BQ51003: bq51003

Hi Raviselvan C R

The thermal rise could be caused by a couple of things.  Can you send schematic and info on coil?

AC fields entering the device could also be a problem, any metal around the RX coil?  If you take the RX coil and extend it with leads, then place only the coil in AC field do you still see the overheating?

Hi Bill Johns,

  Thanks for your quick response.

   As you suggested we kept the device  away from the charging pad by increasing coil lead length but keeping only the Rx coil in the AC field. The coil leads length has been increased to 30mm to keep the far device away from the charging pad. Now the heat generated in the device came down to 37 deg Celsius  from 55 deg celsius, still it's quite large heat for a 30 mm leads, is not it?

Please refer the relevant schematic attached. In the schematic U37 is bq51003 and U14 is bq24072. The coil used in our design is WR222230-26M8-G.

Some times for long time, the OUT1..OUT4 of the bq51003 becomes around 4.6V instead of 5V. This causes the battery to stop charging. Any suggestions?

thanks
Raviselvan C R

Schematic_bq51003 with bq24072.pdf

Hi Raviselvan C R

Extending the coil is a good way to measure the heating due to AC field in device and conversion loss.  It appears you have some of both.

AC loss due to field can be reduce with shielding material that will prevent the AC field from entering PCB, battery and other areas. 

Conversion loss will be due to efficiency of the circuit.  Some of this is thermal rise of BQ24072, linear charger. Some of this is the thermal rise of BQ51003.

Try reducing charge current to battery, this will reduce thermal rise on both devices.

Also tuning of the RX coil does not look right, ( C135, C136, C137 ) are value from EVM but they should be adjusted to matching coil L used in application.

Hi Raviselvan C R

I will close the post for now, if you need any further assistance it can be re-opened.

Hi Bill Johns,
Thanks for your continued support and sorry for our delayed response. As you suggested we extended the leads of the Rx coil and kept the device away from it and noticed that the device is not getting heat. But in the acutal case, the coil and the PCBA are tightly kept together ( PCBA-battery-coil) in the small device ( 68 X 30 X 17).  So we can not avoid the heating issue. We have unsuccessfully tried some other methods to reduce the heat, that is why we could not get back to you soon. This heat issue is stopping us from completing the development.

The heating issue continues even afer we reduced the charging current to the maximum possible level ( 100mA). The output of the bq51003 circuit is configured for 250mA ( R1 = 845 ohms) and the charger IC maximum input current is also reduced to 250mA ( R_ILIM of bq24072 = 6.2K).

We are going to reduce the bq51003 output current and input current of bq24072 further. Also check charging the device with a 2.5W wireless charger. Please suggest us some other way to solve this issue.

thanks
Raviselvan C R

Hi Raviselvan C R

The coil separation test is indicating that some of the heating is from AC field entering the device.  Shielding can be added behind the RX coil to redirect the AC field.

See Wurth Electronics 354002, shielding material.