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BQ51222: thermal shutdown temperature low?

Thijs van Liempd
Prodigy 20 points
Part Number: BQ51222

I have not been able to get the BQ51222 to operate above 85C, while the datasheet lists 125~155 as the shutdown temperature. I did not use an external NTC, but rather a 10k resistor, as per the recommendation from an earlier thread: 'BQ51222: LPRB Excel tool (again)'. I measured the top of the (DSBGA-42) package with an IR camera, as well as a K-type thermocouple, but they both seem to agree. Could there really be a 40~70C difference between the die and the top of the package? or could the internal temperature sensing be off by that much?

(Note: i have currently only tested 2 PCBs, perhaps the small sample size is part of the issue here)

  • 0
    TI__Guru* 75976 points

    Hello

    Can you describe the test to get unit to 85C:

    1.) Is unit in a temp chamber?

    2.) Is the BQ51222 under load?

  • 0 in reply to Bill Johns
    Prodigy 20 points

    outside of a test chamber it only gets to ~85C with a load (Note: it doesn't reach the advertised 1A output, but that may very well be because of bad coil design (high resistance)),

    inside a temp chamber at ~85C it fails even without load. It struggles to maintain an output voltage, and often it will leave the (internal) output-enable off (no voltage on VOUT pin), while reporting a good voltage through I2C (VOUT Status RAM Register).

  • 0 in reply to Thijs van Liempd
    TI__Guru* 75976 points

    We are checking on this, should have an update early next week.

    You have confirmed that the BQ51122 is sending an overtemp fault to the Transmitter, it should shutdown and show fault?

  • 0 in reply to Bill Johns
    Prodigy 20 points

    unfortunately, I cannot confirm whether the transmitter has received an overtemp fault (I do not have access to a Qi TX dev kit). Does the BQ51222 have some way of indicating an overtemp fault? perhaps i can intercept the Qi packets using the header-register or proprietary-packet-registers just before it is sent? (after the TX turns off, these registers are no longer accessible, so it would need to be pretty tight timing).

    I can at least mention that, regardless of load connected to the Vout pin, and regardless of the Qi TX device temperature, the RX appears to shut down when the package radiates ~85C.

  • 0 in reply to Thijs van Liempd
    TI__Guru* 75976 points

    Hello

    I have checked the thermal limit in the lab and with 100mA output unit can at 125C.  Thermal shutdown is above this point.  I verified it will recover.

    Also note that the TX side will set a red fault LED and not restart, recycle power is needed.  Does your TX sent any fault indication (Fault LED)?  To restart does TX require power to be recycled or RX removed and placed back on TX? 

    "Does the BQ51222 have some way of indicating an overtemp fault?" --- TX will shut down and power is removed and BQ51222 will not retain any info.

    It is common for the TX to remain on in a standby state and retain some info.  But this will vary from manufacture to manufacture.

    What can you tell me about the PCB?

    What type of capacitors are you using for resonant capacitors, between coil and AC1/AC2?

    How many of your PCBs show this behavior?

  • 0 in reply to Bill Johns
    TI__Guru* 75976 points

    Can you send a scope capture of the RECT voltage when unit shuts down? 

    Need to see the communication package, if it is thermal we will see a packet followed almost immediately by TX shutdown.

  • 0 in reply to Bill Johns
    Prodigy 20 points

    Good news (and bad):

    My hardware has decided to start working as intended, with 2 different samples both outputting a perfect 5Watts right until shutdown, at ~150C, for which i had to manually heat them using a heat gun.

    It should be noted that my better sample does 5W output at ~70C, while my original sample goes to 90C at 5W. But at least it now reaches that 5W mark, as well as climbing above 85C without the slightest hint of failure. I should probably mention that during these tests, the PCB coil (i designed) heats up to 70~75C, but that is an expected results of switching to 0.5oz copper on this flex-PCB due to manufacturer constraints.

    Unfortunately, this means that TI was never to blame, and there is an issue with my own design and/or manufacturing.

    Below i have included some results for context (like a picture of the PCB, a screenshot of the circuit and Word documents with scope screenshots and thermal images.

    I am terribly sorry to have wasted your time on an issue that turned out to be my own fault.

  • 0 in reply to Thijs van Liempd
    TI__Guru* 75976 points

    Thanks for the update, glad to hear that you have an understanding of the problem.  My experience with PCB coils has not been good.  We tried some designs and they are limited to lower power where size was not critical.

    I will contact you using the PM feature and if needed you can share info in a private message.