unit 1
TRANSCRIPT
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EE2301-POWER ELECTRONICS
UNIT I
Power Semiconductor Devices
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EE2301-POWER ELECTRONICS
Introduction
• What are Power Semiconductor Devices (PSD)? They are devices used as switches or rectifiers in
power electronic circuits
• What is the difference of PSD and low-power semiconductor device?
Large voltage in the off state High current capability in the on state
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Classification
Fig. 1. The power semiconductor devices family
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Important Parameters
• Breakdown voltage. • On-resistance. Trade-off between breakdown voltage and
on-resistance.• Rise and fall times for switching between on
and off states. • Safe-operating area.
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EE2301-POWER ELECTRONICS
Power MOSFET: Structure Power MOSFET has much higher current handling capability in
ampere range and drain to source blocking voltage(50-100V) than other MOSFETs.
Fig.2.Repetitive pattern of the cells structure in power MOSFET
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EE2301-POWER ELECTRONICS
Power MOSFET: R-V CharacteristicsAn important parameter of a power MOSFET is on resistance:
, whereon S CH DR R R R ( )CH
n ox GS T
LRW C V V
Fig. 3. Typical RDS versus ID characteristics of a MOSFET.
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EE2301-POWER ELECTRONICS
Thyristor: Structure• Thyristor is a general class of a four-layer pnpn
semiconducting device.
Fig.4 (a) The basic four-layer pnpn structure. (b) Two two-transistor equivalent circuit.
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EE2301-POWER ELECTRONICS
Three States:Reverse BlockingForward BlockingForward Conducting
Thyristor: I-V Characteristics
Fig.5 The current-voltage characteristics of the pnpn
device.
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EE2301-POWER ELECTRONICS
Applications Power semiconductor devices have widespread
applications:Automotive Alternator, Regulator, Ignition, stereo tapeEntertainment Power supplies, stereo, radio and televisionAppliance Drill motors, Blenders, Mixers, Air conditioners
and Heaters
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EE2301-POWER ELECTRONICS
Thyristors
• Most important type of power semiconductor device.
• Have the highest power handling capability.they have a rating of 1200V / 1500A with switching frequencies ranging from 1KHz to 20KHz.
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EE2301-POWER ELECTRONICS
• Is inherently a slow switching device compared to BJT or MOSFET.
• Used as a latching switch that can be turned on by the control terminal but cannot be turned off by the gate.
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EE2301-POWER ELECTRONICS
Different types of Thyristors
• Silicon Controlled Rectifier (SCR).• TRIAC.• DIAC.• Gate Turn-Off Thyristor (GTO).
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EE2301-POWER ELECTRONICS
SCR
Symbol of Silicon Controlled Rectifier
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EE2301-POWER ELECTRONICS
Structure
G ate Cathode
J 3
J 2
J 1
Anode
10 cm17 - 3
10 - 5 x 10 cm13 14 - 3
10 cm17 -3
10 cm19 - 3
10 cm19 - 3 10 cm19 -3n+ n+
p -
n–
pp+
10 m
30- 100 m
50- 1000 m
30- 50 m
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EE2301-POWER ELECTRONICS
Device Operation
Simplified model of a thyristor
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V-I Characteristics
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EE2301-POWER ELECTRONICS
Effects of gate current
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Two Transistor Model of SCR
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EE2301-POWER ELECTRONICS
1 1
1 1
1 1
1
1
Considering PNP transistor of the equivalent circuit,
, , ,
,
1 1
E A C C
CBO CBO B B
B A CBO
I I I I
I I I I
I I I
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EE2301-POWER ELECTRONICS
2 2 2
2 2
2 2
2
2
Considering NPN transistor of the equivalent circuit,
, ,
2
C C B B E K A G
C k CBO
C A G CBO
I I I I I I I I
I I I
I I I I
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EE2301-POWER ELECTRONICS
2 1
2 1 2
1 2
From the equivalent circuit, we see that
1
C B
g CBO CBOA
I I
I I II
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EE2301-POWER ELECTRONICS
1 2
1 2
Case 1: When 0
1
g
CBO CBOA
I
I II
2 1 2
1 2
Case 2: When 0
1
G
g CBO CBOA
II I I
I
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EE2301-POWER ELECTRONICS
Turn-on Characteristics
on d rt t t
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EE2301-POWER ELECTRONICS
Turn-off Characteristi
c
Anode currentbegins todecrease
tCtq
t
t
Commutation didt
Recovery Recombination
t1 t2 t3 t4 t5
tr r tgr
tqtc
V A K
I A
tq=device off tim etc=circuit off tim e
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EE2301-POWER ELECTRONICS
Methods of Thyristor Turn-on
• Thermal Turn-on.• Light.• High Voltage.• Gate Current.• dv/dt.
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EE2301-POWER ELECTRONICS
Thyristor Types
• Phase-control Thyristors (SCR’s).• Fast-switching Thyristors (SCR’s).• Gate-turn-off Thyristors (GTOs).• Bidirectional triode Thyristors (TRIACs).• Reverse-conducting Thyristors (RCTs).
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• Static induction Thyristors (SITHs).• Light-activated silicon-controlled rectifiers
(LASCRs).• FET controlled Thyristors (FET-CTHs).• MOS controlled Thyristors (MCTs).
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EE2301-POWER ELECTRONICS
Phase Control Thyristor• These are converter thyristors.• The turn-off time tq is in the order of 50 to
100sec.• Used for low switching frequency.• Commutation is natural commutation• On state voltage drop is 1.15V for a 600V
device.
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• They use amplifying gate thyristor.
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EE2301-POWER ELECTRONICS
Fast Switching Thyristors
• Also called inverter thyristors.• Used for high speed switching applications.• Turn-off time tq in the range of 5 to 50sec.• On-state voltage drop of typically 1.7V for
2200A, 1800V thyristor.• High dv/dt and high di/dt rating.
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EE2301-POWER ELECTRONICS
Bidirectional Triode Thyristors (TRIAC)
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EE2301-POWER ELECTRONICS
Mode-I Operation
MT2 Positive, Gate Positive
P 1
N 1
N 2
P 2Ig
Ig
M T 2 (+ )
M T 1 ( )G
V(+ )
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EE2301-POWER ELECTRONICS
Mode-II Operation
MT2 Positive, Gate Negative
P 1
N 1
N 2N 3
P 2
Ig
M T 2 (+ )
M T 1 ( )G
V
F ina lcon du ctio n
In itia lcon d u ctio n
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EE2301-POWER ELECTRONICS
Mode-III Operation
MT2 Negative,Gate Positive
P 1
N 1
N 4
N 2P 2
Ig
M T 2 ( )
M T 1 (+ )G(+ )
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EE2301-POWER ELECTRONICS
Mode-IV Operation
MT2 Negative,Gate Negative
P 1
N 1
N 4
P 2
Ig
M T 2 ( )
M T 1 (+ )
N 3
G(- )
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EE2301-POWER ELECTRONICS
Triac Characteristics
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EE2301-POWER ELECTRONICS
BJT structure
note: this is a current of electrons (npn case) and so theconventional current flows from collector to emitter.
heavily doped ~ 10^15provides the carriers lightly doped ~ 10^8 lightly doped ~ 10^6
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EE2301-POWER ELECTRONICS
BJT characteristics
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EE2301-POWER ELECTRONICS
BJT characteristics
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EE2301-POWER ELECTRONICS
BJT modes of operation
Mode EBJ CBJCutoff Reverse Reverse
Forward active
Forward Reverse
Reverse active
Reverse Forward
Saturation Forward Forward
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EE2301-POWER ELECTRONICS
Cutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch.
Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βf in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.
Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the βf in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.
Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.
BJT modes of operation
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EE2301-POWER ELECTRONICS
BJT structure (active)
current of electrons for npn transistor –
conventional current flows from collector to emitter.
BB
CCEEIIEE IICC
IIBB
--
++VVBEBE VVCBCB
--
++
++-- VVCECE
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EE2301-POWER ELECTRONICS
• A GATE electrode is placed above (electrically insulated from) the silicon surface, and is used to control the resistance between the SOURCE and DRAIN regions
• NMOS: N-channel Metal Oxide Semiconductor
np-type silicon
oxide insulator n
L
• L = channel length
“Metal” (heavily doped poly-Si)
W• W = channel width
MOSFET
SOURCE
DRAIN
GATE
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• Without a gate-to-source voltage applied, no current can flow between the source and drain regions.
• Above a certain gate-to-source voltage (threshold voltage VT), a conducting layer of mobile electrons is formed at the Si surface beneath the oxide. These electrons can carry current between the source and drain.
N-channel MOSFET
n
p
oxide insulatorgate
n
DrainSource
Gate
ID
IG
IS
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EE2301-POWER ELECTRONICS
N-channel vs. P-channel MOSFETs
• For current to flow, VGS > VT
• Enhancement mode: VT > 0
• Depletion mode: VT < 0– Transistor is ON when VG=0V
p-type Si
n+ poly-Si
n-type Si
p+ poly-Si
NMOS PMOS
n+ n+ p+ p+
• For current to flow, VGS < VT
• Enhancement mode: VT < 0
• Depletion mode: VT > 0– Transistor is ON when VG=0V
(“n+” denotes very heavily doped n-type material; “p+” denotes very heavily doped p-type material)
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EE2301-POWER ELECTRONICS
MOSFET Circuit Symbols
p-type Si
n+ poly-Si
NMOS
n+ n+
n-type Si
p+ poly-Si
PMOS
p+ p+
G G
G G
S
SS
S
Body
Body
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EE2301-POWER ELECTRONICS
• The voltage applied to the GATE terminal determines whether current can flow between the SOURCE & DRAIN terminals.– For an n-channel MOSFET, the SOURCE is biased at a lower
potential (often 0 V) than the DRAIN(Electrons flow from SOURCE to DRAIN when VG > VT)
– For a p-channel MOSFET, the SOURCE is biased at a higher potential (often the supply voltage VDD) than the DRAIN
(Holes flow from SOURCE to DRAIN when VG < VT )
• The BODY terminal is usually connected to a fixed potential.– For an n-channel MOSFET, the BODY is connected to 0 V– For a p-channel MOSFET, the BODY is connected to VDD
MOSFET Terminals
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EE2301-POWER ELECTRONICS
VGS
S
semiconductoroxide
GVDS
+ +
D
always zero!
IG
VGS
The gate is insulated from the semiconductor, so there is no significant steady gate current.
IG
NMOSFET IG vs. VGS CharacteristicConsider the current IG (flowing into G) versus VGS :
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EE2301-POWER ELECTRONICS
VGS
S
semiconductoroxide
GVDS
ID
+ +
D
ID
zero if VGS < VT
VDS
Next consider ID (flowing into D) versus VDS, as VGS is varied:
Below “threshold” (VGS < VT): no charge no conduction
Above threshold (VGS > VT): “inversion layer” of electrons appears, so conduction between S and D is possible
VGS > VT
NMOSFET ID vs. VDS Characteristics
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EE2301-POWER ELECTRONICS
The MOSFET as a Controlled Resistor
• The MOSFET behaves as a resistor when VDS is low:
– Drain current ID increases linearly with VDS
– Resistance RDS between SOURCE & DRAIN depends on VGS
• RDS is lowered as VGS increases above VT
NMOSFET Example:ID
IDS = 0 if VGS < VT
VDS
VGS = 1 V > VT
VGS = 2 V
Inversion charge density Qi(x) = -Cox[VGS-VT-V(x)]where Cox ox / tox
oxide thickness tox
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EE2301-POWER ELECTRONICS
ID vs. VDS Characteristics
The MOSFET ID-VDS curve consists of two regions:
1) Resistive or “Triode” Region: 0 < VDS < VGS VT
2) Saturation Region: VDS > VGS VT
oxnn
TGSn
DSAT
Ck
VVL
WkI
where
2
2
oxnn
DSDS
TGSnD
Ck
VVVVL
WkI
where2
process transconductance parameter
“CUTOFF” region: VG < VT
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EE2301-POWER ELECTRONICS
Part I: Bipolar Power TransistorsThe Evolution Of IGBT
• Bipolar Power Transistor Uses Vertical Structure For Maximizing Cross Sectional Area Rather Than Using Planar Structure
EmitterBase
Collector
P
N+
N-
N+
Collector
Base
Emitter
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EE2301-POWER ELECTRONICS
Part II:Power MOSFETThe Evolution Of IGBT
• Power MOSFET Uses Vertical Channel Structure Versus The Lateral Channel Devices Used In IC Technology
n+
P
n-
P
n-
n+
SiO2
Gate
Source
Drain
Gate
Source
Drain
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Lateral MOSFET structure
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EE2301-POWER ELECTRONICS
The Evolution Of IGBT
• Discrete BJT + Discrete Power MOSFET In Darlington Configuration
E
NPN
N-MOSFET
G
B
S
D
C
Part III: BJT(discrete) + Power MOSFET(discrete)
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EE2301-POWER ELECTRONICS
Part IV: BJT(physics) + Power MOSFET(physics) = IGBTThe Evolution Of IGBT
• More Powerful And Innovative Approach Is To Combine Physics Of BJT With The Physics Of MOSFET Within Same Semiconductor Region
• This Approach Is Also Termed Functional Integration Of MOS And Bipolar Physics
• Using This Concept, The Insulated Gate Bipolar Transistor (IGBT) Emerged
• Superior On-State Characteristics, Reasonable Switching Speed And Excellent Safe Operating Area
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EE2301-POWER ELECTRONICS
The Evolution Of IGBT
• IGBT Fabricated Using Vertical Channels (Similar To Both The Power BJT And MOSFET)
n+
n- - drift
p+
p - base
p+ - substrate
Emitter Gate
Collector
E
PNP
NPN
N-MOSFET
G
C
Part IV: BJT(physics) + Power MOSFET(physics) = IGBT
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Device Operation• Operation Of IGBT Can Be Considered Like A PNP Transistor With Base Drive Current Supplied By The MOSFET
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EE2301-POWER ELECTRONICS
DRIVER CIRCUIT (BASE / GATE)
• Interface between control (low power electronics) and (high power) switch.
• Functions:– amplifies control signal to a level required to drive power switch
– provides electrical isolation between power switch and logic level
• Complexity of driver varies markedly among switches. MOSFET/IGBT drivers are simple but GTO drivers are very complicated and expensive.
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ELECTRICAL ISOLATION FOR DRIVERS
• Isolation is required to prevent damages on the high power switch to propagate back to low power electronics.
• Normally opto-coupler (shown below) or high frequency magnetic materials (as shown in the thyristor case) are used.
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EE2301-POWER ELECTRONICS
ELECTRICAL ISOLATION FOR DRIVERS
• Power semiconductor devices can be categorized into 3 types based on their control input requirements:
a) Current-driven devices – BJTs, MDs, GTOsb) Voltage-driven devices – MOSFETs, IGBTs, MCTsc) Pulse-driven devices – SCRs, TRIACs
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EE2301-POWER ELECTRONICS
CURRENT DRIVEN DEVICES (BJT)
• Power BJT devices have low current gain due to constructional consideration, leading current than would normally be expected for a given load or collector current.
• The main problem with this circuit is the slow turn-off time. Many standard driver chips have built-in isolation. For example TLP 250 from Toshiba, HP 3150 from Hewlett-Packard uses opto-coupling isolation.
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EE2301-POWER ELECTRONICS
ELECTRICALLY ISOLATED DRIVE CIRCUITS
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EE2301-POWER ELECTRONICS
EXAMPLE: SIMPLE MOSFET GATE DRIVER
• 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.