Power-supply inductors may be supplanted by piezoelectric energy-storage elements…maybe. And someday.
Today’s step-down DC/DC converters – often converting 48 V down to single-digit voltages — are highly refined topologies, offering efficiencies of 90 percent and higher. Designers can choose among many switched-mode power supply (SMPS) arrangements, each with various subtleties to maximize a desired attribute such as efficiency, transient response, line and load regulation, or output noise. One bill of materials (BOM) aspect that all of these designs share is consistent reliance on magnetics in the form of inductors, to store and release energy as needed during the various operating phases.
But it doesn’t have to be magnetics. A team at University of California at San Diego has developed what they call an “Always-Multi-Path Embedded Flying Capacitor Piezoelectric Resonator-based DC/DC Converter” (that’s a mouthful!) that adds hybrid, multi-path, output-power delivery features to reduce the internal charge-redistribution losses within a piezoelectric resonator.
Their integrated circuit modifies the optimal voltage conversion of the piezo network from 2:1 to 3:1, while adding a switched-capacitor output network and piezoelectric resonators (PRs) to enable continuous multi-path operation. The result is net optimal voltage conversion ratio of 9:1 for the converter. The chip, which is fabricated in a 180-nanometer high-voltage CMOS process, achieves a peak efficiency of 96.2% at a 48-to-4.8 V conversion ratio.
The “flying capacitor” concept itself is not a new development at all; they have been around since the “early days” of electronics. In a classic arrangement, a non-grounded, floating capacitor is first connected to an input source and charged, then it is disconnected for that input and switched to an output to discharge (Figure 1).
Figure 1 The flying capacitor scheme was originally used with electromechanical relays to isolate a signal or power source from the subsequent stage. (Image source: InsightCentral.net)
Also called a switched-capacitor arrangement, it was used for many years to galvanically isolate sensors with electromechanical relays for switching, while modern switching supplies use MOSFETs and other solid-state devices. The switching scheme has also been used in multistage step-up circuits which can deliver thousands of volts from a single-volt source (Reference 3).
What’s wrong with inductors, and why consider using piezoelectric resonators? Inductors are versatile and reliable, but converters using piezoelectric resonators — tiny devices that store and transfer energy using mechanical vibrations — could potentially be smaller, more energy dense, more efficient and easier to manufacture at scale (Figure 2). The UC-SD team claims that inductors have reached a limit in improvement with respect to size and storage density (I suspect inductor vendors would disagree with that assessment).
Figure 2 A piezoelectric resonator (white disk) used by the new chip to perform DC-DC step-down conversion. For comparison, an inductor that is typically used in traditional step-down converters is shown on the left. (Image source: University of California)
Unlike inductors, which store energy in magnetic fields, PRs store and transfer energy through mechanical deformation and piezoelectric effects. They offer several advantages over traditional magnetic devices, including reduced volume due to their thin planar form factors, superior volume-frequency scalability, the ability to be easily batch fabricated, and their potential for direct integration onto silicon chips in future work. The high coupling and quality factor (Q) of PRs makes them attractive when designing high-efficiency, high-performance power systems, especially in the context of next-generation power conversion technologies.
Not surprisingly, an off-the-shelf PR is not suitable for this application. Commercially available units are not optimized for power applications and cannot operate robustly at the high current demands of modern datacenters. Further, the maximum current-carrying capability of a PR is determined by its physical properties such as material, vibration mode, and geometrical design, as well as electrical excitation strategies. For these and other reasons, the team designed a custom PR unit (Figure 3).
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Figure 3 The custom piezoelectric resonator (right) overcomes limitations of commercial ones; the resonator size (left) is shown compared to a penny. (Image source: University of California)
Final performance is characterized by many different parameters under different operating conditions, such as those in Figure 4:
Figure 4 Fabrication and measurement images in abundance augment your knowledge base: a) Silicon die photo of the proposed converter; b) Measured waveform of each side of the PR, its differential voltage (VCP), and output voltage under voltage conversion ratio (VCR) = 10 and VCR = 20; c) Efficiency curve versus load current with fixed VCR (=10); d) Efficiency curve versus VCR with fixed load current (=200mA); e) Output current versus operation frequency, where the frequency operates in the inductive region of the PR. (Image source: University of California)
The team does acknowledge some limitations. Because piezoelectric resonators physically vibrate, they cannot be soldered onto circuit boards using conventional approaches and will require different strategies to integrate them into electronic systems. Although the technology is still in its early stages, the researchers say it represents an important step toward overcoming the limitations of today’s power converters. Future work will focus on improving materials, circuit design and packaging
As project senior author Patrick Mercier, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering noted, “Piezoelectric-based converters aren’t quite ready to replace existing power converter technologies yet. But they offer a trajectory for improvement. We need to continue to improve on multiple areas — materials, circuits and packaging — to make this technology ready for data center applications.”
Will this new approach get some traction? I don’t know, nor does anyone. After all, when optimized magnetic-based converters already have efficiency in the 90-95% range along with other favorable attributes, the pain needed to get another point or fraction of a point of improvement may not be worth the gain. On the proverbial other hand, a reduction in size or cost, even at the same efficiency, may be worthwhile.
Their paper “A hybrid piezoelectric resonator-based DC-DC converter” was published in Nature Communications but is behind a paywall; however, the team has posted a preprint here.
References
- Knowles, “What Are Flying Capacitors?”
- Insight Central, “Flying Capacitor High Voltage Battery Monitor”
- EE World Online, “Generating really high voltages without a tesla coil”
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