Parte 3
Elettronica di potenza:
a. Sistemi elettronici di potenza – cap. 1
M h
Mohan.
b. Interruttori elettronici di potenza – cap.
2 Mohan.
c. Raddrizzatori non controllati – cap. 5
Mohan
Mohan.
d. Raddrizzatori controllati – cap. 6
Mohan.
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by John Wiley & Sons, Inc.
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Chapter 1 Power Electronic Systems
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1
Power Electronic Systems
• Block diagram
• Role of Power Electronics
• Reasons for growth
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1-3
Linear Power Supply
• Series transistor as an adjustable resistor
• Low Efficiency
• Heavy and bulky
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2
Switch-Mode Power Supply
• Transistor as a switch
• High Efficiency
• High-Frequency Transformer
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Basic Principle of Switch-Mode Synthesis
• Constant switching frequency
• pulse width controls the average
• L-C filters the ripple
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3
Application in Adjustable Speed Drives
•
•
Conventional drive wastes energy across the throttling
valve to adjust flow rate
Using power electronics, motor-pump speed is adjusted
efficiently to deliver the required flow rate
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Scope and Applications
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4
Power Processor as a Combination of
Converters
• Most practical topologies require an energy
storage element, which also decouples the input
and the output side converters
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Power Flow through Converters
• Converter is a g
general term
• An ac/dc converter is shown here
• Rectifier Mode of operation when power from ac to dc
• Inverter Mode of operation when power from ac to dc
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5
AC Motor Drive
• Converter 1 rectifies line-frequency ac into dc
• Capacitor acts as a filter; stores energy; decouples
• Converter 2 synthesizes low-frequency ac to motor
• Polarity of dc-bus voltage remains unchanged
– ideally suited for transistors of converter 2
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Matrix Converter
• Very general structure
• Would benefit from bi-directional and bi-polarity switches
• Being considered for use in specific applications
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6
Interdisciplinary Nature of Power Electronics
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Chapter 2 Overview of Power
Semiconductor Devices
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7
Diodes
• On and off states controlled by the power circuit
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Diode Turn-Off
• Fast-recovery diodes have a small reverse-recovery time
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8
Thyristors
• Semi-controlled device
• Latches ON by a gate-current pulse if forward biased
• Turns-off if current tries to reverse
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Thyristor in a Simple Circuit
•
For successful turn-off, reverse voltage required
for an interval greater than the turn-off interval
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9
Generic Switch Symbol
• Idealized switch symbol
• When
Wh on, currentt can flow
fl
only
l in
i the
th direction
di ti off the
th arrow
• Instantaneous switching from one state to the other
• Zero voltage drop in on-state
• Infinite voltage and current handling capabilities
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Switching Characteristics (linearized)
Switching Power Loss is
proportional
p
p
to:
• switching frequency
• turn-on and turn-off times
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10
Bipolar Junction Transistors (BJT)
• Used commonly in the past
• Now used in specific applications
• Replaced by MOSFETs and IGBTs
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Various Configurations of BJTs
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11
MOSFETs
• Easy to control by the gate
• Optimal for low-voltage operation at high switching frequencies
• On-state resistance a concern at higher voltage ratings
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Gate-Turn-Off Thyristors (GTO)
• Slow switching speeds
• Used at very high power levels
• Require elaborate gate control circuitry
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12
GTO Turn-Off
• Need a turn-off snubber
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IGBT
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13
Comparison of Controllable Switches
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Summary of Device Capabilities
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14
Chapter 5
Diode Rectifiers
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Diode Rectifier Block Diagram
• Uncontrolled utility interface (ac to dc)
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15
A Simple Circuit
• Resistive load
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A Simple Circuit (R-L Load)
• Current continues to flows for a while even after the input
voltage has gone negative
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16
A Simple Circuit (Load has a dc back-emf)
• Current begins to flow when the input voltage exceeds the dc back-emf
• Current continues to flows for a while even after the input voltage has
gone below the dc back-emf
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Single-Phase Diode Rectifier Bridge
• Large capacitor at the dc output for filtering and energy
storage
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17
Diode-Rectifier Bridge Analysis
• Two simple (idealized) cases to begin with
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Redrawing Diode-Rectifier Bridge
• Two groups, each with two diodes
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18
Waveforms with a
purely resistive load
and a purely dc current
at the output
• In both cases, the dc-side
voltage waveform is the same
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Diode-Rectifier Bridge Input Current
• Idealized case with a purely dc output current
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19
Diode-Rectifier Bridge Analysis with ACSide Inductance
• Output current is assumed to be purely dc
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Understanding Current Commutation
• Assuming inductance in this circuit to be zero
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20
Understanding Current Commutation (cont.)
• Inductance in this circuit is included
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Current Commutation Waveforms
• Shows the volt-seconds needed to commutate current
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21
Current Commutation in Full-Bridge Rectifier
• Shows the necessary volt-seconds
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Understanding Current Commutation
• Note the current loops for analysis
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22
Rectifier with a dcside voltage
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Diode-Rectifier with a Capacitor Filter
• Power electronics load is represented by an equivalent load
resistance
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23
Diode Rectifier Bridge
• Equivalent circuit for analysis on one-half cycle basis
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Diode-Bridge Rectifier: Waveforms
• Analysis using MATLAB
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24
Diode-Bridge Rectifier: Waveforms
• Analysis using PSpice
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Input Line-Current Distortion
• Analysis using PSpice
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25
Line-Voltage Distortion
• PCC is the point of common coupling
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Line-Voltage Distortion
• Distortion in voltage supplied to other loads
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26
Chapter 6
Thyristor Converters
• Controlled conversion of ac into dc
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Thyristor Converters
• Two-quadrant conversion
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27
Primitive circuits
with thyristors
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Thyristor Triggering
• ICs available
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28
Full-Bridge Thyristor Converters
• Single-phase and three-phase
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Single-Phase Thyristor Converters
• Two groups with two thyristor each
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1-Phase Thyristor Converter Waveforms
• Assuming zero ac-side inductance
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Average DC Output Voltage
• Assuming zero ac-side inductance
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30
Input Line-Current Waveforms
• Harmonics, power and reactive power
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1-Phase Thyristor Converter
• Finite ac-side inductance; constant dc output
current
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31
Thyristor Converter Waveforms
• Finite ac-side inductance
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Thyristor Converter: continuous Mode
• This mode can occur in a dc-drive at light loads
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Thyristor Converter Waveforms
• PSpice-based simulation
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Thyristor Converter Waveforms:
Discontinuous Conduction Mode
• PSpice-based simulation
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33
Thyristor Converters: Inverter Mode
• Assuming the ac-side inductance to be zero
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