DHC Function and Other Considerations for Wireless Power Charging

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.

DHC Function

The DHC function of the LTC1967A would move the frequency of resonance to a preset detuned frequency of 140kHz when the coils have a better coupling factor which means the voltage at VIN is more than 14V and would be tuned to 127kHz when coils have low coupling factor that is VIN is below 14V.

Therefore, when the coil for a transmitter circuit is selected it is important to choose the resonant frequency higher than the receiver frequency set at tuned state. This will ensure same frequency at receiver as transmitter resonance and this circuit will work like double tuned resonant circuit thereby chip will ensure full power transfer.

The below scope capture shows the signal at Ch1, a square pulse at the transmitter frequency. Every time the received signal at VIN is higher than voltage at DHC pin, the DHC pin is pulled down to ensure the VIN (Ch3) does not increase further. The CH2 trace is signal across receiver winding and CH4 signal is current through transmitter.

Summary of Wireless Power Charging Coil Changing Considerations

Higher coupling, lower physical distance between transmitter and receiver and higher turns ratio will ensure higher voltage at receiver and so higher VIN. The DHC function will limit the VIN available to DC to DC converter and will ensure the transmitter operating at wide operating voltage. It has been seen from the experiment (Condition # 2) that the efficiency is higher when the signal received is more sinusoidal. Therefore, when the application demands for wider input voltage higher turns ratio (Approx. 3) may be selected and when higher efficiency is demanded the optimized value of turns ratio (when the signal received is sinusoidal) is recommended.

The transmitter and receiver coil must be selected carefully considering all above listed parameters for high efficiency and/or wider input operation performance of the wireless power transfer demo circuit DC1969A. Various transmitter and receiver power charging coils produced by Würth Elektronik eiSos are available with high Q making the power transfer more efficient.

To learn more about wireless power charging coil changing considerations, take a look at the following resources: