Having recently commissioned a study at Colorado State University to undertake “end-to-end” WiPo system efficiency testing, the WPC has filed a report that appears to be the first salvo aimed at A4WP. With testing criteria based upon (at favorable to) the Qi standard, the CSU group found Qi to be the winner – WAIT, look deeper into the report and it is obvious that the testing methodology was flawed and the WPA report quite self-serving...but efficiency testing is an important issue, so let’s take a closer look:
How the Efficiency of Wireless Power Systems is Measured and Reported…Matters!
Efficiency, the battle cry of the energy conscious consumer
The quest for convenient living and energy conservation poses a ‘Ying/Yang’ dilemma living in the age of electronics. This is certainly the case with wireless power transfer technology. This rapidly emerging technology has the promise of “cutting the cord” and displacing the need for AC/DC wall adapters and perhaps eventually wall sockets.
The question now is; will wireless power increase our carbon footprint by being less efficient than the traditional plug-in AC/DC wall adapters?” Are some wireless systems more efficient than others? These are two fundamental questions that need to be examined in an energy conscious world governed by energy efficient standards.
At present, three different standards for mobile device wireless power (WiPo) systems have emerged – the lower power, tightly coupled Qi standard of the Wireless Power Consortium (WPC), another low power tightly coupled Power Matters Alliance (PMA) standard, and the wide power range, loosely coupled highly resonant Rezence standard of the Alliance for Wireless Power (A4WP). The Qi and PMA standard systems require a well-defined fixed positioning between the power-transmitting coil sets limited to one source and one device, whereas for the loosely coupled A4WP system, the device can be placed anywhere on the source surface and can also accommodate multiple devices. . These fundamental differences between the two main types of standards result in tradeoffs in the way their respective efficiencies can be measured and reported.
A key difference is that systems designed to the Qi / PMA standards can only charge a single device at a time and the power-transmitting coil must be in close proximity (closely coupled) to the receiving device (example: a cell phone). On the other hand, the Rezence standard offers many more conveniences for consumers. For example, systems designed under the A4WP standard can simultaneously charge multiple devices even if they have various receiving coil configurations and power ratings, and the devices are placed with greater separation from the source (loosely coupled) than the current Qi / PMA standards permit.
The significant differences between the various wireless power standards, including traditional AC/DC wall adapters, make it difficult to access and compare the conversion efficiencies between them. To be able to correctly access the conversion efficiency of these systems is important, as these too will eventually need to comply with various energy efficiency standards such as Energy Star to which all AC/DC adapters are required to comply.
Assessing the efficiency measurement challenge
The Qi and A4WP standards have now matured to the point where they are launching criticisms at one another in an effort to bolster their positions to become the “standard of choice.” Recently, the WPC, with their Qi standard, made such a lunge at the A4WP standard with a blog posted on the WPC website. In this blog they reported results of an efficiency study they had commissioned the Colorado State University (CSU) to conduct.
The motivation for this study – find a means by which efficiencies of various wireless power systems can be compared – is admirable. Unfortunately, the fundamental premise holding that systems designed under different standards can be easily compared, as well as the testing methodology employed, were severely flawed. Attempting to settle the claims as to which wireless systems standard produces a system with the highest end-to-end efficiency, the Colorado State group used the Qi standard design parameters to guide the test conditions and evaluate the results. In addition, the study failed to accurately describe the systems evaluated. In the case of the Qi based system, it is assumed an off-the-shelf unit was used whereas the A4WP based system used a demonstration system from EPC. This EPC demonstration system uses A4WP components but is far from compliant to the standard.
The university’s team concluded that an “end-to-end,” power-out over power in, test would be the way to measure and compare results of ALL wireless power transfer systems – regardless of the criteria under which the various systems were designed. This is a sound method for comparing systems WITHIN the same standard, but is without merit and meaningless for comparing systems designed to meet DIFFERENT standards.
Unfortunately, carefully engineered systems do not cooperate with imposed, after the fact, rules; rather well engineered systems produce results for which they were designed. There are tradeoffs when designing systems. Given this maxim, a look at the Colorado State study shows why the efficiency results of the two systems tested cannot be laid side-by-side – they were designed with differing performance objectives!
Efficiency matters…but must be measured and reported correctly
The reporting of the Colorado State report has a number of false and misleading assumptions in the WPC-sponsored Qi/A4WP efficiency study hence they have a vested interest in the outcome, skewed towards their self-interest. Some of the more important egregious assumptions are listed here:
- The EPC wireless charging kit that was used as a representative of the Rezence standard was designed for power up to 38 W, much higher than the power load of 5 W, which is the Qi standard. Given that ALL power electronic systems exhibit lower power conversion efficiencies at lower power than for which they are designed, the efficiency numbers reported does not reflect the true conversion efficiency capability of the EPC system. The EPC system, although using A4WP based components, is not A4WP compliant.
- The device (receiving) board used in the EPC kit was tuned for higher output voltage; so driving to a higher current (twice the design current used in the Colorado State study) will have a negative impact on both the device board and source (transmitting) system.
- The efficiency comparison in the study was made using a 16 W-rated, multi-load coil set operating at one-third the rated power. If the comparison were made using an A4WP class 2-source coil (10 W rated), the efficiency of the EPC system would have been much higher.
- Most noticeably, the study increased the distance between the transmit and receive coils of the EPC system beyond that specified by the A4WP standard, which placed the system at a disadvantage. This matters as the Qi-based system uses a fixed alignment and very tight well-defined spacing between the coil sets. The A4WP standard does specify a very specific distance between the coil sets based on a “use case” scenario. Increasing the distance for the A4WP system effectively placed it outside of the standard to which it was designed and tested.
Upon reflection, the Qi-financed study raises a fundamentally good question about the need to define and measure conversion efficiency. However, because of the difference in structure of the various wireless power standards, intra-standard comparisons of systems are straight forward and easily defined, whereas inter-standard comparison of systems designed under different wireless power standards requires far more diligence to adequately define that will result in a trustworthy comparison
In the case of Qi versus Rezence, additional performance criteria, such as ease-of-use for the consumer, must furthermore be taken into consideration when testing and reporting simple “end-to-end” efficiency testing across the entire operating range. Efficiency must be measured within the expectations of a specific wireless power standard – much as, the top end speed of a Ferrari is not to be compared to that of a Ford truck. After all, the truck was designed for hauling capacity, at the expense of speed.
Need for “objectivity”…or at least a means for “controlled subjectivity”
In order to have “controlled subjectivity,” an efficiency standard and means for measuring it within each wireless power standard is needed prior to comparing systems not only within a standard, but when comparing systems designed under differing standards. Clearly stated design parameters have to be provided and discussed when comparing systems in order to have a context for comparing any performance parameters developed under different standards.
The consumer, through adoption of the WiPo system, will determine the relative importance of efficiency in relation to the other design criteria, such as ease-of-use and reliability. But let’s not forget; to have significant acceptance, the most important efficiency comparison will be a wireless power transfer system to the traditional AC/DC wall adapter…and this will be the “brass ring” for taking market share.
No doubt, there is a need for a universal standard method for determining comparable efficiencies among power transfer systems, but as long as there are different criteria dictating the design conditions for wireless power systems, there will be a need to carefully create the methods by which system efficiencies are measured. And, even more scrutiny will need to be given to the way testing results are reported. Currently, the two wireless power standards – Qi and Rezence – must honor the fact that the common “enemy” is the wall socket and both standards must continue to think hard and creatively so we, the consumer, can finally cut the cord.