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To introduce students to the fundamental principles of power semiconductor devices as switches, and their application for solid-state power conversion and control.
At the end of the course students should be able to:
Discuss the characteristics and operating principles of various power switching devices in terms of:-
Their i-v characteristics
Switching speeds (turn-on and turn-off times, operating frequencies)
Power handling capabilities (wattage, operating voltages and currents)
Gate/base drivers and drive requirements
Switch power losses at turn-on and turn-off transitions
Protection requirements and circuits used for such protection - e.g. snubber circuits
Analyse and explain the operation of the different types of power conversion circuits under different loading conditions, viz:-
Rectifiers (AC to DC converters)
Diode rectifiers under resistive, inductive (RL), RL+E, highly inductive loads
Phase controlled SCR (thyristor) rectifiers under resistive, inductive (RL), RL+E, highly inductive loads
DC choppers (DC to DC converters)
Step down (buck) converter
Step up (boost) converter
Switch mode power supply
Inverters (DC to AC converters)
Apply the principle of power conversion and control for motor speed control (as one example of application)
Power devices: Power diodes, BJTs, MOSFETs, SCR-thyristors, IGBT, GTOs.
Single-phase and three-phase half - and full-wave rectifiers with reactive and active loads.
Power conversion using switch modulation.
Characteristics of switching diodes and thyristors.
AC to DC converters, AC to AC converters, DC to DC converters.
Control of DC power supply system with reversible power flow.
Motor control circuits- phase and PWM control.
Course content (expanded):
Power semiconductor devices
Power transistors (power BJT, power MOSFET, IGBT, GTO, MCT, IGCT, SIT) …fully controlled
Semi controlled switches …SCR thyristor, LASCR, etc.
Look at characteristics, operating requirements (such as voltage and current levels, switching speeds), triggering methods, protection for di/dt, dv/dt, etc.
Power conversion circuits using switch modulation of semiconductor devices (controlled, semicontrolled, fully controlled)
Single-phase (including introd. to three-phase) half-wave and full wave rectifiers, semiconverters, or half-controlled rectifiers (under various loads … R, RL, RL+E, L® ¥), dual converters, freewheeling diodes, source inductance effects.
DC to DC converters (or dc choppers or SMPS).
DC to AC converters (or inverters) …introduction only.
Look at circuit operations, analysis (input - output relations), key waveforms for switched circuits, etc.
NOTE: Fourier series expansion and expression of various waveforms (square wave, quasi-square wave, half-wave rectified, full-wave rectified, etc. is important).*
Application examples … motor control circuits, PWM control, etc.
M. H. Rashid, Power Electronics - circuits, devices and applications, Prentice Hall
N. Mohan and W.P. Robbins, Power Electronics - circuits, applications and design, Wiley
V. R. Moorthi, Power Electronics, Devices, circuits and industrial applications
M. J. Fisher, Power Electronics, PWS-Kent Publishing
C. W. Lander, Power Electronics, Mcgraw-Hill
D. Fewson, Introduction to Power Electronics, Essential Electronics series, Arnold
30% continuous assessment (assignments, test, laboratory)
Three assignments and at least 2 quizzes/test.
70% 2-hour written exam (FIVE questions, attempt THREE)
Teaching and Examination period: 29/08/2011 to 16/12/2011
LECTURER: Dr. George N. Nyakoe, OFFICE: ELB 216 - Faculty office
1.1 WHAT IS POWER ELECTRONICS?
Power electronics deals with electric power (energy) as opposed to electronic signals (such as signals in instrumentation systems, audio signals, HF signals etc…) and their processing (analogue or digital)
Use of semiconductor devices as switches for power conversion from one type of source to another.
Use of solid state electronics for efficient control of the amount of power and energy flow.
Application of control techniques in order to obtain the desired input and output requirements of power utilization.
Thus, power electronics combines power, electronics and control.
Therefore, power electronics is defined as … the application of solid-state electronics (semiconductor devices) for the conversion and control of electric power.
Power electronic circuits convert electric power (energy) from one form to another, using electronic devices. The objective of a power electronics circuit is to match the voltage and current requirements of the load to the source.
Power… deals with static and rotating power equipment for generation, transmission and distribution of electric energy,
Control… deals with the steady-state and dynamic characteristics of closed-loop systems,
Electronics… deals with solid-state devices and circuits for signal processing to meet the desired control objective.
1.2 INTER-DISCIPLINARY NATURE OF POWER ELECTRONICS
Power electronics is a subject of interdisciplinary nature. To design and build control circuitry of a power electronic application, one needs knowledge of several areas:-
Design of analogue and digital electronic circuits, to build the control circuitry.
Microcontrollers and digital signal processors for use in sophisticated applications.
Many power electronic circuits have an electrical machine as their load. In a.c. variable speed drives, it may be a reluctance motor, an induction motor or a synchronous motor. In a d.c. variable speed drives, it is usually a d.c shunt/series motor.
In a circuit such as an inverter, a transformer may be connected at its output and the transformer may have to operate with a non-sinusoidal waveform at its input.
A pulse transformer with a ferrite core is used commonly to transfer the gate signal to the power semiconductor device. A ferrite-cored transformer with a relatively higher power output is also used in an application such as a high frequency inverter.
Many power electronic systems are operated with negative feedback. A linear controller such as a PI controller is used in relatively simple applications, whereas a controller based on digital or state-variable feedback techniques is used in more sophisticated applications.
Computer simulation is often necessary to optimize the design of a power electronic system. In order to simulate, knowledge of software packages such as SPICE and MATLAB, and the know-how to model nonlinear systems may be necessary.
1.3 TYPES OF POWER CONVERTERS:
Converters are generally classified into four types:-
ac-dc converters, also called rectifiers… Convert fixed a.c. power to fixed d.c. (in case of uncontrolled rectifiers) or fixed a.c. to variable d.c. (in case of controlled rectifiers).
ac-ac converters, also called a.c. voltage controllers/regulators or cycloconverters (frequency changers or magnitude controllers)
dc-dc converters, also called d.c. choppers or switched mode power supplies (usually unregulated to regulated or fixed to adjustable).
dc-ac converters, also called inverters…(Constant frequency adjustable magnitude or adjustable frequency and adjustable magnitude)
* * Electronic power converter is the term that is used to refer to a power electronic circuit that converts voltage and current from one form to another. Example follows:
1.4 A POWER ELECTRONIC SYSTEM…
Consists of an input source and a load.
Consists of one or more converters for power conversion.
Consists of power semiconductor devices which are used as switches to perform the power conversion.
Includes a gating/triggering circuits to generate the required gate drive signals (e.g in thyristor switching) and base drive signals (in the case of transistors (BJTs).
Has a feedback control circuit implemented in analog and/or digital electronics.
Incorporates one or more static switches acting as a circuit breaker.
Loads may include inductance and capacitance… (student to review transient behaviour of circuits involving inductance and capacitance… assignment 1.)
Power input… vi, i, input VA, W, f.
From:- Battery, fuel cell, utility, solar, wind, capacitor bank/inductor storage; DC or AC.
Power output…vo, i, output VA, W, f
To loads:- motors (desired voltage and frequency), utility line (AC at line frequency), computers (computer power supply, SMPS), Equipment (several such applications), processes.
**Switching converter = power processor
An example...DC voltage regulator:-
1.5 POWER SEMICONDUCTOR DEVICES
Power devices are the key elements (work horses) of a power converter. The commonly used devices are:-
Silicon-Controlled Rectifiers (SCR) or Thyristors
Gate Turn-off Thyristors (GTOs)
Power Bipolar Junction Transistors (Power BJTs)
Power Metal-Oxide Field-Effect Transistors (Power MOSFETs)
Insulated-Gate Bipolar Transistors (IGBTs)
Mos-Controlled Thyristors (MCTs)
Several other new devices (Give me 5 examples in the next class!)
1.6 APPLICATIONS OF POWER ELECTRONICS
Power electronics has already found an important place in modern technology.
Used in almost all new electrical or electro-mechanical equipment from household air-conditioners and computer power supplies to industrial motor controls.
Difficult to draw the boundaries for the applications of power electronics.
Especially with the trends in the development of (new and compact) power devices and microprocessors, the upper limit is undefined.
1.7 APPLICATION POWER RANGES:
Power levels encountered in high-efficiency converters range from mW to MW.
Less than 1W in battery operated portable equipment,
Tens, hundreds or thousands of watts in power supplies for computers or office equipment,
kW to MW in variable motor speed drives (industrial)
1000 MW in rectifiers and inverters for utility dc transmission lines such as in High Voltage DC (HVDC) transmission and Flexible DC transmission systems (FACTs).
1.8 SOME SPECIFIC POWER ELECTRONICS EXAMPLES:
UNINTERRUPTIBLE POWER SUPPLY (UPS)
Mains 1 supplies the normal power to the load. Mains 2 includes an ac-dc converter that charges the standby battery. The dc-ac converter supplies the emergency power to the load.
AUTOMOTIVE POWERING SYSTEM
The dc-dc converter steps down from 42V to 14V. The static switches distribute either 14 V or 42 V.
MOTOR DRIVE SYSTEM
Includes a feedback control loop, gate drive circuitry, filter, 3-phase diode rectifier and a 3-phase inverter.
TRACTION CONVERTER SYSTEM
A 3-phase dc-ac converter drives the motor. The LC-filter lowers the amount of harmonic injection (Figure below).
DC power from the PV system is converted to other dc voltage for battery bank storage and then further converted to ac to supply ac power to the ac loads.
More examples…Laptop computer power supply system, electric vehicle power and drive system, earth orbiting spacecraft, etc.
1.9 WIDE APPLICATIONS OF PE:
1.10 PERFORMANCE SPECIFICATIONS:
Due to the switching action of power semiconductor devices, power converters generate harmonics and ripples into the input supply and the load.
DC load side: ripple contents and THDs.
DC input side: ripple contents and THD.
AC load side: harmonic contents and THDs.
AC input side: THD and input power factor.
Bi-directional power transfer.
Peripheral effects: EMI.
THD: Total Harmonic Distortion
1.11 ISSUES REQUIRING ATTENTION:
The following decisions need to be made:-
Type of converter topology to select to perform the required conversion.
Type of semiconductor devices to perform the switching functions.
Choice of the base/gate-drive circuitry and the interface between the low-level electronics and the high-power switching converters.
Choice of the base/gate drive strategy to obtain the desired input and output requirements.
Choice of the feedback-loop control and the type of control implementation: analog and/or digital.
1.12 DEVICE USAGE:
2 general modes in which devices are operated:
Linear operation: resistive voltage divider
Switched mode operation
Voltage waveform produced for switched mode operation.
Why switched mode preferred?
„V = High , Iload = 0
„V = High , Iload = High
ƒž Loss = 0
ƒž Loss = High
„V » 0 , Iload = High
ƒž Loss » 0
Handles high power
Simple circuits (min. filtering)
With high efficiency
The ideal power switch:
Used for analysis of a converter so that details of the device operation do not obscure the basic operation of the circuit.
In design stage, real devices must be specified and selected
ƒž non-ideal properties have to be considered.
( Near ideal ) Desired Properties:
High on-state current
High blocking voltage
High power handling capability
Handles inductive loads
Low on-state and switching losses
Small heat sink
High speed switching
High control bandwidth
Easy to turn on/off
Low drive power
Simple control circuitry