Datasheet

0.2 0.4 0.6 0.8 1.0

Load Current (A)

Rectified Voltage (V)

Fs=175

Fs=160

Fs=150

Fs=140

Fs=135

Fs=130

Fs=125

1A Load Step Droop

1A Load Operating PointPing Voltage

bq51011

bq51013

SLVSAT9D –APRIL 2011–REVISED AUGUST 2012

www.ti.com

An example load-line analysis for the Vishay IWAS-4832FF-50 receiver coil is shown in Figure 33:

Figure 33. Vishay IWAS-4832FF-50 Load-Line Results

What this plot 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 1A the dynamic rectifier target is

5.15V. Therefore, the operating frequency will be between 150kHz and 160kHz in the above example. This is an

acceptable operating point. If the operating point ever falls outside the WPC frequency range (110kHz –

205kHz), the system will never converge and will become unstable.

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

1. Rectifier voltage at the ping frequency (175kHz).

2. Rectifier voltage droop from no load to full load at the constant operating point.

In this example, the ping voltage will be approximately 5V. This is above the UVLO of the bq5101x and;

therefore, startup in the WPC system can be ensured. If the voltage is near or below the UVLO at this frequency,

then startup in the WPC system may not occur.

If the max load step is 1A, the droop in this example will be approximately 1V with a voltage at 1A of

approximately 5.5V (140kHz load-line). To analyze the droop locate the load-line that starts at 7V at no-load.

Follow this load-line to the max load expected and take the difference between the 7V no-load voltage and the

full-load voltage at that constant frequency. Ensure that the full-load voltage at this constant frequency is above

5V. If it descends below 5V, 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.

NOTE

Coupling between the primary and secondary coils will worsen with misalignment of the

secondary coil. Therefore, it is recommended to re-analyze the load-lines at multiple

misalignments to determine where, in planar space, the receiver will discontinue operation.

Product Folder Links: bq51011 bq51013