Choosing the inductor
How do you choose the inductor for a buck converter?
| Level | ●● onsite technical |
| Track | Power |
| Really testing | Ripple, saturation, heat — in numbers. |
Free full answer — one of 12 questions answered in full on this site.
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Say this first: Choose the ripple first (ΔI ≈ 20–40% of Iout), compute L = Vout·(1 − D)/(fsw·ΔI), then pick a part with Isat above the peak current (Iout + ΔI/2) with ~20% margin at maximum temperature, and an Irms rating above Iout for heat.
Also consider: core material (ferrite hard saturation vs composite soft saturation), DCR (loss), size vs fsw, shielded vs unshielded (stray field near magnetics-sensitive parts: hall sensors, compasses, audio), acoustic noise.
Numbers to quote: 12 V → 3.3 V, 3 A, 500 kHz, 30% ripple → L ≈ 5.3 µH → 5.6 µH; peak 3.45 A → Isat ≥ 4.1 A.
Going deeper
Full selection, worked — 12 V → 3.3 V, 3 A, 500 kHz.
- L for 30% ripple ≈ 5.3 µH → choose 5.6 µH. Actual ΔI = 0.85 A, peak = 3.43 A.
- Irms = √(I² + ΔI²/12) = √(9 + 0.06) ≈ 3.0 A — the ripple barely adds heat.
- DCR 15 mΩ → 9 × 0.015 ≈ 0.14 W.
- Isat ≥ 4.1 A at the hottest operating temperature. Ferrite Isat can fall 20–30% from 25 °C to 100 °C.
The robust-design check. During a short circuit the inductor carries the IC's current limit (say 5 A). If it saturates below that, current runs away before the limit acts. For rugged designs, choose Isat above the IC's maximum current limit, not just above the normal peak.
Where people go wrong
- Reading Irms (heating) as the saturation rating, or the reverse.
- Using the 25 °C Isat.
- Picking the smallest part that "fits" and finding it whines (see PC-09).
More like this in the book
This is PW-04 from Hardware Engineer Interview Questions. The book has 20 questions on power conversion and 141 in all, each answered the same way: what to say first, how to explain it, the numbers worth quoting and the follow-ups that come next.
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