INTRODUCTION TO
POWER ELECTRONICS I
• Definition and concepts
• Application
• Power semiconductor
switches
• Gate/base drivers
• Losses
• General Information and
Applications
1
Prof. Dr. Ahmet KARAARSLAN
Definition of Power Electronics
DEFINITION:
To convert, i.e to process and control the flow of
electric power by supplying voltage s and currents in a
form that is optimally suited for user loads.
• Basic block diagram
POWER
INPUT
vi , i i
POWER
OUTPUT
Power
Processor
Source
vo , i o
Load
Controller
measurement
reference
• Building Blocks:
– Input Power, Output Power
– Power Processor
– Controller
2
Power electronic converters provide the
necessary adaptation functions to integrate all
different components into a common system.
Classification of power converters
3
The interdisciplinary nature
Control, energy
and circuits are
interrelated with
PE
Relation with multiple disciplines
Power electronics is currently the most active
discipline in electric power engineering worldwide.
4
The History of PE
The thread of the power electronics history
precisely follows and matches the break-through
and evolution of power electronic devices
DISADVANTAGES of PE
•Produce harmonics in the supply system &
controlled system
•Interference with communication system
•Produce low power factor at low voltage
5
Power Electronics (PE) Systems
• To convert electrical energy from one form to
another, i.e. from the source to load with:
– highest efficiency,
– highest availability
– highest reliability
– lowest cost,
– smallest size
– least weight.
• 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
• Drive applications
– intimately contains moving or rotating
components such as motors.
– Examples:
• Electric trains, Electric vehicles, Airconditioning System, Pumps, Compressor,
Conveyer Belt (Factory automation).
6
Application examples
Static Application: DC Power Supply
AC voltage
DIODE
RECTIFIER
DC-DC
CONVERTER
FILTER
AC LINE
VOLTAGE
(1F or 3F )
LOAD
Vcontrol
(derived from
feedback circuit)
Drive Application: Air-Conditioning System
Power Source
Power
Electronics
Converter
Desired
temperature
Desired
humidity
System
Controller
Variable speed drive
Motor
Indoor temperature
and humidity
Air
conditioner
Temperature and
humidity
Building
Cooling
Indoor
sensors
7
Power Conversion Concept
Vs (Volt)
• Supply : 50Hz, 220V
RMS (312V peak).
Customer need DC
voltage for welding
purpose, say.
time
• Sine-wave supply
gives zero DC
component!
• We can use simple
uncontrolled halfwave rectifier. A fixed
DC voltage is now
obtained. This is a
simple PE system.
Average output vol tage :
Vm
Vo =
+
Vs
_
+
Vo
_
Vo
Vdc
time

Average and effective(rms) value of any function:
f avg
1
=
T
f rms =
T
 f (t )
0
1
T
T
 f (t )
0
2
dt
8
Conversion Concept
How if customer wants variable DC voltage?
More complex circuit using controlled switches are required.
vs
ig
t
ia
vo
+
vs
_
+
vo
_
t
ig
Average output vol tage :

t
Vm
1 
(
)
1 + cos  
Vo =
Vm sin t dt =

2 
2
By controlling the firing angle, ,the output DC
voltage (after conversion) can be varied..
Obviously this needs a complicated electronic
system to set the firing current pulses for the SCR.
9
Power Electronics Converters
AC to DC: RECTIFIERS
AC input
DC output
DC to DC: CHOPPERS
DC input
DC output
DC to AC: INVERTERS
DC input
AC output
AC to AC: CHOPPERS
DC input
AC output
10
Current issues
1. Energy scenario
• Need to reduce dependence on fossil fuel
– coal, natural gas, oil, and nuclear power resource
Depletion of these sources is expected.
• Tap renewable energy resources:
– solar, wind, fuel-cell, ocean-wave
• Energy saving by PE applications. Examples:
– Variable speed compressor air-conditioning system:
30% savings compared to thermostat-controlled
system.
– Lighting using electronics ballast boost efficiency of
fluorescent lamp by 20%.
2. Environment issues
• Nuclear safety.
– Nuclear plants remain radioactive for thousands of
years.
• Burning of fossil fuel
– emits gases such as CO2, CO (oil burning), SO2, NOX
(coal burning) etc.
– Creates global warming (green house effect), acid rain
and urban pollution from smokes.
• Possible Solutions by application of PE. Examples:
– Renewable energy resources.
– Centralization of power stations to remote non-urban
area. (mitigation).
– Electric vehicles.
11
Power semiconductor devices
(Power switches)
• Power switches:
I
• Operates in two states:
– Fully on. i.e.
switch closed.
– Conducting state
– Fully off , i.e.
switch opened.
– Blocking state
• Power switch never
operates in linear
mode.
Vswitch= 0
Vin
SWITCH ON (fully closed)
I=0
Vswitch= Vin
Vin
SWITCH OFF (fully opened)
• Can be categorised into three groups:
– Uncontrolled: Diode :
– Semi-controlled: Thyristor (SCR).
– Fully controlled: Power transistors: e.g. BJT,
MOSFET, IGBT, GTO
12
Photos of Power Switches
(From Powerex Inc.)
• Power Diodes
– Single Type
– Stud type
– “Hockey-puck”
type
• BJT
– Single Type
– Module type:
Full bridge and
three phase
• SRC:Thyristor
- Single Type
13
• TRIAC
– Single Type
• MOSFET
– Single Type
– Integrated with its
driver
• IGBT
– Single Type
– Module type: Full
bridge and three
phase
14
GENERAL
INFORMATION
• Quantities and Units
– Units of Measurement
– Scientific Notation
– Metric Unit Conversions
• Voltage, Current, Charge and Resistance
–
–
–
–
–
Atomic Structure
Electrical Charge
Voltage, Current, and Resistance
Resistors
The Electric Circuit
15
• Ohm's Law
–
–
–
–
–
The Relationship of Current, Voltage, and
Resistance
Calculating Current
Calculating Voltage
Calculating Resistance
• Series and Parallel Circuits
• Semiconductor materials
• Diode Applications
16
Quantities and Units
17
Quantities and Units
• SI: Units and Prefixes:
Any measurement can be expressed in terms
of a unit, or a unit with a “prefix”
modifier.
FACTOR
NAME
SYMBOL
10-9
nano
n
10-6
micro
μ
10-3
milli
m
103
kilo
k
106
mega
M
Example: 12.3 mW = ………. W =…… x 10-2 W
18
Charge
• charge is conserved: it is neither created
nor destroyed
• symbol: Q or q; units are coulomb (C)
• the smallest charge, the electronic charge,
is carried by an electron (−1.602×10-19 C)
or a proton (+1.602×10-19 C)
• in most circuits, the charges in motion are
electrons
Current is the rate of charge flow:
1 ampere = 1 coulomb/second (or 1 A = 1
C/s)
19
Current and Charge
• Current (designated by I or i) is the rate of
flow of charge
• Current must be designated with both a
direction and a magnitude
• These two currents are the same:
20
Current and Charge:
i=dq/dt
21
Power: p = v i
The power required to
push a current i (C/s)
into a voltage v (J/C)
is p = vi ( J/s = W).
When power is
positive, the element
is absorbing energy.
When power is
negative, the element
is supplying energy.
22
Example: Power
How much power is absorbed by the three
elements above?
Pa = + 6 W, Pb = +6 W, Pc = -20 W.
(Note: (c) is actually supplying power)
23
Circuit Elements
• A circuit element
usually has two
terminals (sometimes
three or more).
• The relationship
between the voltage v
across the terminals
and the current i
through the device
defines the circuit
element model.
24
Voltage Sources
• An ideal voltage source is a circuit element
that will maintain the specified voltage vs
across its terminals.
• The current will be determined by other
circuit elements.
25
Current Sources
• An ideal current source is a circuit element
that maintains the specified current flow is
through its terminals.
• The voltage is determined by other circuit
elements.
26
Battery as Voltage Source
• A voltage source is an idealization (no limit on
current) and generalization (voltage can be timevarying) of a battery.
• A battery supplies a constant “dc” voltage V
but in practice a battery has a maximum power.
Dependent Sources
Dependent current sources (a) and (b) maintain
a current specified by another circuit variable.
Dependent voltage sources (c) and (d) maintain
a voltage specified by another circuit variable.
27
Ohm’s Law: Resistance
• A (linear) resistor is an element
for which
• v=iR
• where the constant R is a
resistance.
• The equation is known as
“Ohm’s Law.”
• The unit of resistance is ohm
(Ω).
28
Resistors
(a) typical resistors (b) power resistor
(c) a 10 TΩ resistor (d) circuit symbol
29
The i-v Graph for a
Resistor
For a resistor, the plot of current
versus voltage is a straight line:
In this example,
the slope is 4 A /
8 V or 0.5 Ω-1.
This is the graph
for a 2 ohm
resistor.
30
Power Absorption
Resistors absorb power: since
v=iR
p=vi = v2/R = i2R
Positive power means the device is
absorbing energy.
Power is always positive for a resistor!
31
Example: Resistor
Power
A 560 Ω resistor is connected to a
circuit which causes a current
of 42.4 mA to flow through it.
Calculate the voltage across the
resistor and the power it is
dissipating?
v = iR = (0.0424)(560) = 23.7 V
p = i 2R = (0.0424)2(560) = 1.007
W
32
Wire Gauge and
Resistivity
The resistance of a wire is determined by
the resistivity of the conductor as well as
the geometry:
R=ρl/A
[In most cases, the resistance of wires
can be assumed to be 0 ohms.]
33
Conductance
• We sometimes prefer to work with the
reciprocal of resistance (1/R), which is
called conductance (symbol G, unit
siemens (S)).
• A resistor R has conductance G=1/R.
• The i-v equation (i.e. Ohm’s law) can
be written as
i=Gv
34
Open and Short Circuits
• An open circuit between A and B
means i=0.
• Voltage across an open circuit: any
value.
• An open circuit is equivalent to R = ∞
Ω.
• A short circuit between A and B
means v=0.
• Current through a short circuit: any
value.
• A short circuit is equivalent to R = 0
Ω.
35
Battery:
The battery has two terminals
labeled positive (+) and negative (-)
– the most negative voltage region
in the circuit, often the negative end
of the battery, is sometimes called
“ground” or 0 volts – the amount of
push the battery supplies is the
“battery voltage”, often 1.5 V or 9
V or 12 V (with respect to ground)
– as a battery gets worn out (or if it
gets too cold!) its voltage will go
down until the battery is too weak
to continue to push current through
the circuit
symb
ol
36
Wire and Switch:
Wire: provides a path through
which electrical current can
flow – ideally a wire has no
resistance
Switch: place where a current path
can be mechanically opened and
closed, to start or stop the flow of
electrical current – switches are
used to turn things ON and OFF –
place the switch in series with the
component(s) it is meant to
control, like a battery
symb
ol
symb
ol
37
Capacitor:
serves as a place to temporarily
store electrical charge, like a
temporary battery – “charge it
up” (store electrical charge)
then “discharge it”
(temporarily produce electrical
current) – capacitance is
measured in Farads (F) –
electrolytic capacitors are ones
in which it matters which way
+ and – are connected
symb
ol
38
Diode:
Diode: serves as a one-way
valve, only allowing current to
flow one direction under normal
circumstances – an LED (light
emitting diode) is a diode (often
red or green) that glows when
current flows through it – diodes
must be inserted the right way
around for the circuit to operate
correctly
symb
ols
39
Voltage
regulator:
A chip that can be powered by a
range of voltages but uses internal
circuitry to drop the voltage to
output a very stable voltage (e.g. a
“5 V regulator” might be able to
able to be able to run off any
voltage from 6 V up to 20 V, but it
always outputs exactly 5 V) – this
is handy for providing a constant
voltage to components even when
dealing with batteries that can vary
in voltage and circuits that can
vary in overall resistance
40
IC (Integrated Circuit
chip):
IC (Integrated Circuit chip): a
silicon chip with many tiny
transistors on-board which can
be programmed to make
decisions (a microprocessor
chip), to store digital
information (a memory chip),
to convert digital input to
analog form (DAC), or vise
versa (ADC), etc. – connects to
other components through its
multiple legs, called pins – be
very careful never to put a chip
in backward!
41
Breadboard:
a board into which
components can be
plugged and unplugged,
allowing one to build
and check circuits
without having to be as
permanent as soldering
them together
42
PCB (Printed Circuit
Board):
Insulating board onto
which components can
be soldered, with
metallic traces etched
into the board to make
electrical connections
without having to use
external wires
43
Perf. Board (Perforated
(Circuit) Board):
Insulating board onto which
components can be soldered,
with no metallic traces etched
between holes like on a PCB –
using perf. board is more
permanent than using a
breadboard but you need to
connect components with
external wires
44
Transistor:
3-leg device used in logic
circuits so that a small/weak
electrical current at one point
can control a much
larger/more-powerful electrical
current elsewhere in the circuit
45
Sensor:
a device, often powered
using +5 V and ground (+0
V) connections, that has a
third output the voltage of
which varies predictably
and reproducibly as some
physical parameter changes
like temperature or air
pressure – needs to be
“calibrated” (i.e. the output
needs to be checked using
known physical conditions)
so output values can be
correctly interpreted
46
Socket:
a dummy set of receptacles
that matches the pins on a
chip – the socket is soldered
onto the board and the chip
snaps into it so that the chip
can be replaced (carefully!)
without resoldering if it goes
bad
Cable or Jumper:
a wire or set of parallel wires
connecting components
together – for example,
sensors often use a 3-wire
cable with the wires used for
+5 V, ground (+0 V), and
signal (output voltage)
47
Audio jack:
used to make a pull-beforeflight pin to start a flight
computer just before we let go
without having to open up a
payload box
Male and female headers:
used to allow quick
electrical
connections between
sensors, flight
computers, for
programming, etc.
48
Shrink wrap:
plastic insulation tubing one can slide over
exposed metal, like a solder joint, to insulate
it electrically from nearby wires – shrink
wrap contracts (shrinks!) when heated with a
heat gun – think ahead; you might need to put
the shrink wrap on before you do the
soldering
49
Digital Multimeter 1
• DMM is a measuring
instrument
• An ammeter measures
current
• A voltmeter measures the
potential
difference
(voltage) between two
points
• An ohmmeter measures
resistance
• A multimeter combines
these
functions,
and
possibly some additional
ones as well, into a single
instrument
50
Digital Multimeter 2
• Voltmeter
– Parallel connection
• Ammeter
– Series connection
• Ohmmeter
– Without
supplied
any
power
• Adjust range (start from
highest limit if you
don’t know)
51
Ammeter Connection
• Break the circuit so that the ammeter can be
connected in series
• All the current flowing in the circuit must pass
through the ammeter
• An ammeter must have a very LOW input
impedance
52
Voltmeter Connection
• The voltmeter is connected in parallel
between two points of circuit
• A voltmeter should have a very HIGH
input impedance
53
Ohmmeter
Connection
• An ohmmeter does not function with a circuit
connected to a power supply
• Must take it out of the circuit altogether and
test it separately
54
Using a multimeter to make
voltage measurements (AC and DC)
55
Using a multimeter to make
resistance measurements
56
• n type and p type materials.
57
Power Diode
Id
A (Anode)
Id
+
Vd
_
Vr
Vf
Vd
K (Cathode)
Diode: Symbol
v-i characteristics
• When diode is forward biased, it conducts current
with a small forward voltage (Vf) across it (0.2-3V)
• When reversed (or blocking state), a negligibly
small leakage current (uA to mA) flows until the
reverse breakdown occurs.
• Diode should not be operated at reverse voltage
greater than Vr
58
Types of Power Diodes
• Line frequency (general purpose):
– On state voltage: very low (below 1V)
– Large tr (about 25us) (very slow response)
– Very high current ratings (up to 5kA)
– Very high voltage ratings(5kV)
– Used in line-frequency (50/60Hz) applications
such as rectifiers
• Fast recovery
– Very low trr (<1us).
– Power levels at several hundred volts and
several hundred amps
– Normally used in high frequency circuits
• Schottky
– Very low forward voltage drop (typical 0.3V)
– Limited blocking voltage (50-100V)
– Used in low voltage, high current application
such as switched mode power supplies.
59
Thyristor (SCR)
Ia
A (Anode)
Ia
Ig
+
Vak
_
Ig>0
Vr
Ih
Ibo
Ig=0
G (Gate)
K (Cathode)
Thyristor: Symbol
Vak
Vbo
v-i characteristics
• If the forward breakover voltage (Vbo) is exceeded,
the SCR “self-triggers” into the conducting state. !
• The presence of gate current will reduce Vbo.
• “Normal” conditions for thyristors to turn on:
– the device is in forward blocking state (i.e Vak is
positive)
– a positive gate current (Ig) is applied at the gate
• Once conducting, the anode current is latched. Vak
collapses to normal forward volt-drop, typically
1.5-3V.
• In reverse -biased mode, the SCR behaves like a
diode.
60
Thyristor Conduction
ig
vs
ia
+
vs
_
+
vo
_
t
vo
t
ig

t
• Thyristor cannot be turned off by applying negative
gate current. It can only be turned off if Ia 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 current is “diverted” to another
circuitry.
61
Gate turn-off thyristor (GTO)
Ia
A (Anode)
Ia
+
Vak
_
G (Gate)
Ig>0
Vr
Ih
Ibo
Ig=0
I
g
K (Cathode)
GTO: Symbol
Vbo
Vak
v-i characteristics
• Behave like normal thyristor, but can be turned off
using gate signal
• However turning off is difficult. Need very large
reverse gate current (normally 1/5 of anode
current).
• Gate drive design is very difficult due to very large
reverse gate current at turn off.
•
• Ratings: Highest power ratings switch: Voltage:
Vak<5kV; Current: Ia<5kA. Frequency<5KHz.
62
Types of thyristors
• Phase controlled
– rectifying line frequency voltage and current
for ac and dc motor drives
– large voltage (up to 7kV) and current (up to
4kA) capability
– low on-state voltage drop (1.5 to 3V)
• Inverter grade
– used in inverter and chopper
– Quite fast. Can be turned-on using “forcecommutation” method.
• Light activated
– Similar to phase controlled, but triggered by
pulse of light.
– Normally very high power ratings
• TRIAC
– Dual polarity thyristors
63
Controllable switches
(power transistors)
• Can be turned “ON”and “OFF” by relatively
very small control signals.
• Operated in SATURATION and CUT-OFF
modes only.
• No “linear region” operation is allowed due to
excessive power loss.
• In general, power transistors do not operate in
latched mode.
• Traditional devices: Bipolar junction transistors
(BJT), Metal oxide silicon field effect transistor
( MOSFET), Insulated gate bipolar transistors
(IGBT), Gate turn-off thyristors (GTO)
• Emerging (new) devices: Gate controlled
thyristors (GCT).
64
Bipolar Junction Transistor (BJT)
C (collector)
IC
B (base)
IC
+
VCE
_
IB
IB
E (emitter)
BJT: symbol (npn)
VCE (sat)
VCE
v-i characteristics
• Ratings: Voltage: VCE<1000, Current: IC<400A.
Switching frequency up to 5kHz. Low on-state
voltage: VCE(sat) : 2-3V
• Low current gain (b<10). Need high base current
to obtain reasonable IC .
•
Expensive and complex base drive circuit. Hence
not popular in new products.
65
Bipolar Junction Transistor (BJT)
Con’t.
• BJT
– Single Type
– Module type:
Full bridge and
three phase
66
BJT Darlington pair
C (collector)
Driver
Transistor
IC1
IC Output
Transistor
IC2
B (base)
+
VCE
_
IB1
IB2
Biasing/
stabilising
network
E (emitter)
• Normally used when higher current gain is required
I c1 I c 2
b = I c I B1 = (I c1 + I c 2 ) I B1 =
+
I B1 I B1
 Ic2   I B2 
 I B1 + I c1 
= b1 + 
  
 = b1 + b 2  

 I B 2   I B1 
 I B1 
= b1 + b 2  (1 + b1 )
 b = b1 + b 2 + b1b 2
67
Metal Oxide Silicon Field Effect
Transistor (MOSFET)
D (drain)
ID
ID
G (gate)
+
VGS
_
+
VDS
_
+
VGS
_
VDS
S (source)
MOSFET: symbol
(n-channel)
v-i characteristics
• Ratings: Voltage VDS<500V, current IDS<300A.
Frequency f >100KHz. For some low power
devices (few hundred watts) may go up to MHz
range.
• Turning on and off is very simple.
– To turn on: VGS =+15V
– To turn off: VGS =0 V and 0V to turn off.
• Gate drive circuit is simple
68
MOSFET characteristics
• Basically low voltage device. High voltage device
are available up to 600V but with limited current.
Can be paralleled quite easily for higher current
capability.
• Internal (dynamic) resistance between drain and
source during on state, RDS(ON), , limits the power
handling capability of MOSFET. High losses
especially for high voltage device due to RDS(ON) .
• Dominant in high frequency application (>100kHz).
Biggest application is in switched-mode power
supplies.
69
Insulated Gate Bipolar
Transistor (IGBT)
C (collector)
IC
G (gate)
+
VGE _
IC
+
VCE
_
E (emitter)
IGBT: symbol
VGE
VCE (sat)
VCE
v-i characteristics
• Combination of BJT and MOSFET characteristics.
– Gate behaviour similar to MOSFET - easy to turn on
and off.
– Low losses like BJT due to low on-state CollectorEmitter voltage (2-3V).
• Ratings: Voltage: VCE<3.3kV, Current,: IC<1.2kA
currently available. Latest: HVIGBT 4.5kV/1.2kA.
• Switching frequency up to 100KHz. Typical
applications: 20-50KHz.
70
Power Switches: Power Ratings
1GW
Thyristor
10MW
GTO
10MW
1MW
IGBT
100kW
10kW
MOSFET
1kW
100W
10Hz
1kHz
100kHz 1MHz
10MHz
71
(Base/gate) Driver circuit
Control
Driver
Circuit
Circuit
Power
switch
• Interface between control (low power electronics)
and (high power) switch.
• Functions:
– Amplification: amplifies control signal to a
level required to drive power switch
– Isolation: provides electrical isolation between
power switch and logic level
• Complexity of driver varies markedly among
switches.
– MOSFET/IGBT drivers are simple
– GTO and BJT drivers are very complicated and
expensive.
72
Amplification: Example:
MOSFET gate driver
From control
circuit
+VGG
+
R1
Rg
D
G
VDC
Q1
+
LM311
S
VGS
_
_
• Note: MOSFET requires VGS =+15V for turn on
and 0V to turn off. LM311 is a simple amp with
open collector output Q1.
• When B1 is high, Q1 conducts. VGS is pulled to
ground. MOSFET is off.
• When B1 is low, Q1 will be off. VGS is pulled to
VGG. If VGG is set to +15V, the MOSFET turns on.
• Effectively, the power to turn-on the MOSFET
comes form external power supply, VGG
73
Isolation
R1
+
ig
vak
-
Pulse source
R2
iak
Isolation using Pulse Transformer
From control
circuit
D1
Q1
A1
To driver
Isolation using Opto-coupler
74
Switches comparisons (2003)
Thy
BJT
FET
GTO
IGBT
IGCT
Availabilty
Early
60s
Late 70s
Early
80s
Mid 80s
Late 80s
Mid 90’s
State of
Tech.
Mature
Mature
Mature/
improve
Mature
Rapid
improve
Voltage
ratings
5kV
1kV
500V
5kV
3.3kV
Rapid
improvem
ent
6.5kV
Current
ratings
4kA
400A
200A
5kA
1.2kA
4kA
Switch
Freq.
na
5kHz
1MHz
2kHz
100kHz
1kHz
On-state
Voltage
2V
1-2V
I* Rds
(on)
2-3V
2-3V
3V
Drive
Circuit
Simple
Difficult
Very
simple
Very
difficult
Very
simple
Simple
Comm-ents
Cannot
turn off
using gate
signals
Phasing
out in new
product
Good
performan
ce in high
freq.
King in
very high
power
Best
overall
performanc
e.
Replacing
GTO
75
Application examples
• For each of the following application, choose the
best power switches and reason out why.
– An inverter for the light-rail train (LRT) 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.
– A switch-mode power supply (SMPS) for 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.
– A HVDC transmission system transmitting power of
300 MW from one ac system to another ac system
both operating at 50 Hz, and the DC link voltage
operating at 2.0 kV.
76
Power switch losses
• Why it is important to consider losses of power
switches?
– to ensure that the system operates reliably under
prescribed ambient conditions
– so that heat removal mechanism (e.g. heat
sink, radiators, coolant) can be specified. losses
in switches affects the system efficiency
• Heat sinks and other heat removal systems are
costly and bulky. Can be substantial cost of the total
system.
• If a power switch is not cooled to its specified
junction temperature, the full power capability of
the switch cannot be realised. Derating of the power
switch ratings may be necessary.
• Main losses:
– forward conduction losses,
– blocking state losses
– switching losses
77
Heat Removal Mechanism
Fin-type Heat
Sink
SCR (stud-type) on
air-cooled kits
SCR (hokey-pucktype) on power
pak kits
Assembly of power
converters
78
Forward conduction loss
Ion
Ion
+Von-
+Von-
Ideal switch
Real switch
Ideal switch:
– Zero voltage drop across it during turn-on (Von).
– Although the forward current ( Ion ) may be
large, the losses on the switch is zero.
• Real switch:
– Exhibits forward conduction voltage (on state)
(between 1-3V, depending on type of switch)
during turn on.
– Losses is measured by product of volt-drop
across the device Von with the current, Ion,
averaged over the period.
• Major loss at low frequency and DC
79
Blocking state loss
• During turn-off, the switch blocks large voltage.
• Ideally no current should flow through the switch.
But for real switch a small amount of leakage
current may flow. This creates turn-off or blocking
state losses
• The leakage current during turn-off is normally
very small, Hence the turn-off losses are usually
neglected.
80
Switching loss
v
i
v
P=vi
i
Energy
time
time
Ideal switching profile
(turn on)
Real switching profile
(turn-on)
• Ideal switch:
– During turn-on and turn off, ideal switch requires
zero transition time. Voltage and current are
switched instantaneously.
– Power loss due to switching is zero
• Real switch:
– During switching transition, the voltage requires time
to fall and the current requires time to rise.
– The switching losses is the product of device
voltage and current during transition.
• Major loss at high frequency operation
81
Snubbers
+VL-
Vce
Ls
i
+
Vin
-
+
Vce
-
Vce rated
time
Simple switch at turn off
• PCB construction, wire loops creates stray
inductance, Ls.
• Using KVL,
di
vin = vs + vce = Ls + vce
dt
di
vce = vin - Ls
dt
since di dt is negative (turning off)
di
vce = vin + Ls
dt
82
RCD Snubbers
• The voltage across the switch is bigger than the
supply (for a short moment). This is spike.
• The spike may exceed the switch rated blocking
voltage and causes damage due to over-voltage.
• A snubber is put across the switch. An example of a
snubber is an RCD circuit shown below.
• Snubber circuit “smoothened” the transition and
make the switch voltage rise more “slowly”. In
effect it dampens the high voltage spike to a safe
value.
Vce
Ls
+
Vce
-
Vce rated
time
83
Snubbers
• In general, snubbers are used for:
– turn-on: to minimise large overcurrents
through the device at turn-on
– turn-off: to minimise large overvoltages across
the device during turn-off.
– Stress reduction: to shape the device switching
waveform such that the voltage and current
associated with the device are not high
simultaneously.
• Switches and diodes requires snubbers. However,
new generation of IGBT, MOSFET and IGCT do
not require it.
84
Ideal vs. Practical power switch
Ideal switch
Practical switch
Block arbitrarily large
forward and reverse
voltage with zero
current flow when off
Finite blocking voltage
with small current flow
during turn-off
Conduct arbitrarily
large currents with
zero voltage drop
when on
Finite current flow and
appreciable voltage drop
during turn-on (e.g. 2-3V
for IGBT)
Switch from on to off
or vice versa
instantaneously when
triggered
Requires finite time to
reach maximum voltage
and current. Requires
time to turn on and off.
Very small power
required from control
source to trigger the
switch
In general voltage driven
devices (IGBT,
MOSFET) requires small
power for triggering.
GTO requires substantial
amount of current to turn
off.
85