Center for Power Electronics Systems

Power electronics is the "enabling infrastructure technology" that promotes the conversion of electrical power from its raw form to the form needed by machines, motors, and electronic equipment. CPES research activities are dedicated to improving electrical power processing and distribution that impact systems of all sizes - from battery-operated electronics, to vehicles, to regional and national electrical distribution systems.
TECHNOLOGY AREAS
CPES has developed strong expertise in five critical technology areas of power electronics:
1. Power Conversion Topologies and Architectures
Power processing systems have fundamentally transformed in recent years, from centralized power to distributed power.For example, new-generation microprocessors operate at less than 1 V, higher than 100 A, and run at multi-GHz clock rates for maximum speed-power performance. These operating parameters create very fast dynamic loads that demand high current slew rates during transients and have forced a move from the traditional, centralized power supply architecture to a distributed power system (DPS). A dedicated point-of-load converter is placed in each output unit close to the high-speed processor, while the front-end power processing that interfaces with utility lines is performed at a system level.This kind of DPS approach has not only enhanced system performance and improved the design and manufacturing process, but also has opened the opportunity to develop a standardized modular approach to power processing.CPES has been at the forefront of this research, and has developed a number of innovative power conversion technologies based on the modular building block concept. CPES research in this area includes power system architecture, system interface stability and requirements, electromagnetic interference / electromagnetic compatibility at the system level, lter design, single-phase power factor correction circuits, three-phase power factor correction circuits, high-frequency dc-dc PWM converters, as well as resonant converters, and integrated single- phase and three-phase PFC/dc-dc converters.
2. Power Electronics Components
Advanced architectures and topologies require superior power electronics components, including power semiconductor devices, magnetic components and capacitors. Developing these components is a major effort in CPES laboratories.Low profile magnetic components — The design and integration of magnetic components is growing in importance. CPES is studying new high-frequency magnetic materials suitable for high-frequency applications in the multi-MHz range. For accurate characterization and optimal designs, CPES researchers have developed a combination of high- frequency modeling and nite element analysis.With the increased popularity of portable electronics, low-power dc-dc converters are growing more popular. However, bulky magnetic components are a major barrier for integrating a dc-dc converter into a single chip. For example, in a conventional embedded winding with vertical ux, the inductance density will suffer when the core thickness is very thin. CPES is exploring 3-D integrated technology, such as using a low-pro le inductor with a lateral ux pattern as the substrate. This can provide a large inductance density even with very thin core thickness.Test performance surpasses that of commercial surface-mount power inductors of a similar value and outperforms the power handling capability of on-chip inductors designed to operate at similar circuit conditions by a factor greater than ten. To further improve the performance and reduce the size of the inductor, different magnetic structures and ux patterns inside the core, as well as the ux coupling, are being investigated.Switch structures — CPES has been investigating different switch structures since 1997, such as the lateral trench and JFET, and monolithic integration approaches for high-frequency, high-density POL applications. Based on this experience and proprietary tools, CPES is developing a robust analytical loss model for POL applications with proven accuracy.Silicon carbide MOSFETs and JFETs — With the recent developments in wide-bandgap semiconductor devices, silicon carbide (SiC) JFET and power MOSFET have become two candidates for commercialization. Featuring high- blocking voltage, high-workable temperature and low on-state resistance, SiC switches have shown great potential in high–power, high-voltage, high-frequency, and high-density (H4) applications. CPES has been working with device manufacturers to evaluate the performance of these devices, and investigate their use in H4 converters.Early results for SiC MOSFETs show a blocking capability that is at least two times better than Si MOSFETs with a ve-times reduction in on-resistance. SiC MOSFETs also perform well under high temperatures. SiC IGBTs, on the other hand, have exhibited much higher switching speed and lower switching loss compared to similar Si IGBTs.SiC JFETs also show promise and have been tested both with ultra-fast gate drive circuits and with regular switching speed. In both cases, SiC JFETs achieve much higher power density than convention Si devices.
3. Modeling and Control
CPES has historically been at the forefront of power electronics developing advanced modeling and control tools for the design and synthesis of advanced power conversion systems. From its pioneering work in the early 1970’s on space and aircraft applications and the back-then nascent telecommunication industry, to the development of power- electronics-only electrical distribution systems for next and future generation data-processing systems and vehicular power systems, CPES has been a true keystone in the power electronics field.The group has developed a panoply of power conversion models that have unveiled the operation of otherwise complex switching and passive electrical networks, rendering them fully tractable and intelligible for the development of sound time- and frequency-response based control strategies. These models enable the seamless study and analysis of the operation of control strategies and their impact at the device, power converter, and power system levels.
Some key CPES accomplishments in this area are
The PWM-switch model for a variety of current mode controllers.
Average and small-signal models for dc-dc converters under continuous and discontinuous operating modes, for peak, average and one-cycle current controls, and current- and voltage-mode controls, including paralleled and interleaved dc-dc converters valid for low and frequency ranges beyond half of the switching frequency.
Synchronous d-q frame average and small-signal models and controls for multi-phase and multi-level ac-dc, dc-ac and ac-ac PWM power converters and synchronous d-q frame average models of multi-phase multi-pulse ac-dc rectifiers.
Stability theory and prediction of dynamic interactions for dc, hybrid dc-ac, and ac power electronics distribution systems with high penetration of regulated power converters (constant power loads).
Hierarchical modeling of large power electronics conversion systems for data centers, distributed generation and vehicular power electronics systems for the study of power quality, small- and large-signal stability.
Terminal-behavior characterization and black-box modeling of power electronics components for the prediction of low frequency dynamic interaction and converter performance and for the prediction of conducted electromagnetic interference (EMI).
4. EMI and Power Quality
Modern switching-mode power electronics systems generate significant conducted electromagnetic interference (EMI) in a broad spectrum. This EMI can hurt the normal operation of other electronics systems and must be suppressed to an acceptable level before it can propagate. EMI noise is traditionally categorized as either differential mode (DM) or common mode (CM) noise. DM noise is the noise current owing within the power delivery paths, while CM noise is the noise current owing between the ground and the power circuits.Although EMI is a wide and varied eld of study, most of the EMI research at CPES focuses on the generation, mitigation, propagation, measurement of EMI in power electronics systems, as well as the improvement of their power quality.
Recent studies
- Generation and characterization of EMI noise
- Parasitic reduction and cancellation for EMI filters
- EMI noise, separation and measurement techniques
- High power density hybrid EMI filter design for motor drive system
- CM noise reduction with balance and parasitic cancellation techniques
- Active EMI filters
- Integrated EMI filters with planar structure
5. High Density Integration
The emergence of wide-bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN) makes it possible to operate power converters at frequencies beyond 5 MHz and temperatures above 200° C. As the switching frequency increases, switching noise shifts to higher frequencies and can be ltered with small passive components — leading to improved power density.Higher operating temperatures enable not just increased power density, but also the ability for power electronics- based systems to operate in harsh environments, such as military, transportation, and outdoor industrial, and utility systems. High-temperature, high-frequency power electronics systems, however, require more than just better semiconductor devices. Designers must also consider materials, gate drives, controller, passive components, packaging, and cooling.
The scope of work includes
- High-temperature integration — Reliable direct-bond-metal substrate; different die-attach technologies for thermo-mechanical reliability; high-temperature encapsulants for power electronics modules.
- Components: Characterization and modeling of wide-bandgap semiconductor devices; high-frequency magnetics and capacitors.
- Module-level integration: High-temperature packaging of power modules, including gate drives, sensors, and protection.
- System-level integration: High-density power supplies on a chip; high-temperature control components and system integration; integrated packaging of LEDs and drivers.
CPES research in this area follows two coupled paths, leveraging the availability of wide-bandgap power semiconductors and high-temperature passive components and ancillary functions. Both switching frequency and maximum component temperatures are being pushed as high as component technologies, thermal management, and reliability permit.
APPLICATION AREAS
These technology areas area applied to four target applications:
1. Power Management for Computers, Telecommunications & Others
As processor-based systems become more complex, more power is consumed by both active and standby systems. At the same time, more systems are going portable and the emphasis on extending battery life adds more challenges to power management. Designers are now striving for high efficiency at both full and light loads.Since the U.S. Environmental Protection Agency (EPA) added computers to ENERGY STAR specifications, many of the major system manufacturers, such as HP, Dell, Cisco, IBM, and Intel are pushing for even higher energy efficiency. Their current near-term target is 92 percent ef ciency at 20 percent load, 94 percent at 50 percent load, and 92 percent at full load.Historically, the goals of efficiency, thermal management, voltage regulation, reliability, power density, and cost drove power electronics design. These factors remain critical, particularly in the face of falling supply voltages and rising current demands. Yet, as multiple voltages proliferate across the PC board, the challenge becomes distributing and managing power across the board.It is time to revisit the power architectures that are in general use and have evolved over time. These architectures may no longer be the optimal solution.
2. Point-of-Load Conversion
Although they are a subset of power management, point-of-load (POL) conversion for power supplies is becoming a major power electronics application in its own right. POL converters are being used in microprocessors for computers, GPS systems, cell phones, PDAs, and other portable electronics. These applications require higher output current, faster transient response, lower output voltage, and tighter output voltage regulation than conventional technology provides.
POL converters are placed near the processor consuming the power. This system avoids long wiring distances between the converter and processor that is found in conventional power supplies and provides a precise voltage supply that meets low-voltage/high-current needs.
Merely employing POL converters, however, is not enough for tomorrow’s applications. Using today’s POL technology to meet ever-stringent requirements would mean larger output and decoupling capacitors, which would raise the cost and occupy too much space on the motherboard.
CPES researchers are seeking alternative POL technology and are investigating different power system architectures, control methods, power conversion, topologies, packaging, more integration, and improved thermal management solutions.
CPES invented the multi-phase voltage regulator module (VRM) based on paralleling multiple buck converter cells in 1997 and today, every computer with an Intel microprocessor uses that CPES technology. In 1997, the biggest application for VRs was microprocessors for computers; today, applications are growing in quantity and variety.
3. Vehicular Power Converter Systems
Transportation vehicles are becoming more and more electrical, from electric/hybrid cars, electrified trains, to all- electric ships and more-electric airplanes. Power electronics plays a large role in improving the ef ciency of these electricity-based vehicles.
CPES has a long history of research in power-electronics-based vehicular power systems, sponsored by industry partners and agencies like ONR, NASA, AFOSR, DARPA, ARL, DOE, and NSF.
The Center’s vehicular power system research is focused on three major areas:
- High-density Power Converters – use advanced devices, passives, circuits, control and packaging to achieve small volume and low weight converters, which are essential to vehicular system applications.
- Modular Plug-and-Play Power Electronics Building Blocks – develop modular power converters, and corresponding control/communication strategies for multi-functionality, recon guration, better performance, lower cost, and higher reliability.
- System Architecture, Modeling, Analysis, and Control – develop design and evaluation methodology for optimal system architecture; develop multi-level models for characterizing system behaviors from stability, power quality, to EMI; study system control and power management strategy.
4. Renewable Energy Systems
Building upon long-time research experience in space power systems (including solar/photovoltaic sources, battery charger/discharger, and dc distribution), CPES has a strong research program in renewable energy systems. This research focuses on three major areas:
A. Sources — dedicated converters for interfacing renewable sources (photovoltaic, wind, battery, etc.) to the power distribution system
B. Smart and energy-efficient appliances
C. Innovative power distribution systems and microgrid
Sustainable Building Initiative
The agship research project in this area has been the sustainable building initiative (SBI). CPES is developing a dc-based renewable energy system as a testbed for future sustainable home electric power systems. The testbed contains various energy sources, including a 3.5 kW turbine generator, 5 kW PV solar panels, a lithium-ion battery bank for energy storage, and a plug-in hybrid car with bidirectional energy ow. The electrical system has two dc buses: a high-voltage dc bus and a low-voltage dc bus. The high-voltage bus is operated at ~380 V, powering HVAC, simulated kitchen loads, and other major appliances. The low-voltage bus is chosen to be at 48V to coincide with the standard telecom voltage, powering computer loads and LED lighting. The whole system is connected to the utility grid via a bidirectional dc-ac converter.
Nanogrid
Homes that rely on renewable energy may also function as nanogrids. The interconnection between the home and the electric grid can be designed so that the home can operate both as a connection on the grid and as an island on an independent electrical system, managing internal sources and loads. In the independent case, energy storage becomes a critical component. Nanogrids can be further extended from single house systems to multiple homes, buildings, data centers, and neighborhoods.
Alternative energy systems will add complexity to the electrical power system with the coupled dynamics between thousands of distributed actively-controlled generation, storage, and consumption units. This "complexity curse" could be managed by using a single power-electronics-based load/source interface for each nanogrid. Each nanogrid could then be dynamically independent of the grid, but dispatchable by the utility operator.
Over the past few years, the agship research program in this area has been the
SUSTAINABLE BUILDING INITIATIVE (SBI).