BQ51013B: Buzzing noise/FOD limits

Hi Adam,

It looks like one of your photos didn't go through, can you reupload it? 

Would you be able to provide voltage waveforms at the OUT pin, REC pin, and TS pin?

Hi Anthony,

I only intended to upload one image, the second was my initial Ctrl+V, sorry for the confusion.

In the images below I have managed to get waveforms of the RECT, OUT, and TS pins, of course once I connected the probes the system behavior seemed to stabilize and I was able to get consistent charging. I took a variety of horizontal divisions.

CH1 (Yellow): RECT

CH2 (Magenta): OUT

CH3 (Cyan): TS

After some additional testing I should note that I am able to instantly make and maintain a connection when there is no battery to charge, even with the LED on.  I know there is an error in the blocking FETs, I bypassed those.  Once the battery is installed that's when I see failure to maintain a connection, or that's the appearance it gives.  I've not yet been able to grab a good waveform of a failed transfer. The board I soldered wires onto in order to grab the previous waveforms has yet to fail, it is always successful now by itself, which makes me wonder if there's a capacitance issue on one of the pins maybe?

Hi Adam,

Any waveform from the failed transfer would be helpful. Just to confirm, the WPT LED you mention, that is the LED connected to the /CHG pin on the BQ51013B? 

Hi Anthony,

What exactly are you looking for me to catch? I am trying to catch the failed transfer but I'm not sure what I should trigger on or what kind of window you're looking for.

That is the correct LED I was referencing. 

Thanks,

Adam

Hi Anthony,

I used the "Good Start-up sequence" from the troubleshooting guide to grab some failed waveforms.  As far as I can tell everything looks OK on these waveforms, maybe a little noisy but it's hard to tell with my scope. Yellow CH1 is OUT, Magenta CH2 is RECT.  It really just looks like it is just stopping.

Hi Adam,

It looks like the TX may not be receiving a good comm packet. Can you increase the capacitor on the COMM1 pin? We do typically recommend a 22nF capacitor on that pin. You can also try a 44nF capacitor. 

Hi Anthony,

How can you tell that this may be the issue? You mention increasing my COMM1 cap and that you recommend 22nF. I am currently using 22nF, so you think I need to try 44nF? I will try that fix when I get a chance.  I found that by reducing the charge current limit allowed by my charger I can keep a strong connection. This implies to me my issue is when I try to draw too much power through the coil.  Would a bad comm packet only be susceptible to higher power levels?  

Hi Adam,

Generally, if the TX doesn't receive a good comm packet in 1.5 sec, there will be a shutdown and restart. There could be other reasons for the shutdown but these typically don't cycle like in your image. 

I will look and see if the power output could be an issue . 

I made this replacement and it appears that it has made my communications more reliable across a wider range of transmitters.  Other than the stop-start sequencing, how could you tell the comm packet was bad? The levels looked appropriate to me, and I'd like to be able to visually assess the performance of this IC more accurately.

Hi Anthony,

I had several boards built with a 47nF capacitor in the COMMS position and I integrated one into a handle. I have seen that the integration caused my transmission capability to be reduced, I can only connect to about 50% of the transmitters I have, and when I do connect I am hearing that "clicking" noise from my circuit.  Is that indicative of the repeated attempted starts?  Would that mean I need to further increase that capacitance or would I look to increase my BOOT capacitance potentially at that point?

Thanks,

Adam

Hello Adam

The clicking could be due to rectifier capacitors, piezoelectric noise.  This could be at start up or during comm time every 250ms.

But if 50% of the units are dropping out the problem could be a couple of things:

1.) increased distance from the TX coil, added thickness

2.) metal in the handle causing FOD problems.

3.) more metal near the RX coil changing the L and tuning

Also increasing the comm caps may help.

Hi Bill,

Would it be recommended then to try increasing the capacitance on the RECT pin or changing the resistance connected to the pin to eliminate this clicking (C14 or R8)? I've already increased the COMMS capacitance to 47nF, would you suggest an additional 22nF or a different value?

Edit: I was using the wrong schematic image without the updated COMM capacitors.  Would you recommend trying something closer to 75nF?

Hi Bill,

I ran some qualitative tests today around different capacitance values; increasing the COMMS capacitors resulted in no communication across any of my transmitters (I increased by 22nF).  I took that off and looked again at the RECT capacitors and the design guide in the datasheet.  I started to wonder if for the low current draw I am looking for compared to the current draw of the application design I might have too much capacitance. I removed one of the 10uF capacitors and I immediately got sustained communications across all of my transmitters except the quick charge transmitters. Here again I was wondering if there is a difference in communications between a quick charge and a standard Qi charger? I get connection on all transmitters, I have 5, but on the 2 quick charge from Insignia and Samsung don't sustain the power transfer (cut off after ~8 seconds).  

I tried reducing the RECT cap further to 4.7uF from 10uF and increasing to 14.7uF, neither configuration gave as good of communication as the 10uF capacitor (I left the 0.1uF on the board in all cases).

Do these results make sense to you? 

Hello

This makes sense, the depth of modulation will be a function of the Comm Caps and also the filter caps, Cs tuning capacitors.  But the thinking is Comm Caps will dominates over the last two.

Not sure what is happening with the 2 Quick Charge TX.  Have you tested with EVM (Evaluation Board), does it work?

Hi Bill,

I don't have an evaluation board to test with unfortunately.  Part of the issue is the size of my coil I think, I'm using a Wurth 760308101214, which is only a 19mm diameter coil.  I wonder if the small size of that coil has anything to do with the issue and if quick chargers have a different transmission ocil characteristic than standard chargers, for example I dissembled my Samsung QC and found that the transmission coil seemed to be a double coil, two fairly large coils stacked, one smaller than the other.  Would this kind of configuration cause reception problems on my end?

I tried increasing the comms capacitance while the RECT capacitance was lower, and that didn't give me any better results. Is it possible for the coil to be tuned poorly so that it would work on some transmitters and not others? Would it be worthwhile to verify the Ls and Ls' values to look at my series and and parallel capacitors?

Hi Adam,

With a 19mm diameter coil, a 25 uH should work fine, With a smaller coil, position is more of a factor. The Samsung QC transmission coil should be fine if it is Qi certified. It would be worthwile to verify the Ls' values as with the coil inside the case with the battery and on the TX, the value will go up.