Power for a Wireless World

Measuring Wireless Charging EfficiencyIn the Real World

WPC Trade ConferenceTaipei16 June, 2016

Power for a Wireless WorldEfficiency

Matters

• Who cares about efficiency?• IKEA• McDonald’s• EPA/EU/ Gov’t agencies• Auto makers• Consumers• Who doesn’t care?

Power for a Wireless WorldPower for a Wireless World

Industry-Wide Problem:

There is no Standardized test methodology for specifying power-

transfer efficiency of a wireless charging system

Power for a Wireless World

Two

Architectures

Rezence perimeter coil Qi planar coil

Loosely Coupled Closely Coupled

Power for a Wireless WorldArchitecture

Comparison

Architecture Representative Standards

OperatingFrequency

AntennaStructure

Benefits

Loosely Coupled

• Rezence• Qi

• 6.78 MHz• 100 ~ 205 kHz

• Perimeter• Planar

Extended Z-distance

CloselyCoupled

• Qi • 100 ~ 205 kHz Planar • Highly efficient• Low cost

Question: Why not use resonant architecture for all applications?Answer: Efficiency and cost tradeoffs make it inappropriate to do so.

Power for a Wireless World

Use-Case

Examples

Resonant: Under-surface mount

Inductive: Automotive

Inductive: Charging Stand

Inductive: Charging Plate

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Efficiency

Experiment

1. Loosely-coupled, high-frequency wireless charger

– EPC-9112

– Similar to A4WP/Rezence Class 3

– 6.78 MHz operation

2. Closely-coupled, low-frequency wireless charger

– BQ500212

– Qi spec 1.1.2, Type A11

– 110 ~ 210 kHz

Power for a Wireless WorldWhere is Efficiency

Measured?

Li+

Sync

hron

ous

DC

-DC

Buc

k Re

gula

tor

Bat

tery

Ch

arge

r

DC In

AC Adapter

Wireless Transmitter

Wireless Receiver

Efficiency Measurement

Taken at the optimal spatial position and load power (5W, 4.2V @ 1.2A)

DC out

AC

DC

h

Power for a Wireless WorldBattery Model:

2100 mA hr.

Model determines load resistance, voltage

and current test conditions

Power for a Wireless WorldTotal Energy

27 kJ

Energy required for typical (90%) charge-cycle of a 2100 mA hr. battery

Power for a Wireless World

Step 1

Map the Flux Field

Rezence perimeter coil

Loosely Coupled

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Texas InstrumentsEvaluation Kit bq500212

• 100 ~200 kHz operation• CMOS switches• Würth antenna compliant to

Qi A11

Qi

Flux Field (A11)

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Efficiency Test

Results

• Qi is the most efficient system by design

• Efficiency is impacted by:• Switching frequency• Antenna design• Spatial position / Coil-

coupling coefficient• Maximum Power-

Point Transfer

Power for a Wireless WorldBattery Model:

2100 mA hr.

Total energy over 5% to 95% charge cycle:

27 k Joules

Power for a Wireless World

Ideal Definition of Efficiency

Efficiency should be calculated a as spatial average:

“Total joules into the battery divided by total joules into the transmitter averaged over the charge area/volume for a charge

cycle”

Power for a Wireless World

Real-World Energy Use

Qi system used 33% less energy to charge a battery

27 kJBattery

43.8 kJQi

65.7 kJRezence

Power for a Wireless World Conclusions

1. Real-world conditions must be used to measure efficiency (voltage, current, resistance to simulate a battery being charged)

2. Efficiency should be defined as a spatial average

“Total joules into the battery divided by total joules into the transmitter over one charge cycle”

3. There are tradeoffs between different approaches; why not use the most efficient approach?

Power for a Wireless WorldPower for a Wireless World

Thank You