When choosing a receiver coil, TI recommends analyzing the transformer characteristics between the primary coil and receiver coil through load-line analysis. This will capture two important conditions in the WPC system:

• Operating point characteristics in the closed loop of the WPC system.
• Instantaneous transient response prior to the convergence of the new operating point.

An example test configuration for conducting this analysis is shown in Figure 8-16:

Figure 8-16 Load-Line Analysis Test Bench

Where:

• V_IN is a square-wave power source that should have a peak-to-peak operation of 19 V.
• C_P is the primary series resonant capacitor (for example, 100 nF for Type A1 coil).
• L_P is the primary coil of interest (such as, Type A1).
• L_S is the secondary coil of interest.
• C_S is the series resonant capacitor chosen for the receiver coil under test.
• C_D is the parallel resonant capacitor chosen for the receiver coil under test.
• C_B is the bulk capacitor of the diode bridge (voltage rating should be at least 25 V and capacitance value of at least 10 µF)
• V is a Kelvin connected voltage meter
• A is a series ammeter
• R_L is the load of interest

TI recommends that the diode bridge be constructed of Schottky diodes.

The test procedure is as follows

• Supply a 19-V AC signal to L_P starting at a frequency of 210 kHz
• Measure the resulting rectified voltage from no load to the expected full load
• Repeat the above steps for lower frequencies (stopping at 110 kHz)

An example load-line analysis is shown in Figure 8-17:

Figure 8-17 Example Load-Line Results

What Figure 8-17 conveys about the operating point is that a specific load and rectifier target condition consequently results in a specific operating frequency (for the type A1 TX). For example, at 1 A the dynamic rectifier target is 5.15 V. Therefore, the operating frequency will be from 150 kHz to 160 kHz in the above example. This is an acceptable operating point. If the operating point ever falls outside the WPC frequency range (110 kHz – 205 kHz), the system will never converge and will become unstable.

In regards to transient analysis, there are two major points of interest:

• Rectifier voltage at the ping frequency (175 kHz).
• Rectifier voltage droop from no load to full load at the constant operating point.

In this example, the ping voltage will be approximately 5 V. This is above the UVLO of the BQ51013C-Q1 and, therefore, start-up in the WPC system can be ensured. If the voltage is near or below the UVLO at this frequency, then start-up in the WPC system may not occur.

If the maximum load step is 1 A, the droop in this example will be approximately 1 V (using the 140 kHz load-line). To analyze the droop, locate the load-line that starts at 7 V at no-load. Follow this load-line to the maximum load expected and take the difference between the 7-V no-load voltage and the full-load voltage at that constant frequency. Ensure that the full-load voltage at this constant frequency is above 5 V. If it descends below 5 V, the output of the power supply will also droop to this level. This type of transient response analysis is necessary due to the slow feedback response of the WPC system. This simulates the step response prior to the WPC system adjusting the operating point.

See Table 9-1 for recommended RX coils.