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?
A wireless power standard is essential to achieving true ubiquity of wireless charging of electronic devices. This is something wired charging has struggled to achieve just ask anyone who has owned a couple or more laptops. Lets not even get started about the iPhone 5 wired charger!
One of the key questions is what is the magic power level or range that will create the most user friendly ecosystems. Ecosystem being defined as compatible transmitters and recievers. Is there an ecosystem for cellphones / smartphones, another for laptops and so on? Or is the real need to have compatibility across the board for all “general” household consumer devices.
The downside of having a one size fits all type of solution is that you will need to trade-off performance and cost against ecosystem expansion from 0-3.5W to 0-100W. A transmitter that can charge 2 smartphones @ 3.5W each only, will look very different to one that can charge 2 smartphones @ 3.5W each as well as a laptop at 90W.
Most of the smaller consumer electronics devices would only require an ecosystem operating in the sub 10W range – this includes smartphones, cellphones, tablets and the like. On the face of it then, a logical demarcation point for ecosystems might be <10W for smaller devices, and 11W to 100W per receiver for larger capacity devices like laptops?
What do you think?
Read more about wireless power technology.
Coupling is a term widely used in discussing wireless power systems – it refers to a coupling coefficient ‘k’ which defines how well a transmitter and receiver are magnetically “linked” as a percentage. Generally something like transformers have extremely high coupling coefficients approaching 100%.
Technically, tightly coupled systems are interpreted as having high or transformer like coupling coefficients while loose coupling is interpreted as systems with low coupling coefficients. There is some debate around the cut-off for high and low – 50% is one proposed transition point.
From a usability perspective I would define tightly coupled as a system which requires some form of mechanical alignment to fix orientation and transmission distance. This can be done via a magnet or a mechanical alignment feature on the transmitter and receiver. A loosely coupled system would allow complete flexibility of orientation / misalignment in a 2D target zone. A 2D target zone would be a planar area such as a matt (i.e. Proxi-2D), which can wirelessly charge devices imbedded with a receiver that is a few cm above it.
Systems like the Proxi-3D which enables receivers to work in a three dimensional target zone with an omni-directional receiver are what a user may call as beyond loosely coupled!
In my opinion the usability definition is more relevant for anyone buying the system as really the technical definition is really just a means to an end – the end being ensuring customers don’t have to carefully align their devices with a wireless charger.
EMC radiation standards are used to qualify electronic devices against interference with other electronic devices. A piece of wire that has a current running through it will generate electric and magnetic fields – “H-field” emissions measure the magnetic component and “E-fields” emissions measure the electric component. EMC radiation standards ensure that these fields do not interact with other electronic devices to impair their operation.
EMC radiation standards were really designed to control and limit the amount of interference in long distance radio communication. The current standards regulating EMC radiation have been great at regulating these devices, however you now have a new type of electronic device that intentionally generates magnetic fields – those that use wireless power. Such electronic devices were never foreseen when these standards were written.
Consequently, there is one potential aspect of wireless power solutions (read more about wireless power here) which is not currently being regulated. This is the amount of electromotive force (EMF) that can be generated in a device which is in very close proximity. EMF is basically a fancy term for voltage! Electronics are sensitive to over-voltage (some more than others) and over-voltage can cause permanent damage to devices.
Sure there is a limit on how much “H-field” you can emit however these fields are typically measured using an antenna at a few meters or feet away from a device under test. The reading from the antenna and compliance to limits will not really tell you whether your shiny new smart-phone placed on your laptop’s wireless power supply is going to fail.
But hey at least your wireless power supply doesn’t mess with your FM radio…