Qi charging problem

[FrankBuss]

I'm using a BQ51050B and loading works when the coil is in free air over the charging station. But when it is mounted below a battery, it is very unreliable and difficult to start charging, and often it stops charging.

Here is a scope image measured at the coil wires, when it is in free air and charging without problems:



And this is how it looks like mounted below the battery, just captured before it stops charging:



I'm using this coil: https://katalog.we-online.de/pbs/datasheet/760308101214.pdf
The battery is a Varta EZPack S-3.7V: https://products.varta-microbattery.com//applications/mb_data/documents/product_information/VARTA_EasyPack_Design_In_Handbook_en.pdf

The ferrite shield is in the opposite direction of the charger. It looks like this, each line a "layer":

PCB
battery
coil ferrite shield
coil
plastic case
Qi charger

The coil is glued on the bottom side of the battery, half of the coil is overlapping at one side. If in air without the battery, PCB and plastic case, I can charge with the coil in up to 8 mm distance from the Qi charger.

I can't post much more details of the product, because it is a client project and he doesn't want to make it public before release, but maybe someone has already an idea what I can do, measure, modify etc.

[perieanuo]

hi,
you have 2 coils involved, you specified just one.
second, magnetics calculation is a pain in the ass, make shure you respected what datasheet IC says regarding capacitor values.
if you aren't at ease with those calculations, just test C values with caution and current limiting ob BQ board and you'll find the right ones
regards, pierre

[FrankBuss]

It is basically the same circuit as in the application note on page 15:

http://www.ti.com/lit/ug/tidub31/tidub31.pdf

A colleague designed the values for the capacitors, as described in the datasheet with measuring the coil etc. The coil in the Qi charger shouldn't matter, it works without problems with my iPhone and the device should work with any Qi charger.

But I guess the battery and PCB changes the field? I could try to increase or decrease the capacitors in the input resonant circuit.

[NickZA]

Try changing C5 and C21 (on the COMM1 and COMM2 pins) from 22n to 47n.

[FrankBuss]

Try changing C5 and C21 (on the COMM1 and COMM2 pins) from 22n to 47n.

I tried it and it doesn't help.

[FrankBuss]

I think the battery is the problem. If I place the coil in free air above the charger, it works, but as soon as I place the battery on top, it stops charging. Can I shield this somehow with ferrite foil, or maybe a bigger coil would be good? The current coil has only 2 cm diameter, the case would allow maybe a little bit over 3 cm.