Inductor units are fundamentally measured in Henrys (H), but 99% of practical PCB designs rely on microhenrys (µH) for power conversion and nanohenrys (nH) for RF impedance matching. When sourcing physical inductor units for a bill of materials (BOM) or a bench repair, your selection hinges on three hard limits: nominal inductance, saturation current ($I_{sat}$), and DC resistance (DCR). Misinterpreting a surface-mount code or ignoring the self-resonant frequency (SRF) will turn a theoretically perfect LC filter into an unintentional oscillator or a shorted MOSFET.
The Henry Scale: Translating Inductor Units to Real-World Values
The Henry is a massive unit. A 1-Henry inductor capable of handling even 1 Amp of current would be the size of a microwave oven. Therefore, we scale inductor units down for practical electronics. Understanding these prefixes is mandatory before reading component datasheets.
- Millihenry (mH): $10^{-3}$ H. Used in low-frequency audio crossovers, 50/60Hz mains filtering, and low-switching-frequency linear power supplies.
- Microhenry (µH): $10^{-6}$ H. The standard unit for DC-DC buck/boost converters (typically 1µH to 47µH) and EMI chokes.
- Nanohenry (nH): $10^{-9}$ H. The domain of RF design, antenna matching networks, and high-speed digital signal integrity (e.g., 10nH to 100nH).
The reactance of any inductor unit scales linearly with frequency, governed by $X_L = 2 \pi f L$. A 10nH RF inductor presents a negligible 0.6Ω at 10MHz, but acts as a 628Ω block at 10GHz. Conversely, a 4.7µH power inductor unit in a 500kHz buck converter provides the necessary energy storage ($E = \frac{1}{2}LI^2$) to smooth the PWM switching waveform into a clean DC rail.
Decoding Physical Inductor Units: Markings and Color Codes
Unlike resistors, inductor markings are notoriously inconsistent across manufacturers. Misreading an SMD code is one of the most common reasons a prototype power supply fails to regulate.
Surface Mount (SMD) Inductor Codes
Most power SMD inductors use a 3-digit EIA-style code, but the base unit varies by manufacturer and size.
Common SMD Power Code (Base unit = µH):
100= 10 * $10^0$ = 10µH471= 47 * $10^1$ = 470µH2R2= 2.2µH (The 'R' acts as a decimal point)
Common SMD RF Code (Base unit = nH):
101= 10 * $10^1$ = 100nH (0.1µH)3N3= 3.3nH (The 'N' acts as a decimal point)
Through-Hole Radial Color Bands
Radial through-hole inductor units often use a 4-band color code identical to resistors, but the resulting value is read in microhenrys (µH), not ohms. A band sequence of Brown-Black-Brown-Silver translates to 1-0-x10 µH with a 10% tolerance, yielding a 100µH inductor. According to All About Circuits, the physical size of the radial choke usually gives away its current rating; a 100µH unit the size of a grain of rice will saturate at 50mA, while one the size of a AA battery can handle 2A.
Inductor Construction Types: Which Unit Fits Your Circuit?
Selecting the right core material dictates your circuit's efficiency, EMI profile, and thermal stability. The table below maps construction types to their ideal applications.
| Core / Construction | Typical Tolerance | Tempco (ppm/°C) | Saturation Behavior | Typical Use Case |
|---|---|---|---|---|
| Shielded Composite (Metal alloy powder) | ±10% to ±20% | Low / Stable | Soft, gradual roll-off | High-density DC-DC bucks, CPU VRMs |
| Ferrite Drum (Unshielded bobbin) | ±20% to ±30% | High (varies with temp) | Hard, sudden cliff | Cost-sensitive LED drivers, low-EMI general power |
| Iron Powder Toroid (Distributed air gap) | ±10% | Moderate | Extremely high $I_{sat}$ | PFC circuits, high-current switching supplies |
| Ceramic Core (Non-magnetic) | ±2% to ±5% | Ultra-low (<100) | Cannot saturate (air core) | RF matching, VCO tanks, >100MHz filters |
For modern power design, TDK and other major manufacturers heavily favor shielded composite cores. The metal alloy powder provides a "soft" saturation curve, meaning the inductance drops gradually as current increases, giving the control loop time to react before a catastrophic hard-saturation short occurs.
Failure Modes: When Inductor Units Go Bad
Inductors are generally robust, but they are not immune to physics. Diagnosing a failed inductor unit requires looking at both the component and the surrounding circuit.
1. Core Saturation (The Invisible Killer)
Visual Symptom: None on the inductor itself. The downstream switching MOSFET will be visibly cracked, scorched, or shorted drain-to-source.
Mechanism: When current exceeds $I_{sat}$, the magnetic core cannot store more flux. The inductor effectively becomes a piece of wire (DCR only). In a buck converter, this causes an uncontrolled current spike ($di/dt$ limited only by parasitic trace resistance), instantly destroying the high-side FET.
2. Thermal Overload and Insulation Breakdown
Visual Symptom: Darkened or blistered winding enamel, a distinct smell of burning varnish, and sometimes a melted plastic bobbin.
Mechanism: Exceeding the $I_{rms}$ (thermal current limit) causes $I^2R$ heating in the copper windings. The thin polyurethane or polyimide enamel insulation melts, causing adjacent turns to short together. This reduces the total number of active turns, plummeting the inductance value and increasing core losses in a thermal runaway loop.
3. Mechanical Fracture (Ferrite Cracking)
Visual Symptom: A hairline fracture visible on the ferrite core or shield, often accompanied by an erratic or completely open reading on an LCR meter.
Mechanism: Ferrite is essentially ceramic. PCB flexure during board separation (V-score snapping) or severe thermal shock from a nearby hot component can crack the core. A crack introduces an unintended, massive air gap, dropping the inductance drastically and increasing radiated EMI.
The Substitution Matrix: Swapping Missing Inductor Units Safely
When the exact BOM part is out of stock, you cannot simply swap in any inductor with the same microhenry rating. Use this matrix to evaluate substitute inductor units safely.
| Parameter | Substitution Rule | Risk if Violated |
|---|---|---|
| Inductance (L) | Must be within ±10% of original. | Shifts control loop poles/zeros; causes sub-harmonic oscillation or output ripple spikes. |
| Saturation Current ($I_{sat}$) | Must be $\ge$ original part. (Check if datasheet defines $I_{sat}$ at 20% or 30% drop). | Hard saturation during load transients; blown switching FETs. |
| DC Resistance (DCR) | Should be $\le$ original part. | Lower efficiency, higher operating temperature, potential thermal shutdown. |
| Shielding | Never substitute an unshielded drum for a shielded part in noise-sensitive designs. | Magnetic flux couples into nearby feedback traces or high-gain analog nodes. |
According to design guidelines from Coilcraft's inductor library, paying attention to the $I_{sat}$ definition is critical. Manufacturer A might define saturation at a 20% inductance drop, while Manufacturer B defines it at 30%. Always normalize the datasheets to the same percentage before comparing substitute units.
The Final Decision Path: Picking Your Exact Part
Stop guessing. Use this decision tree to terminate your selection process with a concrete, proven part number for the most common hobbyist and professional scenario: a high-efficiency, low-noise DC-DC buck converter.
Scenario: You are designing a 5V to 3.3V buck converter delivering 3A continuous current, switching at 1MHz.
- Step 1: Calculate Required Inductance. Assuming a 30% ripple current ratio ($\Delta I_L = 0.9A$), the math dictates roughly 2.2µH. Decision: Target 2.2µH.
- Step 2: Determine Current Limits. Peak current is $I_{out} + (\Delta I_L / 2) = 3.45A$. Add a 20% safety margin. Decision: $I_{sat}$ must be $\ge$ 4.2A. $I_{rms}$ must be $\ge$ 3A.
- Step 3: Evaluate EMI Constraints. Is this powering a sensitive RF module or audio ADC? Decision: Yes. Must use a fully shielded composite core to prevent magnetic coupling.
- Step 4: Check Footprint. Board space is tight. Decision: 4x4mm or smaller SMD footprint.
The Concrete Pick:
Select the Coilcraft XEL4020-222ML.
- Value: 2.2µH
- $I_{sat}$ (20% drop): 9.4A (Massive margin, virtually impossible to saturate in this circuit)
- $I_{rms}$ (40°C rise): 7.2A
- DCR: 13.5mΩ (Maximizes efficiency)
- Construction: Shielded composite, 4x4x2mm footprint.
By following the hard limits of $I_{sat}$, DCR, and shielding requirements rather than just matching the microhenry marking, you ensure the inductor unit operates invisibly and reliably, exactly as a passive component should.
