Qi Overview
The continual surge of wireless products into the market has created both convenience and a tangle of charging cables and connectors. While we love the freedom of wireless technologies, we are still tied to ugly charging wires. To make matters worse, we usually have a specific charging requirement for every wireless device that we use, creating a drawer full of cables, transformers, adapters, and connectors.
Qi technology is improving the wireless charging method by removing the cables. The fact is that power transfer can be accomplished over short distances without wires. The Wireless Power Consortium consists of independent companies that set international standards for interoperable wireless charging. The main idea is that wireless products designed with Qi-compliant parts will charge on other Qi charging stations, regardless of manufacturer, model, or country, thereby giving end users the confidence to purchase products bearing the Qi logo. In addition, it provides design engineers with an accepted set of interface specifications, freeing them to concentrate on product differentiation and competition.
How it worksShort distance power transmission is usually based on the principle of magnetic induction. Needed are two coils, a transmitter coil and a receiver coil. An alternating current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. This voltage can be used to power a mobile device or charge a battery.
The coupling is determined by the distance between the inductors (z) and
the relative size (D2 /D). The coupling is further determined by the
shape of the coils and the angle between them (not shown).
Total energy consumption
Plugged-in battery chargers are typically left plugged-in continuously, thereby making standby power consumption significant. With the assumption that wireless battery chargers will also be left plugged-in, Qi has continually minimized standby power consumption. Wireless charging efficiency can be as high as 70% and possibly higher if high quality components are used. A wireless power transmitter can be more or less efficient than the wired chargers it replaces, depending on the number of wired chargers that are being replaced, the type of chargers, and the habits of the owner.
The efficiency of the power transfer depends on the coupling (k) between
the inductors and their quality (Q). The losses can be expressed as:
and graphically shown as:
Electromagnetic Fields and ShieldingWPC Qi products use electromagnetic waves within the marked non-ionizing area of the electromagnetic spectrum. Non-ionizing means that the energy of those waves is too low to ionize human tissues.
