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BQ51025: often starts up in low-power mode

Part Number: BQ51025
Other Parts Discussed in Thread: BQ500215,

Dear,

I'm currently trouble shooting an implementation of a colleague of mine but I'm a bit out of options at the moment. The design comprises a wireless power transmitter based on the BQ500215 and a receiver based on the BQ51025. Together these should be able to transfer 10 watts of power. We implemented it into a dedicated system so interoperabiliteit / interchangeability is not of a concern. 

The problem I currently observe is that in 60-70% of the times placing the receiver on the transmitter, in starts as expected but only in low-power mode (PMODE pin is high, transmitter LED is blinking slow). When increasing the load beyond 5 watt the current limit comes in as expected. The rest of the times the receiver enters proprietary mode (PMODE pin remains low, transmitter LED is blinking fast) and the full 10 watt can be used.

What we tried so far:

- Checking all voltage levels and wave forms.

- Tune receivers resonant tank as explained in the datasheet.

Everything seems to be correct, but the behavior described above stays the same. Any ideas where to start?

  • Hello
    To enter the 10W mode RX & TX will exchange packets, handshake. If communications fail during this time unit will enter 5W mode.
    One quick thing to try is increasing the value of the COMM capacitors.
    Also reducing coil to coil distance will help.
  • Thanks for the quick reply.

    Meanwhile we replaced the BQ500215 EVM with a new one and I now have stable start-up and communication with our receiver implementation, indeed using a bit lagere COMM-capacitors. The output is set to 10 V with ILIM to 1.2A (including 20% margin). Drawing 1.1A from the system is no problem, so this seems to work fine.

    Since we use a dedicated setup the coil-to-coil distance shouldn't we a problem. I found that the coupling factor is around 0.8 which should be OK for the system, right?

    Our transmitter implementation, however, rarely goes to proprietary-mode. The BQ51025 EVM and our own receiver implementation are not able to draw 10W from our transmitter. I observed some strange things on the Vcoil, namely:

    - the pings are more frequent, about once every 240 ms where the BQ500215 EVM pings once every 500 ms

    - the ping are shaped differently. I observe a pulse-shape on the BQ500215 EVM of a single peak which is fading out. Our transmitter implementation however shows the same pulse, followed by a second pulse after about 3 ms. The second pluse has the same shape, but has a 160 Vp-p amplitude.

    Do you have any suggestion in where to look? The high amplitude probably has something to do with transmitters resonant tank?

  • In your BQ500215 do you have FOD enabled?
    Resistor between FOD pin 23 and LOSS_THR pin 60. Open circuit between the two pins will disable FOD. But if FOD is disabled unit will not enter 10W mode.
  • The FOD-function was indeed disabled in the design, I don't know why however. Placing a 100k FOD-resistor lets both our own receiver as well as the BQ51025 EVM start-up in proprietary mode (green LED blinks fast).

    However, drawing more than 4 W of output power, leads to an error message at the our own transmitter implementation. Indicating a FOD warning.

    Can this have something to do with the FOD, PMOD, and FOD_CAL resistor settings? If so, can you provide me the TX tuning tool?

  • The 100k resistor between FOD and LOSS_THR will set and FOD fault at 600mW, this can be increased to 850mW with a 205k resistor.

    Another things to check is the Current Sense resistor that feeds I_SENSE pin.  FOD will compare PWR sent by TX with PWR RX receives and the difference is loss. If the TX measures higher current in error the PWR sent is high and tripping FOD is possible.

  • By varying the the FOD resistor value the issue remains the same.

    When looking at the voltage I_SENSE pin and Vrail I noticed a difference in levels. In the picture below our receiver is placed on the BQ500215 EVM, with no load. Channel 1 (yellow) show the voltage on the I_SENSE pin and channel 2 (green) is the Vrail voltage.

    Below the same signals measured on our on transmitter with our own receiver in place, again with no load.

    As you can see there is quite a difference in Vrail voltage. Our transmitter is based on the EVM circuit, therefore I would expect the same behavior. Do you see anything suspicious?

  • It is hard to say this is the problem, no load condition will have a big variation due to higher voltage gain needed on the RX. The top scope capture is about 1.3W of power and the lower is about 1.0W.
    If the difference continues at higher load or increases it would point to something in the TX power section causing a problem.
  • It was indeed increasing over TX power. Looking into the current sense circuit I have found some implementation issues which are concerning me. One of them is that the 4 terminal current sense resistor was connected as a plain 1206 resistor instead of a Kelvin sense connection. I changed this by wiring the connections through the air and this changed the early FOD alarm trigger. The system will now deliver 10 W on the output of the RX.

    The high voltages on the TX coil, however, remain. The test setup show voltages of about 160Vp-p. Another PCBA shows voltages of 230Vp-p. Do you have any idea how these high voltages can arise?
  • I would like to work with you using the private messenger feature of the forum and close out this post.  You should receive a request shortly.