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Test Setup

6.2.2.3 Efficiency

To measure system efficiency, measure the output voltage, the output current, input voltage, and input

current and calculate efficiency as the ratio of the output power to the input power. Connect voltage

meters at the input and output of TX and RX (see Figure 4). Average the input current; the comm pulses

modulate the input current, distorting the reading. See Figure 5 for efficiency. Figure 5 shows efficiency

with standard EVM (C30-COG but C29, C28, and C27 - X7R). Also shown is efficiency with C30, C29,

C28, and C27 all capacitors COG type.

Figure 5. Efficiency versus Power, bq500212AEVM-550 Transmitter and HPA764 Receiver

6.2.2.4 Efficiency Improvements

Efficiency improvements can be made with component changes. The EVM uses an integrated power

stage device that combines both the driver and two MOSFETs into one package, CSD97376. Similar

devices with lower RDSon MOSFETs are available. The CSD97374 reduces RDSon by about 50% and

improves efficiency by 2% to 4%. This device is pin-to-pin compatible and is a drop in replacement.

To reduce cost, the EVM uses a mix on COG and X7R capacitors for the resonant capacitors, C28, C29,

C30 and C27. X7R is lower cost but has higher ESR which results in higher loss. If all COG capacitors are

used, efficiency increases by 2% to 4%.

Note that changing the efficiency of the unit and reducing loss (or increasing loss) changes the FOD

performance and may require re-calibration. This would require FOD_CAL resistor, R99 to change along

with FOD_Threshold resistor, R24. FOD Calibration procedure would need to be repeated.

6.2.2.5 Dynamic Power Limiting

Dynamic Power Limiting (DPL) allows operation from a 5-V supply with limited current capability. Input

voltage is monitored at Pin 46 though a voltage divider network. When input voltage decreases to 4.2 V,

the operating point is adjusted to reduce load and increase input voltage to 4.5 V.

6.2.2.6 Thermal Protection, NTC

Thermal protection is provided by an NTC resistor connected to JP1. At 1 V on the sense side (U1-2), the

thermal fault is set, and the unit is shut down, The status LED, D5, illuminates red. The typical resistor

value for fault is 850 Ω. The system tries to restart in 5 minutes.

SLVU928A–September 2013–Revised October 2013 bq500212A bqTESLA Wireless Power TX EVM

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