BQ51050B: Rough FOD calibration

Austin Petty

Part Number: BQ51050B

We are working on two new products using the BQ51050 for the wireless charging receiver, but do not plan to have them as certified Qi products. We would like for them to be as Qi compliant/compatible as possible, but. definitely do not have enough funding to buy the Avid FOD Tx. We have our first prototype board made, but it when connected to a wireless charger it does not begin power transfer. I suspect this is because the FOD needs to be tuned, but am unsure how to know if that is the problem without buying the expensive FOD Tx. Any advice on troubleshooting this would be greatly appreciated. I saw in another forum thread options for rough FOD tuning were discussed, but the conversation was moved to a private e message before enough useful information was provided.

Thank you,

Austin

10 months ago

0 Austin Petty 10 months ago

Prodigy 110 points

As a follow up, what changes to R-FOD and R-OS could be made to essentially turn off the FOD for testing purposes? I'm thinking if I can just make it so the chip does not use FOD at all to see if it works it would let me know if there is a problem elsewhere in my circuit.

Thank you,

Austin

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

Hello Austin

Austin Petty said:
We have our first prototype board made, but it when connected to a wireless charger it does not begin power transfer.

The voltage at RECT pin is a good indicator of what the problem may be. Would it be possible to send a scope capture of the start up and shutdown?

Austin Petty said:
what changes to R-FOD and R-OS could be made to essentially turn off the FOD for testing purposes?

A safe set of values for testing, R-OS (RECT to FOD) 42.2k, R-FOD (FOD to GND) 226 ohms

Can you send a schematic for review?

Also what Coil are you using?

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

I will work on getting a scope capture done. How would you like me to share the schematic? I am using the TDK WT202012-15F2-ID for the coil.

Thank you,

Austin

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

Hi Austin

The WT202012-15F2 coil has a couple of problems. It is a good TX coil but not a good RX coil. L value is only 6.2uH and diameter is small 20mm.

The TDK WR222230-26M8 is a better solution, 27uH and 22 mm.

Austin Petty said:
How would you like me to share the schematic?

Can you attach as a PDF?

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

I have attached the schematic. The coil was chosen for having the integrated alignment magnet and ferrite shield. The charger will connect similar to the way an Apple Watch charger would, so it should always be nearly perfectly aligned. Is it possible the coil I gave could work or is it a nonstarter?
8321.Battery Charger.pdf

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

The WT202012 will have lower gain and will be harder to start power transfer. The coil to coil distance needs to be kept low.

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

Charger Scope Capture Shutdown.xlsx Charger Scope Capture Startup.xlsx

I have attached scope captures for the startup and shutdown. I don't think the PNG files are very realistic just because the scope program isn't best at displaying data, but the excel files include all data. The capture uses the FOD resistor values in the original schematic I sent. In the mean time I will work on getting them changed out to the values you recommended. Please let me know if you need anything else to help figure out where the problem may be.

Thank you,

Austin

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

Hi Austin

Thanks for scope capture.

Is the voltage correct, amplitude is low less than 1V?

The voltage at RECT needs to increase above 2.7V (UVLO) for the RX to reply to TX.

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

Bill,

Sorry, after a bit more testing I realized the Channel 1 on my scope won't calibrate correctly. My scope is relatively cheap so I'm not 100% certain on its accuracy. I redid the capture on channel 2 and got this data. Peaks are nearly 7V, is that more normal or abnormally high?

Charger scope capture.xlsx

Thank you,

Austin

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

Thanks Austin

Can you get a closer look at one of the pulses? Trying to see if the RX is replying to TX.

Below is what you should see at start up.

0728.bqTESLA FAQ--Start up.pdf

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

Bill,

I think my problem is the transmitter coil I am using. I have ordered a different one that will be here in a week. I will get back to you when I am able to test with that one.

Thank you,

Austin

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

I was able to change Ros and Rfod to the values you recommended and took another scope capture. I am unsure of what the problem is. Power transfer is still not starting, but the charge indicator light on the transmitter turns on for a few seconds then shuts off and will periodically turn on/off again. Please let me know if there is anything else I can do to help figure out the problem.

Thank you,

Austin

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

Hi Austin

Thanks for the scope capture. It looks like the RECT voltage is around 3V which is low. The control packets should command the transmitter operating point to produce 7V at RECT. We see the control packets and some movement in voltage but it is low.

Test is with the WT202012-15F2-ID coil?

0 Austin Petty 10 months ago in reply to Bill Johns

Prodigy 110 points

Yes, test was with the WT202012-15F2-ID. I have a wurth elektronik 760308105214 coil on hand that I could also test with if you think it may work better. Otherwise I will look for a different Rx to order. Unfortunately the TDK WR222230-26M8 coil you recommended is slightly too large for our housing.

0 Bill Johns 10 months ago in reply to Austin Petty

TI__Guru 65746 points

Yes, try the WR222230-26M8

The problem I see in the scope capture is low RECT voltage which is associated with the RX coil L value, higher L will give higher system gain.

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BQ51050B: Rough FOD calibration