A World Without Cords: An Overview of Wireless Charging

Qi (pronounced as "chee") charging is an optimized inductive charging, also known as “wireless charging” allowing you to charge your devices wirelessly. Only compatible devices’ batteries are charged by using induction transfer without requiring separate chargers, cables or adapters... Simply place your compatible device on top of the wireless pad and watch your device charge.

What is Qi Charging?

It is an optimized inductive charging, also known as “wireless charging.” This allows you to charge your devices wirelessly. Only compatible devices’ batteries are charged by using induction transfer and there are no separate chargers, cables or adapters needed while travelling. Simply place your compatible device on top of the wireless pad and let your phone be charged!

In this technical article, we will give a detailed explanation of how this wireless charger works.

Global Standard of Wireless Charging:

Qi is the universal interface standard for wireless charging. It is the Chinese word for “air energy flow” or “spiritual energy flow” and pronounced as “chee”. This standard was developed by Wireless Power Consortium, to transfer inductive electrical power over the distance of up to 4 cm i.e. 1.6 inches.

It was established in December 2008 by WPC (Wireless Power Consumption). The Qi low-power specification was published by WPC in 2009. In 2011, the WPC began to extend the Qi specification to medium power.

System Overview

A Qi system comprises of:

• Base station
• Mobile devices
• Power conversion
• Communication

Let’s take a look at how these devices operate in order to charge a battery.

Base Station:

It contains one or more power transmitters that comprise of a transmitting coil, which generates an oscillating magnetic field. The base stations are typically of flat surfaces i.e. Interface Surface, on which multiple devices can be placed.

Mobile Devices:

These are the devices that consume inductive power. They can be either smartphones or tablets and hold a receiver coil that contains a power receiver. This receiver provides power to the battery. Communication and control units are also there in power receivers.

Power Conversion:

The transmitter in the base station has a power conversion unit that converts electrical power to wireless power signal. Whereas, the receiver in the mobile devices comprises of a power pick-up unit that converts wireless power signal back to electrical power.

Communication:

The communication and control unit are present in power receivers and this communication is always followed by a technique called modulation.

• The receiver in mobile device modulates the load by switching modulation resistor (R_m) or modulation capacitor (C_m).
• The transmitter in the base station demodulates reflected load by sensing primary coil current (I_P) or primary coil voltage (V_P).

Figure 1. Qi Charging Wireless Power Communication System

How it Works?

As we know, an electromagnetic field is used to transfer energy between two objects. You will probably remember magnetic induction technology from your physics class. The same method is used by inductive charging.

As we have discussed above, there are two coils: one in the transmitter and the other in the receiver. A magnetic field is generated with an alternating current in the transmitter coil, and this magnetic field induces voltage in the receiver coil. In a real scenario, the receiver in the mobile device takes power from the electromagnetic field and converts it into electrical current to charge the battery.

Mobile device manufacturers that are working with this standard includes: Asus, HTC, Huawei, LG Electronics, Motorola Mobility, Nokia, Samsung, BlackBerry, and Sony. All the devices with Qi logo are compatible to wireless chargers.

Methods to align the coils:

For power transfer, there are two methods to align the transmitting and receiving coil:

Guided positioning:

Since the transmitter coil is in the base station and the receiver coil is in the mobile device, the mobile device must be placed on a certain position on the surface of the base station.

Free positioning:

In this method, the mobile device is not required to be placed on a certain position on the base station’s surface. There is no need of a direct alignment with the coil. In this way, numerous transmitting coils can be used at the location of the receiving coils, to generate a magnetic field and inductively charge the batteries.

Wireless Charging Modes:

All Qi receivers can be charged in two modes:

1. Inductive Mode
2. Resonant Mode

The operation mode is defined by the transmitter. Let’s discuss how these modes are processed.

Inductive Mode:

For inductive charging, tightly-coupled systems are considered. For best results, the transmitter needs to operate at a frequency that is slightly different from the resonant frequency i.e. of the Qi receiver. In this mode, distance between transmitter and receiver is limited to few millimeters. These tightly-coupled systems helps to achieve high power efficiency.

When the coils are of the same size and the distance between them is less than their diameters, they are said to be tightly coupled (see the figure above). Here, coil distance z is less than coil diameter D.

Figure 2. In Inductive Mode

Resonant Mode:

When the system has loosely-coupled coils, charging is done in resonant mode. When the distance between the transmitter and the receiver coils is larger than their coil diameters, the magnetic coupling between the coils will decrease.

Referring to the figure, the coil distance z is larger than the coil diameters and so it weakens the magnetic field between them. Consequently, systems that have low coupling have to manage at the resonant frequency of receiver and have lower power transfer efficiency.

Figure 3. In Resonant Mode

What is the best choice from inductive and resonant mode?

Tightly-coupled systems are best suitable due to their high-power transfer and their low heat production. This is an advantage for heat-sensitive devices such as smartphones. The downside is that tightly coupled coils are sensitive to misalignment.

Loosely-coupled systems have low-power transfer efficiency. Such types of systems are best suitable for the applications that have tight electromagnetic induction or electromotive force.

The best choice depends on your needs.

Qi Charging Power:

Qi wireless charging with low power is able to deliver up to 5 Watts of power at the receiver output whereas Qi charging with medium power can deliver up to 120 Watts. Also, the communication between transmitter and receiver should be enabled during charging process; the charging process will not start without any interoperable communication.

In comparison to the  traditional charging, wireless charging is considered slower but the good news is that it causes no harm to humans since wireless chargers emit non-ionizing radiations.

Advantages and Disadvantages

Wireless charging has various pros and cons:

Advantages:

• Wireless technology has the biggest advantage of having compatibility with all cell phones and chargers from different manufacturers.
• Non-radiative energy is transferred and so it is not harmful to humans.
• No need to constantly plug and unplug the device for charging purposes.
• It has protected connections with no corrosion.

Disadvantages:

• Loosely-coupled systems have lower efficiency that waste heat.
• Charging is slow as compared to traditional charging.
• Complex and increased cost of manufacturing.