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).
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.