Download Power Electronic Devices

1
Outline
• Introduction to power electronic devices
• Different power transistors
• IGBTs
– Basic structure
– Input, output and switching characteristics
– Other important factors
– Technology
• Comparison with other devices
2
Power Electronic Devices
• Power electronic devices are associated with the
efficient conversion, control and conditioning of
electric power from its available input form to the
desirable output form
• The proliferating demand of controllable power
electronic systems has promoted research on
 Novel device materials
 Structures
 Circuit topologies
3
Power Electronic Devices
• Base material used for power electronics devices –
Silicon
Silicon Carbide (SiC)
Property
Silicon
Silicon Carbide
Breakdown field
2.5 ×105 V/cm
2.2 ×106 V/cm
Energy gap
1.12 eV
3.26 eV (4H SiC)
Thermal
conductivity
1.5 W/cm
4.9 W/cm
Also SiC shows high chemical inertness, high pressure, and radiation resistance and
hence its popular in the recent days.
4
Introduction
• Characteristics of Ideal switch:
 No driving losses: High input impedance so that the drive
current is zero
 Zero on state or forward conduction losses: forward
voltage drop is zero & high operational current density to
make chip small
 Zero off state or reverse blocking losses: Infinitely larger
reverse blocking voltage with zero leakage current
 No switching losses: turn on and turn off times should be
zero. A pulse of small width, tending to zero, must be used
for the operations
 Low Cost: To reduce the cost of the electronic equipment
5
Introduction
Some Practical uses of Switches
 Pulse Width Modulation (PWM) control in DC
motors
 Rectifiers
 Air Conditioners and many more
6
Classification of Power Semiconductor Devices
Ref. 5
7
Parameters with Power Semiconductor Devices
•
•
•
•
•
Breakdown voltage
On-resistance
Rise and fall times
Safe operating area
Thermal resistance
8
Different power Transistors:
 BJT (Bipolar Junction Transistor)
 MOSFET (Metal Oxide Semiconductor Field effect
transistor)
 IGBT (Insulated Gate Bipolar Transistor)
 CoolMOS
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Power transistors
• Controlled Turn-on & Turn-off Characteristics: Power
transistors have controlled turn-on and turn-off
characteristics
• Region of Operation: Transistors used as switches are
generally used in the saturation region, resulting in low onstate voltage drop
• Ratings: These power transistors come with varying
switching frequencies and with different current and
voltage ratings
10
BJTs
Constructions
Input Characteristics
Ref. 5
Output Characteristics
11
BJT’s transfer characteristics
Ref. 5
Circuit diagram of a BJT
(Common Emitter)
Transfer Characteristics of BJT as a
switch
12
Power MOSFET
Vertical diffused
MOS (VDMOS) or
Double-Diffused MOS
(or simply DMOS)
Structure
Verticality of the
Structure
Ref. 5
N.B. Power MOSFETs with lateral structure also exist
13
Characteristics of Power MOSFET
• On-state resistance (Rds_on): In the on state, it behaves like
a resistor between the source and drain terminals
• Breakdown voltage vs. Rds_on trade-off
• Body diode:
source metallization connects both
the N+ and P implantations, although
the operating principle of the MOSFET
only requires the source to be connected
to the N+ zone
Ref. 5
14
Switching characteristics
• Because of their unipolar
nature, the power
MOSFET can switch at
very high speed
• Indeed, there is no need
to remove minority
carriers as with bipolar
devices
• The only intrinsic
limitation in commutation
speed is due to the
internal capacitances of
the MOSFET
Ref. 5
15
Comparisons between MOSFETs and BJTs
MOSFETs
BJTs
Pros
Cons
1. High input impedance
1. Low input impedance
2. Minimal drive power, no DC current
gate
required at
2. Large drive power, continuous DC current required at base
3. Simple drive circuits
3. Complex drive circuits as large +ve and –ve currents are
involved
4. More linear operation, less harmonics
4. More inter-modulation and cross-modulation products
5. Devices can be easily paralleled
5. Devices cannot be easily paralleled
7. Max. operating temp. ~ 200 oC
7. Max. operating temp. ~ 150 oC
9. Very low switching losses
9. Medium to high switching losses (depends on trade-off
with conduction losses)
10. High switching speed, less temp. sensitive
10. Lower switching speed, more sensitive to temp.
Cons
Pros
1. High on-resistance
1. Low on-resistance
2. Low transconductance
2. High transconductance
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IGBT
• It combines the properties of both BJT – Low on
state conduction losses & MOSFET - High input
impedance.
• Various other names are COMFET (Conductivity Modulated FET)
IGT (Insulated Gate Transistor)
IGR (Insulated Gate Rectifier)
GEMFET (Gain enhanced MOSFET)
BiFET (Bipolar FET)
17
IGBT
• Minority carrier device with high input impedance and
large bipolar current-carrying capability
• MOS input characteristics and Bipolar output
characteristics; i.e. a voltage controlled bipolar device.
• IGBT is suitable for scaling up the blocking voltage
capability ; in case of Power MOSFETs the on-resistance
increases sharply with the breakdown voltage due to
an increase in the resistivity and thickness of the drift
region required to support the high operating voltage
• In case of IGBT the drift region resistance is drastically
reduced due to the high concentration of the injected
minority carriers, during on-state current conduction
18
Structure
Ref. 4
Basic Structure of a typical N-channel IGBT based upon the DMOS process
19
Structure
• The Silicon cross-section of an IGBT is almost identical
to that of a vertical power MOSFET, except for the P+
injecting layer
• It shares similar MOS gate structure and P wells with N+
source regions. The N+ layer at the top is the source or
emitter and the P+ layer at the bottom is the drain or
collector
• It is also feasible to make P-channel IGBTs and for
which the doping profile in each layer will be reversed
• IGBT has a parasitic thyristor comprising the four-layer
NPNP structure. Turn-on of this thyristor is undesirable
20
Structure
• Some IGBTs, manufactured without the N+ buffer layer, are
called non-punch through (NPT) IGBTs whereas those with
this layer are called punch-through (PT) IGBTs
• The presence of this buffer layer can significantly improve
the performance of the device if the doping level and
thickness of this layer are chosen appropriately
• Despite physical similarities, the operation of an IGBT is
closer to that of a power BJT than a power MOSFET. It is
due to the P+ drain layer (injecting layer) which is
responsible for the minority carrier injection into the Ndrift region and the resulting conductivity modulation
21
Equivalent Circuit
Ref. 4
Based on the structure, a simple
equivalent circuit model of an IGBT
can be drawn as shown in the Figure .
It contains MOSFET, JFET, NPN and
PNP transistors.
The NPN and PNP transistors
represent the parasitic thyristor which
constitutes a regenerative feedback
loop.
The collector of the PNP is
connected to the base of the NPN
and the collector of the NPN is
connected to the base of the PNP
through the JFET.
22
Equivalent Circuit
• The resistor RB represents the shorting of the
base-emitter of the NPN transistor to ensure that
the thyristor does not latch up, which will lead to
the IGBT latch-up
• The JFET represents the constriction of current
between any two neighbouring IGBT cells. It
supports most of the voltage and allows the
MOSFET to be a low voltage type and
consequently have a low RDSon value
23
Circuit Symbol
• A circuit symbol for the IGBT is shown in
Figure 3. It has three terminals called Collector
(C)/ drain, Gate (G) and Emitter (E)/ source.
24
Main advantages over BJTs and Power MOSFETs
• It has a very low on-state voltage drop due to
conductivity modulation and has superior on-state
current density. So smaller chip size is possible and
thus cost can be reduced
• Low driving power and a simple drive circuit due to the
input MOS gate structure; it can be easily controlled as
compared to current-controlled devices (thyristors ,
BJTs) in high voltage and high current applications
• Wide SOA; it has superior current conduction
capabilities compared to a BJT and also has excellent
forward and reverse blocking capabilities
25
Main Drawbacks
• Switching speed is inferior to that of a Power
MOSFET and superior to that of a BJT; the
collector current tailing due to minority
carriers causes the turn-off speed to be slow
• There is a possibility of latch-up due to the
internal p-n-p-n thyristor structure
26
Comparison between PT and NPT IGBTs
Ref. 4
The physical constructions for both of PT and NPT IGBTs are shown in the figure
27
PT and NPT IGBTs
• The PT structure has an extra buffer layer which performs two
main functions:
(i) avoids failure by punch-through action because the
depletion region expansion at applied high voltage is
restricted by this layer,
(ii) reduces the tail current during turn-off and shortens the
fall time of the IGBT because the holes are injected by the P+
collector partially recombine in this layer
• The NPT IGBTs, which have equal forward and reverse
breakdown voltage, are suitable for AC applications.
• The PT IGBTs, which have less reverse breakdown voltage
than the forward breakdown voltage, are applicable for DC
circuits where devices are not required to support voltage in
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the reverse direction
Characteristic comparisons of PT & NPT IGBTs
Parameter
NPT
PT
Switching loss
Medium;
Long, low amplitude tail
current. Moderate increase
in E_off with Temp.
Low;
Short tail current.
Significant increase in E_off
with Temp.
Conduction loss
Medium;
Increases with Temp.
Low;
Flat to slight decrease with
Temp.
Paralleling
Easy ;
Optional sorting,
recommended share heat
Difficult;
Must sort on Vce(on)
Short-circuit rated
Yes
Limited;
High gain
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Operating Modes
• Forward-Blocking Mode
When a positive voltage is
applied across the collector-toemitter terminal with gate
shorted to emitter the device
enters into forward blocking
mode with junctions J1 and J3
are forward-biased and
junction J2 is reverse-biased. A
depletion layer extends on
both-sides of junction J2 partly
into P base and N- drift region
30
Operating Modes
• Forward Conduction Mode
An IGBT in the forward-blocking state can be
transferred to the forward conducting state by
removing the gate-emitter shorting and applying a
positive voltage of sufficient level to invert the Si below
gate in the P base region. This forms a conducting
channel which connects the N+ emitter to the N- drift
region. Through this channel, electrons are transported
from the N+ emitter to the N- drift
• Result: conductivity modulation
31
Operating Modes
• Reverse blocking mode:
When a negative voltage is applied across the collector-to-emitter
terminal, the junction J1 becomes reverse-biased and its depletion
layer extends into the N- drift region. The break down voltage
during the reverse-blocking is determined by an open-base BJT
formed by the P+ collector/ N- drift/P base regions. The device is
prone to punch-through if the N-drift region is very lightly-doped.
The desired reverse voltage capability can be obtained by
optimizing the resistivity and thickness of the N- drift region
• Reverse blocking IGBT is rare and in most applications, an anti parallel diode (FRED) is used
32
Output characteristics
• The plot for forward
output characteristics
of an NPT-IGBT is
shown in the figure . It
has a family of curves,
each of which
corresponds to a
different gate-toemitter voltage (VGE).
The collector current
(IC) is measured as a
function of collectoremitter voltage (VCE)
with the gate-emitter
voltage (VGE) constant.
Ref. 4
A distinguishing feature of the characteristics
is the 0.7 V offset from the origin.
33
Transfer Characteristics
• The transfer
characteristic is defined
as the variation of ICE
with VGE values at
different temperatures,
namely, 25oC, 125oC, and
-40oC.
• A typical transfer
characteristic is shown in
the figure. The gradient
of transfer characteristic
at a given temperature is
a measure of the
transconductance (gfs) of
the device at that
temperature.
Ref. 4
34
Transfer Characteristics
•
•
•
•
A large gfs is desirable to
obtain a high current
handling capability with
low gate drive voltage.
The channel and gate
structures dictate the gfs
value.
Both gfs and RDSon (on
resistance of IGBT) are
controlled by the channel
length which is
determined by the
difference in diffusion
depths of the P base and
N+ emitter.
The point of intersection
of determines the
threshold voltage (VGE_th)
of the device.
Ref. 4
35
Switching Characteristics
•
•
•
Very similar to that of a
Power MOSFET
Major difference is, it
has a tailing collector
current due to the
stored charge in the Ndrift region
The tail current
increases the turnoff
loss (Eoff) and requires
an increase in the dead
time between the
conduction of two
devices in a half-bridge
circuit
The figure shows a test circuit for switching
characteristics.
Ref. 4
36
Switching Characteristics
•
•
•
The turn-off speed of an
IGBT is limited by the
lifetime of the stored
charge or minority
carriers in the N- drift
region which is the base
of the parasitic PNP
transistor
The only way the stored
charge can be removed is
by recombination within
the IGBT
Traditional lifetime killing
techniques or an N+
buffer layer to collect the
minority charges at turnoff are commonly used to
speed-up recombination
time.
Ref. 4
The Figure shows the current and
voltage turn-on and turn-off waveforms.
37
Switching Characteristics
• The turn-on energy Eon is defined as the integral of IC .VCE
within the limit of 10% ICE rise to 90% VCE fall. The amount of
turn on energy depends on the reverse recovery behaviour of
the free wheeling diode, so special attention must be paid if
there is a free wheeling diode within the package of the IGBT
(Co-Pack).
• The turn-off energy Eoff is defined as the integral of IC .VCE
within the limit of 10% VCE rise to 90% IC fall. Eoff plays the
major part of total switching losses in IGBT.
38
Latch-up
•
•
•
Forward bias of the N+ P junction
and if it is large enough,
substantial injection of electrons
from the emitter into the body
region will occur and the parasitic
NPN transistor will be turned-on.
If this happens, both NPN and
PNP parasitic transistors will be
turned-on and hence the
thyristor composed of these two
transistors will latch on and the
latch-up condition of IGBT will
have occurred.
Once in latch-up, the gate has no
control on the collector current
and the only way to turn-off the
IGBT is by forced commutation of
the current, exactly the same as
for a conventional thyristor.
Ref. 4
The figure shows the paths for current flow in
an IGBT in on-state.
39
IGBTs - Protection
Over-voltages:
• During the blocking state, any applied voltage in excess of the breakdown voltage
between collector and emitter terminals causes avalanche breakdown
• Attach voltage arrestor in parallel to the IGBT
• If the reverse blocking voltage of IGBT is small, a reverse connected diode placed
across the IGBT is helpful
Over-currents:
• Over-current is the current value at which junction temperature becomes more
than 150°C
• When an over-current is sensed, the gate voltage is decreased by switching a zener
diode across the gate terminal within a short time interval of 1μsec
Transients:
Ramp waveform or a two step-waveform replaces the conventional stepped voltage
waveform
40
IGBTs - Technologies
• 1. Resurf Devices: In these, crucial changes takes
place in the electric field distribution near the
surface
– Reduced surface field
– Enhanced current carrying capability of IGBTs
– Resurfed Quasi-lateral IGBTs have a compact size
• 2. Trench Devices: High blocking capability, higher
forward conduction current compared to lateral
and single gate structure
– Deep dry etching (for dielectric isolation)
– Dry etching (high grade of anisotropy)
41
Application domains
• IGBT is suitable in many applications of power electronics,
especially in pulse width modulation (PWM) servo and
three-phase drives requiring high dynamic range control
and low-noise
• It can also be used in uninterruptable power supplies(UPS),
switched-mode power supplies(SMPS) and other power
circuits which require high switch repetition rates
• IGBTs improves dynamic performance, efficiency and
reduces level of audible noise; it is equally suitable for
resonant-mode converter circuits
Optimized IGBTs are available for both low-conduction loss
and low- switching loss
42
CoolMOS
Ref. 4
•
•
•
Recently a new technology for high voltage Power MOSFETs has been introduced CoolMOS
CoolMOS virtually combines the low switching losses of a MOSFET with the on-state losses of an
IGBT. The drastically lower gate charge facilitates and reduces the cost of controllability, the smaller
feedback capacitance reduces the dynamic losses.
Also very less RDS_on state (its dependence on breakdown voltage is made linear)
43
Static Induction Transistors (SITs)
Ref. 4
 Vertical structure with short multi-channels
 Identical to JFET except for vertical and buried gate
 Short channel length –low gate resistance, low gate-source capacitance, small
thermal resistance
44
SIT Output Characteristics
Ref. 4
Its typically an ON device and a negative gate voltage holds it off
45
SITs: Characteristics
• High power, high frequency device
• Turn-on and turn-off times are very small,
typically 0.25 μ secs
• On-state drop as high as 90 V for 180 A device &
18 V for 18 A
• Work on power with 100KVA at 100 kHz or 10 VA
at 10GHz
• Most suitable for high power, high frequency,
applications (audio, microwave amplifiers)
46
Comparison of the different types of
Power Transistors
47
Trade-offs
Ref. 5
48
Comparisons
Switch
type
Base/
Gate
Control
Variable
Switching
Frequency
Onstate
Voltage
drop
Max. voltage
rating, Vs
Max. current
rating, Is
Advantages
Limitations
BJT
Current
Medium
Low
1.5 kV
1 kA
Simple, low-on
state voltage drop
Needs high base
current to turn-on &
sustain on-state
current.
Base drive power
loss.
High switching losses.
MOSFET
Voltage
Very High
High
1 kV
150 A
Higher switching
speed.
Low switching loss.
Facilitates parallel
operation.
Low gate loss.
High on-state
voltage drop, as
high as 10 V.
Lower off-state
voltage capability.
IGBT
Voltage
High
Mediu
m
3.5 kV
2 kA
Low on-state
voltage.
Little gate power.
Lower off-state
voltage capability.
CoolMOS
Voltage
Very High
Low
1 kV
100 A
Low gate drive
requirement;
Low on-state
voltage drop.
Low V & I
ratings
49
References
1) ‘Power Electronics–Circuits, devices, and applications’
, Third Edition 2007, by Muhammad H. Rashid
2) IGBT –Theory and Design, by Vinod Kumar Khanna,
IEEE Press, 2003
3) ‘COOLMOSTM-a new milestone in high voltage power
MOS’ –Paper presented in ‘Power Semiconductor
Devices and ICs, 1999. ISPSD '99. Proceedings’, The
11th International Symposium, publication date: 1999
4) www.google.com
5) www.wikipedia.org
6) ‘Solid State Electronic Devices’, Sixth Edition 2007, by
Ben G. Streetman & Sanjay Kumar Banerjee
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Ref. 5
51