Datasheet

S
2 L
D
Q
R
fp × ×
=
( )
(
)
1
2
C 2 L
1 S S
1
2
1
C 2 L
D
2 S
C
1
'
f
f
é ù
ê ú
ê ú
ë û
é ù
ê ú
ê ú
ë û
-
= × p ×
-
= × p × -
bq51013B
SLUSB62A –MARCH 2013–REVISED OCTOBER 2013 www.ti.com
Figure 42. WPC v1.1 Receiver Coil Test Fixture for the Inductance Measurement Ls’ (copied from System
Description Wireless Power Transfer, volume 1: Low Power, Part 1 Interface Definition, Version 1.1)
The primary shield is to be 50 mm x 50 mm x 1 mm of Ferrite material PC44 from TDK Corp. The gap d
Z
is to be
3.4 mm. The receiver coil, as it will be placed in the final system (e.g. the back cover and battery must be
included if the system calls for this), is to be placed on top of this surface and the inductance is to be measured
at 1-V RMS and a frequency of 100 kHz. This measurement is termed Ls’. The same measurement is to be
repeated without the test fixture shown in Figure 12. This measurement is termed Ls or the free-space
inductance. Each capacitor can then be calculated using Equation 6:
(6)
Where f
S
is 100 kHz +5/-10% and f
D
is 1 MHz ±10%. C1 must be chosen first prior to calculating C2.
The quality factor must be greater than 77 and can be determined by Equation 7:
(7)
where R is the DC resistance of the receiver coil. All other constants are defined above.
Receiver Coil Load-Line Analysis
When choosing a receiver coil, it is recommend to analyze the transformer characteristics between the primary
coil and receiver coil via load-line analysis. This will capture two important conditions in the WPC system:
1. Operating point characteristics in the closed loop of the WPC system.
2. Instantaneous transient response prior to the convergence of the new operating point.
An example test configuration for conducting this analysis is shown in Figure 43:
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