BQ51003: Small coil thermal issues

Neal Jackson

Part Number: BQ51003

Tool/software:

This is a follow up to my previous post about tuning the BQ51003 for a small coil.

We completed the tuning and shielding recommended by the last post and are now noticing significant thermal issues with the coil and battery. I assembled a test unit with longer leads to measure the temperature of each component as well as to get them out of the magnetic field.

When charging a 40mAh battery at 10mAh with the coil not perfectly centered, after 10 minutes I measured the temperature of the battery, the PCB, and the coil with a laser thermometer.

The battery stays around 80°F, the pcb is around 95°F, and the coil is 115°F around the edges, and 110°F in the center.
When the battery is also sandwiched between the coil (with shielding) and the PCB, it also gets hot, close to 110-115°F which is dangerously close to the rated maximum for the battery (45°C or 113°F). I also measured the PCB temperature at 122°F.

Does this suggest poor tuning of the coil, or insufficient shielding? Any ideas on how to fix this? How far beyond the edge of the battery and PCB must the shielding extend?

10 months ago

0 Wyatt Keller 10 months ago

TI__Guru** 105195 points

Hello Neal,

We are reviewing your information and will respond by the end of the day.

Sincerely,

Wyatt Keller

0 Bill Johns 10 months ago in reply to Wyatt Keller

TI__Guru 73611 points

Hi Neal

When you test with the coil extended on lead do you see this type of thermal rise?

Would not think it is a tuning problem.

The Shield should extend beyond edge of battery.

0 Neal Jackson 10 months ago in reply to Bill Johns

Prodigy 20 points

Hi Bill,

when the coil is extended I do not see a thermal rise in the PCB or battery, only in the coil - usually around 100-110°F

0 Bill Johns 10 months ago in reply to Neal Jackson

TI__Guru 73611 points

Does reducing coil to coil distance reduce the temperature?

And this is with a 10mA load?

Magnetic shielding should help the PCB / Battery.

0 Neal Jackson 10 months ago in reply to Bill Johns

Prodigy 20 points

Yes, if the small RX coil is perfectly centered on the charging pad, anecdotally the temperature build up is slowed and reduced.

This is with a 10mA load. Even when charging is terminated due to temperature or finished charging, the TX pad continues outputting power and the system continues to heat up.

I have tried a couple different shielding materials (Wurth 354002, Laird MHLL 12060-000) of different sizes, as well as layering them, however the PCB and battery both heat up to 45°C in only 5-6 minutes. Layering the materials does seem to help slow the build up of heat, but only slightly.

Are there options for thicker shielding that I could try? Maybe something similar to the ferrite shield that is attached to the coil, only larger?

0 Bill Johns 10 months ago in reply to Neal Jackson

TI__Guru 73611 points

Hi Neal

I will contact you and we can discuss this more offline.

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BQ51003: Small coil thermal issues