For 90% of DC-DC buck, boost, and SEPIC converter prototyping, a shielded ferrite core coil inductor with a saturation current ($I_{sat}$) rated at least 20% higher than your peak switch current is the default pick. Specifically, the Würth Elektronik WE-PD series or the Coilcraft MSS1260T series will cover almost all hobbyist and commercial sub-10A power supply needs. If you are building RF filters or impedance matching networks under 100MHz, you abandon ferrite entirely and reach for a multilayer ceramic coil inductor like the Murata LQG15 series. The days of guessing inductor values based purely on microhenry ratings are over; modern power design requires matching the saturation current, DC resistance (DCR), and self-resonant frequency (SRF) to your exact switching topology.

Decoding Coil Inductor Markings and SMD Codes

Unlike resistors and capacitors, surface-mount coil inductors rarely have enough physical real estate for full alphanumeric spec sheets printed on their tops. Manufacturers rely on standardized EIA microhenry codes and, occasionally, color bands for through-hole axial variants. Misreading these codes is the most common reason a newly soldered power supply fails to regulate on the bench.

The standard 3-digit SMD code works exactly like the resistor color code, but the base unit is microhenries (µH). The first two digits represent the significant figures, and the third digit is the multiplier (number of zeros).

Standard SMD Coil Inductor Marking Codes
Printed Code Calculation Inductance Value Typical Application
100 10 × 10^0 µH 10 µH Standard buck converter (1-3 MHz)
101 10 × 10^1 µH 100 µH Low-frequency boost / LED driver
4R7 4.7 µH ('R' = decimal) 4.7 µH High-frequency synchronous buck
R47 0.47 µH ('R' = leading zero) 0.47 µH (470 nH) Very high frequency / RF choke
N10 0.10 µH ('N' = nano range) 100 nH UHF impedance matching
Bench Tip: If an SMD inductor is completely unmarked (common with ultra-cheap unshielded drum cores from bulk marketplaces), do not trust the seller's spreadsheet. Measure it with an LCR meter at 100 kHz. If you only have a standard multimeter, you cannot measure inductance directly, but you can measure the DCR. A 10µH power inductor typically has a DCR between 15mΩ and 50mΩ; if you read 2Ω, you are holding a high-impedance RF choke, not a power inductor.

Core Materials and Construction Types

Selecting the right core material dictates your circuit's efficiency, electromagnetic interference (EMI) profile, and thermal limits. A coil inductor is essentially a copper wire wrapped around a magnetic medium; the medium determines how the component behaves under heavy DC bias.

Coil Inductor Construction and Core Material Comparison
Core Type Construction Tolerance Tempco (ppm/°C) Typical Use Case
Shielded Ferrite Molded magnetic epoxy over drum core ±20% to ±30% Low drift DC-DC converters, noise-sensitive environments
Unshielded Ferrite Exposed copper on a ferrite drum bobbin ±20% to ±30% Low drift Cost-sensitive power supplies, low EMI risk
Metal Alloy / Iron Powder Wire embedded in compressed metal dust ±20% Moderate drift High-current (>10A), high-temp automotive/industrial
Multilayer Ceramic Printed spiral traces in ceramic block ±2% to ±5% Highly stable RF filtering, GHz matching, ultra-low inductance
Air Core Self-supporting copper coil, no magnetic core ±1% to ±5% Zero drift High-power RF transmitters, Tesla coils, audio crossovers

For power applications, the primary trade-off is between shielded ferrite and unshielded ferrite. Shielded parts (like the Coilcraft MSS series) contain the magnetic flux inside the molded housing, preventing stray magnetic fields from inducing noise in adjacent high-impedance analog traces. Unshielded parts (like the Coilcraft DO series) are 20-40% cheaper and offer slightly better thermal dissipation because the copper is exposed to ambient air, but they will fail radiated EMI pre-compliance scans if placed near sensitive nodes.

The Substitution Matrix: Swapping Parts Safely

When the exact bill-of-materials (BOM) inductor is out of stock, you must substitute based on electrical limits, not just the microhenry value. Swapping a 10µH inductor for another 10µH inductor can still destroy your switching regulator if the secondary parameters are mismatched.

Follow these three immutable rules for safe substitution:

  1. The $I_{sat}$ Golden Rule: You can never substitute a part with a lower saturation current ($I_{sat}$). $I_{sat}$ is the DC current level where the inductance drops by 20% (or 30%, depending on the datasheet). If your peak switch current is 3A, and you substitute a 10µH part with a 2.5A $I_{sat}$, the core will saturate during transient loads. The inductor will temporarily become a dead short, spiking current through your MOSFET and destroying the IC.
  2. The DCR Thermal Trade-off: You can substitute a part with a higher DC Resistance (DCR) if it offers a higher $I_{sat}$, but you must calculate the thermal penalty. Power loss equals $I^2 \times DCR$. If you swap a 20mΩ part for a 40mΩ part in a 3A continuous circuit, your inductor dissipation jumps from 180mW to 360mW. Ensure the physical footprint can dissipate that extra heat without exceeding the component's maximum ambient temperature rating (usually 125°C).
  3. The SRF Boundary: The Self-Resonant Frequency (SRF) is where the inductor's parasitic parallel capacitance resonates with its inductance, turning it into a capacitor. Your substitution part must have an SRF at least 10 times higher than your converter's switching frequency ($F_{sw}$). If your buck converter switches at 1 MHz, do not use an inductor with an SRF of 5 MHz.
Warning: Never substitute a standard signal inductor (like the Murata LQH series meant for RF choking) into a DC-DC power path, even if the inductance value and DCR look acceptable. Signal inductors lack the magnetic core volume to handle continuous DC bias and will saturate instantly at currents as low as 50mA.

Failure Modes and Visual Diagnostics

Coil inductors are generally robust, but they are often the silent killers in a failed power supply. When a board comes back to the bench dead, the inductor's physical appearance tells the story of what went wrong.

Inductor Failure Modes and Visual Symptoms
Failure Mode Root Cause Visual / Audible Symptoms Bench Verification
Core Saturation $I_{peak}$ exceeds $I_{sat}$ during load transients Inductor looks pristine. Downstream MOSFET or controller IC has a cracked package, bulged epoxy, or burnt pin. Scope the switch node; look for massive, sharp current spikes at the leading edge of the PWM pulse.
Thermal Runaway Continuous $I_{rms}$ exceeds thermal rating; high DCR heating Discolored PCB pads (brown/black), melted plastic bobbin on unshielded parts, charred enamel smell. Measure DCR with a milliohm meter; if it reads significantly higher than the datasheet spec, the internal copper has annealed or degraded.
Mechanical Fracture Piezoelectric acoustic stress or PCB flexing Micro-cracks visible on the ferrite core edges under 10x magnification. Audible high-pitched 'whine' or 'singing' before failure. Inspect the solder joints and core edges. A cracked core alters the magnetic air gap, unpredictably dropping the inductance value.
Winding Short Voltage spike breaks down thin enamel insulation between windings Slight bulging of the mold compound. Component reads near 0Ω on a standard multimeter. An LCR meter will show a massive drop in inductance and a severe drop in the Q-factor due to the shorted turn acting as a damper.

The most counter-intuitive failure is Core Saturation. Beginners often assume that if the inductor is physically intact, it survived the fault. In reality, when a ferrite core saturates, its permeability drops to near that of air. The inductor stops storing energy and acts as a low-resistance wire directly connecting your input voltage to your switching transistor. The transistor takes the lethal overcurrent hit, explodes, and the inductor cools down looking completely innocent. Always check the inductor's $I_{sat}$ rating when debugging a blown switching regulator.

The Final Decision Tree: Pick Your Exact Part

Stop scrolling through distributor catalogs. Use this decision path to terminate your search and select a concrete, proven part family for your next build.

Coil Inductor Decision Matrix
If Your Circuit Requires... Then Choose This Core Type Concrete Part Family Pick (2026 Standard) Typical Unit Cost
RF Filtering / Impedance Matching
(< 100nH, > 100 MHz, < 500mA)		 Multilayer Ceramic (0402 or 0603) Murata LQG15 or TDK MLG1005 series $0.02 - $0.05
Standard DC-DC Power
(1µH - 47µH, 1-5A, 500kHz - 3MHz)		 Shielded Ferrite (SMD) Würth WE-PD (744774) or Coilcraft MSS1260 $0.80 - $1.50
High-Current Power / Automotive
(> 10A, high ambient temp, strict EMI)		 Metal Alloy / Molded Iron Powder Coilcraft XEL or Würth WE-CHP series $1.20 - $2.50
Low-Frequency Choke / Audio
(> 1mH, < 1A, through-hole)		 Radial Leaded Ferrite Bobbin Bourns 78FR or Fastron SMCC $0.30 - $0.60

For the vast majority of makers and design engineers building embedded systems with integrated switching regulators (like the TI TPS5430 or MP2359), the Shielded Ferrite SMD category is your permanent home. Use the Coilcraft Inductor Finder or the Würth RED EXPERT simulator to plug in your exact switching frequency, input voltage, and output current. These tools will automatically filter out parts where the SRF is too low or the thermal rise exceeds 40°C at your specific $I_{rms}$.

By locking in a shielded ferrite part with a 20% $I_{sat}$ margin and verifying the DCR thermal limits, you eliminate inductor-related EMI failures and transient saturation before you even spin the first PCB prototype.