Centre for Power Electronics

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Centre for Power Electronics

The Centre for Power Electronics (CPE) was established in 1994 to develop and nurture the growing level of

research activities in the Department of Electrical and Computer Engineeringin the field of power

electronics. The centre keeps close links with local power electronics based industries and carries out

advanced research in power electronics of direct relevance to industries.

Power electronics is interdisciplinary in nature and is used in a wide variety of industries from computers to

chemical plants to rolling mills. The importance of power electronics has grown over the years due to several

factors. Two of these are the adve nt of smart power devices and the increasing global concerns about theeffects of environmental pollution. Smart power devices are expected to become ubiquitous and revolutionise

the way power is handled. Electric vehicle is currently looked upon as a pro mising solution to curb urban

pollution. Also, to avoid the pollution due to setting up of new power generating stations, power electronics

has been called upon to ensure better utilisation of existing capacity. This has resulted in research into active

p ower factor correction, harmonic compensation etc., assuming great significance.

CPE is staffed by a team of highly qualified members who have expertise in various aspects of power

electronics. It has adequate laboratory facilities. With the increase in the number of projects handled, more

facilities can be expected to be added in the future.

Besides research projects, the activities of CPE include arranging periodical talks, seminars and short courses

in power electronics both by centre members and distinguished visitors. The centre members can also provide

consultancy in non-research oriente d projects in power electronics.

INTRODUCTION

The Centre for Power Electronics (CPE) has been established for the purpose of promoting advanced

research and development efforts in Singapore in the field of power electronics. A simple definition for

power electronics would be "the control of `raw' input electrical power through electronic means to meet load

e for Power Electronics http://www.ee.nus.edu.sg/CPE

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requirements". Power electronics is interdisciplinary and is at the confluence of three fundamental technical

areas - power, electronics and control.

OBJECTIVES OF THE CENTRE:

Locally: To serve the R&D needs of Singapore based industries in power electronics;

Internationally: To evolve into a world-class research centre of excellence in power electronics and

related application areas.

POWER ELECTRONICS APPLICATIONS

Power electronics applications are very varied and cover virtually all types of industries. A few of the

applications are:

Switch mode power supplies (SMPS)

Uninterruptible power supply (UPS) systems

Photo-voltaic and fuel-cell power conversion systems

Rectifier supplies for electrochemical processes

Heating and lighting, including high frequency illumination control

Induction heating

DC and AC servo drives

High efficiency industrial/commercial drives

Electric vehicle applications

Electric traction

Flexible AC transmission systems (FACTS)

MAIN RESEARCH AREAS

1. Power Supplies

The successful work in this area covers several sub-areas. First, soft-switched converters are studied for use

in high density DC power supplies. High frequency, high efficiency operation and use of parasitics as circuit

elements are some of the concerns addressed. The creation of such converters, their analysis, control, and

design form part of this effort.

Due to the inherent complexity, the control modelling of modern power converters through discrete-time

methods is quite difficult. We have initiated efforts to develop algorithms and software tools to assist theR&D engineer in this area. The scope of the work covers large-signal, steady-state, and small-signal analyses

and design of DC/DC power converters.

The third topic is single-phase active power factor correction. The current industry solution to the proposed

revision of the IEC 555.2 standard on input current harmonics is to use an additional boost stage to shape the

current appropriately. However, th is results in large additional volume, reduction in overall efficiency, and

increased EMI problems. Hence, alternative methods of improving the power factor are being investigated.

In the Uninterruptible Power Supply (UPS) area, our aim is to investigate high efficiency, high power density


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systems for computer and other critical applications. Study of IGBT and MOSFET based AC-to-DC and

DC-to-AC converters, pulse width modulation (PW M) control techniques for input and output harmonic

reduction, battery charging methods, and alternative system configurations all form part of this work.

2. Electric Vehicle (EV)

With increasing concerns over global pollution and the legislative backing provided in California, the time

for practical EVs may have arrived at last. At the Centre, the work of retro-fitting a golf-cart with a chopperfed, IGBT based, DC drive is in pro gress. A project involving the simulation study as well as prototype

development of a complete four wheel drive system for an EV has also been proposed.

3. Power-Semiconductor controlled Variable Speed Drives

Electric drives are considered as the workhorse of modern industries. All most 70% of the total electrical

energy developed in the industrialised nations is converted back into mechanical energy using electrical

machines. In the past most of the drives we re of constant speed type. However, with the advent of power

semiconductor devices and microprocessors, variable speed operation of electric drives is now possible. The

requirements of industrial drives are becoming stringent day by day for example; robus t, rugged,maintenance free running, high accuracy, high efficiency, intelligent etc. Such requirements can be met by the

use of induction motors at medium and high power range and permanent-magnet brushless d.c. motors at low

power range. Recently, the i ntroduction of a new type of motor namely, switched reluctance motor poses

serious competition to induction as well as permanent-magnet brushless d.c. motor drives.

We have initiated a research programme to investigated the control aspects of high performance brushless d.c.

and switched reluctance motor drives: to minimise the electromagnetic torque ripples; improve the transient

performance; making the controller ro bust i.e. insensitive to drive system parameters variation; and

sensorless operation. All these features require the use of the art Digital Signal Processors. In this research it

is intended to develop a general type of experimental platform for the two m otor drives so that various types

of control algorithms can be implemented at ease and their performance can be compared.

4. Power Devices and Smart Power IC's

A variety of simulation and visualising tools have been assembled and used to study different device

structures, such as the Gate Turn-Off Thyristor (GTO), Insulated Gate Bipolar Transistor (IGBT) and Lateral

IGBT, MOS Controlled Thyristor (MCT) and Power MOSFET. Design optimisation of power semiconductor

devices such as a synchronous rectifier used in low output voltage power supplies is also carried out.

Work on "Intelligent Devices" and "Smart Power IC's" are currently undergoing. This includes the integration

of current sensor to the Lateral power devices for control and protection purposes. The performance of apower device can be enhanced in applications such as appliances, air-conditioners and electric vehicles

through the addition of intelligent features such as a current or a temperature sensor. The challenges in the

area of smart power devices include (a) multiple power devices and control circuits in a single chip and (b)

prevention of mal-operations under over-current, over-voltage and over-temperatures protections.

The work on the integration of smart sensors and micromechanic devices is planned in the next few years.

5. Monitoring and Control of Power System Harmonics


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This project aims to develop products with potential commercial values. In the first part, a PC is combined

with a data acquisitions card to form a flexible package for the measurement and analysis of power system

harmonics. In the second part a combinati on of passive and active harmonic filtering techniques are studied

and developed.

6. Flexible AC transmission System (FACTS)

Through power electronics, FACTS aims to improve the quality, reliability and utilisation of large, complex,interconnected utilities. In this proposed project, the potentials and technical implications of applying FACTS

to the ASEAN region will be studie d. This will then be followed by development of FACTS plans and

control strategies for the region.

7. Modern Mass Rapid Transit (MRT) System Research

The operation of modern MRT systems has become very complex due to heavier traffic flows, higher speeds,

and different train characteristics (three phase and DC chopper trains). The research being conducted aims to

improve the MRT operation through (1) i ncreased regenerative braking energy recovery for a reduction of

the operating cost, (2) lowering the amount of electrical noise (or harmonics) generated for a reduction ofinterference on its communication circuits and disturbance to other electricity co nsumers, and (3)

improvement in the MRT's overall quality of service in terms of safety, regularity, passenger comfort, electric

loading and energy efficiency.

FACILITIES

The aim of this Centre is to focus the research efforts of the members of Power Electronics and Machine

Control streams in order to benefit the local power electronics related industries.

The Centre is located at the Power Electronics Laboratory (WS2-05-23) of the Department of Electrical andComputer Engineering. A variety of oscilloscopes including digital, high bandwidth, and isolated channel

scopes are available. Microcontroller systems enhance the capability to perform system development work.

Torque measurement systems with display, loading dynamometers and static loads, flux meters, universal

counters etc. facilitate drives research. DC, brushless DC and AC drive systems are available. Electronic

loads, frequency meters, power supplies etc. are useful particularly in low power converter work. The HP

4194 gain-phase analyser donated by a local company in appreciation of the collaborative research work

performed is an essential tool in high frequen cy power supply research particularly for control loop

frequency response measurements.

In the Power & Machine Division's computing facility housed in the Power Systems Laboratory, seven linkedSun workstations house software particularly suited for power electronics research besides general software

such as word processors and MATLAB. State of the art power semiconductor device simulation packages,

such as MEDICI and DESSIS, are being used. For circuit simulation, SABER is a powerful tool capable of

device primitive level as well as behavioural level simulations, a very useful feature in large power electronic

simulations. POWEREXPRESS tool which has been procured extends SABER for simulation of power

electronic circuits.

CENTRE MEMBERS


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Oruganti, Ramesh (Director)

Chang, Che Sau

Jabbar, M A

Liang, Yung Chii

Liew, Ah Choy

Lock, Kai Sang

Panda, Sanjib Kumar

Hsu, Tar SuTay, Teng Tiow

RESEARCH STUDENTS

Nagaswamy, Kannan

Srinivasan, Ramesh

Yang, Haiqing

Pan, Honglin

Yuen, Xiaolu


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Mr. Stuart Morrall, an M.Sc. student, is working on the Electric Vehicle Project.

Current Research Activities

NEW GENERATION POWER SUPPLY DEVELOPMENT

Contact: Dr. Ramesh Oruganti

Collaboration: Nemic Lambda (S) Pte Ltd

Funding: NSTB-RDAS grant

General

The project has been recently completed successfully. It's main task was the "Evaluation of High Density

Power Supply Topologies". An additional task of "Investigation of Power Factor Correction (PFC) Front

Ends for Power Supplies" was also carried out as part of the project. Based on the work, two patent

applications are being processed.

The motivation for the main task stems from the continued miniaturisation of computer and other electronic

equipment, which has created a demand for the development of ever smaller, lighter and more efficient

switching power supplies. In addition, the sec ond task has become very relevant due to the present green

concerns expressed through revisions to IEC555.2 and other harmonic standards.

High Density Power Supply Topologies

An isolated, soft-switched, two-switch, asymmetrical PWM converter with two variations suitable for high

density power supply applications was developed. The converter effectively utilises the circuit parasitics, the

transformer's leakage and magnetising inductances, and the output capacitance of the power MOSFETs, as

circuit elements. Unlike the quasi-resonant and multi-resonant converters, these soft-switching transition

converters pay relatively less additional penalty in terms of increased device stre sses. Due to the absence of

excess circulating current at reduced loads, the partial and light load efficiencies of his converter remains

high. A prototype 500 W, 48 V output converter has achieved an efficiency of over 94 % at full load and a


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power density of above 50 W/cu.inch. The efficiency remains higher than 90 % even at 20 % of full load. In

addition to prototype development, both steady-state and small-signal analysis have been carried out and

experimentally verified. These results are useful for the engineer in designing these converters.

As a contribution to power electronics fundamentals, the inter-relationships among various two-switch,

soft-switched, PWM converters were also explored. Starting from a generalised two-switch converter

operated under an asymmetrical PWM scheme, the non-is olated and isolated converters of this type were

"derived". All of the converters generally have the advantages of lossless switching, minimum switch voltage

stress, fixed frequency operation and minimum component count. The converters which have been des cribed

in various works were brought together under a common qualitative and theoretical framework.

Development of high density power supplies

Single Phase Power Factor Correction

The aim of this additional task is achieve improvements over the conventional cascaded boost-based PFC

schemes. A novel Boost Bypass Control Scheme has been proposed to realise these aims. Here, the PFC part

processes less power than in the conventional B oost scheme while still meeting the PFC standards. Thescheme results in performance improvements such as increased efficiency, reduced component stresses,

smaller size, reduced EMI problem through higher frequency operation. The viability of the basic co ncept

behind the scheme has been established through experiments. Control schemes to actively control the power

sharing between the boost and the bypass paths have been built and tested.

References

[1] R. Oruganti, C.H. Phua, K.G. Tan and A.C. Liew, "Soft-switched Converter with PWM Control",

Proceedings of the IEEE 15th International Telecommunications Energy Conference, Sept 1993.

[2] C.H. Phua and R. Oruganti, "Family of Two-switch Soft-switched Asymmetrical PWM DC/DC

Converters", Proceedings of the IEEE Power Electronics Specialists Conference, June 1994.

[3] Y.T. Cham and R. Oruganti, "A Novel PFC Scheme for AC to DC Converter with Reduced Losses",

Proceedings of the Twentieth Annual Conference of the IEEE Industrial Electronics Society, Sept. 1994.

ANALYSIS & DESIGN OF POWER SEMICONDUCTOR DEVICES

Cont act : Dr . Yung C. Li ang ( mor e)


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MOS-controlled Bipolar Devices

MOS-controlled bipolar power devices combine MOS and bipolar technology into monolithic structure to tap

the best features from both classes of devices. The bipolar part renders high conduction current capabilities at

low forward voltage while the MOSFET component makes it possible to implement voltage control with

minimum gate drive requirements. In this project, three types of MOS-controlled bipolar devices, namely the

Insulated-Gate Bipolar Transistor (IGBT), the Emitter-Switched Thyristor (EST) and the N-channel

MOS-controlled Thyristor (NMCT), are analysed. Certain features unique to each device are highlighted. Bykeeping their internal parameters as similar as possible, a comparison of their performances was also made

during the study.

Gate Turn-off Thyristor

The maximum controllable anode current is influenced by the current crowding phenomenon observed in

GTO thyristors during turn-off. This phenomenon initiates (1) the formation of the current filament with a

density much higher than its surrounding and leads to the localised thermal instability; and (2) the

conductivity modulation at the conduction channel causing a non-uniform electric field distribution within

the depletion layer and resulting an early punchthrough breakdown. In this project, the failure mechanism isdiscussed. Internal parameters are optimised qualitatively through detailed two-dimensional device

simulations to minimise the current crowding and, at the same time, to maintain a low forward voltage drop

and short turn-off time for a GTO model-cell. It is found that, a lower localised thermal stress and a more

uniform electric field distribution can be obtained.

Vectors of current flow in a MOS-controlled bipolar device

Synchronous Rectifier

Synchronous rectifiers used in high frequency, low output voltage applications are power MOSFETs specially

designed to replace the usual output Schottky diodes in order to reduce converter losses. This project deals

with the analysis and design optimisati on of a synchronous rectifier suitable for applications of 1 to 10 MHz

switching-mode power supplies. Three different MOSFET structures were studied and evaluated through

detailed 2-dimensional device simulations. The internal parameters are optimised aga inst three major


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performance factors, namely (1) the recovery time of the body diode, (2) the product of on-state resistance

and input capacitance, i.e. the loss factor and (3) the breakdown voltage of the body diode. Based on the

evaluation, the UMOS str ucture produces the lowest RC loss factor and the shortest body diode reverse

recovery. The final design optimisation of the UMOS was then carried out and an optimised device is

presented as the final design.

FUZZY CONTROL BY USING FUZZY PROCESSORS

Contact: Dr. Yung C. Liang andA/P C.S. Chang

Collaborators: Dr. S. K. Panda , Dr. S. H. Tan

A 12-bit fuzzy logic controller (FLC) has been successfully developed by using the OMRON FP-3000 fuzzy

processors running at 20 MHz. The controller developed was interfaced to a microcomputer with

man-machine software developed. The justification process for fuzzy rules is currently being developed for

both the SISO and MIMO system control. Controllers of higher proformance by using Togai fuzzy chips is

also under implementation.

Fuzzy logic controller and man-machine interface

ADAPTIVE HARMONIC AND REACTIVE POWER COMPENSATION

Contact: Dr. Lock Kai Sang andA/P A.C. LiewResearch Personnel: Kang Liat Chuan

Harmonics produced by electronically controlled loads have caused serious problems of concern to both the

supply authority and the consumers. Some of these problems include excessive voltages and currents in the

supply network due to resonance, damage to power factor correction capacitors, errors in metering,

overheating of motors and switchboard, interference with ripple control and other electronic equipment.

Conventionally, tuned L-C filters are installed to reduce the harmonics and to increase the power factor.

These passive filters are unable to adapt to changing load conditions as each tune filter is tuned to a particular


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harmonic frequency.

Optimum filters can be achieved by employing a combination of passive and active filters. The passive filter

will provide the "base" in removing the harmonics whilst the active filter plays the role in improving the

overall filtering characteristic s. Both the filters can also be employed to provide reactive compensation in

addition to harmonic compensation. The optimum combination of active and passive filters thus provides an

effective and economical solution to harmonic problems.

The compensator shows excellent response both during steady state and transient state. A prototype

compensator has been constructed and it confirms the feasibility of the proposed compensator.

Other studies on harmonics reflect to the levels put out by various types of equipment, their identification and

its correlation with the requirements of the various standards.

HIGH PERFORMANCE ELECTRIC DRIVES FOR INDUSTRIAL APPLICATIONS

Contact: Dr. S. K. Panda

Collaborators: Dr. Yung C. Liang and Dr. Lock Kai Sang

Research Personnel: H. Q. Yang , S. Rajaram

Variable speed drives play an important role in modern industries either to conserve energy or to improve

quality as well as quantity of the end product by using motion control in factory automation. Traditionally,

conventional proportional--integral (PI) or proportional--integral--derivative (PID) controllers are used for

controlling speed and torque of electric motors. These linear controllers operate reliably so long as the

operating point does not deviate too much from the desired operating range or large parameter variations do

not take place. However, they fail to operate optimally if the operating point varies over a wide range. In

variable speed drives the operating speed and torque could vary from 0 -- 100%. Therefore, it becomes

difficult to ge t high dynamic performance from the drive system using linear PI or PID controllers.

In recent years, due to the development in modern control theory, a couple of nonlinear control techniques

have emerged to tackle the problems of parameter insensitivity, robustness to torque disturbances and

providing high dynamic performance under varyi ng operating conditions. A few of those are: adaptive

control, variable structure system with sliding mode control, feedback linearizing control and fuzzy logic

with neural networks. The aim of this research work is to investigate the possibilities of ex tending some of

the abovementioned nonlinear control techniques to permanent--magnet brushless dc and switched reluctance

motor drives for high performance servo applications. In this research work emphasis will be given to

develop an experimental platfo rm for both type of motor drive system so that the performance of different

types of nonlinear control algorithms can be tested for real--time applications. The variable structure control

and feedback linearization controllers have been designed and simulated for SR motor drive system.

For closed-loop control of the electric drive system it is necessary to measure certain parameters of the

system such as: position, speed, current, flux or torque and feed back to the controller to take necessary

corrective control actions.

The use of direct sensors leads to several disadvantages such as : increase in the drive system cost; increase in

the size of the system; reliability problems arise in certain type of industrial environment; mechanical

alignment problems arise in the plac ement of position sensors and so on. In order to overcome some of the

abovementioned drawbacks, the physical parameter of Indirect rotor position sensing in VR stepper motors


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interest is indirectly sensed from the measurement and further processing of motor terminal parameters such

as: phase voltage and current.

In this research the waveform detection technique is used to monitor rise and fall times of the phase current

waveform to extract rotor position information of the Variable or Switched Reluctance motor. Both current

rise and fall times of the active phase as well as inactive phase can be used to monitor the position

information. However, from the mathematical analysis as well as experimental verification it has been found

that fall time of the diagnostic current in the inactive phase provides reliable pos ition information and hence

is recommended. Figure 1 shows an experimental set-up of such a scheme.

Figure 1. Indirect rotor position sensing in VR stepping motors

DESIGN OF SHIELDING SYSTEMS

Contact: S. R. H. Hoole, Harvey Mudd College, Claremont, CA 91711

Sponsorship: Southern California Edison Co.

The design of effective shielding systems is important. This is especially so as high-speed computers are

developed. Even in power devices, shielding systems are important because of the large currents involved

and the increasing use of high-frequency pow er supplies.

Traditional approaches to the design of shielding systems involve repeated analysis. Following each analysis,

the results are checked with the desired performance, and then the parameters of the devices geometry as well

as permeability and conductivity of the shield are modified by the expert designer so as to make the

electromagnetic fields fall below the maximum acceptable threshold. This procedure for modifying the


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descriptive parameters of the shield is expensive and requires an expert.

This project develops new methodologies in shield design where shape-optimisation techniques from civil

engineering are used with mathematical optimisation methods to yield an accurate, efficient and automatic

design process that does not rely on an exper t.

The new optimisation process is iterative and relies upon finite element analysis to evaluate the

"performance" of the shield at each stage of the design cycle, the "performance" being whether the

electromagnetic fields in the zone to be shielded are acce ptably low. The design problem is posed as a

problem of constrained mathematical optimisation where the volume of the shield has to be minimised

subject to the constraint that the electromagnetic fields must fall below prescribed limits in the shielded re

gion and that dimensions must be within certain tolerances. The method investigates the use of the

derivatives of the electromagnetic fields (which are intrinsic to finite element solutions), in employing fast

and efficient optimisation strategies.

The project further investigates how this method can be combined with other disciplines such as artificial

neural networks and fuzzy logic to bring about first an approximate design which may then be refined by the

accurate process described above, thereb y reducing the cost of design.

A QRC CONTROLLER FOR DISK DRIVE SPINDLE MOTORS

Contact: Dr. M. A. Jabbar

The proliferation of portable computers, like lap tops, note books and palm tops have brought great changes

in disk drives as well. The area where the greatest change is demanded is the disk drive power management.

This is mainly because all the portables are battery-powered systems, and dry cells do not last very long in

computers.

The disk drive spindle motor is a brushless d.c. system, which operate on a fixed supply voltage. The controlat start and run conditions are through linear mode operation of the inverter system. This is wasteful of

energy. A PWM technique on the supply s ide with fully switch-mode operation of the inverter system will

improve the situation, but still incur switching losses at the PWM stage.

This new invention is based on a Quasi-Resonant Converter. The switching can be achieved at either zero

current or zero voltage, thus eliminating virtually all the switching losses in the system. A switching

frequency of 1 MHz, has be en used, thus reducing the component size, cost and weight.


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Centre for Power Electronics