Power Inductor Design Calculator
Size a power inductor for a buck (step-down) topology. Calculates minimum inductance, peak saturation current, and stored energy for core selection.
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Power Inductor Design Guide
The inductor is the energy storage element in a switching power supply. Selecting the right inductor requires calculating the minimum inductance for your ripple target, ensuring the core does not saturate at the peak current, and verifying the RMS current rating for thermal performance.
Minimum Inductance Formula
L = (Vin − Vout) × D / (f × ΔIL)
Where D = Vout/Vin (duty cycle), f is the switching frequency in Hz, and ΔIL is the peak-to-peak current ripple. A 20% ripple means ΔIL = 0.2 × Iout. This gives the minimum inductance for CCM (Continuous Conduction Mode) at full load.
Peak Current and Saturation
Ipeak = Iout + ΔIL / 2
Always select an inductor whose saturation current (Isat) rating exceeds Ipeak, ideally with 20–30% margin. If the inductor saturates, inductance collapses, current spikes, and the switch may be damaged. Check the L vs. I curve in the datasheet — some inductors show a "soft" saturation (gradual rolloff) while others saturate abruptly.
Stored Energy — Core Size Estimation
E = 0.5 × L × Ipeak²
The stored energy determines the physical core size. A larger energy requires a larger core volume. Use this value to filter inductors in component selection tools (Mouser, DigiKey) by energy handling capability.
RMS Current and Winding Losses
For CCM with triangular ripple, the RMS inductor current ≈ Iout (the ripple contribution is small). Select an inductor with an RMS rated current above Iout. The RMS rating is thermally limited — exceeding it causes winding temperature rise and copper losses (I²R × DCR).
Design Example: 24V to 5V, 3A, 400kHz
- D = 5/24 = 0.208
- With 30% ripple → ΔIL = 0.3 × 3A = 0.9A
- L = (24−5) × 0.208 / (400e3 × 0.9) = 11.0 μH → round up to nearest standard value, 12 μH
- Ipeak = 3 + 0.9/2 = 3.45A → select Isat ≥ 4.3A (25% margin)
- E = 0.5 × 12e-6 × 3.45² = 71.4 μJ — narrows the search to compact shielded power inductors in the 12–15μH range
At this point, cross-reference against a manufacturer's selector tool (Coilcraft XAL, Würth WE-PD, TDK SPM) filtering on inductance ≥ 12μH, Isat ≥ 4.3A, and RMS current ≥ 3A — usually 2-3 candidate parts survive, and DCR (winding resistance) becomes the deciding factor for efficiency.
Common Mistakes in Inductor Selection
- Selecting Isat with zero margin above Ipeak. Datasheet Isat is usually defined at a fixed inductance drop (typically 10–30%), measured at 25°C. At elevated ambient temperature, saturation current drops further — 20–30% margin at room temperature is a starting point, not a guarantee.
- Confusing Isat with the RMS/thermal current rating. These are two different limits on the same part. Isat is about core saturation (a sudden failure mode); the RMS rating is about winding heating (a gradual one). An inductor can satisfy one and violate the other — check both.
- Designing at the nominal inductance value and ignoring inductance tolerance. Power inductors commonly have ±20% (or worse, ±30%) inductance tolerance. At the low end of tolerance, ripple current and peak current both increase — re-check Ipeak against Isat using L at −20%, not the nameplate value.
- Assuming DCM never happens. At light load, the inductor current can fall to zero mid-cycle (Discontinuous Conduction Mode), which changes the ripple and control loop behavior assumed by the CCM formulas above. If your design must operate efficiently across a wide load range, verify DCM behavior separately or budget for it in the control loop compensation.
Frequently Asked Questions
What is a good current ripple percentage?
20–40% of Iout is the standard design target. Below 20% gives a larger, more expensive inductor with minimal benefit. Above 40% increases core losses and output voltage ripple, and may cause DCM at light load.
How do I pick between a toroid and a shielded drum core?
Shielded drum cores (fully enclosed) have lower EMI radiation and are preferred for switching supplies. Toroid cores are more efficient and have lower core losses, but require manual winding and emit more field radiation. For most PCB designs, off-the-shelf shielded inductors (e.g., Würth, TDK, Coilcraft) are the practical choice.
Does inductance change with temperature?
Yes. Ferrite cores have a significant temperature coefficient. Check the L vs. Temperature curve. Many ferrites show a dip near their Curie temperature (~100–150°C). Ensure the inductance remains adequate across the full operating temperature range.