1 Introduction 2009

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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3

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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4

PE - Scope and Applications

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5

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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6

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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7

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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9

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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12

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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13

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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15

PE Application-Electric

vehicles

World fastest

electric car

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16

PE – wind & tidal wave

energy

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17

PE – Home Appliances

Inverter microwave

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18

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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19

Interdisciplinary Nature of

Power Electronics

Application of electronic semiconductor

devices and circuits in the conversion

and control of electrical power.

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20

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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21

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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22

Photo of Power Switches

SITH

THYRISTORS

(SCR)

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23

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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24

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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26

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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30

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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31

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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32

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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33

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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34

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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35

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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36

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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37

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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38

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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39

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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40

Switches idealized

characteristic

DIODE SCR

BJT MOSFET

MCT

IGBT

GTO

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41

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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42

Power Switches: Device

capabilities

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43

(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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44

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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45

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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46

Driver Circuit

Some gate drive circuit for IGBTs

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47

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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48

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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50

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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53

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.

Documents

1 Introduction 2009