This webinar will compare the power-transfer efficiency of two wireless charging architectures. We will also propose a standard method for measuring total system efficiency and provide some insight on the major power losses of the two systems.

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Power for a Wireless World
Industry-Wide Problem:
There is no standardized test
methodology for specifying power
efficiency of a wireless charging system
Synchronous DC-DC
Buck Regulator
Battery Charger
½ Bridge Driver
DC
Input
DC Out
C
WPT Transmitter
WPT Receiver
DC Out
A
DC Out
B

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

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1. DC out A: Not a valid representaFon of real-world applicaFon
2. DC out B: A good proxy, if the right load range is selected
3. DC out C: The real-world view, also allows complete energy
analysis
No Measurement
Standard
Synchronous DC-DC
Buck Regulator
Battery Charger
½ Bridge Driver
DC
Input
DC Out
C
WPT Transmitter
WPT Receiver
DC Out
A
DC Out
B

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Two Architectures
Rezence perimeter coil Qi planar coil
Resonant InducFve

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Architecture
Comparison
Architecture RepresentaGve
Standards
OperaGng
Frequency
Antenna
Structure
Benefits1
Resonant
AKA: Loosely
Coupled
• Rezence
• Qi
• 6.78 MHz
• 105 kHz
• Perimeter
• Planar or
coil-array
• Extended Z-distance
• MulF-device
InducFve
AKA: Closely
Coupled
• Qi
• PMA
• 110 ~ 205 kHz
• 200 ~ 300 kHz
• Planar or
coil-array
• Highly efficient
• Low cost
QuesGon: Why not use resonant architecture for all applicaFons?
Answer: Efficiency and cost tradeoffs make it inappropriate to do so.
1. There are no Rezence or Qi-resonant products in the market, so benefits are as per the promoFonal
materials and prototypes

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Use-Case
Examples
Resonant: Under-surface mount
InducFve: AutomoFve
InducFve:
Charging Stand
InducFve: Charging Plate

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Ideal DefiniGon of
Efficiency
Efficiency should be calculated as spaGal average at “DC out C”
“Total joules into the ba/ery divided by total joules into the transmi/er averaged
over the charge area/volume for a charge cycle”
Synchronous DC-DC
Buck Regulator
Battery Charger
½ Bridge Driver
DC
Input
DC Out
C
WPT Transmitter
WPT Receiver

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Where is Efficiency
Measured?
Li+
Synchronous DC-DC
Buck Regulator
Battery Charger
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
η

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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

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Ba@ery Model:
2100 mA hr.
Model determines load resistance, voltage and current test
conditions

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Total Energy
27 kJ
Energy required for typical (90%) charge-cycle of a
2100 mA hr. bagery

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6.78 MHz
GaN
Driver
5Vout
VDD
8V – 36V
Gate Drive
& Control
7.5V
Synchronous buck
pre-regulator
LDO
GaN
Zero Voltage Switching Class D Amplifier
Efficient Power Conversion
EvaluaGon Kit EPC9112
• 6.78 MHz operaFon
• GaN switches
• ZVS, Class D amplifier
• NuCurrent antenna system compliant
to Rezence class-3
Rload
EPC Device Board
(Receiver)
High-Frequency Wireless Charger Efficiency
Experiment

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Efficiency vs. posiGon for 10 ohm load
“Open-Loop”
SpaGal PosiGon
Ma@ers
Rload
EPC Device Board
(Receiver)

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Two ConfiguraGons
Rload
EPC Device Board
(Receiver)
Rload
EPC Device Board
(Receiver)
Synchronous DC-DC
Buck Regulator
Battery Charger
ResisFve Load
ConfiguraFon
Bagery Charging
ConfiguraFon
0V - >40V
5.0V
0V - 28V

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Texas Instruments
EvaluaGon Kit bq500212
• 100 ~200 kHz operaFon
• CMOS switches
• Würth antenna compliant to
Qi A11
Low-Frequency Wireless Charger Efficiency
Experiment
½ Bridge Driver
WPT Transmitter
Synchronous DC-DC
Buck Regulator
Battery Charger
WPT Receiver

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Efficiency vs. posiGon for 5 ohm load
Full System
SpaGal PosiGon
Ma@ers

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Efficiency Test
Results
• Qi is the most efficient system by
design
• Efficiency is impacted by:
• Switching frequency
• Antenna design
• SpaFal posiFon / Coil-
coupling coefficient
• Maximum Power-Point
Transfer

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Ba@ery Model:
2100 mA hr.
Total energy over 5% to 95% charge cycle:
27 k Joules

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Real-World Efficiency
Total Energy Efficiency
Qi = 59.4%
Rezence = 39.6%
27 kJ
η= 100%
43.8 kJ
η= 59.4%
65.7 kJ
η= 39.6%

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Conclusions
• Real-world condiFons must be used
• Efficiency should be defined as a spaFal average based on real-world use
– “Total joules into the ba/ery divided by total joules into the transmi/er,
averaged over area, for one charge cycle”
• Qi (low-frequency system) total charge efficiency ~60%
• High-frequency total charge efficiency ~40%
• A wireless charging standard that meets all market needs and use-cases must be
dual-mode (resonant & inducFve)

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Aimee Kalnoskas
Editor & Moderator
EE World Online
akalnoskas@wtwhmedia.com
@DW_aimee
Questions?
John Perzow
VP of Market Development
WPC
administrator@wirelesspowerconsortium.com
@qipower
Measuring Wireless Charging Efficiency in the Real World

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Thank You
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