BQ51013B: Low Vrect and Vout.

I’m using TDK WR303050-15F5-G as receive coil.

Hello

If you are seeing overheating a couple of things to check?

1.) The heating could be AC inductive heating due to PCB in AC field from the TX coil.  As a test extend the coil with wire so only the coil is in the AC field not PCB.

2.) Check output current see if it is high.  Can use the Voltages at ILIM as a proxy for output current.  The divide ratio is about 400 to 1.

3.) Is the heating coming from the BQ51013B ?  Problem could be the current limit set point as a test increase current limit by reducing R14.

And a one question for the old PCB with same circuit: if I place RX coil on the center of TX coil, Vrect is above 6V, BQ51013B overheating, then Vrect jumps for a second to 8.6V, and then BQ51013B turns off. Vout=Vrect=0V and it all starts over.----Bill J - This sound like the unit starts OK (6V) then output turns off (jump to 8.6V).  If unit restarts in about 40mS or so this would be a loss of communication.  If shutdown was due to thermal fault TX should stay off for longer.

But if I place RX coil on the edge of TX coil, Vrect=5.5V and all works fine. What should I do to make everything work fine in the center of the coil? -- Bill J - This may be caused by coil tuning. 

Have you measured the coil L with coil in the case?

Thank you for answer!

Today I soldered a new board without varnish. And all works. BQ51013B is getting warm while Li-Po battery charging, but not critical. I think the varnish is to blame

But the following happens periodically: Vrect = 6.3V and charge is in progress (Vout = 5.0V). Vrect = 8.6V and Vout = 0.0V. Power transfer from TX stops. And after half a second power transfer starts again, Vrect = 6.3V and Vout = 5.0V. Again and again. Could this be a communication error?

Bill Johns said:

Have you measured the coil L with coil in the case?

Hi

"But the following happens periodically: Vrect = 6.3V and charge is in progress (Vout = 5.0V). Vrect = 8.6V and Vout = 0.0V. Power transfer from TX stops. And after half a second power transfer starts again, Vrect = 6.3V and Vout = 5.0V. Again and again. Could this be a communication error?"

This could be a comm issue or a command from the RX to turn off TX.

If it is a comm issue, TX has not received a good packet from the RX in 1.5 sec the TX turns off.  Signature of this is the TX just stops and then restarts.  You probably would not see Vrect jump to 8.6V and output go to 0V.

If the RX commands TX to shutdown you will seen a comm packet on Vrect then voltage will drop.  This would be the end power transfer.  In this case you may see the 8.6V on Vrect (this is the output turning off and no load is higher V).

In some case the RX / TX cannot get to the operating point needed, Vrect does not converge on target and the output turns off.  Signature of this would be TX operating at max or min point and RX sending packets every 30mS.