By Paul O'Shea
Sr. Technical Editor
Wireless charging, as it is commonly known today, is similar to another type of cordless charging called Surface Energy or Open Dots charging. Neither wireless nor Open Dots use cords connected directly to a device to charge. The wireless charging techniques are popular in the consumer market today. They use inductive charging (coils that heat up to provide energy to the area in contact and uses electromagnetic fields), have a slow charge time, and can only charge one device at a time.
Surface Energy uses conductive charging and the Open Dots technique so devices don’t get hot from charging. Additionally, there are no electromagnetic fields, the charging process is as fast as if the device were connected to a wall outlet, and you can place multiple and various types of devices on a pad at the same time.
See Surface Energy vs. wireless charging video
Surface energy allows you to place your smartphone, tablet, laptop and other devices down on a charging pad without ports or cords. Major auto manufacturers have already incorporated the pads into their latest models. (See photos below).
Typical examples of Surface Energy applications.
Recently, I had the opportunity to ask Mitch Randall, the inventor of the Surface Energy concept, and the co-founder of Zii Energy, about surface energy, what it is, how it is different from wireless charging and where it is used.
What is Surface Energy?
Surface Energy is not inductive, but conductive. The only practical form of Surface Energy is called Open Dots . As it says on the conductive energy informational website, Open Dots is an open source specification describing a wire-free power technology that delivers many benefits over competing technologies.
See Surface Energy – how it works video
It is called ‘Open’ because the specifications and compliance information is available to the public. It is called ‘Dots’ because products employing the standard use a distinctive pattern of contact ‘dots’ to transfer power. The basic principle-of-operation of the Open Dots standard is a pattern of connection points on the bottom of an Open Dots device making contact randomly with metal strips on the Open Dots pad. The contact strips on an Open Dots pad are alternately connected to plus and minus. By the nature of the geometry of the contact points on the device (the power receiver), at least one contact will be connected to plus, and at least one contact will be connected to minus no matter where the device rests on the pad.
Because it is not possible to know which contact(s) will land on a positive strip and which contact(s) will land on a negative strip, a four-way bridge rectifier is used to right the random polarity. The contact points connect to the signals A, B, C, and D shown in the diagram, and the output of the rectifier is labeled “+” and “-“.
Contact points connect to A, B, C, and D, and the output of the rectifier is labeled plus and minus.
The Open Dots standard calls for a potential of 15 to 20 V on the pad electrodes. Thus, the output of the four-way bridge rectifier is approximately 15 to 20 V. For many applications, such as for cell phones, the rectifier output is regulated down to 5 V with a switching regulator.
A power supply provides power for the system. The power is conditioned by a sensing circuit and brought to the pad electrodes. The contact points make direct electrical connection to the pad electrodes. The contact signals are brought through a rectifier to provide a positive and negative power signal into the device-side regulator. Finally, the regulated output powers a device.
The Open Dots block diagram
The pad voltage is 15 V for pads that can supply up to 50 W, and 20 V for pads that can supply up to 160 W. Thus a device resting on the pad can determine if it is on a low-power pad or a high-power pad.
What is the efficiency of surface energy charging?
Surface Energy is basically a connector or sometimes we call it a “plug-less power strip”. The portion of the technology that brings power into a device is >98% efficient including the necessary safety electronics (sensing) for the power surface, and the active bridge rectifier needed within the device that rests on the power surface. Of course, the input requires a DC power supply, and the output sometimes requires a DC/DC converter. Those components have whatever efficiency is the state of the art.
What does a battery powered device need to be able to charge using the surface energy product? Any special parts designed into the device?
The device that rests on the power surface requires four contacts and a four-way bridge rectifier. Various devices may require power conditioning (regulator) to produce a specific operating voltage.
How many devices can realistically charge at the same time?
About 100, though this depends on how physically large the power surface is and its power capability. For example, we have worked with companies who need to charge dozens of personal police cameras every evening. We can easily handle this with a 2' X 2' power surface. In the case of power tool batteries, we can charge four batteries at once with a 90 W power surface.
Compare how long Surface Energy takes to charge versus wireless, as well as AC-based charging.
Surface Energy can provide up to 160 W, while all inductive technologies are stuck at 5 W (because of safety and efficiency), making Surface Energy the faster charge.
There is no charging time difference between Surface Energy and plugging a device in, because for all practical purposes, there are no power limitations.
How can engineers implement this in their designs?
Engineers can find out exactly how the technology works and get example schematics and app notes from http://opendotsalliance.org. Alternatively, Zii Energy sells modules and chips to make things a lot easier.
Because Surface Energy it is conductive, there are no mysteries. Engineers can wrap their heads around this and knock out a design with confidence. No need to worry about thermal management, parasitic RF rectification, and shielding.
The power supply for the surface energy charging pad – what are the requirements (connector type, AC voltage, DC voltage)
There are two DC power supply requirements in the open dots specs. For power surfaces that are rated at less than 50 W, the voltage is 15 V. For power surfaces rated at >50 W, the voltage is 20 V. The AC input requirements are as per the target country, or it can be international tolerant. There is no specific connector as this is a product requirement, not a technology requirement.
Anything else engineers need to know?
The open dots alliance has partnered with UL to provide verification of the open dots spec. This ensures that any device or power surface one company makes will be compatible with any device or power surface another company makes. This service further takes risk out of any new product development.
Zii Energy is also working on a development kit that engineers can use to get familiar with the technology and also gain familiarity with modules that Zii Energy offers. Engineers may be comforted to know that Surface Energy was the first in-vehicle charging solution debuting in the Dodge Dart in 2012. It is by far the easiest technology to pass the very stringent EMC requirements for vehicle cabins. Since then, Surface Energy has been introduced in more vehicle models than all other technologies combined because of its versatility and diverse ecosystem. Also, anyone can become more familiar with the technology by purchasing power surfaces and enablement from http://www.shopsurfaceenergy.com
Any list of companies that are members of the Open Dot alliance?
There is a list available at http://opendotsalliance.org/alliance-program/ Please note, however that companies do not intend to publish their membership until shortly before their product releases.
Are any Asian automobile manufacturers using open dot charging?
The Open Dot Alliance is an international standard, and world-wide use is encouraged. Presently there are vehicles in China that are outfitted with the technology, although we do not know the official launch dates. Toyota has implemented this standard. We do not have specific knowledge of Korean participation.
Can these chargers be installed after-market for cars?
Yes, Surface Energy bins are available as after-market. These can be purchased at http://www.shopsurfaceenergy.com, or as our kickstarter rewards. The kickstarter campaign has an excellent list of vehicles and model years.
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