LTC4120 Receiver and Transmitter Coil and Frequency

It is important to understand the operation and features of wireless power charge controller LTC4120 and the specification of the coils to fit in the demo circuits. The transmitter coil is driven by a current fed source in order to have a good sinusoidal signal transmitted from the transmitter.

As indicated in the datasheet of LTC4120 the selection of Lx (Transmitter coil inductance) and Lr (Receiver coil inductance) is ideally to obtain 1:3 turns ratio. The inductance values may be selected such that the size of the coil required is not too large (In case too small value of capacitor at transmitter end) and circulating current in the transmitter end is not too high (in case too small value of inductance at primary). For the right selection of resonance inductors and capacitors the backward analysis is followed below.

Receiver Coil and Frequency

The receiver side coil currently used in DC1969A has 47 µH, it is the embedded 4 layer PCB coil used with a ferrite base. Würth Elektronik eiSos has a coil exceeding the PCB specification by far to achieve better efficiency, the part number is 760 308 101 303. For detailed specification the datasheet can be downloaded from\wirelesspower.

The receiver frequency changes from 127kHz to 142KHz. At tuned condition both C2P and C2S would affect the resonant frequency and at detuned condition only C2S would affect the resonant frequency.

When calculated using the above formula for resonant frequency when detuned (FD = 142 kHz) the required capacitance will be C2S = 26.7 nF

  • Paralleling of 22 nF and 4.7 nF provides the capacitance required.
  • The detuned frequency 142 kHz instead of 140 kHz is due to limitation of available capacitance.

Similarly, for the resonant frequency when tuned (FT = 127 kHz), the required capacitance will be-

C2P = 6.75 nF,

  • The closest value for this capacitance is 6.8nF.

Note: The parts used in DC1969A are 1.8 nF and 4.7 nF in parallel and with this value the frequency will be 131 kHz.

Transmitter Coil and Frequency

Now that the receiver coil is selected and its inductance value is 47 µH, the transmitter coil can be selected to meet the turns ratio of 1:3 as recommended in LTC4120 datasheet.

For the above requirement Würth Elektronik eiSos has a part 760308101302, A brief specification is 5.3 µH, 6 A, 33 mΩ & Q = 100

Frequency of resonance aimed

With the above formula to meet the transmitter resonant frequency of 130 kHz, the Cx required will be 283 nF. As this value is a non-standard part we should be able to choose 180 nF and 100nF to achieve close to the frequency intended for operation. These two capacitors will share the circulating current. More precise values can be selected to achieve the exact resonance frequency desired.

Therefore, the actual capacitance will be 280 nF and for this capacitance the resonant frequency will be:

F0 = 130.71 kHz

  • F0 is 0.5% higher than original required frequency.

However, the chosen capacitance value in the DC1969A is 2 x 0.15 µF

The part number = ECHU1H154GX9

The new resonant frequency with the above value will be 126.3 kHz (unloaded)

To learn more about wireless power charging coil changing considerations, check back with our blog or take a look at the following resources: