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BQ24650 overheating -> thermal shutdown, likely bug in pcb design

Other Parts Discussed in Thread: BQ24650

Hello,


I am turning to you all for help with BQ24650 charger design that has proven to be difficult lately and was unable to debug it with significant effort.

The problem is very simple, using this charger even with low currents of about 500mA the IC itself overheats and falls into thermal shutdown. Since there should not be any power conversion path via the IC itself, this should not be happening without charging.

We have made two revisions of the PCB to exclude any manufacturing induced errors as well as assembly/soldering bugs with several prototypes.

All the operating parameters as defined in the datasheet and reference design BQ24650EVM are as specified and the regulated output voltage is stable. Power consumption of the charging circuit in standby is 10mA, no battery connected.

Design is published at: https://github.com/IRNAS/ServalRuggedBatteryPack/tree/master/electronics/V3 Note that in testing the rest of the design has not been populated, just the Charging section tested.

I am happy to provide any other information if required, currently I do not have the prototype at hand to perform additional measurements but can do that shortly. Any other thoughts on the design are welcome.

  • Upon examining your schematic, it looks like R8 (sense resistor between SRP and SRN pins of the device) is too small. Currently, the schematic has 4mOhms. The BQ24650 employs a cycle-by-cycle current limiting that terminates each cycle when the SRP-SRN voltage differential reaches 40mV. 

    Following Equation 3 on page 13 of the device datasheet, a 4mOhm resistor will result in a 10A charge current. If you want to limit the fast charging current to 500mA, then a 80mOhm resistor must be used for R8.

    Therefore, if the charge current is set to 10A with a 4mOhm resistor, and the PCB layout is not designed for high current, it is very likely that there is no thermal dissipation for the external high-side and low-side FETs. This would cause the IC to go into thermal shutdown due to the close proximity of the overheating external FETs to the IC.

  • Thank you for the reply. The circuit is designed for 10A, however we have performed the testing also with 20mOhm resistor, limiting the current significantly and same problem persists.

    However even the MPPT input catches the charging current and limits it to keep the regulation voltage, limiting the current with this method does not solve the problem either.


    Two revisions of the PCB have been made, increasing the separation of mosfet to IC and improving thermal conductivity, however the problem persists. The analysis with thermal camera confirms that the IC heats up much faster and much more then the mosfet.

  • Can you verify that the thermal pad of the IC is soldered onto the PCB well? Looks like you have 4 vias in the thermal pad for heat dissipation, and if the thermal pad is making a good connection to the pad, this should help.

    Beyond this. could you share some scope captures of the nodes on your PCB? Waveforms of PH, HIDRV, LODRV, and VBAT will be a good start.

  • Yes, the thermal pad is soldered and connected with 4 vias to ground plane.


    I will post the scope captures as soon as possible. When previously tested, the waveforms were matching the ones in the datasheet.