BQ51013B: BQ51013B high temperature of chip and coil

Michal1994

Part Number: BQ51013B

Hello.

We are developing small wearable devices and we going to charge it wirelessly. We created PCB with all components, and also with BQ51013B chip and all passive components. PCB is circle shaped with diameter of 24mm. We used WE 760308101214 (https://pl.mouser.com/ProductDetail/Wurth-Elektronik/760308101214?qs=SUJESEQH53AiltUeJALBqQ%3D%3D) coil due to small amount of space with capacitor C1=100nF and C2=10nF - capacitors was choosen experimentaly to make device fit to some choosen chargers (also to reduce voltage amplitude of TX coil). Device is closed in small casing with diameter of about 28mm and height of about 12mm. On the top of main PCB there is battery. We are charging battery using external chip with current of 75mA (battery has 85mAh).

Charging waorking just fine except of temperature. When housing is fully closed it has temperature on top of up to 40 degree of C. Also wireless charger and bottom of device is hot - up to max 55 or 60 degree C. When we open housing right after charging whole PCB is hot and BQ51013B has temperature up to 70 degree C and also coil is heating up. Battery has temperature of 60 degree of C at max.

Thermal pad of BQ51013B is connected to GND plane around chip but there were not space for vias under chip (we are using QFN version). in open air there is no problem with temperature. Chip becoms a little bit hot but i think no more then 45 degree of C.

Can you tell me what could cause this kind of problems?

Best Regards

over 5 years ago

0 Gautham over 5 years ago

TI__Genius 12485 points

Hello,

The problem could be caused by several things or combination of problems.

A good first test is to separate the Coil from PCB / Battery and see how much the temp changes. The coil would be extended on leads and only the coil is in magnetic field, not PCB and Battery. This would isolate the heating from AC field from Conversion loss.

thanks,

Gautham

0 Bill Johns over 5 years ago in reply to Gautham

TI__Guru* 75646 points

Hello

Any update on this problem, any additional test?

0 Michal1994 over 5 years ago

Prodigy 40 points

Hello,

I tried to take coil out of device and temperature is much lower - only chip getting hot a little bit - i think to max 45 degree of C. The problem is that in this device i cannot move coil aside in the way that the electronic arent in magnetic field due to small size of device. I tried also on smaller charger (for Galaxy Watch) and in this case tenperature is lower (there is smaller coil in this charger - mayby this is important). I have two more ideas - to build custom charger with coil which diameter is simmilar to receiver coil and to add additional ferrite shield between receiving coil and electronic in my device. Can you tell me if any of this can improve someting and helps with temperature?

Best Regards

0 Bill Johns over 5 years ago in reply to Michal1994

TI__Guru* 75646 points

Shielding between the RX coil and PCB / battery has been used before to reduce temp. The AC field from TX will generate currents in metals resulting in heating.

Some material to try, Wurth Electronics 364002 available from Digikey.

0 Michal1994 over 5 years ago

Prodigy 40 points

Hello,

I also found out that one of the hottest part on PCB is metal emi shield on ble module. My coil is smaller then PCB so its ferrite shield do not covering all parts of device. I will try to found ferrite shield and cut it to size of PCB or larger and then i will give you feedback about results. This test can take more time, because i need to order this shield