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Design News, March 2013

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Electronics & Test that directly affect the efficiency on the wireless power system. The secondary coil Voltage is the first circuit to receive the transmitted Rectification conditioning power as magnetic flux. Due to Ohms law, the current going through the coil during power transfer causes I2R losses. In order to Power reduce these Ohmic losses, higher-inductance coil with fewer turns are required so excessive resistance is not introduced. Good Controller Communication 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 higherFigure 2: Power receiver block diagram — Power losses within the receiver. 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 Battery Battery due to resistive losses from the integrated Rx charger 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. Figure 3: Discrete charging solution. Also, external FETs reduce the Rds(on) of the overall Rx. Receiver The voltage conditioning stage typically is Power realized by a low drop out (LDO) or linear regulator. It provides a constant DC output Voltage voltage regardless of the load change or Rectification Coil conditioning input voltage, as long as they are within the specifications of the part. The efficiency of Communication LDO regulators is limited by the quiescent current and input/output voltages. Quiescent, or ground current, is the difference Charger Controller Battery between input and output currents. Low quiescent current is necessary to maximize the current efficiency. Also note that LDO Figure 4: Block diagram of a direct charging solution. near dropout voltage is almost always more efficient than a buck converter. Dynamically controlling the rectified such as a cell phone. The Rx itself consists of multiple voltage from the rectification stage helps maintain low key circuits as shown in figure 2. The secondary coil is difference between input voltage and the output voltresponsible for receiving the transferred power from the age of the voltage conditioning stage. This allows the transmitter as magnetic flux. The rectification circuit is LDO to operate near drop out at any load condition, used to convert the received AC-to-DC power. A voltwhich allows significant increase in the efficiency of the age conditioning circuit buffers the unregulated received receiver. This feature is implemented on several TI wireDC power to a regulated and clean DC output power less power receivers, like the bq51013A, bq51013B, and ready for down system use. The communication circuit is bq5105xB. responsible for all communications with the transmitter and unidirectional from receiver to the transmitter. Direct Charging Solutions Each of these subsections within the receiver has losses After the power conditioning stage in the receiver Coil Design News | march 2013 | www.d esign n ews.com –42–

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