why the rectifier voltage of RX decrease as load current increases?

Regarding 1, this is due to the dynamic rectifier control function where the target voltage of the dynamically controlled rectifier varies as a function of the output current. This is achieved by providing enough voltage headroom across the internal regulator at light loads in order to maintain regulation during a load transient. The Dynamic Rectifier Control allows for a 2 V change in rectified voltage before the transient response will be observed at the output of the internal regulator (output of the bq5101xB). The behavior is illustrated in Figure 8 and 9 in the datasheet.

Regarding 2, please refer to the communictaions protocol section in "An Introduction to the Wireless Power Consortium Standard and TI's Compliant Solution" in the link below. The 'Using the bq5101x as a wirless power supply' section and 'Details of a Qi Wireless Power System and bq5101Xb Power Transfer Flow Diagrams' section in the bq51013 datasheet page 17 to 19 also explains this.

http://www.ti.com/lit/an/slyt401/slyt401.pdf

http://www.ti.com/lit/ds/symlink/bq51013b.pdf 

Hi Jing Zou , Thank you for your reply. I have another question,
1.Does the bq51013EVM-725 work normally if the transmitter vary which the input voltage may be 5V,12V and so on ? Why?
2.Why the WPC defines the different input voltage Tx(19V,5V,12V...)? What are they different?
Best Regards, wenzi

Regarding 1, the bq51013EVM-725 will work with other Qi certified TX.  Transmitter TX Coil design will change with input voltage. 

- 5V input TX coil inductance is 6.3uH

- 12V input TX coil inductance is about 12uH

- 9V input TX coil inductance is about 24uH

Note that as the coil inductance changes the resonant capacitors value change so that the resonant point remains at 100kHz. Changes in the system with voltage are not limited to the above. Design for the TX is tightly controlled by the Qi standard.

Regarding 2, the reason for different input voltage is that customer can have different design options.

Hi Jing Zou,

              Thank you for your reply. I have another question,

 As for the Rx coil design, the coupling coefficient K between TX and RX is a very important parameter.

1.As you said "the bq51013EVM-725 will work with other Qi certified TX.  Transmitter TX Coil design will change with input voltage. 

- 5V input TX coil inductance is 6.3uH

- 12V input TX coil inductance is about 12uH

- 9V input TX coil inductance is about 24uH

Note that as the coil inductance changes the resonant capacitors value change so that the resonant point remains at 100kHz. Changes in the system with voltage are not limited to the above. Design for the TX is tightly controlled by the Qi standard."

So, I think that the important reason for the bq51013EVM-725 could work with other different Qi certified TX is the coupling coefficient K between Qi certified TX and RX are different. Do I understand correctly?

2.The coupling coefficient K is so important. Is there any equation  to caculate the K. Could you recommend some useful materials or information about coupling coefficient K.

Best Regards,

          wenzi 

Wenzi,

Regarding on the coupling coeffienct K, please refering to the "Designing a Qi-compliant receiver coil for wirless power systems, Part 1" application note in the link below.

http://www.ti.com/lit/an/slyt479/slyt479.pdf

Regards,

Jing

Also, please note that the bq51013 is NRND. We recommend you to change to the bq51013B (WPC v1.1). Or, the bq51013A which is only v1.0.

Hi Jing Zou, Thank you very much for you reply. Now,I have another question,
1.As we know, bq51020 or bq51003 have adopted dynamic rectifier. What is dynamic rectifier? Are there any different to other kind of rectifier?
2.Why bq51020 or bq51003 adopt dynamic rectifier? Are there any advertages?
looking forward to your reply.thank you again.best regards, wenzi.

1. Basically, the dynamic rectifier will adject the rectified voltage based on output current. As I mentioned before, this is achieved by providing enough voltage headroom across the internal regulator at light loads in order to maintain regulation during a load transient. As the power increased, the amount of a load step decreases so the rectifier voltage gets closer to dropout.Table 1 in bq51020 datasheet summarizes how the rectifier behavior is dynammically adjusted based on RILIM settings.
2. The advantage is this algorithm enhances the transient response of the power supply while still allowing for very high efficiency at high loads.

Hi,

    I'm sorry to trouble you again. And I have some questions, as A1 coil

1.How much the peak current through the A1 coil?

2.How to evaluate the B and H of the A1- transmitter transmit.

3.Is the current through the A1, A6 or A11 coil same. 

    we have make some tests and find that when a battery is on the back of the ferrite the battery will be hot seriously( base on A1 transmitter 19V A1 coil), the systerm could not work normally.  Then we change the transmitter to A612V , the battery is not hot ,the system could work normally. why?

looking forward to your reply.

     Thank you.

                                                    wenzi