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BQ51050B: Charging suddenly stops or I(chg) is very low

Daniel_U
Prodigy 235 points
Part Number: BQ51050B
Other Parts Discussed in Thread: BQ50002A

Hello,

I have a problem using the BQ51050B wireless charging receiver. I'm using it to charge a standart 500mAh Li-Ion battery.

Here is my schematic with all used values for caps and resistors (in brackets are the recommended values taken from the datasheet):

As I don't know the exact data from the receiver-coil I went for C1 and C2 in the recommended range stated in the datasheet for a "common" coil.

My used components are like the ones from this set (I removed the electronics from the receiver-coil and only used the coil itself)

In my first try I used 47nF capacitors as CCOMM. The CHG-Pin went low and high every second or two. My charger indicated connection to a receiving device while the pin was low and no connection when it was high. I then changed to 33nF caps for COMM with similar results (only the interval of low/high was longer). Then I changed the caps to 15nF and got it to work and charge the battery properly. The output voltage was ok and the current flowing to the battery was more or less as calculated and constant (CC phase of charging). But after 2 minutes (and no external interference) the mess started again and charging stopped. The CHG-Pin started toggling again. I still measured about 80mA flowing to the battery most of the time and sometimes little spikes of current. But the voltage on the battery was not higher than V(bat) without the charger, so there was no proper charging anymore. I restarted the transmitter-coil but wasn't able to reproduce the stable charging scenario anymore.

Playing around with different additional C1 and C2 capacitors there still was no improvement. Though after reverting back to the previous design without those additional caps it suddenly worked really good. So after all it seems that the design is not very reliable at the moment.

 I'm not that much into Qi-charging, so I can't estimate the influence of different cap-values in the design and how much they effect it.

Where could my problems come from? 

Regards

Daniel

  • 0
    TI__Guru 72461 points
    Hello Daniel
    Could you send a scope capture of the RECT voltage at start up?
    This would give an indication of coil tuning.

    Also do you have a measurement of the RX coil inductance in position on TX coil?
    What is your coil to coil distance?
  • 0 in reply to Bill Johns
    Prodigy 235 points

    Hello Bill,

    I have edited my original post in the minute of your reply. Right now it seems to work relatively good. I don't know why and wether it will be stable in the future but apart from noise in the currentflow (about 40mA at 100-300mA charging current) it looks ok. Maybe it's just luck that the last few tests went positive.

    Here is a scope shot of the RECT-Voltage(1V / 1s):

    After this startup the device worked properly. Currently the error doesn't appear.

    Unfortunately I am not able to measure the coil inductance at 100kHz as described in the datasheet. My multimeter only measures at 100Hz.

    The measured values are: on top of the surface inductance: 150µH; free-space inductance: 60µH; coil-resistance: 500mOhm

    The inductances seem pretty high compared to the example ones in the datasheet.

    The coil to coil distance is about 3mm (in the final design it will be about 6mm).

    Regards

    Daniel

  • 0 in reply to Daniel_U
    TI__Guru 72461 points
    Hi Daniel
    The start up waveform look good, do not see any problems with it.

    The comm capacitor value can be adjusted to fit the system performance. Typical value is 10% of the Cs value (C1). The Cs value will change with RX coil and tuning.
  • 0 in reply to Bill Johns
    Prodigy 235 points

    Hello Bill,

    ok, that sounds good.

    Speaking of C1 and C2 tuning: How serious can the measured values for the inductances be taken (because of the measurement-frequency)?

    With those values C1 calculates to 16.8nF and C2 0.43nF. This seems very low.

    Regards

    Daniel

  • 0 in reply to Daniel_U
    TI__Guru 72461 points

    Hi Daniel
    "The measured values are: on top of the surface inductance: 150µH; free-space inductance: 60µH; coil-resistance: 500mOhm" -- Bill J - This does not look right, too high. If you have some way to check the multmeter at 100Hz, the typical TX coil is 6.3uH.  Depending on the size you should be in the 10uH to 30uH range.

    ---Updated---

  • 0 in reply to Bill Johns
    Prodigy 235 points

    Bill Johns said:

    This does not look right, too high. If you have some way to check the multmeter at 100Hz, the typical TX coil is 6.3uH.  Depending on the size you should be in the 10uH to 30uH range.

    Unfortunately I don't have the possibility to check the multimeter at the moment.

    For now I will assume the values to be the ones from a standart coil and use the corresponding capacitor-values (the ones in my original schematic).

    So in general: Can I expect the problems to come from the C1/C2/C(COMM) values or is there any other design-flaw that could cause such problems? I noticed that the device doesn't deliver high enough I(chg) in CC-phase. Though this current changes with R1 setting but is never as calculated. It's about half the calculated current. Also charging doesn't terminate at the right moment. It goes down to about 10mA before stopping (no matter what R1 is with R4 always set for 10% I(chg)). Maybe it's a hint for the main problem, otherwise I will open another post regarding this specific problem after collecting some more data on it.

    Regards

    Daniel

  • 0 in reply to Daniel_U
    Prodigy 235 points

    Today the old problem suddenly occured again. This time I was able to capture what RECT does (2V / 0.5s):

    There obviously is something severely wrong with the voltage going from 5V to 8.5V.

    Increasing the distance to the transmitter coil a few millimeters results in V(RECT) to be at a pretty constant 4V. But then no charging is going on despite the CHG-Pin beeing low.

  • 0
    Prodigy 40 points
    I have encountered a similar problem. Have you solved this problem? How to solve it?
  • 0 in reply to LiJia Pei
    TI__Guru 72461 points

    Hi Daniel

    I have captured a typical start up waveform, below.

    CH1 / Yellow is RECT voltage.

    CH2 / Red is CHG pin voltage

    CH3 / Blue is BATT voltage

    CH4 / Green is BATT current

    Test were done with standard EVM and BQ50002A / 511A TX EVM.

    In your waveform the voltage variations are large, possible oscillations or communications packets I cannot tell.

    How many units are you seeing this problem on?

  • 0 in reply to Bill Johns
    Prodigy 235 points
    Thanks for the effort!

    The curves are roughly what I expected a working system to behave.
    As the current circuit is build on a single development PCB there is only one unit I am testing with. Because of the difficulties in debugging at the moment I ordered a new PCB a few days ago to make changing capacitors and so on easier. Then I can better research on whether something is wrong with the resonant circuit.
    If true, I am still confused about the CHG-Pin beeing low then (no charging is going on). Because in an error-case I wouldn't expect it to do so.
    I am also working on a better measurement of the coil inductance. This will take a few days.

    I will let you know about the outcome of this coming tests.

    Regards
    Daniel
  • 0 in reply to Daniel_U
    TI__Guru 72461 points
    Thanks for the update.

    This sounds like a good approach, improved PCB.

    OK to close this post and work with you off line?