IEEE Power Electronics Magazine - September 2022 - 59

well defined and controlled, which results in little test
variability [5].
For the purposes of efficiency measurement, wireless
power transmitters can best be compared to external
power supplies. They are both used in a voltage-source
configuration and are both used to
supply power to charging and noncharging
circuits (task lights, etc.),
just as ac-USB wall-warts are used.
The problem is, it is an external
power supply broken into two pieces;
where the primary-side electronics
are in the transmitter and the secondary-side
electronics and magnetics
are in the receiver. Consequently,
wireless power-transfer efficiency
is poorly defined and difficult to
measure. Wireless power transmitters
have been characterized by different
manufacturers as voltage or
current sources, efficiency has been
described in the industry as " coil-to-coil, " " dc-in to dcout, "
" power-in to power-out, " etc. The " dc out test point
may be located after rectifier (dc out A), after the dc-dc
regulator (dc out B) or at the actual load (dc out C) " (Figures
3 and 7).
Seldom do efficiency claims also include conditions
For the purposes
of efficiency
measurement,
wireless power
transmitters can best
be compared to
external power
supplies.
transmitters in this class use near-field techniques and
are intended to support charging distances (z-distance)
up to 50 mm. The goal was to establish a uniform, repeatable,
fair, and generally accepted method for measuring
power-transfer efficiency where test labs could look at
the unit under test (UUT or transmitter)
as a " black box, " and where
low-cost, off-the-shelf test equipment
could be employed. The WPC
task force used the Energy Star test
guidelines [4] as the starting point
of the test protocol design: That is,
the test must be; repeatable, not
overly burdensome, unambiguous,
reproducible, anticipate technology
changes, harmonize with related
EPA/DOE test procedures, representative,
discourage circumvention,
and be consistent with legal authority.
like
load resistance or how the relative position between
the receiver and transmitter is controlled (x-, y-, and
z-axis). Additional factors like random user placement
of the receiver on the transmitter (for systems that offer
open placement) and the fact that the Qi standard does
not strictly define the receiver further complicate test
approaches.
To address the lack of an industry-wide definition
and method for wireless power system-efficiency testing,
the WPC initiated a task force to draft an efficiency
test protocol that would represent all inductive wireless
power systems regardless of underlying operating
principle. The scope of the test protocol was limited
to magnetic induction-based wireless power transmitters
designed to supply up to 100 W and intended to
power or charge consumer electronics products like cell
phones, tablets, audio equipment, task lights, etc. All
- Table 1. Qi receiver power classes, 2018.
Spec Version
TBD
Qi 1.1
Qi 1.2
TBD
TBD
Power Class
2.5W
7.5W
15
30
100
Fixed DC Output
5V
5V
12V
12V
19.5V
Overview of WPC Proposed Efficiency Test Protocol
The WPC test protocol compares dc power into the load
to dc power into the transmitter. Four key independent
variables that need to be controlled to ensure accuracy
and fairness were identified as follows.
1) Power Class: The transmitter's stated output power
rating is used to place the device into one of five
classes. These classes tend to correspond to usecases,
which are useful in determining the load characteristics
for the test setup (Table 1). The classes are
2.5 W (wearables), 7 W (phones), 15 W (phones and
tablet computers), 30 W (small notebook computers),
and 100 W (large notebook computers).
-50.0
-40.0
-30.0
-20.0
-10.0
0.0
10.0
20.0
30.0
40.0
50.0
Test Receiver Location Template
Peak Coil
Coupling (kmax)
Active
charging
area
-50.0
-30.0
-10.0
10.0
X-axis (%)
FIG 6 Polar grid generator, a tool used to assign receiver test
locations.
September 2022 z IEEE POWER ELECTRONICS MAGAZINE 59
30.0
50.0
Y-axis (%)
IEEE Power Electronics Magazine - September 2022

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