Each of these subsections within the receiver has losses that directly affect the efficiency on the wireless power system. The secondary coil is the first circuit to receive the transmitted power as magnetic flux. Due to Ohms law, the current going through the coil during power transfer causes I2R losses. In order to reduce these Ohmic losses, higher-inductance coil with fewer turns are required so excessive resistance is not introduced. Good shielding prevents magnetic flux losses and provides a low-impedance path so that very few flux lines impinge upon surrounding metallic objects, thus permitting a higher-inductance coil to be realized. Higher permeability shields offer a larger inductance on a per turn basis, but suffer from lower saturating points.
The rectification stage losses are mainly due to resistive losses from the integrated power FETs. Reducing Rds(on) of the power FET is an effective method of improving rectification losses. Off-loading the power dissipation to external FETs helps reduce the heat dissipation on the IC. Also, external FETs reduce the Rds(on) of the overall Rx.
Figure 3: Discrete charging solution.
Figure 4: Block diagram of a direct charging solution.
The voltage conditioning stage is typically realized by a low drop out (LDO) or linear regulator. It provides a constant DC output voltage regardless of the load change or input voltage, as long as they are within the specifications of the part. The efficiency of LDO regulators is limited by the quiescent current and input/output voltages. Quiescent, or ground current, is the difference between input and output currents. Low quiescent current is necessary to maximize the current efficiency. Also note that LDO near dropout voltage is almost always more efficient than a buck converter.
Dynamically controlling the rectified voltage from the rectification stage helps maintain low difference between input voltage and the output voltage of the voltage conditioning stage. This allows the LDO to operate near drop out at any load condition, which allows significant increase in the efficiency of the receiver. This feature is implemented on several TI wireless power receivers, like the bq51013A, bq51013B, and bq5105xB.
Direct charging solutions
After the power conditioning stage in the receiver module, the power is ready to be used. In portable applications such as cellphones, the output usually is used to charge a Lithium-Ion (Li-Ion) battery. Thus, a discrete battery charger is required. The charger uses the constant voltage from the receiver as an input power supply (figure 3).
As described earlier, every sub-circuit within the wireless power receiver system contributes to power losses and affects efficiency. Design engineers are challenged by the efficiency of a wireless receiver, as well as thermal performance, board size, and BOM price of the portable device as a whole system. This implies that the battery charger within a portable device contributes to these challenging requirements, as well.
One practical approach to lowering these requirements is to combine the rectification stage, voltage conditioning, and battery-charging circuits into one single IC for a highly efficient solution, versus a solution that uses a wireless power receiver followed by a separate downstream charger IC (figures 4 and 2) and expressed by equations 3 and 2, respectively.
Efficiency (discrete solution %) = [(AC input Power)/(Vout*Iout)] * [(Vout*Iout)/(VBAT*IBAT)]
Efficiency (direct charging %) = (AC input Power)/(VBAT*IBAT)
For example, the bq51050B provides all these functions in a single IC. This eliminates the need for a separate charger IC and provides potential cost savings in a small space-saving package.
Tahar Allag is an analog power applications engineer with TIs single-cell battery management group where he is responsible for supporting external customers with detailed technical support for wired charging and wireless power-related problems.