Download Lesson 1 - Introduction

Electrical Energy
Conversion and
Power Systems
Universidad
de Oviedo
Power Electronic Devices
Semester 1
Power Supply
Systems
Lecturer: Javier Sebastián
Outline

Review of the physical principles of operation of semiconductor
devices.

Thermal management in power semiconductor devices.

Power diodes.

Power MOSFETs.

The IGBT.

High-power, low-frequency semiconductor devices (thyristors).
2
Electrical Energy
Conversion and
Power Systems
Universidad
de Oviedo
Lesson 5 – The Insulated Gate Bipolar
Transistor (IGBT).
Semester 1 - Power Electronic Devices
3
Outline
• The main topics to be addressed in this lesson are the following:
 Introduction.
 Review of the basic structure and operation of bipolar junction
transistors (BJTs).
 Internal structures of IGBTs.
 Static characteristics of the IGBTs.
 Dynamic characteristics of the IGBTs.
 Losses in the IGBTs.
4
Introduction (I).
• Power MOSFETs are excellent power devices to be
used in power converters up to a few kWs.
• They have good switching characteristics because
they are unipolar devices.
• This means that the current is due to majority
carriers exclusively and that it does not pass
through any PN junction.
• Due to this, conductivity modulation does not take
place.
• This fact limits the use of these devices for high
power applications, because high-voltage devices
exhibit high RDS(ON) values.
• The challenge is to have a device almost as fast as
a MOSFET, as easy to control as a MOSFET, but with
conductivity modulation.
Channel
Gate
Source
- +
N+
P
N-
N+
Drain
Drain Current
5
Introduction (II).
• On the other hand, Bipolar Junction Transistors (BJTs)
are devices in which the current passes through two PN
junctions.
• Although the current is due to the emitter majority Base Current
carriers, these carriers are minority carriers in the base.
Therefore, the switching process strongly depends on
SiO2 B
E
the minority base carriers.
N+ P • Due to this, BJTs (bipolar devices) are slower than
NMOSFETs (unipolar devices).
• Moreover, the control current (base current) is quite
N+
high (only 5 -20 times lower than the collector current)
in power BJTs.
C
• However, as the collector current in BJTs passes
through two PN junctions, they can be designed to
Collector Current
have conductivity modulation.
• As a consequence, BJTs have superior characteristics
in on-state than MOSFETs.
6
Introduction (III).
• Summary of a comparison between BJTs and MOSFETs
Switching
Control Conductivity
modulation
Losses in on-state in
high voltage devices
BJT
Slow
Difficult
Yes
Low
MOSFET
Fast
Easy
No
High
• Could we have the advantages of both types of devices together in a
different device?
• The answer is that we can design a different device with almost all the
advantages of both BJTs and MOSFETs for medium and high voltage (from
several hundreds of volts to several thousand of volts).
• This device is the IGBT (the Insulated Gate Bipolar Transistor).
• To understand its operation, we must review the structure and operation
of the BJT.
7
Review of the basics of BJTs (I).
PNP transistor: Two P-type regions and a N-type region
NPN transistor: Two N-type regions and a P-type region
Collector (N)
Collector (P)
Base
(N)
PNP
Base
(P)
Emitter (P)
NPN
Emitter (N)
Conditions for such device to be a transistor:
• The emitter region must be much more doped than the base region.
• The base region must be a narrow region (narrower than the diffusion
length corresponding to the base minority carrier).
8
Review of the basics of BJTs (II).
• Example: a PNP-type silicon low-power transistor
(the actual geometry is quite different)
P
+
Emitter
-
P
N
Base
1m
Collector
• The emitter region must be much more doped than the base region.
NAE=1015 atm/cm3
NDB=1013 atm/cm3
• The base region must be narrower than the diffusion length
corresponding to the holes in the base region.
WB = 1 m << Lp = 10 m
9
Review of the basics of BJTs (III).
• Operation in active region: E-B junction is forward biased and
B-C junction is reverse biased.
• The concentration of minority carries when the junctions have
been biased can be easily deduced form slide #80, Lesson 1.
V2
V1
C
B
E
P+
+-
N-
-+
WB
High gradient  High
current due to holes in
the E-B junction
High gradient  High
current due to holes in
the B-C junction
Low gradient  low reverse
current due to electrons in
the B-C junction
Low gradient  low forward
current due to electrons in
the E-B junction
Electrons in the emitter
P
0-
0+
x
WB-
Holes in the base
WB+ Electrons in the collector
10
Review of the basics of BJTs (IV).
• Currents passing through the transistor in active region.
iE
Minority carrier concentration
Linear
scale
pB3
pB2
0
nE
Currents
nC
Base contact
VEB -i
B
B
V2
VEB1 < VEB2 < VEB3
iE3
-iC3
iE  IS·evEB/VT
iE2
-iC2
-iC  iE·a
(a 0.98-0.995)
iC b·iB
(b 20-200)
iE1
0
pB1
C
E
-iC
-iC1
11
Review of the basics of BJTs (V).
• Operation in cut-off region: E-B and B-C junctions are reverse biased.
Minority carrier concentration
nE (active)
nE (cut-off)
Linear
scale
pB (active)
V1
nC
pB (cut-off)
0
Currents
IE (active)
IE (cut-off)
0
-iC
iE
V2
Active region
Base contact
-IC (active)
-IC (cut-off)
iE
-iC
V1
V2
Cut-off region
12
Review of the basics of BJTs (VI).
• Operation in saturation region: E-B and B-C junctions forward biased.
Minority carrier concentration
pB (saturation)
Linear
scale
V1
nC (sat.)
nE
0
Currents
pB (active)
-iC
iE
V1
iE (saturation)
iE (active)
0
-iC (saturation)
-iC (active)
V2
Active region
nC (active)
Base contact
-iC
iE
V2
Saturation region
• However, the operation in
saturation usually takes place in
other type of circuits.
13
Review of the basics of BJTs (VII).
• Usual circuit to study the saturation region.
• We are going to increase the
value of V1.
Minority carrier concentration
pB (boundary)
nE
0
pB (sat.)
pB (active)
Linear
scale
nC
• The transistor will be in active
region while vCB < 0. When vCB >
0, it is in saturation.
As the collector current is
approximately
constant,
these concentration profiles
have the same slope.
Currents
iE (saturation)
iE (boundary)
iE (active)
0
-iC
-iC (saturation)
-iC (boundary)
-iC (active)
V2/R
vCB +
R
-
-iB
V2
V1
iE
14
Review of the basics of BJTs (VIII). Very important!!!
• We can increase the height of point pB1 as much as we want, because we can
increase V1 indefinitely.
• However, the collector current ( emitter current) is limited to the maximum
possible value of V2/R (otherwise, the transistor would behave as a power
generator, which means that energy is generated from nothing).
• As the current passing through the transistor (from emitter to collector) is limited,
then the slope of pB is also limited.
• As a consequence, pB2 must also increase to maintain the current constant, which
implies that the base-collector junction becomes forward bias.
The transistor becomes saturated.
Minority carrier concentration
pB1
pB (sat.)
Linear scale
R
Not possible
pB1
vCB +
-
pB2
nE
pB (bound.)
pB2
-iC
nC
-iB
0
V2
V1
iE
15
Review of the basics of BJTs (IX).
• Output characteristic curves.
Voltage and current references
Output curves
iC
vCE
+
-
+
iB
vBE
iC [mA]
-40
iB=-300A
-
iB=-200A
-20
iB=-100A
Saturation
Active
iB=-400A
0
-2
iB=0A
vCE [V]
-4
-6
Cut-off
16
Review of the basics of BJTs (X).
• The on-state of bipolar transistors is quite good, because the
voltage drop between collector and emitter is quite low.
• However, the turn-off is quite slow (next slide).
-iC
iC
R
-iB
0.5 V
-
+
P
-
0.2 V
N
P
0.7 V
R
iB
+
0.5 V -N
+
+
P
N
iE
V2
0.7 V
0.2 V
-iE
V2
17
Review of the basics of BJTs (XI).
• The longest time in the switching process of a bipolar transistor
is the one corresponding to eliminating the excess of minority
carriers in the base region when the transistor turns-off.
Transistor in saturation
Concentration
P+
nE
0
N-
P
pB (sat.)
pB Cut-off
nC
These excess carriers
(holes in this case)
must be eliminated to
turn-off the transistor
Transistor in cut-off
18
Review of the basics of BJTs (XII).
• A good trade-off between switching speed and voltage drop in on-state can
be reached using anti-saturation circuitry (circuits to maintain the transistor
just in the boundary between active region and saturation).
-iC
Excess carriers to be eliminated
when the transistor turns-off
(lower than in saturation).
R
-iB
Concentration
P+
N-
0
pB Cut-off
+
P-
0.7 V
N
P
P
pB (boundary)
nE
0V
-
nC
0.7 V
+
iE
V2
Voltages just in the boundary
between active region and
saturation
19
Review of the basics of BJTs (XIII).
• Hard-saturation circuits
(the voltage across the transistor terminals is the same).
0.7 V
P
N
-iB 0.5V
R
R
+
0.2 V
P
+
V2
0.7 V
-iB
P +
-
0.2 V
N
R
-iB
-
0.5V
P
+
-
V2
P +
0.2 V
N
P
-
V2
0.5V
20
Review of the basics of BJTs (XIV).
• Soft-saturation circuit
(anti-saturation circuit).
R
0.7 V +
P +
-iB N
0.7 V
R
S1
0.7 V +
P +
-iB N
0.7 V
P
-
V2
P
-
V2
• In soft-saturation (boundary),
when S1 closed.
• In cut-off, when S1 open.
21
Review of the basics of BJTs (XV).
• As a bipolar transistor is a “bipolar device”, conductivity modulation
can take place if the transistor is properly designed.
B
SiO2
E
N+
Drift region
P
NN+
P+
N-
N+
Structure needed to have conductivity
modulation
(from slide #100, Lesson 1)
C
22
Principle of operation and structure of the IGBT (I).
• The IGBT (the Insulated Gate Bipolar Transistor) is based on a structure that
allows:
 Conductivity modulation (good behaviour for high voltage devices when
they are in on-state).
 Anti-saturation (not so slow switching process as in the case of complete
saturation).
 And control from an insulated gate (as in the case of a MOSFET).
R
R
P
P
N
N
D
S1
P
V2
G
P
V2
S
23
Principle of operation and structure of the IGBT (II).
Collector
Collector (C)
P E
N
D
Gate (G)
B
P
C
Emitter (E)
G
Gate
S
Emitter
Schematic symbol for a N-channel IGBT.
Simplified equivalent
circuit for an IGBT.
Another schematic symbol also used.
24
Principle of operation and structure of the IGBT (III).
Emitter
• Internal structure (I).
Gate
Emitter (E)
Gate (G)
Gate
Emitter
Collector
Collector (C)
N+
N-
N+
P
N+
P+
Collector
25
Principle of operation and structure of the IGBT (IV).
• Internal structure (II).
Emitter
Emitter
Gate
Gate
Gate
Emitter
Rdrift
N+
N+
Collector
Simplest model
for an IGBT.
Rdrift
N-
P
N+
P+
Collector
Collector
Model taking into
account the drift
region resistance.
26
Principle of operation and structure of the IGBT (V).
• The IGBT blocking (withstanding) voltage.
Depletion
region
Emitter
Gate
Gate
V2
Emitter
V2
N+
N+
P
NN+
P+
Collector
Rdrift
R
R
Collector
27
Principle of operation and structure of the IGBT (VI).
Conductivity
modulation
• The IGBT conducting current (a first approach).
Transistor
effect
Emitter
V1
Gate
V1
Gate
V2
Emitter
Rdrift
NN+
P+
Collector
V2
N+
N+
P
Rdrift
R
R
Collector
28
Principle of operation and structure of the IGBT (VII).
• A more accurate model.
• However, there is another parasitic transistor.
Emitter
Gate
Gate
Emitter
Rbody
Rbody
N+
N+
Rdrift
NN+
P+
Collector
Model taking into account the
MOSFET-body resistance.
N+
P
Rdrift
P
NN+
P+
Collector
Model taking into account the
parasitic NPN transistor.
29
Principle of operation and structure of the IGBT (VIII).
Emitter
Emitter
Gate
Gate
N+
Rbody
Rdrift
Rbody
P
NN+
Rdrift
P+
Collector
Model taking into account the
parasitic NPN transistor.
Collector
• The final result is that there
is a parasitic thyristor.
30
Principle of operation and structure of the IGBT (IX).
• The basics of the thyristor: the PNPN structure (I).
Emitter
Gate
Rbody
N+
P
N-
N+
P
N
N+
P+
P+
E1
E1
N+
B1 P
P
C1 N
C2
N B2
P+
B1
E2
C1
C2
B2
E2
Collector
31
Principle of operation and structure of the IGBT (X).
• The basics of the thyristor: the PNPN structure (II).
+
N
E1
P
N+
B1
C1 C2
B2
N
P+
E2
N
-
Forward
bias
VDC
+
Reverse
bias
-
P
+
-
Forward
bias
+
R
P+
• There are two junctions forward biased and one is reverse biased.
• As a consequence, the PNPN device can block (withstand) voltage
without conducting current.
• However, it will be able to conduct current as well,
as it is going to be shown in the next slide.
32
Principle of operation and structure of the IGBT (XI).
• The basics of the thyristor: the PNPN structure (III).
N+
Forward
bias
+
P
N
Reverse
bias
+
VDC
-P
bias
R
+
N+
P+
+
-
VB
VDC
P
N +
N
Forward
-
Forward
bias
+
- -+
N
Forward
bias!!
-
Forward
bias
+-
P
R
+
P+
iR
• If VB is high enough (0.6-0.7 V in a silicon device), then the NPN transistor
becomes saturated.
• As a consequence, the base-collector junctions corresponding to both the NPN
and the PNP transistor become forward biased. Both transistors are saturated.
• Therefore, all the junctions are forward biased right now and the voltage across
the device is quite low (e.g., 0.9-1.2 V). The current passing through R can be
33
quite high (approximately VDC/R).
Principle of operation and structure of the IGBT (XII).
• The basics of the thyristor: the PNPN structure (IV).
N+
Q1
N
P
-i
VB
+
+
iB_1
iB
VDC
N+
Q1
C_1=iB_2
-
-N
+
P
iC_2
Q2
+
P+
N
R
iR
P
-i
VB
+
+
iB_1=iC_2
C_1=iB_2
-
-N
+
P
Q2
+
VDC
P+
R
iR
• Initially, the current needed for transistor Q1 to start conducting (active
region) comes from the voltage source VB.
• When iC_1 increases, iC_2 strongly increases because iC_2 = b2·iB_2 = b2·iC_1.
Therefore, current iB_1 will be mainly due to iC_2.
• As iC_2 is the main current needed to maintain both transistors saturated, the
situation does not change if we remove VB.
34
Principle of operation and structure of the IGBT (XIII).
• The basics of the thyristor: the PNPN structure (V).
• A PNPN structure has two different stable states (so, it works as a flip-flop):
 As a short-circuit (IR  VDC/R).
 As a open-circuit (IR = 0).
N+
Q1
N
-
Forward
bias
P
+
VDC
+
-Reverse
bias
+
-
P
N
Forward
bias
P+
+
R
-
P +
Q1
N
Q2
-
N+
-
iB_1=iC_2
+
Forward
bias
iC_1=iB_2 iR = 0
-N
+
P
Q2
+
VDC
R
P+
iR  VDC/R
• The device state at a specific moment depends on whether Q1 emitter-base
junction has been forward biased previously.
• The only way to turn-off the device is by decreasing IR up to zero.
35
Principle of operation and structure of the IGBT (XIV).
• The IGBT conducting current (actual paths).
Emitter
-
Gate
N+
Rbody
Channel
P
Rbody
+
N-
N+
P
N
N+
R
+
N+
body
P
P+
P+
N
P
Q2
Q1
N
P+
Collector
BJT current
MOSFET current
BJT current
BJT current
• The voltage across Rbody must not be high enough to turn-on the PNPN
structure, which is called latch-up.
• Else, the total device cannot be turned-off by the gate voltage any more. 36
Principle of operation and structure of the IGBT (XV).
• To avoid the IGBT latch-up, Rbody must be as low as possible.
Rbody
Emitter
Emitter
Gate
Gate
N+
P
N+
Channel
P
P+
N-
N-
N+
N+
P+
P+
Collector
BJT current
Channel
Collector
BJT current
• The new P+ region decreases Rbody, thus increasing the value of the current
needed to reach the voltage drop on Rbody corresponding to latch-up.
37
Principle of operation and structure of the IGBT (XVI).
• The IGBT cannot conduct reverse current when vGE = 0 (it is not as the MOSFET).
C
C
P
D
P
N
Parasitic
diode
G
N
P
G
G
S
E
Reverse current
P
Reverse current
External
diode
E
Reverse current
• This means that it is able to block reverse voltage.
• Symmetrical IGBTs are especially designed for blocking reverse voltage.
However, they have worse forward voltage drop than asymmetrical (standard)
IGBTs.
• To conduct reverse current when vGE = 0, an external diode must be added.
38
Principle of operation and structure of the IGBT (XVII).
• Asymmetrical versus symmetrical IGBT structures.
Emitter
Emitter
Gate
Gate
N+
N+
P
P+
P
P+
N-
N-
N+
P+
P+
Collector
• Asymmetrical IGBT
(also called punch-through IGBT).
Collector
• Symmetrical IGBT
(non-punch-through IGBT).
39
Static output characteristic curves of a IGBT.
vEB_BJT +
C
-
iD [A]
6
6
vGE = 10V
vGE = 8V
E
vGE = 5V
vGS = 6V
2
4
vGE = 4V
vGS = 5V
vGE < VGE(th) = 3V
2
0
vGS = 4V
vGS < VGS(TO) = 3V
0
vGE = 6V
4
G
vGS = 10V
vGS = 8V
iC [A]
2
4
vDS [V]
• Static output characteristic curve
of a MOSFET.
• It is also the one corresponding to
the MOSFET part of a IGBT.
2
4
vCE [V]
vEB_BJT
• Static output characteristic curve of a IGBT.
• It can be easily obtained from the MOSFET
characteristic curve by adding the voltage
drop vEB_BJT corresponding to the emitter-tobase junction of the BJT part of the IGBT.
40
General characteristics of the IGBTs (I).
• We will use a specific IGBT to address the general IGBT characteristics.
41
General characteristics of the IGBTs (II).
• General information regarding the IRG4PC50W.
42
Static characteristics of the IGBTs (I).
43
Static characteristics of the IGBTs (II).
IC_max @ T = 50 oC: 55 A
IC_max @ T = 75 oC: 48 A
44
Static characteristics of the IGBTs (III).
Asymmetrical IGBT
45
Static characteristics of the IGBTs (IV).
• Static output characteristic curve for a given vGE voltage.
iC [A]
6
vGE = 15V
4
2
0
2
4
vCE [V]
vEB_BJT
• As in slide #40 of this lesson.
vEB_BJT 1V
46
Static characteristics of the IGBTs (V).
Thermal behaviour
like a MOSFET
Thermal behaviour
like a BJT
47
Dynamic characteristics of the IGBTs (I).
vGE
• Turn-off in a IGBT with inductive load and ideal diode
(see slide #32, lesson 4).
C
vGE(th)
G
iC
E
MOSFET-part turn-off
IL
BJT-part turn-off
IGBT tail
vCE
RG
A
VG
iC
B
C
G
+
V’G vGE
-
VDC
+
vCE
E
48
Dynamic characteristics of the IGBTs (II).
• Comparing IGBT and MOSFET Turn-off.
vGE
• IGBT turn-off
• MOSFET turn-off
C
D
vGE(th)
G
G
iC
E
MOSFET-part turn-off
vGS
S
vDS(TO)
iD
BJT-part turn-off
IGBT tail
vCE
Period with
switching losses
Switching losses
vDS
49
Dynamic characteristics of the IGBTs (III).
• Turn-on in a IGBT with inductive load and ideal diode
(see slides #32-39, lesson 4, for comparison).
vGE
C
vGE(th)
G
iC
E
Period with
switching losses
IL
RG
A
vCE
VG
MOSFET-part turn-on
iC
B
C
G
+
V’G vGE
-
VDC
+
vCE
E
-
BJT-part turn-on
50
Dynamic characteristics of the IGBTs (IV).
• Actual turn-on and turn-off waveforms with inductive load, taking into account
the diode real behaviour (recovery times) and the stray (parasitic) inductances.
51
Dynamic characteristics of the IGBTs (V).
52
Dynamic characteristics of the IGBTs (VI).
• Parasitic capacitances and gate charge.
53
Losses in IGBTs.
• Conduction losses can be computed from the static output
characteristic curve (see slide #46 of this lesson).
• Switching losses can be computed from the information given by the
manufacturer.
54