BQ51020: IC stops providing charging voltage on output

Hi Vignesh

The freq in the scope capture looks like 110kHz which would be the high end.  If the TX is variable freq type (110kHz to 205kHz) then 110kHz is the high output power side.

Is the operating freq 110kHz?

Also on scope capture:

1.) Working Board -- I output turned on, 5V out?

2.) No working Board -- is output ON?

Coil look OK, good one --- What is the spacing between RX and TX coil?

Schematic looks good.

Hi Johns,

1. We have captured the frequency on the receiver coil for your reference.

2. Working board output voltage waveform.

3. Non-working board output voltage waveform.

And all the non-working boards have different floating voltage as mentioned in the below sheet.

4. The exact spacing b/w Rx and Tx coil is 6mm.

Please let me know if you need any more information's will reply back.

Best Regards,

Vignesh S

Hi Vignesh

With the freq at 118kHz this points to a low gain situation where the system has trouble increasing voltage at RECT pin to 7V.

This could be the coil or spacing between units.

Do you have a scope capture  of the RECT pin at start up?

Should look similar to the attached.

7178.TX Start up and Comm Packets Guide.pdf

Hi johns,

I had probed the RECT pin at the startup for the working board and below attached is the captured waveform.

I observed that the RECT pin is going more than 10V, and please refer the schematic again because the capacitor that is connected to RECT pin is rated for 10V ,Is there any chances to CAP to get failed.

Also I had an another observation, while I start probing on RECT pin the IC starts charging and I tried removing the battery and re-inserted it again the issue is re-covered.

Best Regards,

Vignesh S

Hi Vignesh

The working board looks good we can see the different start up phases.

How does this compare to a problem board?

Sorry but not sure what to recommend if the board changes while being probed .  The BQ51020 is a Ball Grid Device and solder issue are possible.

Hi Bill,

Can you please recheck the schematic once?? It has been shared above.  What about the voltage ratings of the caps used?? look like IC is not able to feed to rect pin which leads to charging failure. What about FOD Resistor. Currently this is 56.2K do we need to change this resistor?? 

I have tried replacing this IC before and device started charging. Concern is initially device started working but over the period of time customer is facing issue with charging device. If it is soldering issue it should not charge at first place only right???

One more thing....i just tried checking the receiver power on P9243 evm i observed below. Power on Rx side is more than 5W and on primary side it is showing power loss in negative. So it means receiver is not calibrated properly for 5W power, is it taking more than 5W??? Is it true??? I can see some register setting for 0x01 and 0x02....is that use to set receiving power

Hello

Dhananjay Nemade said:

Can you please recheck the schematic once

RECT pin voltage will typically be below 10V, OVP is 15V.  Higher voltage rated capacitor is recommended.  Voltage rating on other capacitors look good. 

Dhananjay Nemade said:

ook like IC is not able to feed to rect pin which leads to charging failure.

The RECT is unregulated voltage from rectification of AC1 and AC2. RECT is input to OUT LDO.  RECT pin is rated for 20V.  Are you seeing any fails of the LDO?

Dhananjay Nemade said:

If it is soldering issue it should not charge at first place only right???

If the soldering joint is not good, marginal it will fail during thermal cycling over time.  Will test good then fail later.  BQ51020 will heat up during normal use and can stress the solder joints.  

Have you tried re-soldering problem units, do not remove but just reflow solder?

Hello

Dhananjay Nemade said:

showing power loss in negative. So it means receiver is not calibrated properly for 5W power, is it taking more than 5W??

The Power Loss should be negative, this is correct.  Loss error budget for the RX is about 350mW, this is added to the power RX is reporting it has received, it will over report received power.

If the RX is calibrated properly Loss will be between -25mW and -350mW over the load range.  Adjust R-FOD and  R-OS to shape the loss vs load curve.

If the RX is calibrated properly Loss will be between -25mW and -350mW over the load range.  Adjust R-FOD and  R-OS to shape the loss vs load curve

What should be the Rfod and Ros values??? Should we increase it or decrease it further??

The RECT is unregulated voltage from rectification of AC1 and AC2. RECT is input to OUT LDO.  RECT pin is rated for 20V.  Are you seeing any fails of the LDO?

Looks like input to LDO is not proper hence we not getting any output of 5V.

Dhananjay Nemade said:

If the RX is calibrated properly Loss will be between -25mW and -350mW over the load range.  Adjust R-FOD and  R-OS to shape the loss vs load curve

What should be the Rfod and Ros values??? Should we increase it or decrease it further??

TX --- 4.35W

RX - 5.82W

PL -  -1.435W

The RX is reporting too much received power needs to come down by about 1W.  

R29 = 643 ohms, reduce this value to about 499 ohm this will reduce RX power.  Will need to adjust the value to your design, 499 is probably not final value.

Hi John,

I was thinking in other way to increase the value of R29 rather than decreasing it to reduce receive power as  per below mentioned in datasheet. So RILIM has to be increase to reduce IIlim current right??

Hello

The FOD and ILIM are related.

Current limit is set by R28 ILIM and R29 FOD, larger the resistor values the lower the current limit point.

Received Power reported to the TX is set by the voltage at FOD pin, higher R-FOD value the higher the voltage and more power is reported to TX.

The ILIM pin is a current source that is proportional to current at OUT.  ILIM also has a voltage comparator that monitors the voltage when it reaches the ILIM point unit enters current regulation point.

Because the current at ILIM is proportional to output current it is used to drive the FOD pin voltage and indicate output current for received power calculation.

If FOD is decreased (643 to 499) then ILIM resistor would need to increase to keep ILIM at same point.

, 

Hi Bill,

We are charging the battery in 5V /890mA = 4.45W which is almost closer to 5W which BQ51020 can deliver , Because of this after a period of time the IC lifecycle would come down?

Best Regards,

Vignesh S

Hi Vignesh

The IC can operate at 5V / 890mA, this should not reduce the life cycle of the device.

Note the device has internal thermal shutdown, to prevent overheating and damage.  

Also BQ51020 is a thermal enhanced design with larger lower loss AC rectifier mosfet to reduce thermal rise..

Have you identified any problems with devices removed, shorts or opens on pins?