When you need to pass high DC current through a switching node but cannot find a single inductor with the required inductance and saturation current, parallel inductance is your way out. Placing inductors in parallel reduces the total equivalent inductance ($L_{eq}$) while increasing the total current handling capacity and lowering the effective DC resistance (DCR). The governing math is identical to parallel resistors: the reciprocal of the sum of reciprocals. For two identical inductors, $L_{eq}$ is exactly half the value of one.
This guide breaks down the physical topology, maps out exactly what happens when components drift or fail, and walks through a real-world buck converter design using off-the-shelf power inductors.
The Parallel Inductance Topology: Node Labels and Core Behavior
In a standard parallel inductor configuration, the circuit is defined by two primary junctions. Node A is the common input junction where the source current enters and splits into the individual inductor branches. Node B is the common output junction where the branch currents recombine before flowing to the load. Because both inductors share Node A and Node B, the voltage across $L_1$ and $L_2$ is identical at any given instant ($V_{L1} = V_{L2}$).
The formula for total equivalent inductance ($L_{eq}$) for $n$ inductors (assuming zero mutual inductance) is:
$$ \frac{1}{L_{eq}} = \frac{1}{L_1} + \frac{1}{L_2} + ... + \frac{1}{L_n} $$
Below is a data-dense reference table showing real-world combinations using standard E-series inductor values. Notice how placing a small inductor in parallel with a large one drags the $L_{eq}$ down close to the smaller value.
| Branch 1 ($L_1$) | Branch 2 ($L_2$) | Resulting $L_{eq}$ | Combined DCR (Assuming Equal) | Current Split Ratio |
|---|---|---|---|---|
| 10 µH | 10 µH | 5.00 µH | $0.5 \times$ Single DCR | 50% / 50% |
| 22 µH | 47 µH | 15.01 µH | Depends on specific parts | 68% / 32% |
| 47 µH | 47 µH | 23.50 µH | $0.5 \times$ Single DCR | 50% / 50% |
| 100 µH | 10 µH | 9.09 µH | Depends on specific parts | 9% / 91% |
The formula above assumes the magnetic fields of the inductors do not interact. If you place two unshielded inductors less than 5mm apart on a PCB, their flux lines will couple. If their magnetic fields align, $L_{eq}$ will be higher than calculated; if they oppose, $L_{eq}$ drops. Always use magnetically shielded inductors (like ferrite drum or molded types) or space unshielded cores at least one core-diameter apart and rotate them 90 degrees relative to each other.
Element Change Behavior and Failure Extremes
Unlike series circuits where an open component kills the entire path, parallel topologies exhibit graceful degradation—up to a point. Understanding how the circuit reacts to component drift or catastrophic failure is critical for power supply reliability.
Element Change Behavior Matrix
| Variable Change | Effect on $L_{eq}$ | Effect on Current Division | System-Level Consequence |
|---|---|---|---|
| $L_1$ increases (e.g., core gap shifts) | Increases slightly | Current shifts toward $L_2$ | $L_2$ runs hotter; potential early saturation |
| $L_1$ DCR increases (thermal drift) | No direct change | Current shifts toward $L_2$ | Unequal heating; thermal runaway risk in $L_2$ |
| $L_1$ goes OPEN (solder joint fracture) | $L_{eq}$ becomes $L_2$ (Increases) | 100% of current flows through $L_2$ | Ripple current drops, but $L_2$ likely saturates and overheats |
| $L_1$ goes SHORT (internal winding fault) | $L_{eq}$ approaches 0 | Massive current spike through $L_1$ | Catastrophic failure; trace vaporization or source trip |
The most dangerous extreme is a shorted winding. Because inductors pass DC based purely on their DCR, a shorted inductor drops its DCR to near zero. It will hog almost all the DC current, rapidly overheat, and potentially catch fire or melt the PCB pads. Conversely, an open inductor forces the remaining inductor to handle the full load current. If the remaining inductor's saturation current ($I_{sat}$) is lower than the total load current, its core will saturate, inductance will collapse, and the switching FETs in your converter will blow from excessive peak currents.
Parallel vs. Series: Why Choose This Topology?
Why put inductors in parallel instead of just wiring them in series to add their values together? The decision comes down to current handling, physical footprint, and DCR.
- Choose Parallel When: You need high current handling and low DCR. Two 47µH, 5A inductors in parallel yield 23.5µH at 10A capacity with half the DCR of a single unit. This drastically reduces $I^2R$ copper losses and improves thermal distribution across the PCB.
- Choose Series When: You need high inductance for low-ripple, low-current applications (like EMI filtering or high-impedance RF chokes) and want to block high voltage spikes. Series inductors add linearly ($L_{total} = L_1 + L_2$), but their DCR also adds linearly, which is disastrous for high-current power rails.
For a deeper theoretical breakdown of how magnetic fields interact in these configurations, the All About Circuits textbook chapter on inductors provides excellent foundational math, while Electronics Tutorials offers great visual phasor diagrams for AC applications.
Design Walkthrough: Sizing for a 5A Buck Converter
Let’s design the output filter for a 12V-to-5V synchronous buck converter running at 500kHz with a maximum DC load of 5A. Our target inductance to maintain a 30% ripple current ratio is roughly 22µH. Finding a single, compact 22µH inductor with a >6A saturation current and low DCR can be difficult or expensive.
The Solution: Use two identical 47µH inductors in parallel.
- Select the Component: We choose the Coilcraft XEL4030-473ME. It is a 47µH shielded power inductor with a 4.2A $I_{sat}$ (20% drop), a 5.1A $I_{rms}$ (40°C rise), and a tight 10.5mΩ max DCR.
- Calculate $L_{eq}$: $47\mu H / 2 = 23.5\mu H$. This is perfectly within our 22µH target tolerance.
- Calculate Current Capacity: Two 4.2A $I_{sat}$ inductors in parallel theoretically handle 8.4A. Derating by 20% for thermal overlap and PCB hotspots gives us a safe 6.7A limit, which easily covers our 5A load.
- Calculate Combined DCR: $10.5m\Omega / 2 = 5.25m\Omega$. At 5A, our DC copper loss is $I^2 \times R = 25 \times 0.00525 = 0.13W$. This is exceptionally low and will run cool.
Breadboard Testing Protocol: Step-by-Step Verification
Never trust a schematic over a physical measurement, especially when prototyping parallel magnetics on a breadboard. Breadboard contact resistance can easily skew DCR measurements. Here is how to verify your parallel inductor bank on the bench using a standard LCR meter (like a Keysight U1733C or a basic Der EE DE-5000).
- Configure the LCR Meter: Set the test frequency to 100kHz (standard for power ferrite cores) and the test voltage to 1Vrms. Set the measurement mode to series equivalent circuit (Ls + Rs) to capture both inductance and DCR accurately.
- Short and Zero: Short the test leads together at the exact point where they will touch the breadboard contacts. Press the 'Zero' or 'Null' button on the meter to subtract the lead resistance and inductance from the baseline.
- Measure Individual Branches: Before wiring them in parallel, measure $L_1$ and $L_2$ individually. Record both the inductance and the DCR. If the DCR differs by more than 10% between the two parts, expect an uneven current split in your final circuit.
- Wire the Parallel Topology: Insert both inductors into the breadboard. Use a solid jumper wire to bridge the left legs together (Node A) and another to bridge the right legs together (Node B). Keep the inductors at least 2cm apart to prevent magnetic coupling from skewing the reading.
- Measure the Bank: Place your zeroed probes across Node A and Node B. The meter should read your calculated $L_{eq}$ (e.g., ~23.5µH).
- Verify DCR: Switch the LCR meter to DC Resistance mode (or use a high-precision bench multimeter). Measure across Node A and Node B. The reading should be exactly half the DCR of a single unit. If it reads significantly higher, you have poor breadboard contact resistance; press down firmly on the jumper wires and re-measure.
By validating the $L_{eq}$ and combined DCR on the bench before committing to a PCB layout, you eliminate the most common failure modes in parallel magnetics: unexpected core saturation from coupled flux and thermal imbalance from asymmetric routing. For further component selection, the Coilcraft Inductor Finder remains the industry standard for filtering by exact $I_{sat}$ and DCR requirements.
