Measuring the performance of the wireless power demo kit - an update

When measuring the performance of the wireless power demo kit, I forgot to look at the efficiency. So I repeated my test series, but also measured the current consumption of the transmitter board.

I also ordered a cheap Qi receiver off eBay. Since my son has more than just a single toy I intended it to be used for other ones. Since it comes from China I did not expect to it get before a month or so, but shipping was really fast this time. So I found it in my mail on Thursday, and added it to the test series.

Measuring efficiency

First, here are the updated measurements. For the transmitter current, I measured the range of current drawn while moving the coil around. Typically the lowest current consumption is when the receiver coil is centered, and the highest right before the transmission breaks off. The efficiency calculation uses the same range, so it goes from higher to lower percentage. And since I don’t know the power consumption of the LED board, there are no efficiency values for it.

  Load   Distance   Offset (charge)   Offset (startup)   Tx current  Efficiency
0 mA 1 mm 13 mm 13 mm  105-230 mA 0 %
0 mA 3.5 mm 14 mm 13 mm  145-400 mA 0 %
0 mA 10 mm 13 mm 9 mm  525-1100 mA 0 %
LED board 1 mm 17 mm 8 mm  270-415 mA  
LED board 3.5 mm 18 mm 9 mm  285-570 mA  
LED board 10 mm 16 mm 8 mm  525-1240 mA  
100 mA 1 mm 13 mm 12 mm  260-360 mA 38-28 %
100 mA 3.5 mm 15 mm 13 mm  300-540 mA 33-18 %
100 mA 10 mm 14 mm 8 mm  670-1180 mA 15-8 %
250 mA 1 mm 14 mm 12 mm  390-510 mA 64-48 %
250 mA 3.5 mm 15 mm 13 mm  400-660 mA 62-38 %
250 mA 10 mm 14 mm 9 mm  660-1320 mA 38-19 %
500 mA 1 mm 13 mm 12 mm  720-820 mA 69-61 %
500 mA 3.5 mm 15 mm 13 mm  740-960 mA 68-52 %
500 mA 10 mm 13 mm 8 mm  980-1650 mA 51-30 %

The first number of these ranges are measured with no displacement at all, and the second one is right before the output voltage starts to drop.

Its noticeable that there is a significant quiescent current flowing even when there is no load attached to the receiver. Even the transmitter just trying to detect a receiver draws about 60 to 100 mA (pulsing, about twice a second). But with higher loads, the efficiency gets better.

Also, moving the coils apart from each other increases the current the transmitter uses, regardless of the axis. Interestingly, a 100 mA load at 10 mm coil distance needs more current than a 250 mA load (and I verified that twice…). Maybe there is some interference with the automatic power adoption which causes this.

Another receiver

Since I wanted to equip more than just one toy, I looked for other places to buy Qi / WPC receivers. First, the receiver EVM is quite large and won’t fit everywhere. Second, with 200 EUR its not exactly cheap. And third, designing my own board is tricky since the TI chips all come in QFN packages which are rather difficult to solder.

A quick Google search revealed that there is a ton of mobile phone chargers available off eBay. They all seemed to be more or less equal, except for their different form factors, depending on the phone they are intended for. So why not testing these? They are small and cheap, so even if they don’t work its not a big loss. I ordered one designed for the Samsung Galaxy S4, since it was the smallest one I could find:

cheap eBay Qi receiver

As it can be folded, the final size can be as small as 54 by 39 mm (the large part contains the coil, and the small one the electronics). This should fit into most toys.

I don’t know how they manufacture and ship these things for just about $5 (about 3.85 EUR), but I received mine after just one week. So I added it to the test series I needed to run anyway, to find out how it stacks up against the EVM:

  Load   Distance   Offset (charge)   Offset (startup)   Tx current  Efficiency
0 mA 1 mm 13 mm 10 mm  90-125 mA 0 %
0 mA 3.5 mm 15 mm 12 mm  90-220 mA 0 %
0 mA 10 mm 17 mm 12 mm  250-770 mA 0 %
0 mA 13 mm 12 mm 6 mm  500-1050 mA 0 %
100 mA 1 mm 11 mm 9 mm  235-260 mA 42-38 %
100 mA 3.5 mm 14 mm 11 mm  220-355 mA 45-28 %
100 mA 10 mm 15 mm 11 mm  300-900 mA 22-10 %
100 mA 13 mm 15 mm 7 mm  540-1150 mA 18-7 %
250 mA 1 mm 12 mm 9 mm  375-390 mA 67-64 %
250 mA 3.5 mm 14 mm 11 mm  375-480 mA 67-52 %
250 mA 10 mm 16 mm 11 mm  450-1050 mA 55-24 %
250 mA 13 mm 15 mm 7 mm  600-1350 mA 42-19 %
500 mA 1 mm 10 mm 9 mm  760-810 mA 66-62 %
500 mA 3.5 mm 13 mm 11 mm  750-880 mA 67-57 %
500 mA 10 mm 15 mm 11 mm  880-1320 mA 57-38 %
500 mA 13 mm 14 mm 6 mm  1050-1600 mA 48-31 %

First thing to note: it has slightly larger vertical range - it can run with 13 instead of just 1 mm distance between the coils. So I added this as an additional test point. But on the other hand this receiver allows a slightly smaller horizontal offset (even though its just one millimeter). More noticeable is that effect when trying to start a power transfer - the distances one can use are about 20 % less than with the EVM receiver.

I also noticed during the measurements that the power adoption with this receiver takes longer to stabilize again. Every time the receiver coils moves, the output voltage drops and takes one to two seconds to stabilize again. And its even longer with higher load and larger distances - then it sometimes take 5 seconds to stabilize again. Also, in contrast to the TI receiver, the transmitter current increases significantly (sometimes up to 30 %) during this phase.

OTOH, this receiver shows a better efficiency. This can be seen on lower loads (up to 250 mA), and for higher distances. Only starting with 500 mA load, and small distances, the TI receiver can keep up.

Some other observations, and conclusions

In the end, I see both receivers on par. The TI one has better range under load (except for vertical distance) and especially when starting a transfer. The cheap eBay one has somewhat better efficiency, but the TI one better ranges when starting a transfer. So I will make sure that both work properly in my setup.

I’m not sure how serious I should take the quiescent current of the transmitter. Its a pulsing 0.5 W load (not taking into account the efficiency of the transmitter power supply). Maybe I can add a tactile switch to detect that something has been placed on the transmitter pad.

I also noticed that sometimes (when the horizontal offset of the coils is right at the edge) the transmitter sometimes signals a successful transmit process, but there is no voltage at the outputs. That happened with both receivers. This again calls for a startup delay to avoid starting into load. I will also think about a second detection mechanism for a successful transfer - maybe check the current the transmitter takes?

When looking at the current the transmitter needs, one can see that the currents can get quite large. A load of 250 mA at the maximum distance is more than the supplied USB power adapter can deliver (its specified for 1200 mA). So this limits the maximum current I can draw from the receiver for my application.

And my project now a little bit larger. I will not only try to enhance the RC controlled snow groomer, but also another RC car, just to prove that it can be done.

Promise: next time will not be about even more measurements. Instead, I will outline my basic solution for this project.

Posted by Hendrik Lipka at 2014-03-30 (Google)
Categories: electronics projects contests