Qi transmitter coil assembly: the WPC A11 coil in a charging pad, 6.3 µH on ferrite, tuned with 400 nF to 100 kHz
The charging-pad side of Qi · WPC type A11 coil of 6.3 µH ±10% wound from 105-strand Litz on a ferrite sheet, fixed by the standard and resonated with 400 nF at 100 kHz
In a Qi charger the transmitter coil is not a free design choice. The Wireless Power Consortium specifies transmitter types by letter and number, and for each one the coil, its shielding and its resonant capacitor are fixed so that any certified phone can couple to any certified pad. Buying a transmitter coil assembly therefore starts with the WPC type the controller is built for, such as the A11 type that TI's 5 V bq500212A drives, and then checks that the coil meets its inductance, Q and resistance limits and survives the reliability tests.
Transmitter (primary) coil assemblies for Qi wireless chargers: the wound Litz coil, its ferrite shield sheet and adhesive layers, as specified for WPC transmitter types such as A11, A6, A29 and MP-A5, and the resonant capacitor they are tuned with. Receiver coils in phones and wearables are covered on the Receiver coil assembly for Qi wireless charging page; general power inductors and chokes for switching converters on the Power inductors page; AGV charging pads on the Inductive charging module for AGVs and mobile robots page; electric-vehicle pads on the Vehicle assembly (VA) for wireless EV charging page.
CNFX editorial research · Sources checked October 12, 2026 · 3 sources
WPC A11 coil: 6.3 µH ±10%, 10 turns of 105 × 0.08 mm Litz, Q at least 70
TDK's product specification for its WT-505060-10K2-A11-G transmitter coil lists the electrical values an A11 coil is checked against, measured at 100 kHz and 1.0 Vrms on an LCR meter. [1]
The assembly is a stack, not just a coil: TDK's material list names a PET tape, a ferrite sheet, adhesive tape, the air-wound coil, glue and marking ink. [1] TI explains the role of the ferrite: shielding behind both transmitter and receiver coils directs the field into the coupled zone and keeps it from coupling into neighbouring parts of the product. [2] TDK also notes that the part is RoHS and halogen-free and must not go through reflow or dip-flow soldering, [1]
Editorial note: these are one maker's values for one A11 part; ask each supplier for its own specification sheet showing inductance, Q and DC resistance measured at 100 kHz, and compare them line by line rather than by part name.
Ask for the WPC type the coil is built to, inductance and Q at 100 kHz, DC resistance, Litz construction, ferrite sheet material and thickness, lead length and termination, and whether the part tolerates any reflow.
02
400 nF in the tank and resonance at 100 kHz: why the coil value cannot drift
TI's bq500212A, a controller for WPC type A5 or A11 transmitters at 5 V, states that the transmitter coil and matching capacitor are set by the WPC standard and cannot be changed: a total of 400 nF rated 50 V is required in the resonant tank for 100 kHz resonance. Power is then regulated by moving the operating frequency between 110 and 205 kHz — further from resonance gives less power — with the duty cycle held at 50% until 205 kHz is reached. [2]
Editorial calculation: resonance at f = 1/(2π√(LC)) with C = 400 nF and f = 100 kHz gives L ≈ 6.33 µH, matching TDK's 6.3 µH A11 coil. [2][1] At the ±10% inductance tolerance (5.67–6.93 µH) the same 400 nF resonates at about 106 kHz and 96 kHz, which is why the tolerance on coil inductance and on the capacitors matters together.
TI makes up the 400 nF from three 100 nF X7R capacitors in parallel with one 100 nF C0G, which its demodulation allows instead of four C0G parts; it calls for 1% current-shunt resistors and a precision current-sense amplifier because foreign object detection relies on measuring the power lost in the field. Its default loss threshold is 400 mW, found empirically with the WPC disc, ring and foil test objects. [2] TI also notes the WPC practical coil-to-coil distance of less than 5 mm, which allows for the housing and interface surfaces. [2]
Editorial note: the pad's top cover thickness eats into that 5 mm; agree the cover material and thickness before the coil is mounted, because a thicker cover lowers coupling for the same coil.
State the transmitter controller and WPC type, the total tank capacitance and its make-up, and the cover thickness above the coil when ordering coils.
03
Which coil goes with which transmitter type: A11, single- and three-coil A6, A29, MP-A5
TI's list of transmitter coils it tested with its evaluation modules (SLUA649, revised July 2016) pairs controllers with WPC transmitter types: bq500212A and bq50002/bq500511 with A11; bq500412 and bq500414Q with three-coil or single-coil A6; bq500215 with A29; and bq501210 with the medium-power MP-A5. [3]
The same document's 2016 revision removed A1 and A5 coil references because no new products could be certified with them, [3] even though the bq500212A datasheet still describes A5 or A11 operation. [2] Editorial note: confirm with the WPC certification lab which transmitter types are currently accepted for new certifications before choosing a coil type for a new pad.
Reliability tests on the A11 coil: thermal shock, 60 °C/90% RH under 1 A at 100 kHz, vibration and drop
TDK's specification lists the conditions its A11 coil is qualified under; they show what a pad maker can expect to request from any coil supplier. [1]
Editorial note: a car-mounted charging pad sees more heat and vibration than a desk pad; the −30 to 70 °C operating range in this specification should be checked against the cabin location before a desk-pad coil is reused in a vehicle accessory.
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Transmitter: [WPC type — A11 / A6 / A29 / MP-A5; controller part number]
Coil: [inductance and tolerance at 100 kHz; minimum Q; maximum DC resistance; Litz construction]
Shield: [ferrite sheet material and thickness; adhesive; outline]
Mounting: [lead length and termination; soldering method; cover thickness above the coil]
Environment: [operating temperature; desk or vehicle use]
Evidence: [specification sheet; reliability test report; samples for tank tuning]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What inductance is a Qi A11 transmitter coil?
TDK's A11 coil is specified at 6.3 µH ±10% at 100 kHz, with Q of at least 70 and DC resistance of at most 0.06 Ω. [1]
Why can't the transmitter coil value be changed?
TI states the WPC fixes the transmitter coil and capacitance; 400 nF in the tank sets resonance at 100 kHz, and power is regulated between 110 and 205 kHz. [2]
What is the coil assembly made of?
TDK lists PET tape, a ferrite sheet, adhesive tape, the air-wound Litz coil, glue and ink. [1]
Can A5 coils still be used in new chargers?
TI's 2016 coil list removed A1 and A5 coil references because no new products could be certified with them. [3]
Sources and what still needs a direct check
Sources checked 2026-10-12. Coil values, construction and reliability tests come from TDK's A11 coil product specification; resonance, regulation range, tank capacitors and foreign object detection from Texas Instruments' bq500212A datasheet; the pairing of transmitter types and controllers from TI's tested coil list SLUA649. The WPC specification itself defines the transmitter types.
