Problems setting up the STWLC38

[ggeorge_42]

Hello,

We plan to use the STWLC38 for power in a new wearable design.

We purchased the STEVAL-WLC38RX and STEVAL-WBC86TX kits to get familiar and experiment with the ICs.

Now we are trying to adapt the Rx of the STEVAL-WLC38RX as used in a wearable device and followed the application diagram from the STWLC38 datasheet (Figure 13. STWLC38 basic application diagram in Rx mode), using the recommended coil (Wurth 760308101219). We also changed the Cs and Cd capacitors as recommended.

The STEVAL-WBC86TX board was not modified.

The problem is that the power reception from the STEVAL-WBC86TX is not enough to reach 5V at the output.

We tried changing the Cs according to STDES-WLC38WA (TN1440) as well as some other values with better results, but not enough. In the  chart of WPSTUDIO Vrect looks unstable, with lots of ripple and Vout is OFF.

Do you have any suggestions on how to tune the Rx circuit with the new coil?

Thank you,

G.Giannopoulos

[willzhou]

Hi
1.What is your distance between TX and RX?

2.About STDES-WLC38WA (TN1440) recommend CS,

"We also changed the Cs and Cd capacitors as recommended." what does Cs and Cd changed?

STDES-WLC38WA recommend CS=247nF
CS1, CS2 = 100nF
CS3 = 47nF
CS4 = N.M.

STDES-WLC38TWS recommend CS=300nF
CS1, CS2, CS3 = 100nF
CS4 = N.M.

RX coil specifications use Inductance 11.8 μH, dimensions Φ15 mm

What is your rx coils?

3.How about you set high voltage in TX, usually you set to 5V, can you change to 6V or higher to check?

4.What is RX fw version, please give below picture to me.

5.what is the volume of wearable design? if volume is large, i can contract local FAE to send STDES-WLC38WA board for test.

[ggeorge_42]

Hi, here are some answers:

1. The distance between Rx and Tx is 3-5mm.

2. The Cs values as you say.

The Rx coil is Wurth 760308101219, https://www.we-online.com/components/products/datasheet/760308101219.pdf

Inductance and dimensions as you say.

3. I will try to set the Tx voltage and report the results.

4. My Rx fw version is:

5. The wearable will be a small watch, but we have not built it yet. We are experimenting with the evaluation boards to finalize the design.

[willzhou]

Thank you

About chart of WPSTUDIO Vrect looks unstable, with lots of ripple and Vout is OFF
1.Please save GUI charts data and i can see vrect value and other values.

2.So you modify board and use Wurth coils put on tx and test, can you share your test picture, so i can see tx and rx board.
3.Please use attach cfg file and use GUI tool program it to test again.

[ggeorge_42]

Dear willzhou,

There has been a strange development...

I tried the Rx again without changing anything and suddenly it worked!!!

BUT... the Tx board (STEVAL-WBC86TX) has problems:

- It only works for about 1 minute. I do not know if it is due to an EPT or IC problem.

- The communication of the Tx with the GUI stopped and I cannot connect to the board.

I get the message "Unable to find any [TX]WBC86 device. Please check your connection again"

I am sure that there is no problem with the cables and I can see that the PC recognizes the USB device.

- When it is not working, the IC gets hot.

So, now I suspect that the Rx is fine and it was a Tx problem from the start.

Is there something that I can do to restore communication with the Tx, or I must conclude that it is damaged HW?

[willzhou]

Hi

About "When it is not working, the IC gets hot."

Please check TX IC current, you can use power supply with limited current to check.

Also for RX side, you can not draw high current at first, you need add current step by step.

If TX IC current is not very high may be TX is not damaged.

You can use "Empty" to connect again.

[ggeorge_42]

Hi,

Unfortunately "Empty" does not work.

It is not wrong config id or patch. It is like the WBC86 is not there at all.

Maybe it is not damaged because it is still transmitting power for a while, but I cannot connect to see any information.

I suspect that the problem is in the USB/I2C bridge.

But first I will try to control the current as you say and see what happens...

[ggeorge_42]

Hi again,

I was able to check the current of the Tx today.

The STEVAL-WBC86TX draws 7mA when it is idle (no Rx near it).

When the coil of the STEVAL-WLC38RX come near, the STEVAL-WBC86TX has an in-rush current of ~950mA and then draws 735mA. This is with no load on the Rx.

When I limit the current from the power supply, the Tx stops.

Is this normal? (I suppose not, because the Tx chip gets hot).

Best regards

[willzhou]

Hi It is abnormal, TX is hot and power loss is very high, i suggest you change other STEVAL-WBC86TX or change tx chips

[ggeorge_42]

Hi again,

There has been a new development for the better.

I tried the STEVAL-WBC86TX on two other PCs. The GUI was run on them for the first time.

On one of them I was able to connect without problem. On the other I could not.

The difference is that the PC where it worked has a lot of ST SW in it from other projects, so maybe it has a different driver of something that works. Is there any special driver that I should have installed???

Anyway, I was able to check why the Tx stops transmitting and I saw an EPT due to TX SS TO (Signal Strength or Ping Timeout). This probably has to do with the coupling of Tx/Rx coils (very different diameters), because when I change to the original Rx coil it has no problem.

So, for now I am back to investigate the Rx to Tx coupling and I plan to order a new Tx coil (Wurth 760308101214 as per Tx datasheet) to see if I can make them cooperate. I will let you know when I have the new coil.

Thank you