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15/01/2009 SEE 4433/5433
Dr. Awang /Dr. Zainal
1
Chapter 1
INTRODUCTION TO
POWER ELECTRONICS
SYSTEMS
• Definition and concepts
• Application
• Power semiconductor switches
• Gate/base drivers
• Losses
• Heat sinks
• Snubbers
• Safe operating area (SOA)
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2
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.
• Basic block diagram
• Building Blocks:
– Input Power (Source), Output Power (Load)
– Power Processor
– Controller
Power
Processor
Controller
Load
measurement
reference
POWER
INPUTPOWER
OUTPUTvi , ii vo , io
SourceControl
signal
* power electronic converter
*
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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, Electronic Welding, Heating, Lighting, Cooling, Electronic ballast, PV and Fuel Cell Conversion, VAR and Harmonic Compensation
• Drive applications
– intimately contains moving or rotating components such as motors.
– Examples: • Electric trains, Electric vehicles, Air-
conditioning System, Pumps, Compressor, Conveyer Belt (Factory automation).
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PE - Scope and Applications
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Application examples
Static Application: DC Power Supply
FILTER LOADDC-DC
CONVERTER
DIODE
RECTIFIER
AC voltage
AC LINE
VOLTAGE(1 or 3 )Φ Φ
Vcontrol
(derived from
feedback circuit)
System
Controller
Power
Electronics
Converter
Motor Air
conditioner
Power Source
Building
Cooling
Desired
temperature
Indoor
sensors
Indoor temperature
and humidity
Temperature and
humidity
Desired
humidity
Variable speed drive
Drive Application: Air-Conditioning System
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WHY POWER ELECTRONIC IS
SO IMPORTANT TODAY?
• Electrical power conversion and control at
high frequency
• Apparatus at low cost, small size, high
reliability and long life
• Very important element in modern electrical
power processing and industrial process
control
• Fast growth in global energy consumption
• Environmental and safety problems by fossil
and nuclear power plants
• Increasing emphasis of energy saving and
pollution control by PE
• Growth of environmentally clean sources of
power that are PE intensive (Wind, PV and
Fuel Cell)
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PE vs. Linear Electronic
e.g. DC Power Supply
• Simple system
• Series transistor works as an adjustable resistor
• Low Efficiency (high losses) – can drop up to 40%
• Suitable for low power
• Heavy and bulky (transformer & magnetic components)
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• Transistor works as a switch (ON, OFF)
• High Efficiency ( > 90 %)
• High-Frequency transformer
• Multi-output voltage application
• Low cost, small size , light weight
PE vs. Linear Electronics
e.g. DC Power Supply
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Power Conversion concept:
example
• Supply from TNB:
50Hz, 240V RMS
(340V peak).
Customer need DC
voltage for welding
purpose, say.
• TNB sine-wave
supply gives zero DC
component!
• We can use simple
half-wave rectifier. A
fixed DC voltage is
now obtained. This is
a simple PE system.
time
Vs (Volt)
Vo
time
Vdc
+
Vo
_
+
Vs
_
πm
oV
V =
:tageoutput vol Average
340 V
Simple power
processor
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10
Conversion Concept
+
vo_
+
vs_
ig
ia
( ) [ ]απ
ωωπ
π
αcos1
2sin
2
1
:tageoutput vol Average
+== ∫ mmo
VtdtVV
How if customer wants variable DC voltage?
More complex circuit using SCR is required.
By controlling the firing angle, α,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.
ωt
vo
α
ig
ωt
ωt
v s
πα2π
A K
G
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11
Power Electronics Converters
AC input DC output
DC input AC output
1. AC to DC: RECTIFIER
2. DC to DC: CHOPPER
3. DC to AC: INVERTER
DC input DC output
4. AC to AC: CYCLOCONVERSION
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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:
Fact: 65% generated energy consumed by electrical drives – mainly by pumps and fans; and 20% used by lighting
– Variable speed compressor air-conditioning system controlled by PE more efficient: 30% savings compared to thermostat-controlled system.
– Lighting using electronics ballast (high freq. lamp) boost efficiency of fluorescent lamp by 20%.
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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.
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PE Application - BIPV
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PE Application-Electric
vehicles
World fastest
electric car
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PE – wind & tidal wave
energy
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PE – Home Appliances
Inverter microwave
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PE growth
• PE rapid growth due to:
– Technology advancement in power
(semiconductor) switches
– Advances in microelectronics (DSP, VLSI,
microprocessor/microcontroller, ASIC)
– New ideas in control algorithms
– Demand for new PE applications with
smaller size and lighter weight, new VSD
motors
• PE is an interdisciplinary field:
– Digital/analogue electronics
– Power and energy
– Microelectronics
– Control system
– Computer, simulation and software
– Solid-state physics and devices
– Packaging
– Heat transfer
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Interdisciplinary Nature of
Power Electronics
Application of electronic semiconductor
devices and circuits in the conversion
and control of electrical power.
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Power semiconductor devices
(Power switches)
• Power switches:
work-horses of PE
systems.
• 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.
POWER SWITCH
SWITCH OFF (fully opened)
Vin
Vswitch
= Vin
I=0
• Can be categorised into three groups:
– Uncontrolled: Diode :
– Semi-controlled: Thyristor (SCR).
– Fully controlled: Power transistors: e.g. BJT,
MOSFET, IGBT, GTO, IGCT
SWITCH ON (fully closed)
Vin
Vswitch
= 0
I
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Photos of Power Switches
(From Powerex Inc.)
• Power Diodes
– Stud type
– “Hockey-puck”type
• IGBT
– Module type: Full bridge and three phase
• IGCT
– Integrated with its driver
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Photo of Power Switches
SITH
THYRISTORS
(SCR)
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Power Diode
• 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
A (Anode)
K (Cathode)
+
Vd
_Id
Diode: Symbol v-i characteristics
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Reverse Recovery
• When a diode is switched quickly from forward to
reverse 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, trr(reverse recovery time) to recombine with opposite
charge and neutralise.
• Effects of reverse recovery are increase in switching
losses, increase in voltage rating, over-voltage
(spikes) in inductive loads
IF
IRM
VR
t0
t2
trr= ( t2 - t0 )
VRM
dif/dt
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Softness factor, Sr
IF
VR
t0
t2
Sr= ( t2 - t1 )/(t1 - t0)
= 0.8
t1
IF
VR
t0
Sr= ( t2 - t1 )/(t1 - t0)
= 0.3
t1 t2
Snap-off
Soft-recovery
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Types of Power Diodes
• Line frequency (general purpose):
– On state voltage: very low (below 1V)
– Large trr (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.
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THYRISTOR
• SCR was invented 1956 – Bell Telephone
Lab, then it was commercialized by GEC 1957
•It has a 4-layer p.n.p.n structure with 3
terminals ( anode, cathode, gate)
•Anode and cathode connected to main power
circuit. Gate terminal connected to low level
gate current
• Operation- VAK positive, J1 & J3 forward
biased, but J2 reverse biased- no conduction
(off-state) until VAK> VBO
• Applied gate voltage Vg to gate- lowered the
level of J2 reversed biased, so thyristor will
turn-on (conducting) . Current flows in
forward direction.
J1
J2
J3
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Thyristor (SCR)
• If the forward break over 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. (temperature dependant)
• In reverse -biased mode, the SCR behaves like a diode. Voltage blocking is bi-directional.
v-i characteristics
A (Anode)
K (Cathode)
+
Vak
_
Ia
Thyristor: Symbol
G (Gate)
Ig
Ia
Vak
Vr
Ig=0Ig>0
IhIbo
Vbo
IL
Forward break
over voltage
Reversed break
over voltage
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Thyristor Conduction
• 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/line commutation),
• Another method of turning off is known as “forced commutation”, ( i.e when supply is DC, e.g. Inverter )
– The anode current is “diverted” to another circuitry.
– Auxiliary energy in capacitor used to force anode current to zero.
+
vo_
+
vs_
ig
ia
ωt
vo
α
ig
ωt
ωt
v s
πα
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Types of thyristors
• Phase controlled
– rectifying line frequency voltage and current for ac and dc motor drives
– large voltage (up to 7 kV) and current (up to 4 kA) capability
– low on-state voltage drop (1.5 to 3V)
• Inverter grade
– used in inverter and chopper
– Quite fast. Can be turned-on using “force-commutation” method.
• Light-activated
– Similar to phase controlled, but triggered by pulse of light guided by optical fibre.
– Normally very high power ratings (HVDC)
• TRIAC
– Dual polarity thyristors ( 2 SCRs integrated in inverse-parallel )
– Frequently used in many low-power applications such as juice maker, blenders and vacuum cleaners
Give other factors that can
turn-on SCR !!!
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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). Or MCT (MOSFET
controlled Thyristor)
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Bipolar Junction Transistor (BJT)
• BJT invented in 1948, by 1960 substantial power handling capability
• Current Control Device (base current, Ib control Ic). It has 3-layer p.n.p or n.p.n structure with 2 junctions.
• Ratings: Voltage: VCE<1000, Current: IC<400A. Switching frequency up to 5kHz. Low on-state voltage: VCE(sat) : 2-3V
• Low current gain (β<10). Need high base current to obtain reasonable IC . (Increase current gain by Darlington Pairs – 100s)
• Expensive and complex base drive circuit. Hence not popular in new products. SOA- second breakdown-requires snubber )
IC
VCE
IB
v-i characteristics
VCE (sat)
BJT: symbol (npn)
+
VCE
_
IC
IB
C (collector)
B (base)
E (emitter)
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BJT Darlington pair
• Normally used when higher current gain is required
• Triple Darlington can be selected for more current gain, but with higher Vce drop and slow switching speed
( )
( )
2121
121
1
1121
1
2
2
21
1
2
1
11211
1
βββββ
βββ
βββ
β
++=⇒
+⋅+=
+⋅+=
⋅
+=
+=+==
B
cB
B
B
B
c
B
c
B
cBccBc
I
II
I
I
I
I
I
I
I
IIIIII
+
VCE
_
IC2
IB2
C (collector)
E (emitter)
IC
IB1
B (base)
IC1Driver
Transistor Output
Transistor
Biasing/
stabilising
network
β1
β2
ic= βib
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Metal Oxide Silicon Field Effect
Transistor (MOSFET)
• Available in 1970
• Three terminal device (GSD). Voltage control device
( Vgs control ID). No reverse voltage capability.
• 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 0 V to turn off.
• Gate drive circuit is simple
v-i characteristicsMOSFET: symbol
(n-channel)
+
VDS
_
ID
D (drain)
G (gate)
S (source)
+
VGS
_
ID
VDS
+
VGS
_
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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.
• Has positive temperature coefficient, results in non-existence of second breakdown. (avoiding the creation of hot spot)
• Has high input impedance, so easily connected to CMOS or TTL logic.
• 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 (>100kHz) and low power application . Biggest application is in switched-mode power supplies.
• CoolMOS ? (Rdson half of the NORMAL MOSFET for the same V & I rating - higher efficiency)
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Insulated Gate Bipolar
Transistor (IGBT)
• Developed 1980,widely available 1990s
• 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 Collector-Emitter voltage (2-3 V).
• Ratings: Voltage: VCE < 3.3 kV, Current,: IC < 1.2kA currently available. Latest: HVIGBT 4.5kV/1.2kA.
• Switching frequency up to 100 kHz. Typical applications: 20-50 kHz.
IC
VCE
VGE
v-i characteristics
VCE (sat)
IGBT: symbol
+
VCE
_
IC
C (collector)
G (gate)
E (emitter)
+
VGE _
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Gate turn-off thyristor (GTO)
• Behave like normal thyristor, but can be turned off using gate signal (applied negative voltage across VGK)
• However turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current) and longer off time (tail current). E.g. a 2500V, 1000A GTO requires a peak negative gate current of 250 A.
• Gate drive design is very difficult due to very large reverse gate current at turn off.
•
• Ratings: Highest power ratings switch: Voltage:Vak<5 kV; Current: Ia<5 kA. Frequency<2kHz.
• Very stiff competition:
Low end-from IGBT. High end from IGCT
G (Gate)
A (Anode)
K (Cathode)
+
Vak
_
Ia
GTO: Symbol
Ig
v-i characteristics
Ia
Vak
Vr
Ig=0Ig>0
IhIbo
Vbo
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Insulated Gate-Commutated
Thyristor (IGCT)
• Among the latest Power Switches.
• Conducts like normal thyristor (latching), but can be turned off using gate signal, similar to IGBT turn off; VGT of 20V is sufficient (fast rising turn-off pulse with very short duration)
• The gate drive requirement decrease by a factor of 5 compared to GTO.
• Power switch is integrated with the gate-drive unit.
• Ratings:
Voltage: Vak< 6.5 kV; Current: Ia< 4 kA. Frequency<1KHz. Currently 10kV device is being developed.
• Very low on state voltage: 2.7 V for 4 kA device
A (Anode)
IGCT: Symbol
K (Cathode)
+
Vak
_
Ia
Ig
IGCT
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MOS-CONTROLLED
THYRISTOR (MCT)
• New recently commercially available (Harris SemiCon.)
• Basically a Thyristor with two MOSFETs built into the gate structure (ON-FET turns ON MCT and OFF-FET turns OFF MCT)
• Turn on MCT by turning on M1
- Apply positive gate-cathode voltage
• Turn off MCT by turning on M2
- Apply negative gate-anode voltage
• Low on-state losses with high current capability
M1
M2
pnp
npn
a) Equivalent cct b) symbol
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Switches idealized
characteristic
DIODE SCR
BJT MOSFET
MCT
IGBT
GTO
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Power Switches: Power Ratings
10Hz 1kHz 1MHz100kHz 10MHz
1kW
100kW
10kW
10MW
1MW
10MW
1GW
100W
MOSFET
IGBT
GTO/IGCT
Thyristor
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Power Switches: Device
capabilities
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(Base/gate) Driver circuit
• 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
– Minimise switching losses ( by fast switching
transition)
• Complexity of driver varies markedly among
switches.
– MOSFET/IGBT drivers are simple
– GTO and BJT drivers are very complicated and
expensive.
Control
Circuit
Driver
Circuit
Power
switch
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Amplification: Example: Simple
MOSFET gate driver
• Note: MOSFET requires VGS = +15V for turn on and 0 V to turn off. LM311 is a simple op-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 from external power supply, VGG (inject (sourcing) enough current to charge MOSFET capacitor Cgs)
+
VDC
_
DG
S+
VGS
_
From control
circuit
+VGG
R1
Rg
LM311
Q1
(Comparator)
B1
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Isolation
+
vak-
iak
Pulse source
igR1
R2
Isolation using Pulse Transformer
From control
circuit To driverQ1 D1 A1
Isolation using opto-coupler
(optically isolated control)
Photo-transistorLED
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Driver Circuit
Some gate drive circuit for IGBTs
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Opto-Coupler Isolated
MOSFET Drives
Qsw
VGG+
Vd
Io
VGG-
CGG-
CGG+
RG
TB+
TB-
Df
Opto-coupler
AC
power
in
Signal
from control
electronics
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Switches comparisons (2003)
Thy
BJT
FET
GTO
IGBT
IGCT
Avail-
abilty Early
60s
Late 70s Early
80s
Mid 80s Late 80s Mid 90’s
State of
Tech. Mature Mature Mature/
improve
Mature Rapid
improve
Rapid
improvem
ent Voltage
ratings 5kV 1kV 500V 5kV 3.3kV 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
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Device Applications
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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
10 kHz.
– 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 100 kHz.
– 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.
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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
– Gate drive losses
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Heat Removal Mechanism
SCR (stud-type) on
air-cooled kits
Fin-type Heat
Sink
SCR (hokey-puck-
type) on power
pak kits
Assembly of power
converters
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Forward conduction loss
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 (between 1-3 V, depending on type of switch) during turn on.
– Losses (on state) 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
+Von−
Ion
Ideal switch
Ion +Von−
Real switch
Mosfet: I2.Rdson.D BJT: ic.Vce(sat).D+ib.Vbe(sat)
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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.
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Switching loss
• 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. For inductive load, the switch loss can be given as
• Major loss at high frequency operation
vi
time
Ideal switching profile
(turn on)
v i
time
Real switching profile
(turn-on)
P=vi
Energy
PL = 0.5VsIL(tr+tf)fs
where fs is switching frequency ,IL = load current, tf & tr = rise and
fall time of load current
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Switching power loss is proportional to:
• switching frequency
• turn-on and turn-off times
Switching Characteristics
(linearised)
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An example of heat sink
design
Say that a MOSFET is mounted on
heat sink. So what is the correct
heat sink size (ie Rhs-a)?
Let say Tj=100oC, Ta = 25oC, and total power
loss is PT = PD
PT = ∆T/Rth =(Tj-Ta)/Rth
Where Rth = total thermal resistance, = Rj-c+Rc-
hs+Rhs-a
Tj= Junction Temp,. Ta = ambient temp.
Rj-c and Rc-hs are given in the device(M)
datasheet, then the Rhs-a can be calculated
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An example of heat sink
design
Say that two diodes are mounted on
the same heat sink. So what is the
correct heat sink size (ie Rhs-a)?
D1 D2
Tj
Tc
Tins
Ta
Ths
Let say Tj=100oC, Ta = 25oC, and total power
loss is PT = PD1 + PD2
PT = ∆T/Rth =(Tj-Ta)/Rth
Where Rth = total thermal resistance,
Tj= Junction Temp,. PD = diode power loss
aRhshs)Rcc2(Rj
hs)Rcc(RjRth
2
−+−+−−+−
=
Rj-c and Rc-hs are given in the device
datasheet, then the Rhs-a can be calculated
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Snubbers
• PCB construction, wire loops creates stray
inductance, Ls.
• Using KVL,
time
Vce
Vce rated
+
Vin
−
Ls
+
Vce
−
+VL−
i
dt
diLvv
dtdi
dt
diLvv
vdt
diLvvv
since
since
cescesin
+=
−=
+=+=
off) (turning negative is since
Simple switch at turn off
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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 – turn-off snubber
• 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. Or an electrical circuit used to suppress ("snub") electrical transients.
+
Vce
−
Ls
time
Vce
Vce ratedC
RD
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Snubbers
• In general, snubbers are used for:
– turn-on: to minimize large over currents
through the device at turn-on (reduce dI/dt )
– turn-off: to minimize large over voltages across
the device during turn-off ( reduce dV/dt).
– 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.
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Snubber Circuit
e.g. Turn-off RCD snubber
dV/dt
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Safe Operating Area(SOA) - BJT
log ( v )CE
BVCEO
2nd
breakdown
2nd
breakdown
T j,max
ICM
10 sec
10 sec
10 sec-3
-4
-5
dc
switching trajectory of diode-
clamped inductive load circuitC
log( i )
Second Breakdown – when high voltage and high
current occurs simultaneously during turn-off, a hot
spot is formed & device failed by thermal runaway
Pmax = Vce x Ic
DC
Pulses (non-
shaded)
SOA is defined as the voltage, current and power
(temperature) conditions over which the device
can be expected to operate without self-damage.
Current limit
Voltage limit
Power limit
2 Second
Breakdown limit
(shaded)
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Safe Operating Area(SOA) -
MOSFET
DC
10 sec-3
10 sec-4
10 sec-5
IDM
BVDSS
Tj,max
iD
log ( )
vDS
log ( )
DC
pulses
Voltage
limit
Current limit
Power
limit
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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.