Measuring efficiency for 2D wireless charging zones – which metric is best?

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:

  • You have system A with a 100cm2 transmitter coil with a charging area of 25cm2 at an efficiency of 65-70%
  • You have system B with a 100cm2 transmitter coil with a charging area of 50cm2 area at efficiency of 55-60%

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:

  • System A has a 100cm2 transmitter coil with a charging area of 25cm2 area at an efficiency of 70% everywhere in this charging area
  • System B with a 100cm2 transmitter coil with a charging area of 50cm2 area at an efficiency of 60% everywhere in this charging area

In this case the charging area weighted efficiency is:

  • System A 25/100 x 0.7 = 17.5%
  • System B 50/100 x 0.6 = 30%

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?

Kunal Bhargava is Engineering Manager for PowerbyProxi