Wireless Power Charging Systems Span Time And Frequency Domains

May 23, 2012
Tektronix' Scott Davicson explores the test implications of the Qi standard for wireless charging of portable devices.

The use of magnetic induction as a way to move power from a source to a device is nothing new. It works, it’s efficient, and it’s safe. So why aren’t we all wirelessly charging our mobile phone, tablet, and laptop batteries? Certainly consumers would appreciate the convenience of just placing their smart phone or tablet down somewhere and watching it automatically charge up.

Like most things in the electronics industry, technologies require standards to achieve widespread adoption. For consumers, standards provide the confidence that the technology works and they won’t be locked into a dead end. For the industry, standards lead to the critical mass needed to drive down component costs. Without standards, even promising technologies fail to gain much traction.

At long last, wireless charging appears to be on its way thanks to the Qi (pronounced “chee”) standard published by the Wireless Power Consortium (WPC). Its more than 100 members range from phone vendors to chipmakers to wireless carriers. At the 2012 International CES, more than 75 products and prototypes included Qi technology. A few other wireless power efforts, most notably WiTricity, are in the works. But with its broad industry backing, Qi seems to be the most probable winner, at least in the short term.

For the embedded or systems engineer, this momentum likely means that you should plan on having to implement wireless charging in your designs at some point in the next few years. Initially, the charging of mobile electronic devices will be the most popular application area for wireless power. But there are many other potential applications in medical devices and the industrial and transportation segments. For instance, instead of plugging in electric vehicles, drivers could simply drive into wireless charging bays when they need to juice up.

New Test Challenges

Unlike wireless telecommunication systems such as radio or cellular phones, wireless power transmission depends more on the efficiency of transfer than signal-to-noise ratio. From a measurement perspective, the chargers present many design challenges.

In its current spec, a Qi wireless charger is designed to produce 5 W of charging power. The efficiency of power transfer depends on system design including both transmitter and receiver, specifically the interaction between each one. Designs typically target greater than 70% efficiency for a 5-W system. The selection of coils, shielding, components, and physical design influence the overall system efficiency.

This is more complicated in a wireless charging system than in a typical charger, since the wireless system requires both a transmitter and a receiver. Other complications exist due to the shielding requirements, which are necessary to protect sensitive electronics and the battery from the RF fields. And, the system must be able to detect foreign objects so they don’t get hot or reduce the system’s efficiency.

A Qi system includes low-frequency modulated RF, digital, and analog circuits all on a single board (see the figure). The charging system uses digital communication for JTAG debugging and to transfer data between the secondary and primary circuits across the resonant link. A secondary-side microcontroller monitors the charger’s output voltage, generates signals, and uses modulation techniques to transfer information to the primary side.

Qi wireless charging systems use low-frequency modulated RF, digital, and analog circuits all on a single board. They also use digital communication for JTAG debugging and to transfer data between the secondary and primary circuits across the resonant link.

The information is demodulated on the primary side, where the primary-side microcontroller interprets it. The modulated information is organized into information packets that have preamble bytes, header bytes, message bytes ,and checksum bytes. Per the WPC specification, information packets can be related to Identification, Configuration, Control Error, Rectified Power, Charge Status, and End of Power Transfer information.

Wireless Everywhere

The emergence of wireless power dovetails with another trend in embedded systems: the use of wireless everywhere. More than 60% of oscilloscope users also use a spectrum analyzer. These engineers are troubleshooting embedded system designs with integrated wireless modules, requiring them to work in both the time and frequency domain. This has led to the need for more capable oscilloscopes such as the Tektronix MDO4000 that can provide time-correlated views of analog, data, and RF signals. The MDO4000 is the world’s first oscilloscope with this ability.

Qi designs illustrate the importance of mixed-domain capabilities. When paired with appropriate accessories such as near-field probes and bench power supplies, the mixed-domain oscilloscope (MDO) can monitor digital control signals, track the RF received output with a spectrum view, and show RF amplitude versus time. This allows the designer to see a signal at its point of origin, within the RF link signal, and at the point of receipt across the transmitter winding. It can also measure the analog step load performance of output regulators and evaluate electromagnetic interference (EMI) emissions.

With a strong push from the WPC, it’s a solid bet that cable clutter will be a thing of the past before too long. For engineers tasked with adding wireless battery charging to their designs, the ability to look at time-correlated analog, digital, and RF signals will be critical to efficient troubleshooting of system-level issues. By integrating a mixed-signal oscilloscope with a modern spectrum analyzer, the MDO4000 for the first time delivers that capability. Goodbye wires.

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