"Wonderful resonance"

The word "resonance" has intrigued me since before I could define it. "Resonance" has many different meanings, most of which are related in some way. As an electronics engineer, my first thought when I hear the word now is a tuned LC circuit.

But then I think of many other things, like pendulums, playground swings, or tuning forks, which are all examples of resonance in the real world. I have asked other people how they understand resonance, and the responses I get range from blank stares to "reminds me of poetry."

According to Wikipedia, resonance is "the tendency of a system to vibrate with greater amplitude at some frequencies than at others." While this blunt, technical definition may not seem very poetic, it does become poetic when we think about the true meaning of the concept of "natural frequency." After all, there is something elegant about precise tuning and high efficiency.

So what does resonance have to do with battery management? Resonant power converters are at the heart of wireless power technology, which can improve the utility and efficiency of many battery-powered devices. As we rely more and more on these devices in our daily work and life, we need a way to keep them as charged and available as possible.

Standards are being developed to enable compatible operation between wireless power transmitters and portable devices (wireless power receivers) from various manufacturers.

In the coming months and years, we can expect to see more and more wireless charging stations in public places, offices and transportation. More handheld and portable devices will also be equipped with wireless charging capabilities to take advantage of the various charging power sources available. Users can just place the phone on the table to fully charge the battery.

The most commonly used wireless power sources currently use the principle of electromagnetic induction. A basic explanation of this can be found on the Wireless Power Consortium website: Inductive Power Transfer .

Mechanical resonance was studied in the 1500s, and electrical resonance in the 1800s. In recent years, resonant converters have finally become a common method for wireless power transmission. This resonant technology is favored in these designs because it allows a system to maintain good conversion efficiency over a relatively wide range of power levels. For an overview of resonant coupling, see: Resonant Coupling .

Figure 1: Block diagram of a wireless power system using TIbqTESLA.

Generally speaking, today's consumers install wireless routers at home for multiple computers, smartphones and other devices, and use wireless hotspots in multiple locations to connect their devices to the Internet when traveling. In contrast, the introduction of wireless power standards will allow future mobile users to use wireless charging bases at home to charge multiple devices, and also save the trouble of carrying chargers when commuting or traveling. As more people begin to experience the convenience and practicality of wireless power, the concept of wireless charging should really begin to resonate with the public.

Figure 1 shows a basic block diagram of a wireless power system designed with the TI bqTESLA  chipset. The duty cycle of the transmitter (charging base) is fixed, but the operating frequency varies. The receiver uses electromagnetic induction, as used for forward power transfer, to transmit the desired power level back to the transmitter.

Figure 2: Received power varies with transmitter frequency.

As shown in Figure 2, the received power varies as the transmitter frequency changes. If the receiver needs to increase or decrease power, the transmitter can change its frequency accordingly. The receiver is tuned to the same resonant frequency as the transmitter to achieve maximum power transfer at the resonance point. When the receiver does not need maximum power, the transmitter increases its frequency, away from the resonance peak.

To learn more about resonant converters and wireless power, read more information on the TI website: www.ti.com/bqtesla-ca .

Upal Sengupta is a senior application engineer in  TI's  Battery Management Solutions Division. Since joining  TI  in  2003  , he has worked as an application engineer and technical marketing to support portable power and battery management. Prior to joining  TI  , Upal worked as a system design engineer for several OEMs  developing mobile phones, portable computers and consumer products . Upal graduated from the University of  Illinois  and Michigan State University with a bachelor's degree in electrical engineering and a master's degree in electrical engineering, respectively  .