9.2.1.2.2 Series and Parallel Resonant Capacitor Selection

Shown in Figure 32, the capacitors C1 (series) and C2 (parallel) make up the dual resonant circuit with the receiver coil. These two capacitors must be sized correctly per the WPC v1.2 specification. Figure 33 illustrates the equivalent circuit of the dual resonant circuit:

Figure 33. Dual Resonant Circuit With the Receiver Coil

Section 4.2 (Power Receiver Design Requirements) in Part 1 of the WPC v1.2 specification highlights in detail the sizing requirements. To summarize, the receiver designer will be required take inductance measurements with a fixed test fixture. Figure 34 shows the test fixture.

Figure 34. WPC v1.2 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 1.2)

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 (that is, 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 34. This measurement is termed Ls or the free-space inductance. Each capacitor can then be calculated using Equation 6:

Equation 6.

where

• f_S is 100 kHz +5/-10%
• 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:

Equation 7.

where

• R is the DC resistance of the receiver coil. All other constants are defined above.