Filter inductor (output smoothing and EMI filter inductor)

01

Size inductance and current rating

TI’s buck-converter application report gives the first-pass estimate L = VOUT × (VIN − VOUT) ÷ (ΔIL × fS × VIN) where the controller data sheet supplies no inductor range. Its estimate uses ripple current of 20–40% of maximum output current; fS is the minimum switching frequency. TI also requires an inductor current rating above the maximum switch current and gives priority to the controller’s recommended inductance range. The former Revision B link now redirects to Revision C. [1]

Editorial calculation: assuming a continuous-conduction ideal buck stage with 12 V input, 5 V output, 3 A maximum output and minimum switching frequency of 500 kHz, choosing the midpoint ripple fraction of 30% gives ΔIL = 0.9 A and L = 5 × 7 ÷ (0.9 × 500,000 × 12) ≈ 6.48 µH. The corresponding ideal peak is 3 + 0.9 ÷ 2 = 3.45 A. These are assumed design calculations, not manufacturer-published circuit performance or measured operating data; verify the controller’s current limit and permitted inductance. [1]

Editorial note: ask the supplier for inductance tolerance, inductance versus DC bias (saturation) curve, DC resistance and temperature rise at your RMS current, and self-resonant frequency.

The Bourns SRR1260-6R8Y row lists 6.8 µH ±30%, SRF 28.2 MHz (typ.), DCR 18.0 mΩ maximum, Irms 6.30 A in the table’s maximum column and Isat 6.50 A (typ.). The series defines temperature rise as 40 °C (typ.) at rated Irms and inductance drop as 25% (typ.) at Isat. Operating temperature is −40 to +125 °C including self-heating; nominal package dimensions are 12.5 × 12.5 × 6.0 mm. [3]

Editorial calculation: assuming a nominal 6.8 µH part with only its stated −30% tolerance applied, inductance is 6.8 × 0.70 = 4.76 µH. With voltages and switching frequency fixed in the ideal buck model, ripple increases by a factor 1 ÷ 0.70 ≈ 1.429, or 42.9%, rather than by 30%. Separately, assuming constant 3 A RMS and winding resistance of 18.0 mΩ, with temperature and AC-resistance effects excluded, I²R = 9 × 0.018 = 0.162 W. These are illustrative calculations, not manufacturer-published loss/temperature results or measured operating data, and do not establish thermal suitability. [3] [1]

The Bourns sheet also has different test-frequency entries: the SRR1260-6R8Y electrical-table row shows 7.96 MHz, while the General Specifications block assigns SRR1260-1R0Y through -8R2Y an inductance test at 100 kHz/0.1 V. Confirm which test frequency applies to the ordered part and parameter with Bourns. [3]

Give the supplier the circuit (buck, boost, PFC, inverter output, line filter), inductance and tolerance, switching frequency, DC and peak currents, ripple, allowed temperature rise, DC resistance limit, core preference, size and mounting (SMD, through-hole), withstand voltage and insulation class, safety approvals, and operating temperature.

02

Choose core type, insulation and mounting

Hensiron describes ferrite UU filter inductors wound with enamelled copper wire, including UU9.8, UU10.5 and UU15.7. It calls 1–100 mH its “common inductance” range and recommends UU types below 500 kHz versus toroidal types above 500 kHz; these are that supplier’s selection statements. Its UU10.5 table gives 1,500 V/60 s/2 mA withstand testing, a 30 W power-rating label and Class F insulation. The same table labels “Input Voltage” as 15 mH and “Output Voltage” as 75 Ts. Those units do not describe voltage; confirm their intended parameter meanings with Hensiron rather than treating them as verified inductance and turn-count fields. [2]

Specify the noise mode and winding arrangement as well as inductance. Current-compensated coupled windings and single-winding energy-storage parts require different current and circuit information. Hensiron lists filter and common-mode product families; the TI example concerns a buck output inductor. [2] [1]

Editorial note: for mains-connected filter inductors, give the supplier the safety standard and hipot test required, and confirm the insulation class against the expected winding temperature.

03

Separate ripple-current and EMI winding requirements

Send the circuit and current waveform with the inductor request. The buck design in TI’s note and the UU filtering family described by Hensiron have different application conditions. [1] [2]

04

Read each inductor rating together with its test conditions

Coilcraft's application note Document 469 (Selecting the Best Inductor for Your DC-DC Converter, revised 07/22/17) recommends two current ratings: one for core saturation from the peak current and one for heating from the average current. Its catalogue example, the XAL4020-102, lists 1.0 µH ±20%, 14.6 mΩ maximum DC resistance, 79 MHz (typ.) self-resonant frequency, 8.7 A Isat and 9.6 A Irms, with inductance tested at 1 MHz and 0.1 Vrms, Isat at a 30% (typ.) inductance drop, Irms at a 40 °C (typ.) temperature rise, and all parameters tested at 25 °C. [4]

The note gives copper's temperature coefficient of resistance as approximately +0.4% per °C, so a part rated 0.009 Ω maximum would need a 0.011 Ω maximum rating at 85 °C. Above 200–300 kHz it recommends asking the supplier for loss versus frequency data, and it warns that catalogues using a 50% inductance-drop definition may give a misleading impression of usable current. [4]

Bourns uses a 25% inductance drop (typ.) at Isat, while Coilcraft’s catalogue example uses 30% (typ.). Ask for DC-bias curves and temperature conditions before comparing the listed currents; the numbers use different criteria. [3] [4]

05

Core, winding, insulation and board connection

06

Inductance, bias current and insulation test schedule

07

Send the circuit, current and safety data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Circuit: [buck; boost; PFC; line filter]
Electrical: [L; tolerance; fS]
Current: [DC; peak; RMS; ripple]
Limits: [DCR; temperature rise]
Core: [UU; toroid; powder]
Safety: [withstand V; insulation class; approvals]
Mounting: [SMD; THT; size]

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