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BQ51050 : Unable to load battery

Other Parts Discussed in Thread: BQ51050B, TIDA-00762, BQ500511

Hello, I've a small design based on BQ51050 for charging AA Battery, Li-Ion 3.6V

I've design a board based on BQ51050, after reading datasheet and Evaluation board : my trouble is the following :

Load Current doesn't exceed 68mA, even my battery is very low, and frequency mesured on Coil is 165kHz, and not 100kHz (Qi standard)

You'll find attached schematic with value computed for the design, I've done few times measures around coil to fit Cs and Cd.

Could you help me find the trouble?

best regards7870.Design_BQ51050.pdf

  • Hi,

    The schematic looks good.  I would recommend a total of three capacitors on the AC1 line (C10 + C13 + Cx).  This will improve thermal performance (efficiency) but I don't think it is causing the issue you are seeing.  How was Ls' measured?  In most cases, I see an increase in inductance for the Ls' value.

    The Qi standard sets the resonant frequency at 100kHz.  The operational frequency is 110kHz to 205kHz, so this system is operating in the middle of the range.

    How low is your battery?  The bq51050B has limits for battery short and precharge.  The 68mA you are seeing does not correlate to either value, but we need to verify the limits.

    What voltage rating do your capacitors have?  In particular, the capacitors on the AC1 and AC2 pins.



  • We have do some test with 3 capacitors on AC1 : same result
    Ls' was measured : charging a capacitor (200nF), and discharging it in coil while monitoring oscillations in LC Circuit : freq = 1/(2*pi()*sqrt(L*C))
    Battery is around 3.5V
    We have just do a test by replacing R16 by a 0R, in order to have only charging current (no termination current) : same result 68mA measured
    Capacitors rating on AC1 and AC2 : 50V, 10%,X7R

    We have tried on different tranceiver always same load current.....

    Thanks for your help, do you have other idea or test we can do?

  • You have certainly done the right debugging.  One more thing I will throw out.  Please check for PCB issues (solder joints, wrong components and such).  I have had several users report that once they did a reflow, or replaced a bad component, the system started working.

    Are you using an evaluation transmitter or a transmitter purchased online?

    It's strange to me that the frequency is in the middle.  Generally if the power level not enough, the RX will request more power and the frequency will shift down.  Once it hits the 110kHz, it can't go any lower so that's where it stays.

    At 3.5V, you should clearly be in the high current charging phase.

    Do you have a source meter you could use to replace the battery?  I generally use one for debugging.  Set the voltage to 3.5V and the current limit to 1A.  That will eliminate the battery and anything else.



  • Hello,

    We work with few transmitters :

    - one from NXP (sample send for free for evaluation : NXQ1TXH5DB1401)

    - one from IKEA0576.Design_BQ50002.pdf

    - one we are building based on TI ref design (schematic attach)

    We always have trouble with current value which doesn't match with value expected. After a lot of test and verification, we find a problem with mulitmeter (current measure)...

    We are using a new one, results are OK with IKEA and NXP transmitter but we need to use our design (based on TI IC's) : Red led is blinking : we removed FOD protection and it works....

    In our case, coil is the same for tansmitter and receiver : wurth : 760308102207 : is it a problem or not? a student has done test and it seems to work, maybe it's wrong, or not???

    What are your recommandations? I have a hard mechanical contraint 40*40mm for tansmitter coil....

    Thanks for your support.


  • For the TX coil we only recommend 6.3uH +/-10% the wurth 760308102207 is 8uH. Wurth also makes a 30mm TX coil 760308101103, this has been used on TX Reference Design TIDA-00762 and others with good results.
  • Hello,

    It doesn't work with TX Coil 760-308-101-103.....

    When I receive TX Coil, I mesure again Ls an Ls' for Rx coil (couple and not) with Tx coil :

    Ls' = 8.753uH and Ls = 8.523uH

    I've modified Capacitor (see attache schematic for Tx and Rx) - doesn't work, never arrive to 600mA, goes in FOD

    So I change FOD THR (R22 = 100k and R25 = 200k), not enter in FOD but no more current

    Coils are separated from around 3mm (2*1.5) : 3D printers : laser sintering of polyamide

    Do you have an idea of what can be wrong? I'l become crazy !!!

    4621.Design_BQ51050-20160811.pdfThanks for your support



  • The FOD feature will require calibration to correct for coil changes, circuit changes, friendly metal in the case and others. The RX and TX FOD values will probably not work for your design.
    For test I would recommend disabling FOD by pulling bq500511 pin 35 high, remove R25.

    If that does not get you more power what is the operating freq when at 600mA?
    110kHz is the min freq and max gain--this would indicate a coil or tuning issue.
    Freq higher than this would indicate something is tell the system to not deliver more power.

  • Before removing R25 and R22 and tied High pin 35 of BQ50011, i've check frequency and signal on the coil : frequency moving around 115 - 130kHz (see attach AvecFOD.jpg)

    After removing, same measure, signal seems to be more "beautiful", and frequency moving around 120kHz (see attach SansFOD)

    Current is around 400 / 450mA, sometimes higher when coil aligned but very sensitive few mm only? is it normal?

    How to calibrate FOD in my enclosure now, that it seems to work better? What are the measurement to do? I don't find any documentation app notes?

    Thanks for your support


  • For coils in the 30mm range they will show sensitivity to alignment.

    FOD calibration for Qi certification is complicated. Will the device be used with other transmitters / receivers, interoperability required?
    If you only plan to operate your transmitter & receiver the FOD set up is more flexible.

    I have an FOD presentation in the FAQ section of the forum, see button in upper right.
  • Hi,

    I've do more test in two different configurations (different coils), with FOD removed

    Results are attached to this reply

    Frequency isn't around 110kHz, does it mean something is wrong? It seems schematic send before, and capacitor values are OK?

    What can I do to reach 110kHz?

    Thanks for your support


  • Can you clarify which traces are shown in the attachment?  The blue trace (Oscilloscope 1) with the pulses is what signal? What are the levels seen?  Same question with Oscilloscope 2 (red trace).

    The traces appear to be the normal communication packets with nothing unusual.

    The frequency should be between 110kHz and 205kHz.  Anywhere in that range is okay with the WPC specification.  Lower frequencies indicate the transmitter is working harder to transfer more power.  If you bump up against 110kHz that generally indicates your system is not operating with enough margin.  130kHz to 140kHz that you see in configuration 1 is a good operating range.

    In configuration 2, the communication pulses are requesting more or less power (lower or higher frequencies from the TX).

    The smaller TX coil in configuration 2 is not coupling as well as the larger coil used in configuration 1.



  • For each configurations
    - first screenshot :
    - blue trace is current measurement (entrace of battery), Voltage measured correpond to 650mA (home maid tool)
    - red trace : not applicable
    - second screenshoot :
    - blue trace : voltage on coil 1 (side 1 of BQ51050B)
    - red trace : voltage on coil 2 (side 2 of BQ51050B)

    So, best configuration is with the same coil for receiver and transmitter, frequency is more stable between 130 and 140 kHz

    No problem to work with same coil? My product is specific, receiver and transmitter will align the two coils each time the'll be together.

    Now, it works, I've to configure and calibrate FOD? Is there an application note or procedure to do this?

    Best regards
  • I'm ready to build 50 products using BQ51050 architecture described in this post, using same coil for receiver and transmitter (Wurth ref : 760308102207 ) because in my configuration and mechanical design it's the way to get best result.

    One point is to be fixed quickly now, how to calibrate FOD, removed during test?

    Best regards

  • Go to the Tools & software tab on the bq51050B product page.  The Software section has a file "Foreign Object Detection (FOD) Calibration/Tuning Tool".  That is a GUI to help with the calibration.  The GUI also includes a PDF User Guide that details the FOD measurements.

    Additionally, if you go to the Support & training tab, there are a couple of FOD videos showing the introduction to the tool and the lab collection.  The videos use the GUI and walk through the steps shown in the User Guide.



  • Hi Dick,
    You said,

    "How low is your battery? The bq51050B has limits for battery short and precharge. The 68mA you are seeing does not correlate to either value, but we need to verify the limits."

    Can you help by posting what the limits are for the bq51050B limits for battery short and precharge?

    Thanks so much!
  • Jamie,

    By limits, I mean the "VLOWV" specification for the PRECHARGE. Below VLOWV (3V nominal), the bq51050B will be in precharge mode which limits the current to 20% of fast charge. Below VBAT(SC) (0.8V nominal) the bq51050B is in short mode and limits the current to about 18mA.