Pint-sized Power Conversion with Piezoelectrics

Many of the microsystems we use every day, embedded in everything from smartphones to automobiles, need power converters to shift electricity from one voltage to another. A large amount of the system is taken up by the inductors and capacitors required to power the device and, as Patrick Mercier, who heads up the Energy Efficient Microsystems lab at University of California, San Diego noted, due to the growing power demands of applications such as AI and more complicated wireless standards, the problem is only getting worse.

As computational demands continue to increase, an increasingly large fraction of the phone's volume is occupied by power electronics.

“The difficult part that we face here is that inductors, which are the workhorse of power electronics, don't have a scaling law,” he said. “Moore's Law exists for semiconductors, and as a result our processors continue to get better every year. But there's no equivalent law for inductors. We're limited by fundamental physics when it comes to using inductors.”

Mercier and his team may have a solution: a piezoelectric-based chip that relies on minuscule vibrating parts to regulate electricity. The device could miniaturize and revolutionize DC-DC power conversion.

Piezoelectric resonators are crafted to vibrate at a specific frequency, much like a tuning fork, which resonates most loudly at a particular pitch, called the resonant frequency. When the right amount of external vibration is applied, the resonator can efficiently convert that mechanical energy into electrical energy. Conversely, applying an electric signal at the resonant frequency can also trigger the resonator to convert that electrical energy back into mechanical energy in the form of acoustic waves.

The challenge for the team in using piezoelectric resonators was reimagining a circuit design that could perform efficiently DC-DC power conversion, especially at a low voltage ratio.

“A two-to-one conversion ratio is the most efficient, so if the input voltage is 100 volts, then it works best if the output voltage is 50 volts,” Mercier said. “But if we want the output voltage to be 10 volts, these piezoelectric resonators won't work as well. We addressed this in our research by introducing switch capacitor circuits into the piezoelectric resonator-based DC-DC converter, which allows it to be more efficient at those larger voltage conversion ratios.”

The team’s initial demonstration showed promising results, achieving a 310 percent loss reduction, but there are still challenges ahead, including how to mechanically mount and package the converters reliably and at-scale and still maintain their high performance.

“How do you get good mechanical contact with your resonators?” Mercier asked. “Inherently we are converting between electrical and mechanical energy, and so those need to be well coupled and this is where mechanical engineers can come in and make a huge impact.”

The team’s next step is to continue to refine the circuit. They’ve been exploring other techniques to further improve performance. They are also looking to the industry for real-world applications in various sectors, including high-power computing servers, automotive systems, USB chargers, and battery-powered devices.

Discover the Benefits of ASME Membership

Mercier is also interested in exploring how to design and optimize the piezoelectric materials themselves for power applications.

“The piezoelectric resonators that we're using are just devices that we can buy off the shelf. And because use of piezoelectric resonators for power applications is new, these resonators are not optimized for power applications, so they don't support the higher peak currents that we would need for those sorts of applications,” he said.

As the team continues to optimize the circuit, industry professionals can begin to imagine how this small converter can revolutionize their own designs.

Cassandra Kelly is a technology writer in Columbus, Ohio.