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Electrical and Computer Engineering

Research

Power Group Research Areas

Non-Linear Analog and Digital Control of Power Converters

  • Power converters that have generally been controlled using linear analog feedback loops are simple and affordable, yet lack speed due to limited bandwidth. It is necessary for converters to react faster to changing load conditions, as digital devices are being rapidly advanced and require a much higher power density. Therefore, intelligent non-linear controlled power supplies are expected to play a larger role in communication, automobile, computer and aerospace industries in the near future.
  • Modern control theories, such as fuzzy logic control, neural network control, sliding mode control, and adaptive control have been developed using low cost digital signal possessors (DSPs) and field programmable gate arrays (FPGAs) to improve the large-signal dynamic characteristics of both DC-DC and AC-DC converters.
  • Research being conducted includes the development of advanced analog and digital controllers for high-performance, high-frequency DC-DC converters. Using the principle of capacitor charge balance, linear/non-linear controllers are being developed which minimize the output voltage deviation and settling time of a power converter undergoing changing input voltage or load current conditions. The increased controlled reaction speed permits the use of fewer bulk capacitors, allowing the converter to be more compact and cost-efficient.

High Efficiency Low Output Voltage Converters

  • In order to improve the dynamic response and decrease the filter size of power converters, there has been a continuous push to increase its switching frequency However, as switching frequencies increase beyond 1MHz, gate loss and switching loss at turn-on and turn-off in power MOSFETs become increasingly significant. Traditionally, gate-drive methods use a voltage source to charge and discharge the MOSFET gate through a resistive switch and an external gate resistor. Energy is taken from the supply voltage to charge the gate and then sent to ground when discharging the gate; thus, gate energy is wasted.
  • Furthermore, due to the relatively slow charging curve of an RC MOSFET gate circuit, switching losses can be significant as the MOSFET operates in saturation mode for a greater period of time. Thus, research is currently being conducted on the development of "current source" MOSFET drivers. Through the use of a small inductor, the proposed drive scheme is capable of charging and discharging MOSFET gates with a relatively constant current. This drive scheme allows for much shorter switching transitions and gate energy recovery, significantly decreasing the gate loss and switching loss of a switching power converter. Thus, it is possible to increase the switching frequency (and subsequently decrease the size of the converter) without sacrificing efficient operation.

Current Source MOSFET Driver Techniques

  • The switching frequency of power converters increases into the MHz range as power density and response speed increases, however output voltage requirements have been reduced to sub 1V levels while current requirements are steadily increasing, which poses a challenge to the power supply industry.
  • Topologies that possess zero voltage switching (ZVS) are being researched rather than the widely used Buck topology, which is not feasible at MHz-level switching frequencies due to its hard-switching and narrow duty cycle properties. These ZVS topologies are able to extend the effective steady-state duty cycle of the converter.
  • Research is being done on novel topologies based on single- and multi-phase full bridge converter topologies for use in both isolated and non-isolated applications in communication and computer power systems, such as voltage regulator modules (VRMs) for future microprocessors.

Power Factor Correction (PFC) Improvements

  • In order to improve power line quality, standards have been proposed to limit the harmonic content generated by AC-to-DC power supplies. Traditionally, a diode bridge and a bulk capacitor are used to achieve AC-to-DC conversion. Unfortunately, the power factor of these circuits are very low (around 70%). Various power factor correction circuits have been proposed to achieve a power factor greater than 99%, however further work needs to be done to improve the performance of these methods.
  • The research is focused on finding new converter topologies to achieve high power factor, high efficiency, low EMI and eliminate the diode reverse recovery problem. In addition, research will be done on how to use DSP techniques for PFC control while achieving significant cost reduction and performance enhancement.

Modern Control Techniques in Electric Motor Drive Systems

  • Modern control theories, such as fuzzy logic control, neural network control, sliding mode control and adaptive control have been developed for electric motor drive systems. These techniques make the motor drive systems more robust against any possible disturbances so that the speed, or position of the motor accurately tracks the reference.
  • The research in this area is focused on modern control theory applications and DSP based control systems design for induction motors, permanent magnet synchronous motors, brushless motors and switched reluctance motors.

EMI Filter Design

  • In communication systems, "off the shelf" power modules are used to convert the input voltage to a lower voltage to power digital and/or optical circuits. However, these power modules normally cannot meet the FCC, or CISPR regulatory standards for conducted EMI. As a result, additional EMI filters, normally both common mode (CM) and differential mode (DM), are needed at the input of these power modules in order to filter out switching noise and eliminate electromagnetic interference to other equipment.
  • The research in this area is focused on new designs for EMI filters for both DC-DC and AC-DC switching power supplies.

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