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Understanding wireless charging basics

May 02, 2019

Kuckoo-May 02, 2019

The notion of untethered wireless charging has long been considered a key smartphone differentiator for making cables and power banks obsolete. It’s finally happening now as wireless transmitters can now be found in furniture, airport terminals, and restaurants.

According to Yole Développement, technology developers have efficiently handled the technical complexity of putting a smartphone into a strong electromagnetic field. The market research firm projects that wireless charging in mobile phones will s urpass 1.2 billion receivers by 2024.

A wireless charging system comprises two parts: the transmitter that acts as the actual charging station and the receiver integrated into the device it’s going to charge. Silicon vendors are offering single-chip solutions for both transmitter and receiver parts of the wireless charging design value chain.

Fig. 1: A view of a plug-and-play wireless charging design kit. (Image: STMicroelectronics)

For a start, chipmakers are providing solutions around both inductive charging and resonant charging, which use different physical mechanisms but share similar technological content. That, as Yole points out, includes inverters, rectifiers, drivers, buck converters, and coils. Next, chipmakers are simplifying the implementation of wireless power transmitter systems while steadily reducing the number of discrete components.

In wireless charging designs, much of the limelight has been captured by foreign object detection (FOD) systems and related issues as such accuracy, safety, and power draw. Here, the transmitter chip linked to the receiver chip via bidirectional communications must ensure the safe detection of the charging device.

The role of FOD is critical in providing detection accuracy and thus ensures the security of the charging operation. It also plays a vital role in preventing metal objects from overheating when placed between the transmitter and mobile device like the smartphone.

In the wireless charging standards realm , the Wireless Power Consortium (WPC), which manages the Qi specification, has introduced the Extended Power profile for faster charging by raising the maximum charging power from 5 W to 15 W.

This new profile dramatically shortens charging time by enabling devices to be charged up to three times more quickly. It’s worth mentioning here that wireless charging systems support most of the proprietary wireless fast-charge extensions of leading smartphone manufacturers.

Smartphone designs, craving for technology differentiators, are starting to offer Qi-based wireless charging pads. The following section will provide design examples of how new smartphone models are reinvigorating this otherwise slow-moving technology initiative.

Wireless charging smartphones

In a wireless charger transmitter, a DC/DC converter and a controller generate the input power and the control signals for an external half-bridge power stage that drives the charging transmitter antenna. Case in point: STMicroelectronics’ 15-W wireless battery-charger chip STWBC-MC that supports the Qi 1.2.4 specification.

The transmitter chip controls multiple charging coils, making charging less dependent on the precise position of the battery-powered device. At the same time, however, STWBC-MC facilitates extended FOD through bidirectional communication with the receiver to detect a valid receiver and select the most efficient coil for power transfer.

Next, STWBC-MC utilizes the digital DC/DC controller to deliver the required power to the coil at a fixed frequency, which also minimizes interference with other systems inside smartphone and tablet designs. Additionally, the transmitter chip monitors coil current and voltage to ensure fast and stable charging for all types of devices.

The transmitter chip comes with the STEVAL-ISB047V1 evaluation kit and the STSW-STWBCGUI graphical user interface (GUI) that can quickly download the STSW-ISB047FW dedicated firmware.

Samsung has recently unveiled the Duo wireless charger pad that can simultaneously charge the Galaxy S9, Galaxy Note 9, and other Samsung smartphones as well as the Galaxy smartwatch. The 7.5-W wireless charger utilizes IDT’s wireless power transmitter based on the WPC Qi specifications. IDT’s wireless power receiver complements the transmitter chip, and both wireless charging ICs are based on a 32-bit Arm Cortex-M0 processor.

>> Read the complete article originally published on our sister site, Electronic Products: "Wireless charging: basic design considerations."

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