BQ51013B not reaching VRECT threshold 4

Luis,

What is the RILIM value (total resistance from ILIM to ground)?

The bq51013B output is set to 5V.  If the output collapses, the power is too high.  The bq51013B is set to 5W (at 5V, that's 1A and you are 20% over that).

Try reducing the charge current to see the results.

Regards,

Dick

Dear Dick,

RILIM value is 75R + 187R.

I think that I wasn't clear when I referred to the 1.18A current value. I referred to charging current from a charger IC (bq24100) not Iout from the BQ51013B. Therefore actual BQ51013B Iout is lower.

For instance real measurements I took:

With 3.1V battery voltage @ 1.2A charging current (during the CC cycle) I measured 4.1V Vout (BQ51013B).

With 4V battery voltage @ 1.2A charging current (during the CC cycle) I measured 4.6V Vout (BQ51013B).

Best regards,

Luis Olvera.

Dear Dick,

Haven't heard from you, I hope everything is alright.

I have compared using two different wireless charging stations (since we are testing a beta prototype transmitter as well) but the same behaviour occur. Both TX stations are of the WPC A1 type.

I measured net efficiency i.e. from TX DC supply power to actual charging power and it is as follows:

Output power 3.72W
3.1V battery voltage @ 1.2A charging current . BQ51013B's Vout 4.1V-4.3V (BQ51013B)
Input power 7.6W
20V DC supply voltage @ 0.38A.
Efficiency 49%

Efficiency gets better as the battery gets charged towards the CV cycle:

Output power 4.92W
4.1V battery voltage @ 1.2A charging current . BQ51013B's Vout 4.9V-5V (BQ51013B)
Input power 8W
20V DC supply voltage @ 0.41A.
Efficiency 60%

Our target efficiency is between 60% to 70% which seams feasible according to the graphs I've found in the reference documents.


Please advise,

Luis Olvera.

Luis,

Thank you, everything is fine, I have been working on another project.

I am still a bit concerned about the bq51013B voltage being down to 4.1V.  The only time I see that is when the output current limit is reached or when the input power is limited.  The coil you mention is specified as 11uH for Ls.  What was the Ls' value?  This would be measured in your system.

Can you get a scope plot of the voltage at RECT and the voltage at OUT of the bq51013B?  It would be very interesting to see these voltages at different current levels.  

What can you tell me about the distance between the TX and RX coils?  If the TX is WPC compliant, it is a standard thickness from the coil to the user surface.  What is the thickness from your surface to the coil?

Regards,

Dick

Dear Dick,

I'm glad everything is okay.

I used a HP 4763B LCR meter to measure Ls and Ls':

- Ls 10.88 uH (taking battery, PCB and back cover into account).

- Ls' 11.17uH (taking battery, PCB and back cover into account and using an actual WPC A1 TX station).

This is the scope capture you asked for, purple curve is RECT cyan curve is VOUT. Battery voltage was about 3.7V-3.8V @ 1.2A charging current:

Distance between the RX coil and the TX station surface is between 6mm-6.5mm.

I was wondering about the resistance value of the resistor from VRECT to the FOD pin, I'm not sure if it has an impact. Right now we have a 100K resistor from VRECT to the FOD pin.

Best regards,

Luis Olvera.

Luis,

The WPC spec calls for 5mm between the coils.  if you are 6mm from the RX coil to the surface of the TX station then you are definitely pushing the limits of the configuration and this may be causing some of the issues.  The can be investigated by pulling the coil out of the system and reducing the distance.

The resistor from RECT to FOD is referred to in much of the documentation as Ros.  This resistor, along with the resistance from FOD to GND (RFOD is used for the Foreign Object Detection (FOD) which is required as part of the WPC v1.1 specification.  The bq51013B web page has a link to "Tools & Software".  There you will see a link under "Software" for the Foreign Object Detection (FOD Calibration/Tuning Tool.  This is a GUI that will help work through how to calibrate for FOD.  The first screen has a link to a "User Manual" that will cover the how the data should be taken.  The GUI will then process the data and aid with selecting the correct values for Ros and RFOD.

And one last measurement request.  Can you measure the frequency of the power transfer?  This can be done on AC1 of the RX or TX.

Dick

Dear Dick,

Our tablet features a book cover with a thickness of 3.5mm-4mm removing it and placing the tablet on the TX station makes little difference on VRECT. For instance with a battery voltage of about 3.1V-3.3V @ 1.2A charging current:

- With book cover: VRECT 4.1V DC @130 KHz (frequency measured at the RX coil terminals with a differential scope).

- Without book cover: VRECT 4.3V DC @125 KHz (frequency measured at the RX coil terminals with a differential scope).

I'd like to see the BQ51013B working as intended within WPC specs (frequency, TX-RX Z distance and such).

BTW Ls and Ls' measurements were done with the book cover in place.

Please advise,

Luis Olvera.

Dear Dick,

As a side note, I measured the voltage across the RX coil and it is about 36V pk-pk however across AC1 and AC2 it drops to 12V pk-pk.

I wonder why having these AC voltage levels (specially the RX coil voltage) VRECT is so low.


Luis Olvera.