Measuring Ls' (a Qi parameter)

Upon review of the spec, 1Vrms @ 100kHz is to be applied directly to the secondary coil, and the LCR meter supplies this signal.

Unfortunately I get different results when I use different charging stations.  The description in the TI datasheet says to use "50 mm x 50 mm x 1 mm of Ferrite material PC44 from TDK Corp".  Can anybody help me find this material?

Kyle,

Glad you got the first part figured out.  I'm checking into the Ferrite.  As an alternative to using this ferrite, you can use a WPC transmitter - as long as it is one without a magnet in its transmitter coil.

For instance, the TI transmitter EVM for the bq500212A that you are using, has a Qi-compliant A11 coil which does not use a magnet.

Regards,

Dick

Thanks for the response, Dick.

I'm able to take the measurement using the bq500212A board and a spacer of 3.4mm from the ferrite surface to the Rx device's interface surface.  However, when I place the bq500212 in a charging case which contains aluminum, and perform the test I get a different reading.  So I'm wondering if I should use the Ls' value measured in the configuration specified in the TI documentation, or if I should use the value measured when the transmitter's aluminum case is present since that how my mobile device will often be charged.

The reason I'm so concerned about getting the correct value is that we have had a lot of issues with reliability of the charging.

Thanks,

Kyle

Kyle,

That's a good question. The ultimate calibration is the final configuration.  I would suggest using the measurement from the final configuration. What values do you get for each measurement of Ls'?  And, what are the values of the Cs that your calculation shows?

Can you describe more about your "reliability of charging"?

Regards,

Dick

I am using a TDK secondary coil (WR-383250-17M2-G, datasheet shows Ls=11.1 μH Ls'=12.3 μH(1)).  I measure Ls = 11.67uH.  For Ls' I measure 13.27uH on bq500212 @ dz = 3.4mm.  I measure 11.06uH when the bq500212 is placed in my charging station.  The Cs values are 191nF and 229 nF, respectively.  With Cs = 200nF, we were experiencing every almost all units fail with this coil.  Then after dropping from 200nF to 168nF, they all started working again.  I realized my method for arriving at 168nF was flawed and perhaps I should have increased the value to 229nF instead.

WRT "poor reliability"
On many units, charging seems to work fine.  But some of our units have troubles and the effect we see is that the power LED of the mobile device goes on (output of bq51013a) and then turns off.  We have seen many different types of behavior including variable time the LED will stay on (2 seconds, 5 seconds, many minutes).  Sometimes when the LED goes out, it never comes back on.  Sometimes it sits there undisturbed and comes back on seemingly randomly.  We've tried combinations of 2 different secondary coils, various power supplies for the TI eval module (power supplies sometimes make a difference), third party charging stations, and tuning values for Cs & Cd.  We have yet to find a combination that is 100% reliable.

Kyle,

Thank you for the details.  I've got a lot of questions that might help move things along.

You mention 229nF.  Have you run with that value?   How did you configure the values?

You mention 168nF and all working.  Can you share the capacitor values used to get to 168nF?

What capacitance was used for the results you shared?

Based on you results, there is something else I'd like you to investigate.  The bq51013A and the bq500212A use "PMOD" to check for Parasitic Metal Object Detection.  It's possible the aluminum is absorbing energy being created by the transmitter.  A quick test can be to remove the PMOD resistor from the bq500212A which disables this function.  On the EVM, this would be removing R27.

A good application note on this is:  http://www.ti.com/lit/an/slua698/slua698.pdf.

I expect this is known, but just to reiterate - the bq51013A is not Qi V1.1 compliant.  If Qi certification is required, consider the bq51013B.  The difference between the  A and the B is PMOD is changed to FOD (Foreign Object Detection).  The functions are quite similar, but the FOD is more accurate and is specified in V1.1.

I'd like a bit more information about the testing you've done.  Specifically, are you using the EVMs as shipped or have they been modified?  Is the coil you're using attached to the bq51013A EVM or is it a board you've built?

Regards,

Dick

Dick,

I'm putting together a lot of detail about our tested configurations.  I probably shouldn't put this information on the internet.  Is this something I could send to you offline?

Thanks,

Kyle

Kyle,

Thanks.  We can complete this offline.

Regards,

Dick

Dick,

We are adding Wireless Charging to one of our system and also need help with the Ls' value. (I am using BQ51050B) It is a bit unclear to me, when you measure Ls' on the actual transmitter:
1. Should the coil of the transmitter be disconnected?
2. Should the TX not have a magnet in, even if the final transmitter has one in?
3. Should you add the 3.4mm spacer between the TX and RX or just put them straight on?

Thank you
Johann

Great questions, Johann.

I hate this part of the Qi spec because it indicates to take the measurement while placing the secondary coil on a power transmitter.  But you shouldn't be designing a device to be compatible with one specific transmitter, you need your device to work reliably on all chargers.  The spec also neglects to mention the materials in the wireless device, it just says to place the "coil" on a transmitter.  But the properties of the coil could be affected by your device's housing, circuit boards, ... any material in your device really.  I ended up using the values from the coil installed in a representative device.  I had to sacrifice a unit to drill holes in the aluminum case and run two wires out for the measurement.  I can't remember anymore if I left the coil in-circuit during the measurement... I think I did.

You might have to take measurements in several scenarios (different transmitters, bare coil, coil installed in device, etc) and play around with the fs and fd tolerances from the Qi spec.  Maybe see if you can get your values to converge such that regardless of which Ls/Ls' measurements you use, those tolerances are still satisfied?  And I forget which one, but one of the two values (Cs or Cd ) is typically more important.  The other is only there for when the device needs to work on a transmitter that has primary coils that physically move to perform alignment on a motorized stage.

Also, TI has a USB Interface Adapter that plugs into the transmitter eval kit and they have a software package that gives Qi info such as efficiency and error codes.  The tool is a bit sketchy, but definitely get set up with it.

Good luck!

-Kyle

Johann,

The reply from Kyle pointed out some very good points.  I'll add a bit to hopefully clear things up a bit.

The Cs capacitors are critical for setting the resonant frequency to 100kHz (which matches the frequency setting for the Transmitters).  The Cd is secondary and not nearly as sensitive.  Setting both the TX and RX to the same resonant frequency ensures the interoperability between different RX and TX systems.

The key to measuring the Ls' is using the final RX solution you will have.  This can get difficult since you will need to disconnect the RX coil from the system and get the connections to the external meter.  In most cases, this requires additional wire connections.  This will rarely impact the Ls' measurement, but can impact the resistance - but I have not seen significant changes in the resistance.

If you are using a custom TX, then you should place the RX system on the TX system for the final Ls' measurement.  This will be with the full systems in place.

As you know, changes in metal, magnetic fields and spacing can have a great impact on the final inductance measurement.  

The spacing should be as close to your final system as possible.  The 3.4mm discussion is for standard systems.  It tries to limit the TX and RX sides of the equation to aid in interoperability.

The resonant frequency is set to 100kHz.  The TX can change frequencies down to 110kHz.  The extra 10kHz can be considered a guardband for tolerances between systems.  Most RX systems do not operate over the full WPC range (110kHz to 205kHz).  If the RX is tuned and operates at 175kHz (the ping frequency) and around there, the 110kHz frequency (maximum power deliver) will never reach that level.

Let me know if this helps and we can continue the discussion as needed.

Regards,

Dick

Thanks Dick and Kyle for your answer.

So what you are saying is that I have to measure the Ls' on the actual TX and calculate the Cs. Once I place the RX (complete system) on the TX (complete system) I have to measure the freq on the RX coil and tune Cs to get the working freq to 100kHz?

Thank you

Johann,

The Ls' needs to be measured when the complete RX is placed on the TX.  That value is used to calculate the Cs.  Once the proper Cs is chosen, the TX / RX system will take care of the rest.  You do NOT tune Cs to get working frequency at 100kHz. 

When you tune your system with the Cs chosen from the measured Ls', the system should operate above 110kHz.  The TX will not operate at a frequency below 110kHz.  

When your system is tuned properly, I would suggest measuring the frequency at light, medium and full loads for your system.  This will show you how much margin you have in your system.  In typical cases, the maximum spread is 20-30kHz.  If you see that your maximum power level gives a frequency close to 110kHz (minimum frequency - maximum power delivery), you may consider tuning the Cs to keep away from the extremes.  Same would be true if your system is operating at 205kHz for light (or no) load.  

In general, proper tuning will not result in operation near the extremes.  

Note that alignment of the RX on the TX will also have an impact on the operating frequency.  

Regards,

Dick

Dick,

Thank you, I think I am starting to understand how the system works. I will do more tests and see what happens and ask if I have more questions.
Just a random question. I see TI is also doing more and more PMA chips, which one do you think is better, PMA or Qi? I like the new bq51221, its just a bit expensive because you still need to add a charger IC as well.

Thanks