/boost converters where EMI must be contained |
| Toroidal Powdered Iron | Insulated iron powder compressed into a ring shape | ±10% to ±15% | +20 to +100 | High-current, low-frequency filtering, PFC chokes, audio crossovers |
| Air Core | Self-supporting copper coil, no magnetic core material | ±5% to ±10% | ~0 (Linear) | RF circuits, high-frequency resonant tanks, VHF/UHF filters |
| Multilayer Ceramic | Alternating layers of ceramic and conductive paste | ±10% to ±20% | +100 to +400 | Ultra-compact signal filtering, high-frequency decoupling (< 100mA) |
Selection Criteria: Choose shielded drum cores or molded ferrite for switching power supplies to prevent magnetic flux from coupling into nearby sensitive traces. Reserve air core and ceramic types for RF and signal paths where core saturation and hysteresis losses would distort high-frequency waveforms.
Decoding the Markings: What Do Those Numbers Mean?
Reading inductor markings is a common stumbling block, especially with surface-mount devices (SMD) where space is limited. Unlike resistors, inductor codes usually denote microhenries (µH).
SMD Inductor Codes
- The 'R' Decimal System: A marking of
4R7means 4.7µH. The 'R' acts as the decimal point.R47means 0.47µH. - The Three-Digit System: Similar to capacitors, the first two digits are significant figures, and the third is the multiplier (number of zeros) in µH.
100= 10 × 10^0 = 10µH (Note: this is a frequent trap; beginners read this as 100µH).101= 10 × 10^1 = 100µH.472= 47 × 10^2 = 4700µH (4.7mH).
Through-Hole Color Bands
Axial through-hole inductors use a four-band color code identical to resistors, but the values are read in microhenries (µH), and the tolerance band is often silver (10%) or gold (5%).
Bench Scenario: The Melted Buck Converter Inductor
To understand what happens when you misunderstand inductor specifications, let's look at a real-world bench failure involving a standard LM2596 buck converter module.
The Setup: A hobbyist was designing a 12V to 5V step-down converter to power a 3A LED strip. They selected a 33µH unshielded drum-core inductor from a generic kit. The inductor's datasheet listed a maximum DC resistance (DCR) of 120mΩ, an RMS current rating of 2A, and a saturation current ($I_{sat}$) of 2.5A.
The Numbers: In a buck converter, the inductor must handle both the DC load current and the AC ripple current. For a 3A load with a typical 30% ripple, the peak current is:
I_peak = I_out + (ΔI_L / 2) = 3A + (0.9A / 2) = 3.45A.
The Outcome: Upon applying the 3A load, the inductor emitted a high-pitched whine and became too hot to touch within 30 seconds. The output voltage sagged from 5.0V down to 3.2V, and the LM2596 chip began thermal cycling.
What Went Wrong: The builder confused RMS current (which dictates wire heating) with saturation current (which dictates core magnetic limits). The 3.45A peak current vastly exceeded the 2.5A $I_{sat}$ limit. When an inductor core saturates, its magnetic permeability drops to that of air. The inductance effectively collapses from 33µH down to near zero. With no inductance to limit the rate of current rise ($di/dt$), massive current spikes hammered the LM2596's internal MOSFET, triggering its overcurrent protection and causing severe $I^2R$ heating in the inductor's copper windings.
The Fix: The inductor was replaced with a Bourns SRP1265A-330M, a shielded SMD power inductor rated for 33µH, with a 4.5A RMS current rating and a 5.5A saturation current limit. The output stabilized at 4.98V, and thermal imaging showed the inductor running only 15°C above ambient.
Never operate a power inductor near its saturation current limit in a switching regulator. If the core saturates, the resulting current spike can instantly destroy the switching MOSFET. In high-voltage or mains-connected SMPS designs, a blown MOSFET can create a dead short across the AC line, potentially causing a fire or explosive component failure if the primary fuse is improperly rated.
Failure Modes and Visual Symptoms
Inductors are generally robust, but they do fail. When troubleshooting a dead power supply, check for these specific failure modes using a multimeter and visual inspection.
1. Open Circuit (Wire Break)
- Cause: Thermal cycling, mechanical shock, or excessive current melting the thin copper wire.
- Visual Symptoms: Cracked epoxy coating, bulging plastic base, or a distinct burnt resin smell. On SMD parts, the end caps may look scorched.
- Measurement: Multimeter in resistance mode reads
OL(Over Limit) or infinite resistance. A healthy power inductor should read between 10mΩ and 500mΩ.
2. Shorted Turns (Insulation Breakdown)
- Cause: Overheating melts the thin enamel insulation between adjacent wire windings, allowing current to bypass turns.
- Visual Symptoms: Discolored copper wire (turns dark purple/black), melted plastic bobbin, or deformed core halves.
- Measurement: The DC Resistance (DCR) drops significantly below the datasheet specification. For example, an inductor specified at 45mΩ might measure 18mΩ. The inductance value (measured with an LCR meter) will also drop proportionally.
3. Functional Saturation (Thermal Runaway)
- Cause: Operating beyond the $I_{sat}$ limit, as described in the bench scenario above.
- Visual Symptoms: Often none to the naked eye, but a thermal camera will show the component running 40°C to 80°C above ambient board temperature.
- Measurement: DCR is normal, but the circuit exhibits severe voltage ripple, switching noise, and erratic regulator behavior.
Safe Substitution: When You Don't Have the Exact Part
When prototyping or repairing a board, you rarely have the exact OEM inductor in your bin. According to TDK Electronics and general magnetics design principles, you can safely substitute an inductor if you strictly follow these four rules:
- Match the Inductance (±20%): For power supply filtering, a 22µH inductor can usually substitute for a 20µH part. However, for RF tuning or precise EMI filters, you must match the value exactly (±5% or better).
- Exceed the Saturation Current ($I_{sat}$): Your substitute's $I_{sat}$ must be higher than the peak current of the circuit ($I_{DC} + I_{ripple}/2$). Never substitute a lower $I_{sat}$ part.
- Exceed the RMS Current ($I_{rms}$): The substitute's thermal rating must exceed the maximum continuous DC load current to prevent the copper windings from overheating.
- Match or Lower the DCR: A substitute with a higher DC Resistance will waste more power as heat ($P = I^2R$) and reduce the overall efficiency of the power supply. Always aim for a lower DCR if physical size permits.
The Shielding Trap: Never substitute an unshielded inductor for a shielded one in a mixed-signal circuit. Unshielded inductors radiate magnetic flux that can induce noise into nearby high-impedance analog traces or feedback loops, causing erratic switching frequencies or audible noise. If you must use an unshielded part, orient the core gap away from sensitive traces and verify the output ripple with an oscilloscope.
By treating the inductor not just as a passive loop of wire, but as a complex magnetic component with strict thermal and saturation boundaries, you can design power circuits that run cooler, quieter, and survive the inevitable fault conditions of the real world. For deeper mathematical modeling of inductor ripple and core losses, refer to the comprehensive inductor guides on All About Circuits.
