Tracking the losses

Hello Radoslav

Overall system efficiency from input of transmitter to output of receiver will be lower at light loads of 100mA output current and improve to about 70%-75% at higher loads of 1A.

The loss takes a couple of forms depending on operating point:

Light loads – to power the transmitter and generate a minimum magnetic field will require 500mW to 1W.  This is with little or no power out of the receiver.  Some of the power is used in the logic and control section but most is in the power section.  Wireless power transmitters will use a resonant converter topology and will have high currents in the coil even at light load.  Transmitter power section losses are in switching losses in MOSFETs, DCR loss in coil and ESR of resonant capacitors.

Receiver losses at this point are low.

Medium loads (500mA) – This is the start of the peak efficiency region in this area the transmitter losses have not increased a great deal.  Receiver losses are higher and centered in, Coil DCR, ESR of Cs capacitors, AC to DC rectifier and Output LDO.

High Load (1A) – Highest power dissipation on the receiver due to voltage drop in the rectifier and LDO.  Efficiency has not changed much but output current will drive losses.  Losses in the TX are also up but again only modestly.

Is the energy radiated? (If we use smaller receiver coil comparing to transmitter one, are there radiative losses?)---BJ—Ratio of RX to TX coil size will impact coupling and system efficiency.  Closer to 1:1 the better.  Lower the coupling the bigger a field the TX will have to generate to meet receiver output power requirements.  To do this the coil current is increased and conversation losses in the SW MOSFETs, coil DCR and Resonant capacitors increases. Also note if the line of flux is not broken by the RX coil it will return and no energy is lost. 

Is it lost in the rectifier at the receiver side or some other component?—BJ—It is complicated and depends on the operating point.  In your case 100mA output current most of the losses are on the transmitter side.

Or is it just a resistive loss in the resistor which we use to determine the battery charging current.---BJ—The measurement of output current is very low loss and does not add a significant amount to the losses.