Hi there!
We are developing a small wearable product with a coil footprint of ~16x28mm, modulo some space for case walls and fastener holes.
We have tuned our design based on the FAQ and are still having issues getting RECT to reliably reach 7V. Our design works pretty reliably with the coil outside of the case, directly against the charging pad. Our prototype case has 1mm of material between the coil and the outside of the case. When the coil is within the case, startup (getting RECT to initially reach 7V and the output of the BQ51003 to turn on) is hit or miss. Pressing the case against the charging pad can get the power output to start, but simply placing the case on the pad is frequently not successful. Once the output does start, the case doesn't need to be pressed into the pad anymore.
I am using an Anker 313 charging pad.
We have selected WR221230-36M8-G as our RX coil, which has Ls = 27.9µH. The measured inductance at 100kHz within the case and on top of an unpowered TX Qi pad is Ls' = 28.6µH.
Given this, we have calculated C1=88.6nF and C2=917pF.
Our initial prototype, before we made a coil selection, used reference values. C1=66nF, C2=900pF, C_comm = 22nF. We identified issues with communications with our coil and C_comm = 22nF, so we reduced C_comm = 4.7nF (what we had on hand). This improved communication substantially as the TX pad is now repeatedly attempting to power our device.
We have tried tuning C1 by replacing one of our 22nF resistors with a 39nF to get closer to our calculated value above, and now have C1 = (22 + 22 + 39) = 83nF. This change does not seem to have improved operation. We can see that RECT only reaches 6.7V when the device is placed on the pad.
When placing the device on the pad, RECT behaves like the below trace, periodically increasing to a steady 6.2V.
Rarely, the communication will fail (I think), and the TX pad will blink green and the COMM signal appears as follows, with coinciding spikes on RECT to 2.7V. This is the behavior we experienced every time we placed the device on the pad before changing the COMM capacitors.
When pressing the case firmly into the charger, RECT does successfully and reliably reach 7V. This is presumably because the coil distance is lowered just enough.
I was hoping that tuning C1 would get us the rest of the way there and have relatively reliable wireless charging operation with this coil, but that is not the case. Are there other any other suggestions? Should we abandon this coil for a different one? Any help is much appreciated!
Thanks!
- Neal
