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Lithium Niobate Surface-Acoustic-Wave Resonator for Very-High-Frequency Isolated Power Conversion
Liyang Jin, Zhiyang Cao, Zichen Xi, Jun Ji, Yizheng Zhu, Liyan Zhu, Linbo Shao
TL;DR
VHF power electronics need alternatives as conventional magnetics incur increasing core losses, while existing piezoelectric power devices face constraints in frequency, power handling, and isolation. This work demonstrates an application-specific lithium-niobate SAW resonator and converter prototypes. The resonator handles 25.7 W with 2.17 kV isolation, and DC-AC conversion reaches 93.8% efficiency; DC-DC conversion reaches 52.7%.
Problem
Conventional magnetic components face rising core losses at VHF, while piezoelectric power devices have limitations in operation frequency, power density, heat dissipation, spurious modes, and isolation requirements.
Source: §I · p. 1Method
The paper designs a 40-MHz lithium-niobate SAW resonator with isolated IDTs and Bragg mirrors and demonstrates DC-AC and rectified DC-DC converter configurations.
Source: §I · p. 1, §III · p. 3Results
93.8% peak DC-AC efficiency and 52.7% peak DC-DC efficiency were demonstrated; the resonator handled 25.7 W and measured 2.17 kV isolation.
Source: Ⅳ. DISCUSSION AND CONCLUSION · p. 3, §III · p. 3, §II · p. 3Takeaways & Limitations
The results demonstrate SAW resonators as a candidate for very-high-frequency isolated power conversion, with both DC-AC and DC-DC operation shown.
Source: §I · p. 1, §III · p. 3, Ⅳ. DISCUSSION AND CONCLUSION · p. 3DC-DC efficiency is significantly lower than DC-AC efficiency because of diode forward-voltage drop and varying impedance seen by the SAW resonator.
Source: §III · p. 3
Abstract
from arXiv · showhide
Piezoelectric devices offer a promising magnetic-free alternative for high-frequency power electronics, enabling compact energy transfer with inherent galvanic isolation. Here we demonstrate a lithium niobate surface acoustic wave (SAW) resonator that enables isolated power transfer and addresses the challenges in existing piezoelectric devices for power applications, including operation frequencies, power delivery figure of merit (FOM), power handling, and electrical isolation. Our application-optimized 40-MHz SAW resonator, with the two electrically isolated interdigital transducers (IDTs) and acoustic Bragg mirrors, achieves a 25.7 W output power at a less than 15 {\deg}C temperature rise with only passive cooling through the back mounting side, and an isolation voltage of 2.17 kV. In DC-AC power conversion, we demonstrate a peak efficiency of 93.8 %, a power delivery FOM of 13.47 mW/V$^2$ and a peak chip power density of 81.7 W/cm$^3$. DC-DC conversion is also demonstrated by connecting a diode rectifier to the output IDT, despite efficiency degrading to 52.7 %. Our results advance SAW devices as one of the core components for next-generation very-high-frequency, miniaturized, high-density, magnetic-free, isolated power converters.
I. INTRODUCTION
VHF power conversion is increasing as GaN enables faster switching, but conventional magnetics face rising core losses. The introduction presents SAW devices as a possible magnetic-free alternative and motivates an application-specific resonator for power conversion.
I. INTRODUCTION
- VHF switching is enabled by GaN, while conventional magnetic components face exponentially increasing core losses from hysteresis and eddy currents.VHF spans 30 to 300 MHz.Source: §I · p. 1
- Piezoelectric devices transfer energy through mechanical elastic waves and offer higher power density and Q factors than electromagnetic counterparts at VHF.Prior BAW converters span low-, medium-, and high-frequency bands, and an LN thickness-extensional resonator has reported 5.7 kW/cm3 mechanical power density.Source: §I · p. 1
- Existing power-conversion BAW devices have lower operating frequencies and power densities, while VHF scaling risks fragile sub-100-μm structures and difficult fabrication and packaging.Suspension packaging also limits heat dissipation, and thickness-mode spurious modes complicate control and reduce efficiency.Source: §I · p. 1
- SAWs are proposed to address these constraints through surface confinement for direct mounting, Bragg-mirror mode definition, and lithographically set resonant frequencies.The lithographic frequency definition permits multiple frequencies on one die.Source: §I · p. 1
- Power-conversion SAWs require high efficiency, power handling, isolation voltage, and low impedance, but existing SAWs do not meet these requirements.The introduction gives example targets of >90% efficiency, >10 W, and >1 kV, and defines power delivery FOM as Pout/Vin2.Source: §I · p. 1
II. DEVICE DESIGN AND CHARACTERIZATION
The device uses a lithium-niobate SAW resonator with two IDTs and Bragg mirrors. Characterization reports resonant behavior, power handling, and electrical isolation relevant to its power-conversion use.
II. DEVICE DESIGN AND CHARACTERIZATION
- The 40-MHz design uses a 500-μm-thick 128° Y-cut bulk LN substrate, 50-μm-pitch IDTs, and 50-μm-pitch metal Bragg mirrors.The IDTs are separated by 500 μm, yielding a design isolation voltage above 2 kV; the chip measures 1.9 cm × 1 cm × 0.05 cm.Source: §II · pp. 1–2, Figure 1 · p. 2
- 39.35 MHz series resonance and 39.72 MHz parallel resonance define an inductive operating range with BVD Q of 715 and no significant spurious interference.The BVD fit gives static input capacitance Cp = 34.28 pF and mechanical resonance equivalents Lm = 24.75 μH, Cm = 0.66 pF, and Rm = 8.54 Ω.Source: §II · p. 2, Figure 2 · p. 2
- -1.66 dB maximum S21 transmission was measured within the series-to-parallel resonance range, with low S11 indicating most input electrical power became surface acoustic waves.The two-port measurements use a standard 50 Ω network.Source: §II · p. 2, Figure 3 · p. 2
- 25.7 W output power was reached, and the measured isolation breakdown voltage was 2.17 kV, limited by air breakdown between electrodes.The reported 25.7 W measurement used a 99.8 Ω load; the resonator hotspot reached 37.5 °C.Source: §II · pp. 2–3, Figure 4 · p. 2
III. DEMONSTRATION OF SAW-BASED POWER CONVERSION
The paper demonstrates DC-AC conversion with a load connected directly to the output IDT and DC-DC conversion through a diode rectifier. DC-AC efficiency reaches 93.8%, while DC-DC efficiency is lower because of diode drop and varying resonator impedance.
III. DEMONSTRATION OF SAW-BASED POWER CONVERSION
- 93.8% peak DC-AC conversion efficiency was measured with the load resistor directly connected to the output IDT and a custom GaN full-bridge inverter.Reported efficiency divides load-resistor power by measured input power to the GaN bridge; gate-driver power is excluded.Source: §III · p. 3
- DC-DC conversion was demonstrated by connecting a diode bridge full-wave rectifier and load resistor to the output IDT.The authors sweep input voltage and switching frequency while measuring output power and efficiency.Source: §III · p. 3
- DC-DC efficiency is significantly lower than DC-AC efficiency because of diode forward-voltage drop and varying impedance seen by the SAW resonator.The authors state that further circuit development is needed to improve DC-DC efficiency.Source: §III · p. 3
Ⅳ. DISCUSSION AND CONCLUSION
The application-specific SAW resonator demonstrates high-frequency isolated power conversion, with strong DC-AC performance and lower DC-DC efficiency than the state of the art. Further work could target converter circuits, application-specific optimization, and copackaging with GaN HEMTs.
Ⅳ. DISCUSSION AND CONCLUSION
- 93.8% DC-AC efficiency at 9-V input, 7.76 W peak output at 24-V input, and 81.7 W/cm^3 chip power density demonstrate the converter's DC-AC performance.The work also reports a 25.7 W resonator power-handling capability and 2.17 kV isolation voltage.Source: Ⅳ. DISCUSSION AND CONCLUSION · p. 3
- 52.7% peak DC-DC efficiency is significantly below the state of the art, while peak power reaches 1.66 W in the 14-V to 18.2-V conversion.The peak efficiency is reported at the 13-V to 11.9-V condition.Source: Ⅳ. DISCUSSION AND CONCLUSION · p. 3
- The device achieves the highest switching frequency and power delivery FOM among the compared devices; its power-density calculations use both chip dimensions and acoustic mode volume.Table I compares reported DC-AC piezoelectric power devices, while Table II compares reported DC-DC piezoelectric power devices.Source: Ⅳ. DISCUSSION AND CONCLUSION · p. 3, Table I · p. 3
- Future work could develop SAW converter circuits, optimize for power level, voltage ratio, and load, and copackage or integrate SAW devices with GaN HEMTs.Source: Ⅳ. DISCUSSION AND CONCLUSION · p. 3
- Figure 5 presents the conversion setup, voltage waveforms at 10 V input, and output AC power and efficiency across loads at 8 V input; Figure 6 characterizes output power and efficiency across input voltages, switching frequencies, output powers, and loads.Source: Figure 5 · p. 3, Figure 6 · p. 3