Receiver coil assembly for Qi wireless charging: spiral coil on a ferrite sheet, measured in the device and tuned to 100 kHz

01

Free-space Ls against Ls′ measured with the battery and back cover on the WPC fixture

TI's BQ51013B receiver datasheet, following the WPC v1.2 power receiver requirements, measures the coil twice: Ls′ with the coil placed as in the final system, including back cover and battery, on a 50 × 50 × 1 mm TDK PC44 ferrite primary shield with a 3.4 mm gap, and Ls in free space without the fixture, both at 1 Vrms and 100 kHz. Its recommended receiver coils show how far the two differ; Ls′ was measured with a standard battery behind the coil except for the Vishay part, and TI asks designers to repeat all inductance measurements in their own system. [1]

The later BQ51013C datasheet, written to WPC v2.0, lists Würth 760308103215 and XFMRS XFMCC483201-143 coils of 48 × 32 mm at 14.3 µH, the TDK WR483265-15F5-G at 13.3 µH, all for 50–1,000 mA, and a 22 mm round TDK WR222230-26M8-G at 27 µH for 50–500 mA. [2]

Give the supplier the space available (outline, thickness, cut-outs), the output current, the Qi version the receiver must meet, what sits behind the coil (battery, metal frame, NFC antenna), the housing material and thickness over the coil, and the termination (leads or flexible circuit and connector).

02

Series and parallel resonant capacitors from Ls′ and Ls, and a quality factor above 77

TI sizes the series capacitor C1 from Ls′ for a resonance fS of 100 kHz +5/−10%, then the parallel capacitor C2 from Ls and C1 for a second resonance fD of 1 MHz ±10%, and requires a quality factor, 2π·fD·Ls/R with R the coil's DC resistance, above 77. In its worked example a coil with Ls 11 µH, Ls′ 16 µH and 191 mΩ gives C1 = 158.3 nF (range 144–175 nF), built as 68 + 47 + 39 = 154 nF, and C2 = 2.3 nF, built as 2.2 nF plus 100 pF, both rated at least 25 V; Q comes out above 500. [1]

Editorial calculation: C1 = 1 / ((2π × 100 kHz)² × 16 µH) ≈ 158 nF, matching TI's figure; with Ls′ of 18.8 µH, as for the TDK coil above, the same formula gives about 135 nF, so a capacitor bank sized for one coil does not fit another. [1]

03

Ferrite backing between the coil and the battery or electronics

An ITMO University study (arXiv 2408.09852) built a Qi receiver into an AA-size rechargeable battery from a commercial Qi planar coil of 40.5 × 30.5 mm, 14 turns of two parallel 0.4 mm enamelled copper wires, bent to a 7 mm radius and backed with a 41 × 28 mm flexible ferrite sheet 0.1 mm thick to decouple the battery's other components from the transmitter. The coil measured 12 µH and was resonated at 100 kHz with four 68 nF, 25 V capacitors, with a 2.2 nF second-resonance capacitor; its simulations showed that, with the ferrite layer in place, adding the battery's internal components left the field close to that of the ferrite-backed coil alone. [3]

The study's highest measured charging efficiency was 28%, at zero distance and an empty cell, and it fell with distance and state of charge. [3] Editorial note: that figure belongs to a curved coil on a 14 mm cylinder, not to a flat coil in a phone; ask the coil supplier for efficiency or coupling measured on your transmitter and in your housing.

04

Send the space, current and stack-up behind the coil together

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

Space: [outline; thickness; cut-outs]
Electrical: [Ls target; output current; Qi version]
Stack-up: [battery; metal frame; NFC antenna; housing material]
Ferrite: [sheet thickness; material]
Termination: [leads; flexible circuit; connector]
Test: [Ls and Ls′ on WPC fixture; DC resistance]

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