Integration and Miniaturization Trends of High Voltage Power Supply for MEMS Device Driving
Microelectromechanical systems devices have revolutionized sensors, actuators, and various micro scale systems. Many MEMS devices require high voltage for operation, including electrostatic actuators, resonators, and tunable capacitors. The trend toward integration and miniaturization of the high voltage driving electronics enables complete systems in small packages, expanding the applications of MEMS technology.
MEMS devices use electrostatic forces for actuation and sensing. Comb drive actuators use interdigitated fingers that attract or repel when voltage is applied. Parallel plate actuators use the attraction between plates. Resonators use electrostatic forces to sustain mechanical vibrations. These devices typically require tens to hundreds of volts for operation.
The high voltage for MEMS has traditionally been provided by external power supplies, limiting the portability and increasing the system complexity. Integration of the high voltage generation with the MEMS device or its control electronics enables standalone operation and reduces the system size.
Integrated high voltage generation uses charge pump circuits that multiply the input voltage through switched capacitor networks. Dickson charge pumps use diodes and capacitors to generate high voltage from a lower input. These circuits can be fabricated in standard integrated circuit processes, enabling monolithic integration with control electronics.
Switched capacitor circuits use MOSFET switches instead of diodes for more efficient operation. The switches are actively controlled to transfer charge between capacitors, building up voltage on the output capacitor. The efficiency of these circuits depends on the switch on resistance and the switching frequency.
Piezoelectric transformers can generate high voltage through the electromechanical coupling in piezoelectric materials. An AC voltage applied to the input section causes mechanical vibration, which induces a higher voltage in the output section. Piezoelectric transformers can achieve high voltage ratios in a small volume, suitable for miniaturized applications.
Miniaturization challenges include the high voltage isolation requirements. The small geometries in integrated circuits have limited breakdown voltages. Special design techniques, such as increased spacing, thick oxide layers, or silicon on insulator technology, enable higher voltage operation in integrated circuits.
Capacitor integration for high voltage circuits requires significant area. The charge storage capacitors in charge pump circuits must be large enough to provide the required charge with acceptable voltage droop. Integrated capacitors use MOS gate oxide, intermetal dielectric, or trench structures. The capacitor value per area limits the achievable miniaturization.
Power dissipation in integrated high voltage circuits causes heating that can affect the MEMS device performance. The efficiency of the high voltage generation determines the power dissipation. Higher efficiency reduces the heating but may require more complex circuits or larger components. Thermal design must ensure that the temperature remains within acceptable limits.
Packaging for integrated MEMS and high voltage electronics must provide both the mechanical protection for the MEMS devices and the electrical isolation for the high voltage. Wafer level packaging encapsulates the MEMS devices while providing electrical interconnections. System in package approaches combine MEMS dies with integrated circuit dies in a single package.
The trend toward higher integration continues with development of processes that can fabricate both MEMS devices and high voltage electronics on the same substrate. Monolithic integration eliminates the interconnections between separate dies, reducing parasitics and improving reliability. The process development must accommodate the different requirements of MEMS structures and high voltage circuits.
Application specific integrated circuits for MEMS driving integrate the high voltage generation, the drive electronics, and the control logic in a single chip. These ASICs provide complete solutions for specific MEMS devices, simplifying system design and enabling rapid development of MEMS based products. The ASIC design optimizes the circuit for the specific MEMS device requirements, achieving the best performance in the smallest package.
