When you need a specific inductance value for a power filter or RF choke, the default move is to find a single component that matches your target. But in high-frequency switching regulators and sensitive analog rails, a single large inductor often becomes a liability due to parasitic capacitance and physical height constraints. This is where an inductance series topology solves the problem.
In a pure series configuration (assuming zero mutual coupling), total inductance adds linearly: L_total = L1 + L2 + ... + Ln. The current through every inductor is identical, while the voltage drop across each divides proportionally to its inductance and the rate of current change (di/dt). Below, we break down exactly how to design, test, and troubleshoot series inductor networks for real-world PCB and breadboard applications.
Topology and Node Behavior
Consider a standard two-inductor series network. We define three critical nodes to analyze voltage and current behavior:
- Node A (V_in): The input source or upstream switching node.
- Node B (Midpoint): The junction between L1 and L2. This node is critical for probing and fault isolation.
- Node C (V_out): The output rail feeding the load or the next filter stage.
Because the components are in series, the instantaneous current i(t) is identical through L1 and L2. However, the voltage at Node B will fluctuate based on the impedance ratio of the two inductors at your operating frequency. If L1 and L2 are identical, Node B sits at the exact midpoint of the AC voltage drop.
The math L_total = L1 + L2 only holds true if mutual inductance (M) is zero. If you place two inductors side-by-side with their magnetic axes aligned, they will couple, altering your total inductance. Always rotate series inductors 90 degrees relative to each other on the PCB, or maintain a physical gap of at least one inductor-body-length between them.
Behavior Matrix: What Changes When Parameters Shift
| Parameter Changed | Effect on L_total | Effect on Node B Voltage | Effect on Network SRF |
|---|---|---|---|
| Increase L1 value | Increases linearly | Shifts closer to V_out (larger drop across L1) | Decreases (higher L means higher parasitic C impact) |
| Increase frequency | No change (ideal) | Node B AC amplitude increases (higher X_L) | N/A (until SRF is reached, then L drops) |
| Replace L2 with lower DCR part | No change | DC offset at Node B shifts slightly closer to V_in | Depends on new part's parasitic capacitance |
Why Series Over a Single Large Inductor?
If you need 4.4 µH, why buy two 2.2 µH inductors instead of one 4.7 µH inductor? The answer usually comes down to Self-Resonant Frequency (SRF) and physical profile.
Every physical inductor has parasitic parallel capacitance. At the SRF, the inductor acts like a pure resistor; above the SRF, it acts like a capacitor, completely defeating your filter design. Larger inductance values require more wire turns, which increases parasitic capacitance and lowers the SRF. By splitting the required inductance across two smaller physical packages in series, you push the SRF of the network much higher, maintaining inductive impedance well into the VHF band.
Furthermore, a single high-inductance, high-current shielded inductor might be 4mm to 6mm tall. If your enclosure constraint limits component height to 2mm, placing two low-profile 2.2 µH inductors in series solves the mechanical problem while maintaining the electrical target.
Failure Modes at the Extremes: Opens and Shorts
Understanding how the circuit breaks is just as important as how it works. Here is what happens when the extremes hit:
L1 Opens (Catastrophic Failure)
If L1 fractures or its solder joint fails open, the series path is broken. Current drops to zero. Node B and Node C float to 0V (pulled down by the load). The downstream circuit loses power entirely. Diagnostic signature: V_in is present at Node A, but Node B reads 0V under load.
L2 Shorts (Degraded Performance)
If L2 suffers an internal winding short (or is accidentally bypassed by a solder bridge), its inductance drops to near zero. L_total becomes equal to L1. The filter loses half its designed attenuation. More dangerously, if L2 was acting to limit di/dt during a transient spike, L1 now takes the full brunt of the voltage spike and may saturate, leading to downstream overvoltage. Diagnostic signature: Node B and Node C show identical waveforms on an oscilloscope.
Design Walkthrough: 2MHz Buck Output Filter
Let's design an output filter for a 5V, 2A rail powered by a 2.25 MHz switching regulator (like the TI TPS562201). We need to suppress the 2.25 MHz fundamental ripple and the 4.5 MHz harmonic to power a sensitive 12-bit ADC.
- Target Inductance: The regulator datasheet recommends 2.2 µH for stability. To create a steeper roll-off for the analog rail, we want to double this to 4.4 µH using a series network.
- Component Selection: A single 4.7 µH inductor in a compact package (like 2020 metric) might have an SRF of 25 MHz. This is dangerously close to our 4.5 MHz harmonic when considering PCB trace capacitance. Instead, we select two Coilcraft XEL4020-222 inductors (2.2 µH each, SRF 45 MHz, I_sat 3.5A, DCR 13 mΩ).
- Network SRF Check: With two 45 MHz parts in series, the combined network maintains high impedance well past the 4.5 MHz harmonic.
- DCR and Thermal Check: Total DCR is 26 mΩ. At 2A DC load, the voltage drop is 52 mV (1% of 5V), and power dissipation is I²R = 4 * 0.026 = 104 mW. This is easily handled by the 4x4mm footprint pads without thermal vias.
- Layout Execution: Place L1 and L2 in a straight line but rotate L2 by 90 degrees to orthogonalize their magnetic flux paths, eliminating mutual coupling.
Decision Tree: Single vs. Inductance Series
Use this framework to decide whether to populate one footprint or two on your next board spin.
| Design Constraint | Action | Concrete Pick Example |
|---|---|---|
| Switching freq < 500kHz, no strict height limit | Use Single Inductor | Coilcraft MSS1260-473 (47 µH) |
| Switching freq > 1MHz, need low SRF margin risk | Use Inductance Series (2 parts) | 2x Coilcraft XEL4020 (Split value) |
| Max component height < 2.0mm | Use Inductance Series (Low profile) | 2x Wurth 74404054 (Split value, 1.8mm tall) |
| Need exact non-standard value (e.g., 11.5 µH) | Use Inductance Series (Tuning) | 10 µH + 1.5 µH standard values |
How to Breadboard and Verify the Series Chain
Do not trust the datasheet values blindly; parasitic effects on a breadboard can alter your network. Follow this exact verification sequence before committing to a PCB layout.
- Prepare the LCR Meter: Set your LCR meter (e.g., Keysight U1733C or a reliable DER EE DE-5000) to measure inductance (L) in Series Equivalent mode (Ls). Set the test frequency to 100 kHz. Testing at 100 Hz or 120 Hz will yield massive errors for small RF inductors.
- Measure Individual Baselines: Measure L1 and L2 separately. Record the values. (Expect a 10-20% variance from nominal due to breadboard contact resistance and lead inductance).
- Build the Chain: Insert L1 and L2 into adjacent breadboard rows. Use a short jumper wire to connect them (Node B). Keep the jumper under 10mm to minimize added series inductance.
- Measure the Network: Probe across the free ends of L1 and L2 (Node A to Node C). The reading should equal L1 + L2 ±5%. If the reading is significantly higher, your inductors are magnetically coupling. Pull them apart or rotate one 90 degrees.
- AC Ripple Verification: Power the circuit. Connect an oscilloscope probe to Node B. Critical: Remove the long alligator ground clip and use a ground spring attached to the nearest ground plane. Probe Node B and Node C simultaneously. The AC ripple amplitude at Node B should be roughly halfway between the V_in ripple and the V_out ripple. If Node B looks identical to Node C, L2 is shorted or bypassed.
By treating the inductance series topology as a deliberate high-frequency design tool rather than just a math exercise, you can bypass component shortages, meet strict mechanical constraints, and push your filter attenuation far beyond what a single magnetic component can achieve. For deeper magnetic theory, the Electronics Tutorials guide on series inductors provides excellent baseline math, while the Coilcraft Inductor Finder is indispensable for matching SRF and I_sat specs to your specific series network needs.
