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BQ25504: BQ25504 - MPPT and OCV issue

LzBone
Intellectual 770 points
Part Number: BQ25504

Hi,

I had made custom PCB for BQ25504  and asked fabrication vendor to fabricate PCB for me.

I also make sure i followed all guidelines from the datasheet PCB layout recommendation.

BQ25504 chip i bought are from element14 "https://sg.element14.com/texas-instruments/bq25504rgtt/battery-charger-li-ion-qfn-16/dp/2144306?scope=partnumberlookahead&ost=2144306&searchref=searchlookahead&exaMfpn=true&ddkey=https%3Aen-SG%2FElement14_Singapore%2Fw%2Fsearch"

I tested my 2 PCB boards with same harvesting source and both of my boards have all same components.

My issues are as follows:

Board 1:

- When i measure input voltage to PM, i found out that OCV and MPPT 90% seems ok. However, after OCV sampling, input voltage keep dropping and creating saw tooth pattern. I have tested BQ25504 evaluation boards before and if i recall correctly, after OCV sampling the voltage should remain that level until next OCV sampling, right?

- So going down like that, is it a problem? What could be the possible issue?

Board 2:

- Then i test my board 2, which has the exact same components as well as same input source with board 1.

- I noticed board 2, OCV doesn't even close to 2V, whereas, board 1 OCV is ~2.4V.

- Besides OCV difference, board 2, MPPT seems to be way off too. I set it to be 90%, as like i did for board 1.

For both of the board, I make sure that during both testing, VBAT voltage is pre-charged to abotu 2.4V to avoid cold start state.

I will really appreciate if you can suggest me regarding with above mentioned issues i encounter. Thank you.

Regards,

LB

  • 0
    TI__Guru**** 166176 points
    Hi LB,

    For board 1, if VIN_DC is drooping after sampling, the capacitor on VREF_SAMP may too much leakage or the solder flux may be creating a leakage path. Can you try removing the capacitor, cleaning the board then replacing the capacitor? Alternatively, you could apply an external power supply to VREF_SAMP at your MPPT voltage to confirm there is no other issue with your board.

    For board 2, it appears that the either the capacitor on VREF_SAMP is shorted or the MPPT resistors have the solder flux issue mentioned above.

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Intellectual 770 points

    Hi jeff,

    Thanks a lot for your reply. I will try and report back.

    Thanks and Regards,

    LB

  • 0 in reply to LzBone
    Intellectual 770 points

    Hi Jeff,

    Below are my testing results.

    -----------------------------------------------------------------------------------------------------------------------------------------------------------------

    (Test : 1) I removed only Cref capacitor. I only clean Cref existing solder and flux, and provide power supply of 1 V to VREF_SAMP pin. Result is as below. Both seems to be working well.

    Q1: As per my above result, can i assume my boards has no other issues, even for MPPT resisters soldering and fulx? (I have not removed/resolder MPPT resistors solder and flux yet,i would like to get your opinion on this first)

    -----------------------------------------------------------------------------------------------------------------------------------------------------------------

    (Test : 2) Now I solder Cref with another capacitor (MCU0805R103KCT -  SMD Multilayer Ceramic Capacitor, 10000 pF, 50 V, 0805 [2012 Metric], ± 10%, X7R, MCU Series , https://sg.element14.com/multicomp/mcu0805r103kct/cap-0-01-f-50v-10-x7r-0805/dp/9406352?scope=partnumberlookahead&ost=9406352&searchref=searchlookahead&exaMfpn=true&ddkey=https%3Aen-SG%2FElement14_Singapore%2Fw%2Fsearch ) . In my previous PM BQ25504 board I made, about a year ago, i used above mentioned capacitors and I did not had any leakage issue. So, initially, i assumed they should work, however, result as below and leakage problem still seems to be there.

    Q2: Would you recommend any particular capacitor for me to try? The exact type of capacitor that is suggested in BQ25504 evaluation board user manual is no longer available to purchase.

    -----------------------------------------------------------------------------------------------------------------------------------------------------------------

    Q3. Is there any other thing would you recommend me to check in my PM board for the issue?

    Thanks a lot. I really appreciate your kind help.

    Regards,

    LB

  • 0 in reply to LzBone
    TI__Guru**** 166176 points
    Hi LB,

    Regarding Q1, if the IC is properly regulating to 1.0V when you use external power supply at VREF_SAMP, the next step is to check for solder flux issues with the resistors by removing, cleaning and replacing. When you replace, try standing the resistors on their side to reduce the likelihood of parasitic resistors from flux.

    Regarding Q2, unfortunately, I do not at this time. A similar sized and rated capacitor (especially the lowest impedance point on the impedance curve which is 0.07ohm for the EVM's capacitor) as the one on the EVM should be good enough.

    Regarding Q3, see Q1. Also, to rule out the IC pin itself, is it possible for you to measure current in nA range from the source attached to VREF_SAMP?

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Intellectual 770 points

    Hi Jeff,

    Thanks again.

    As per your suggestion, now i tried to solder them by the side as you can see below (I understand u correctly right). As of now, apart from this 3 components (2 x MPPT resistor, 1x cref), the rest of the components are soldered by the pcb vendor.

    After soldering that 3 components by the side, measurement results as below. 

    What i meant by cleaning in my previous post is, i used solder wick to remove the original solders used by vendors. Then, I use clean cotton bud to rub a little bit to pad and area between two pads.

    Q4. Should i be cleaning with some special solution or something?

    ---------------------------------------------------------------------------------------------------

    As for current measurement of the external source attached to VREF_SAMP, i use NI DAQ card NI-9207 to capture the current. It is fluctuating, but on average, it is about 5 uA for both of the boards. (Note: for this measurement, Cref capacitor is removed)

    Q5. Should it be lesser than that? Any other suggestion for me for further troubleshooting?

    Thank you.

    Regards,

    LB

  • 0 in reply to LzBone
    TI__Guru**** 166176 points
    LB,

    It appears that the MPPT resistors are not the problem. 5uA is very high for this device but your meter may not be able to measure that into the nA range. Try cleaning the entire board with alcohol, especially the IC.

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Intellectual 770 points

    Hi Jeff,

    Today I have done followings:

    (1) Desolder two mppt resisters, and Cref capacitor

    (2) Clean their pads and in between pad areas with alchol

    (3) Re-solder back them 

    (4) Wiped all 16 pins of the chip with alchol too.

    Results as follows:

    It still seems like Cref capacitor leakage still seems to be there.

    However, one improvement i noticed is that, for board 2, previously, it wasn't charging the Vbat at all. But now, it is charging. And board 2, MPPT sampling voltage level is now higher than before.

    Q6. Should i be de-soldering and cleaning the rest of the resistors and capacitors too on the board?

    Q7. For BQ25504 chip, after MPPT sampling, if Vin is keep dropping as like in my board, is it actually a concern from operational point of view?

    Thank you.

    Regards,

    LB

  • 0 in reply to LzBone
    TI__Guru**** 166176 points
    LB,

    Regarding Q6, my next recommendation would be to remove and resolder the IC itself.

    Regarding Q7, the charger's MPPT circuit is not functioning properly but you have confirmed that the IC appears function with external reference voltage at VREF_SAMP. The IC will not be damaged but it will charge very slowly if at all.

    Regards,
    Jeff
  • 0 in reply to Jeff F
    Intellectual 770 points

    Hi Jeff,

    As per your suggestion, i have de-solder ic chip, then wiped, then re-solder back. It works well now. Result as below:

    Thanks a lot indeed for helping me giving good advises for my boards troubleshooting. That's means a lot to me. I really appreciate your kind help. Have a nice day to you.

    :)

    Regards,

    LB