For those who like to experiment with wireless power transfer this circuit can deliver almost 50 W at an efficiency of 88 % (supply voltage 12-24 V).

Here we present a circuit for high power wireless power transfer for experimental purposes designed by Würth Elektronik eiSos GmbH & Co. KG. Most information about the theory of operation is documented in their application note ‘High Power Wireless Power Transfer for the Industrial Environment’ (ANP032e). Also quite a number of simple and more complex example circuits are presented. The basis for the circuit is a resonant converter, the Zero Voltage Switching converter (ZVS converter). We used a circuit with faster gate driving which uses a separate power supply (IC1) for the gate drive  circuits, derived from the input/output voltage. Two identical circuits (and PCBs) are used, transmitter and receiver are the same. The circuit looks very much like an astable multivibrator with two switching transistors. Instead of using a cross-coupled feedback network with two time delay capacitors a LC parallel resonant circuit is used and so the oscillation frequency is almost constant. The frequency shifts a little bit when the two coils of the transmitter and receiver are coupled.
The original circuit didn’t have any protection and so we added a PTC resettable fuse and a simple adjustable electronic fuse using a high-side current monitor (IC2). A discrete thyristor (T11/T12) disables the voltage regulator and the converter is shut down. R27 limits the maximum current through D8 and T11. A reset button (S1) is added to restart the converter. With both circuits coupled at power-on the electronic fuse is most likely triggered and you can see the blue led light up for a brief moment and then go out. Just push S1 to switch on the converter. The blue led should light continuously when the converter is working. D8 prevents R25/R26 from influencing the under-voltage setting by R9/R13. IC1 switches off at 1.09 V and on again at 1.18 V. The output voltage of IC1 is a little over 9.6 V.
IC1 is a ZXCT1107SA-7 from Zetex. It is a unipolar current sense monitor which means it should  only be mounted on the transmitter PCB. On the receiver PCB the output current flows in the wrong direction and only the current sense resistor R18 should be mounted on both PCBs. The current monitor has a wide supply and common-mode voltage range from 2.5 to 36 V. Quiescent current is only 3 µA. The monitor has a current output which has a fixed relation with the voltage across R18: Iout (pin 1) = 0.004 * UR18. The voltage across R21+P1 will trigger T11/T12 at about 0.6 V. The current can roughly be set between 1.6 and 3 A (UR21+P1/(0.004*R18). The highest current setting is not recommended. When testing at higher currents (2.7 A) the two MOSFETs failed after a few minutes and short circuited. Since these are not the easiest parts to replace we strongly recommend to set the current limit not higher as 2 A, keep the supply voltage of the transmitter below 20 V and so limit the power  output to 40 W. The circuit can deliver 50 W in the receiver but not indefinitely. We destroyed the MOSFETs more than once. So keep the power limited! At almost maximum power (> 40 W) efficiency was 88 %. For the test setup we placed a 6.5 mm piece of wood between the transmitter and receiver coils. A good power source for the converter can be a notebook adapter of at least 70 W or more and an output voltage of 19.5 V or so.
At the input of the current monitor we placed  a RF-filter (R19/C28). If C28 is not mounted and R19 replaced by a 0 ohm resistor  in the receiver a ZTX1109SA-7 can be used (it has the sense inputs swapped) and the rest for the electronic fuse can be mounted. But we think this is not necessary since the current at the receiver is nominally lower than the transmitter side. Due to the large amount of decoupling capacitors of the voltage regulator the electronic fuse can’t disable the converter fast enough to prevent components failing in case of a short circuit or overload.
The maximum current through R18 (0.5 W) is 4.77 A, a current that should never occur! The maximum voltage across the sense inputs of IC2 is +0.8 V, but that would mean 36 A and R18 and IC2 going up in smoke….

Bill of materials
R1,R2,R5,R6,R12 = 4.7 kΩ, 1 %, 0.25 W, SMD 1206
R3,R4 = 4.7 Ω, 1 %, 0.25 W, SMD 1206
R7,R8 = 1 kΩ, 1 %, 0.25 W, SMD 1206
R9 = 100 kΩ, 1 %, 125 mW, SMD 0805
R10 = 180 kΩ, 1 %, 125 mW, SMD 0805
R11,R26 = 33 kΩ, 1 %, 125 mW, SMD 0805 (R26 transmitter only)
R13,R20 = 11 kΩ, 1 %, 125 mW, SMD 0805 (R20 transmitter only)
R14,R17 = 3 kΩ, 1 %, 125 mW, SMD 0805
R15,R16 = 100 kΩ, 1 %, 0.25 W, SMD 1206
R18 = 0.022 Ω, 5 %, 0.5 W, SMD 1206
R19 = 1 kΩ, 1 %, 125 mW, SMD 0805 (transmitter only)
R21 = 2.2 kΩ, 1 %, 125 mW, SMD 0805 (transmitter only)
R22 = 47 kΩ, 1 %, 125 mW, SMD 0805 (transmitter only)
R23,R25 = 470 kΩ, 1 %, 125 mW, SMD 0805 (transmitter only)
R24 = 1 MΩ, 1 %, 125 mW, SMD 0805 (transmitter only)
R27 = 100 Ω, 1 %, 125 mW, SMD 0805 (transmitter only)
P1 = 2 kΩ, 25 %, 0.2 W, SMD trimmer (3364W-1-202E Bourns) (transmitter only)
C1,C18,C29 = 100 nF, 10 % 50 V, X7R, SMD 0805  (C29 transmitter only)
C2,C3 = 330 µF, 20 %, 35 V, 0.15 Ω, 10x16 mm, radial
C4,C5,C12-C15,C17,C25,C26 = 4.7 µF, 10 %, 50 V, X7R, SMD 1210
C6,C7 = 68 µF, 25 V, 0.015 Ω, 8x12 mm, Radial Can - SMD
C8,C9,C10,C11 = 100 nF, 10 %, 310VAC, polypropylene, lead spacing 10 mm
C16 = 22 nF, 10 %, 50 V, X7R, SMD 0805
C19,C20,C24 = 4.7 nF, 5 %, 50 V, NP0, SMD 0805
C21,C22,C23,C27 = 22 µF, 20 %, 16 V, X5R, SMD 1210
C28 = 10 pF, 5 %, 50 V, C0G/NP0, SMD 0805  (transmitter only)
L1,L2 = 68 µH, 20 %, 7.5 A, 0.0273 Ω, SMD (74435586800 Würth)
L3 = 10 µH, 20 %, 1.2 A, 0.322 Ω, SMD (74438336100 Würth)
L4 (not on PCB) = 24 µH, 10 %, 6 A, 0.1 Ω, Wireless Power Charging Coil (Würth)
D1,D2 = BYM12-200, DO-213AB (MELF)
D3,D4,D5 = BYM13-40, DO-213AB (MELF)
D6 = WE-TVSP Diode 15V/1500W, DO-214AB
D7 = Led Blue, SMD 0805
D8 = BAT54, SMD SOT-23  (transmitter only)
T1,T2,T3,T4 = DZT5551, SMD SOT-223
T5,T6 = FDD86250, SMD TO252
T7,T8,T9,T10 = PBHV9115Z, SMD SOT-223
T11 = BC850C, SMD SOT-23  (transmitter only)
T12 = BC860C, SMD SOT-23  (transmitter only)
IC1 = WE_VDRM_Power-Module, 171032401 Würth, SMD TO263-7
IC2 = ZXCT1107SA-7, SMD SOT-23  (transmitter only)
K1,K2 = Terminal block, 2way, pitch 5 mm
J1 = Pin header, 3way, pitch 2.54 mm
S1 = Pushbutton, tactile switch, SPST-NO (FSM4JRT) (transmitter only)
F1 = PTC resettable fuse, 0.048 Ω, Ihold 2.5 A, 30 V, MF-R250-0-10
J1 = Shunt jumper, 2.54 mm spacing
2 x PCB 160119-1 v1.1