It was interesting to see these comments from Qualcomm in this recent Computerworld article by Lucas Mearian: Samsung uses Qi charging for Galaxy S4, but sees A4WP as the future.
“The frequencies at which tightly coupled solutions operate are not that far from the frequencies that are used for conductive cooking,” he said. “The tightly coupled solutions today have a problem where they can heat the metal surfaces in the smartphone & or metal objects. The result is that a lot of times [with] the tightly coupled solutions, the foreign object detection either dials back the power or simply turns the power off”
Let’s get a few things clear first of all:
You can have tightly coupled systems operating at high frequencies and loosely coupled systems operating at low frequencies.
And the good news here at PowerbyProxi is that in none of these cases do we design wireless charging systems like you would design induction cookers! With an induction cooker inefficiency is the target, the less efficient the better – it’s how you create heat. The opposite is true for any respectable wireless power supply. High efficiency is the target.
PowerbyProxi continues to demonstrate real wireless power solutions that prove loosely coupled systems operating at low frequencies, when designed properly, actually have better thermal performance to tightly coupled systems (when measured on all key areas of the phone like the LCD, back cover and battery as well as the transmitter surface area). This is because loosely coupled systems operating a low frequencies have superior average efficiency. Average efficiency is what the user experiences day to day (peak efficiency is what only test engineers experience). Please see Kunal’s blog on average efficiency if you don’t know what I am talking about.
Let’s remember that the user does not care about how you achieve loosely coupled or what the frequency is. The user wants to place his or her phone and other electronics devices anywhere on the pad without any thought and have it recharge as fast as a wired charger. Furthermore they want to know that is is safe to use, will not cause interference with other devices and is environmentally friendly.
The average efficiency of PowerbyProxi’s Proxi Smartphone pad (loosely coupled and operating at low frequencies) is almost triple that of a loosely coupled system operating at high frequencies. I know which one our customers call the induction cooker.
If you would like more information please contact us directly at firstname.lastname@example.org.
In my previous blog I talked about efficiency and using it to measure “how loose” a loosely coupled system actually is. The next question is how much does an end-user actually care about the efficiency of sub 20W consumer device charging solutions. When was the last time you checked the efficiency of your wall wart for your smartphone or your laptop for that matter? Is this data even easily available to curious end-users?
To get an appreciation for how close to the thermal edge smartphones operate at today, you only need to play music or stream a video over 3G / WiFi on a sunny day and see how long it takes before smartphone goes into self-preservation mode. It is said that computer design is more like refrigerator design these days to see who can design the best heat sinks. For a long time Apple did not put i7 processors in their MacBook Pros due to the inability to get heat outside the slick Aluminium shell.
To ensure that wireless charging for consumer devices is widely adopted (such as smartphones & tablets), the technology should not limit the usability of devices while charging is taking place. In my view efficiency is actually a means to achieving thermal performance which is the “end”, and NOT the “end” itself. Other parameters that matter are; cost, Human RF Exposure, EMC performance, Rx size, and how quickly the device charges.
Efficiency is a commonly referenced performance metric for Wireless Power systems so I thought I would take the opportunity via blog to examine:
a) how it is measured, and
b) how it is analysed to compare the performance of various systems – specifically ones with two-dimensional charging zones i.e. charging pads.
Having been in the industry for almost a decade now I have seen multiple dimensions ranging from transmitter only efficiency, receiver only efficiency, efficiency of coils and so on and so forth. Our efficiency measurements here at PowerbyProxi are more simplistic. We define efficiency as total DC power out divided by DC power in @ the rated load “fast charging” load. This rated fast charging load tends to be 3.5W to 5W for most smartphones. If there are multiple devices, the DC power out is the sum of DC power to all devices.
Looking at the voltage and max current stated on your wall wart wired charger you can calculate the rated load. Wired power supply manufacturers rarely over-spec the current rating of their power supplies, in order to keep cost and size down.
With this methodology you will be able to measure efficiency at any given point.
So how do you analyse a system with a 2D charging area?
What we think really matters is not just how efficiency changes as you move a receiver around the charging area of a transmitter, but also the size of the transmitter coil(s) vs. the area in which the the centre of the receiver coil can move while the receiver still receives rated power (charging area).
Take the following scenarios for example:
Which system is better?
According to a basic analysis of minimum and maximum efficiency, system A wins – hands down!
However if we apply weighting for the charging area, we will get a better insight into overall system performance. Let us walkthrough the above example using really simple numbers:
In this case the charging area weighted efficiency is:
So actually system B is better if you use the charging area weighted efficiency.
The charging area weighted efficiency helps define “how loose” a loosely coupled system actually is.
We believe this is the right metric for comparing efficiency of wireless power systems. What are other people’s views?