Overview to Power Electronics Mohd Shawal Bin Jadin Ext : 2321 A01-2074
[email protected]
Learning Outcomes • At the end of the lecture, student should be able to: – Identify the function of electroni electronics cs switches,, hence to select a proper switches switching for certain applications. applications. – Outline the principles of energy recovery and also to calculate the power for non-sinusoidal periodic waveform
Chapter 1 •
•
•
•
Definition of Power Electronics Multidisciplinary Nature of the Field Block Diagrams of Power Electronic Systems The Need for Power Electronics
•
Future Trends
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Types of Power Conversion
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Electronic Switch Switch
•
Switch Selection
•
Energy Recovery
y1
x1 x2 x
Load1
y2
Power Electronic "Power" Circuit
Load2
yn
Loadn
m
f 1
Electrical Inputs "Sources"
f 2
f k
Feedback "ControlCircuit"
(input) Source Side
Electrical or Mechanical Output"Loads"
(output) Load Side
Power Processing circuit (P loss )
Load
What is Power Electronics?
• Generally:
– Electronics: Electronics: Solid State Electronics Devices and their Driving Circuits. – Power: Power: Static and Dynamic Dynamic Requirements equirements for Generation, Conversion and Transmission of Power. – Control: Control: The Steady State and Dynamic Stability of the Closed Loop system.
• POWER ELECTRONICS may be defined as the application of Solid State Electronics for the Control and
Definition of Power Electronics
• DEFINITION:
To convert, i.e to process and control the flow of electric power by supplying voltages and currents in a form that is optimally suited for user loads.
Power Electronics (PE) Systems
• To convert convert electrical electrical energy from one form form to another, another, i.e. from the the source to load with: with : • highest efficiency, • highest availability • highest reliability • lowest cost, • smallest size • least weight
Detailed Block Diagram of Power Electronics System Pre-stage Input Form of electrical energy
Filter & Rectify
Power proc. stage
Post stage Filter & Rectify
PE Circuit
Output
Electrical Mechanical
Load
Could generate undesirable waveforms
Mostly unregulate d dc voltage h s c e t v i i w r S D
Mostly ac line voltage (single or three phase)
Form of elec. or mechan. energy
Control Circuit
Electrical Variable Feedback MechanicalVariable Feedback
Interface between control and power circuits
Process feedback signals and decide on control
Applications • Static applications – involves non-rotating or moving mechanical components. – Examples: • DC Power supply • Un-interruptible power supply, Power generation and transmission • (HVDC), Electroplating, Welding, Heating, • Cooling, Electronic ballast
Applications • Drive applications – intimately contains moving or rotating components such as motors. – Examples: • Electric trains, Electric vehicles, Airconditioning System, Pumps, Compressor, Compressor, • Conveyer Belt (Factory automation).
Application examples • Static Application: DC Power Supply
Application examples • Drive Application: Air-Conditioning System
Other Applications Electroplating, Welding
Photovoltaic Systems.
Heating, cooling, CFL
eV (fuel cell, Solar)
Wind-electric systems.
Conversion concept: example1 •
•
•
Supply from TNB: 50Hz, 240V RMS (340V peak). Customer need DC voltage for welding purpose, say. say. TNB sine-wave supply gives zero DC component! We can use simple halfwave rectifier . A fixed DC voltage is now obtained. obtained. This is a simple PE system.
•
Average output voltage
•
Conversion concept: example (Cont)1
How about if customer wants variable DC voltage? – More complex circuit using SCR is required. Average output Voltage
•
By controlling the firing angle, angle, α, the output DC voltage (after conversion) can be varied.
Advantages of Power Electronics •
High energy conversion efficiency – Instead of using 50/60Hz motor-generator
•
Higher Reliability and cost effective – Less maintenance, longer longer lifetime, light and small size, fast recovery time, unlimited range of conversion
•
Environmentally clean and safe – produce no hazardous waste products – Burning of fossil fuel emits gases such as C,0,, CO (oil burning), S02, NOx (coal burning) etc. Creates global warming (green house effect), acid raill and urban pollution h-oll)
•
Quite operation – has no moving parts, suitable for residential, hotels etc
•
reduce dependence on fossil fuel (coal, natural gas, oil) and nuclear power resource (uranium). –
•
Effort to tap renewable energy resources such as solar, wind, fuel-cell etc. need to be increased.
Special effort is needed to reduce pollution in cities by enforcing the use of electric vehicle.
PE growth • PE rapid growth due to: – Advances in power (semiconductor) • switches
– Advances in microelectronics (DSP, VLSI, microprocessor / microcontroller, microcontr oller, ASIC) – New ideas in control algorithms – Demand for new applications
PE is an interdisciplinary field: – – Digital/analogue electronics – – Power and energy • – – Microelectronics – – Control system – – Computer, simulation and software – – Solid-state physics and devices P k i
Power Electronics Converters AC to DC: RECTIFIER
DC to DC: CHOPPER
DC to AC: INVERTER
AC to AC: CYCLOCONVERTER
Power semiconductor devices (Power switches)
• Can be categorised into three groups: Uncontrolled:: Diode – Uncontrolled Semi-controlled:: Thyristor (SCR) – Semi-controlled – Fully controlled: controlled: Power
Photos of Power Switches Powerex Inc.)
(From
Power Electronics Converters AC to DC: RECTIFIER
DC to DC: CHOPPER
DC to AC: INVERTER
AC to AC: CYCLOCONVERTER
The Need For Switching In Power Electronic Circuits •
The need to use semiconductor switching devices in power electronic circuits is based on their ability to control and manipulate very large amounts of power from the input to the output with relatively very low power dissipation in the switching device .
• Implication of low efficiency: efficiency: – The cost of energy increases due to increased consumption. – Additional Additional design complications might be imposed, especially regarding the design of device heat sinks
Example Investigate the efficiency of four different power electronic circuits whose function is to take power from a 24 V dc source and deliver a 12 V dc output to a 6Ω resistive load. load. In other words, the task of these circuits is to serve as dc transformers with a ratio of 2 : 1. The four circuits are shown in Fig. 1 (a), (b), (c), and (d) representing a voltage divider circuit, zener regulator (assume IZ is 10% of load current), transistor linear regulator, and switching circuit, respectively respectively.. [Hint: For circuit (d), Vo=Vin*D]
Example (Cont)
(e) Zener diode i-v switching characteristics. (f) Switching waveforms for circuit
Example (Cont) • Cicuit (a) : Voltage Voltage Divider dc Regulator • Since Vin=24V and RL=6Ω and desired Vo=12V.
Hence, R = RL=6Ω . η
P out
=
Thus,
P in
=
%
R L
R L
P L
=
R
P in
%
%
=
+
6
6 6
%
50%
=
+
• Cicuit (b) : Zener dc Regulator • Since desired Vo=12V, hence the blocking voltage for zener diode, VZ I T = I L + I z = 12V. 2 + 0.2 =
Since, RL=6Ω. Thus IL=2A. Assume that, IZ = 0.2A (10% of load current) Thus,
2.2 A = P in
2.2 × 24 = 52.8W =
P out
2× 12 = 24W = P out
η =
P in
%
24 =
52.8
%
45.5% =
Example (Cont) • Cicuit (c) : Transistor dc Regulator Regulator
For Vo=12V, it is clear that VCE must be around 12V. Hence, the control circuit must provide base current, IB to put transistor in active mode with VCE=12V. For given Vo=12V and RL=6Ω, thus IL=2A =2A.. Thus, IC = 2A since IB too small in such that to turn on transistor. P = V I = 2( 24) = 48W in
P diss
in
c
= V I C CE ≈ V I C CE
P out
η ∴ =
P in
%
+ V I B BE
12 × 2
≈
24
=
48
%
24W
=
50%
=
Example (Cont) • Cicuit (d) : Switching dc Regulator Assume the switch is ideal and periodically turn on and off. From figure (f), Vo is given by V o , ave
1 =
T s D
T s
V ∫
in
d t
= V in D
0
=12V) • For Vo,ave=12V, hence D=0.5 (Vo,ave=24 x 0.5 =12V) Since V in
24V , =
P in
I L × V = in
2× 24 = 48W =
Due to switching ( assume ideal ), P out
P = in
P out , η % = P in
48
=
48
%
100 100% =
Ideal Switching Characteris Characteristics tics •
•
• •
No limit on the amount of current that the device can carry when in the conduction state (on-state) No limit on the amount of device voltage (known as blocking voltage) when the device is in the non-conduction state (offstate) Zero on-state voltage drop when in the conduction state Zero leakage curr current ent when in the nonconduction state
Ideal Switching Characteris Characteristics tics
Power loss
The Practical Switch • The practical switch has the following switching and conduction characteristics: – Limited power po wer-handling -handling capabilities – Limited switching speed – The existence of forward voltage drop in the on state, and reverse current current flow (leakage) in the off state – Because of characteristics 2 and 3, the practical switch experiences power losses in the on and off states (known as conduction loss) and during switching transitions (known as
Power Diodes
• When diode is forward biased, it conducts current with a small forward voltage (V f ) across it (0.2-3V) • When reversed (or blocking state), a negligibly small leakage current (uA to mA) flows until the reverse breakdown occurs. occurs. Diode should not be operated at reverse voltage greater than V r .
Power Diode (Reverse Recovery) • When a diode is switched quickly from forward to reverse bias, bias, it continues to conduct due to the minority carriers which remains in the p-n junction. • The minority carriers require finite time, i.e, t rr (reverse recovery time) to recombine with opposite charge and neutralize. • Effects of reverse recovery are increase in switching losses, increase in voltage rating, over-voltage (spikes) in inductive
Power Diode (Reverse Recovery)
Types Types of Power Power Diodes •
Line frequency (general purpose) :
• Fast recovery • very low trr (<1us). • Power levels at several hundred volts and several hundred amps • Normally used in high frequency circuits
• on state voltage very low (below 1V) • large t rr (about 25us) • very high current (up to 5kA) and voltage (5kV) ratings • Used in line-frequency (50/60Hz) • applications such as • Schottky rectifiers • very low forward voltage voltage drop (typical 0.3V) • limited blocking voltage (50-100V) • Used in low voltage, high current • application such as switched mode power supplies.
Thyristor based • Thyristor refers to the family of power semiconductor devices made of three pn junctions (four layers of pnpn) pnpn) that can be latched into the on state through an external gate signal that causes a regeneration mechanism in the device. • Thyristor family currently used in electronic circuits: – The silicon-controlled rectifier (SCR), – gate turn-off thyristor (GTO), – triode ac switch (triac), – static induction transistor (SIT), – static induction thyristor (SITH), and MOS-controlled thyristor (NICT).
power
Thyristor (SCR)
•
Unlike the diode, the SCR has a third terminal called the "gate" used for control purposes. • The holding current is the minimum forward current the SCR can carry in the absence of a gate drive. • The forward breakover voltage, VBO, is the voltage across the anode-cathode terminal that causes the SCR to turn on without the application of a gate current. current. • Reverse avalanche (breakdown) occurs when V AK is negatively large.
Thyristor (SCR) • Thyristors can only be turned on with two conditions: – the device is in forward blocking state (i.e V ak is positive) – a positive gate current (Ig) is applied at the gate •
Once conducting, the anode current is LATCHED (continuously flowing).
•
In reverse - biased mode, the SCR behaves like a diode. It conducts a small leakage current which is almost dependent of the voltage, but increases with temperature.
•
When the peak reverse voltage is exceeded, breakdown occurs, occurs, and the large current will flow.
•
In the forward biased mode, with no gate current present (i.e. in the untriggered state), the device exhibits a leakage current.
•
If the forward breakover voltage (V BO) is exceeded, the SCR “self-triggers” into the conducting state and the voltage collapses to the normal forward volt-drop, typically 1.5-3V. The presence of any gate current will reduce the forward breakover
avalanche
Thyristor Conduction
How to turn off thyristor ?
Thyristor Conduction • Thyristor cannot be turned off by applying negative gate current. current. It can only be turned off if I A goes negative (reverse)
– This happens when negative portion of the of sine-wave occurs (natural commutation). • Another method of turning off is known as “forced commutation”,
The anode
is
“diverted”
Controllable switches (power transistors)
• Can be turned “ON”and “OFF” by relatively very small control signals. signals. • Operated in SATURATION and CUTOFF modes only. only . No “linear region” operation is allowed due to excessive power loss. • In general, power transistors do not operate in latched mode. mode. • Traditional devices: Bipolar junction transistors (BJT), Metal oxide silicon field effect transistor ( MOSFET), MOSFET), Insulated gate bipolar transistors (IGBT), Gate turn-off thyristors (GTO)
Bipolar Junction Transistor (BJT)
• Ratings: Voltage: V CE<1000, Current: IC<400A. Switching frequency up to 5kHz. Low on-state voltage: V CE(sat) : 2-3V. • Low current gain (β). Need high base current to obtain reasonable IC . (Current driven). Expensive and complex base drive circuit. • Not popular in new n ew products
BJT Conduction • The level of IB in the active region just before saturation must be I I Bmax max
>
c sat
β dc
• At saturation, the current IC is quite high and the voltage VCE very low. The resistance across the terminals V determined by CE sat R = sat
Saturation conditions and the resulting terminal resistance
I C sat
Cutoff conditions and the resulting terminal resistance
Metal Oxide Silicon Field Effect Transistor (MOSFET)
• Ratings: Voltage V DS<500V, current IDS<300A. (Voltage driven) • Very fast device: >100KHz. For some low power devices (few hundred watts) may go up to MHz M Hz
MOSFET characteristics • Turning on and off is very simple. simple . Only need to provide V GS =+15V to turn on and 0V to turn off. Gate drive circuit is simple. • Basically low voltage device. High voltage device are available up to 600V but with limited current. current. Can be paralleled quite easily for higher current capability. • Internal (dynamic) resi resistance stance between drain and an d source during on state, RDS(ON), limits the power handling capability of MOSFET. MOSFET. High losses especially for high voltage device due to RDS(ON) . • Dominant in high frequency application (>100kHz). Biggest application is in switched-
Insulated Gate Bipolar Transistor (IGBT)
• Combination of BJT and MOSFET characteristics. Compromises Compromises include:
–
Gate behaviour similar to MOSFET - easy to turn on and off.
–
Low losses like BJT due to low on-state Collector-Emitter voltage (2-3V).
Insulated Gate Bipolar Transistor (IGBT) • Ratings: Voltage: V <3.3kV, Current,: I <1.2kA CE
C
currently available. Work in under progress for 4.5kV/1.2kA device. Constant improvement in voltage and current ratings. • Good switching capability (up to 100KHz) for newer devices. Typical application, IGBT is used at 20-50KHz. • For very high power devices and applications, frequency is limited to several KHz. • Very popular in new products; practically replacing BJT in most new applications.
“Snubberless” operation is possible. Most new
Gate turn-off thyristor (GTO)
• Behave like normal thyristor, but can be turned off using gate signal • However turning off is difficult. difficult. Need very large reverse gate current (normally 1/5 of anode current)
Gate turn-off thyristor (GTO) • Ratings: Voltage: V ak <5kV; Current: Ia<5kA. Highest power ratings switch. Frequency<5KHz. • Gate drive design is very difficult. difficult. Need very large reverse gate current to turn off. Often customtailored to specific application. • Currently getting very stiff competition from high power IGBT. IGBT. The latter has much simpler and cheaper drivers. • GTO normally requires snubbers. snubbers. High power snubbers are expensive. • In very high power region (>5kV, >5kA),
Switches comparisons (2000) Device Type
Year made
Rated Voltage
Rated Current
Switching Frequency
SCR
1957
6k V
3.5k A
500Hz
GTO
1962
4.5kV
3k A
BJT
1960s
1.2kV
MOSFET
1976
IGBT
1983
Rated Power
Drive Circuit
Comments
100s MW
Sim ple
Cannot turn-off using gate signal
2k H z
10s MW
Very Difficult
400A
5k H z
1 MW
Difficult
Phasing out in new product
500V
200A
1MHz
100 k W
Very Simple
Good performance in high frequency
3.3kV
1.2k A
100kHz
100s k W
Very Simple
Best overall performance
King in very high power
Application examples • For each of the following application, choose the best power switches and reason out why. 3. An inverter inverter for the light-r light-rail ail train train (LRT) (LRT) locomotive locomotive operating from a DC supply of 750 V. The locomotive is rated at 150 kW. The induction motor is to run from standstill up to 200 Hz, with power switches frequencies up to 10KHz. 5. A switch-m switch-mode ode power power supply supply (SMPS) (SMPS) for remote remote telecommunication equipment is to be developed. The input voltage is obtained from a photovoltaic array that produces a maximum output voltage of 100 V and a minimum current of 200 A. The switching frequency should be higher than 100kHz. 7. A HVDC transmi transmission ssion system system transmitti transmitting ng power power of 300 MW from one ac system to another ac system both
Questions?????
