1MOTOROLA
C0065C0078C0049C0053C0052C0049
C0073C0110C0116C0114C0111C0100C0117C0099C0116C0105C0111C0110 C0116C0111 C0073C0110C0115C0117C0108C0097C0116C0101C0100 C0071C0097C0116C0101 C0066C0105C0112C0111C0108C0097C0114 C0084C0114C0097C0110C0115C0105C0115C0116C0111C0114C0115
Prepared by,Jack Takesuye and Scott Deuty
Motorola Inc.
INTRODUCTION
As power conversion relies more on switched applications,
semiconductor manufacturers need to create products that
approach the ideal switch,The ideal switch would have:
1) zero resistance or forward voltage drop in the on–state,
2) infinite resistance in the off–state,3) switch with infinite
speed,and 4) would not require any input power to make it
switch.
When using existing solid–state switch technologies,the
designer must deviate from the ideal switch and choose a
device that best suits the application with a minimal loss of
efficiency,The choice involves considerations such as
voltage,current,switching speed,drive circuitry,load,and
temperature effects,There are a variety of solid state switch
technologies available to perform switching functions;
however,all have strong and weak points.
HIGH VOLTAGE POWER MOSFETs
The primary characteristics that are most desirable in a
solid–state switch are fast switching speed,simple drive
requirements and low conduction loss,For low voltage
applications,power MOSFETs offer extremely low
on–resistance,R
DS(on)
,and approach the desired ideal
switch,In high voltage applications,MOSFETs exhibit
increased R
DS(on)
resulting in lower efficiency due to
increased conduction losses,In a power MOSFET,the
on–resistance is proportional to the breakdown voltage raised
to approximately the 2.7 power (1).
MOSFET technology has advanced to a point where cell
densities are limited by manufacturing equipment capabilities
and geometries have been optimized to a point where the
R
DS(on)
is near the predicted theoretical limit,Since the cell
density,geometry and the resistivity of the device structure
play a major role,no significant reduction in the R
DS(on)
is
foreseen,New technologies are needed to circumvent the
problem of increased on–resistance without sacrificing
switching speed.
R
DS(on)
C0084 V
2.7
DSS
(1)
ENTER THE IGBT
By combining the low conduction loss of a BJT with the
switching speed of a power MOSFET an optimal solid state
switch would exist,The Insulated–Gate Bipolar Transistor
(IGBT) technology offers a combination of these attributes.
The IGBT is,in fact,a spin–off from power MOSFET
technology and the structure of an IGBT closely resembles
that of a power MOSFET,The IGBT has high input impedance
and fast turn–on speed like a MOSFET,IGBTs exhibit an
on–voltage and current density comparable to a bipolar
transistor while switching much faster,IGBTs are replacing
MOSFETs in high voltage applications where conduction
losses must be kept low,With zero current switching or
resonant switching techniques,the IGBT can be operated in
the hundreds of kilohertz range [1].
Although turn–on speeds are very fast,turn–off of the IGBT
is slower than a MOSFET,The IGBT exhibits a current fall time
or,tailing.” The tailing restricts the devices to operating at
moderate frequencies (less than 50 kHz) in traditional,square
waveform” PWM,switching applications.
At operating frequencies between 1 and 50 kHz,IGBTs offer
an attractive solution over the traditional bipolar transistors,
MOSFETs and thyristors,Compared to thyristors,the IGBT is
faster,has better dv/dt immunity and,above all,has better gate
turn–off capability,While some thyristors such as GTOs are
capable of being turned off at the gate,substantial reverse
gate current is required,whereas turning off an IGBT only
requires that the gate capacitance be discharged,A thyristor
has a slightly lower forward–on voltage and higher surge
capability than an IGBT.
MOSFETs are often used because of their simple gate drive
requirements,Since the structure of both devices are so
similar,the change to IGBTs can be made without having to
redesign the gate drive circuit,IGBTs,like MOSFETs,are
transconductance devices and can remain fully on by keeping
the gate voltage above a certain threshold.
As shown in Figure 1a,using an IGBT in place of a power
MOSFET dramatically reduces the forward voltage drop at
current levels above 12 amps,By reducing the forward drop,
the conduction loss of the device is decreased,The gradual
rising slope of the MOSFET in Figure 1a can be attributed to
the relationship of V
DS
to R
DS(on)
,The IGBT curve has an
offset due to an internal forward biased p–n junction and a fast
rising slope typical of a minority carrier device.
It is possible to replace the MOSFET with an IGBT and
improve the efficiency and/or reduce the cost,As shown in
Figure 1b,an IGBT has considerably less silicon area than a
similarly rated MOSFET,Device cost is related to silicon area;
therefore,the reduced silicon area makes the IGBT the lower
cost solution,Figure 1c shows the resulting package area
reduction realized by using the IGBT,The IGBT is more space
efficient than an equivalently rated MOSFET which makes it
perfect for space conscious designs.
Order this document
by AN1541/D
C0077C0079C0084C0079C0082C0079C0076C0065
SEMICONDUCTOR APPLICATION NOTE
Motorola,Inc,1995
C0065C0078C0049C0053C0052C0049
2 MOTOROLA
FORWARD DROP (VOLTS)
Figure 1a,Reduced Forward Voltage Drop of
IGBT Realized When Compared to a MOSFET
with Similar Ratings
40
35
30
25
20
15
10
5
0
1086420
PEAK CURRENT
THROUGH DEVICE (AMPS)
V
CE(sat)
MGW20N60D IGBT
V
DS
MTW20N50E
MOSFET
When compared to BJTs,IGBTs have similar ratings in
terms of voltage and current,However,the presence of an
isolated gate in an IGBT makes it simpler to drive than a BJT.
BJTs require that base current be continuously supplied in a
quantity sufficient enough to maintain saturation,Base
currents of one–tenth of the collector current are typical to
keep a BJT in saturation,BJT drive circuits must be sensitive
to variable load conditions,The base current of a BJT must
be kept proportional to the collector current to prevent
desaturation under high–current loads and excessive base
drive under low–load conditions,This additional base current
increases the power dissipation of the drive circuit,BJTs are
minority carrier devices and charge storage effects including
recombination slow the performance when compared to
majority carrier devices such as MOSFETs,IGBTs also
experience recombination that accounts for the current
“tailing” yet IGBTs have been observed to switch faster than
BJTs.
Thus far,the IGBT has demonstrated certain advantages
over power MOSFETs with the exception of switching speed.
Since the initial introduction of IGBTs in the early 1980s,
semiconductor manufacturers have learned how to make the
devices faster,As illustrated in Figure 2,some trade–offs in
conduction loss versus switching speed exist,Lower
frequency applications can tolerate slower switching devices.
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
Figure 1b,Reduced Die Size of IGBT Realized
When Compared to a MOSFET with Similar Ratings
0.10
0.05
0
1
AREA
(SQ,INCHES)
IGBT DIE SIZE
(0.17 X 0.227)
MOSFET DIE SIZE
(0.35 X 0.26)
é
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
Figure 1c,Reduced Package Size of IGBT Realized
When Compared to a MOSFET with Similar Ratings
0.60
0.20
0
1
AREA
(SQ,INCHES)
IGBT PACKAGE
SIZE (TO–220)
MOSFET PACKAGE
SIZE (T0–247)
é
0.40
Because the loss period is a small percentage of the total on
time,slower switching is traded for lower conduction loss,In
a higher frequency application,just the opposite would be true
and the device would be made faster and have greater
conduction losses,Notice that the curves in Figure 2 show
reductions in both the forward drop (V
CE(sat)
) and the fall time,
t
f
of newer generation devices,These capabilities make the
IGBT the device of choice for applications such as motor
drives,power supplies and inverters that require devices rated
for 600 to 1200 volts.
Figure 2,Advanced Features Offered by the Latest Motorola IGBT
Technologies for Forward Voltage Drop (V
CE(sat)
) and Fall Time (t
f
)
t
f
(μs)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1.00.80.60.40.20 0.90.70.50.3
1ST GENERATION COMPETITOR 1985
2ND GENERATION COMPETITOR 1989
1ST GENERATION MOTOROLA 1993
3RD GENERATION COMPETITOR 1993
2ND GENERATION MOTOROLA
DEMONSTRATED
V
CE(sat)
(VOL
TS)
HIGH SPEED
SERIES
LOW SATURATION
SERIES
0.1
C0065C0078C0049C0053C0052C0049
3MOTOROLA
CHARACTERISTICS OF IGBTs,
DEVICE STRUCTURE
The structure of an IGBT is similar to that of a double
diffused (DMOS) power MOSFET,One difference between a
MOSFET and an IGBT is the substrate of the starting material.
By varying the starting material and altering certain process
steps,an IGBT may be produced from a power MOSFET
mask; however,at Motorola mask sets are designed
specifically for IGBTs,In a MOSFET the substrate is N+ as
shown in Figure 3b,The substrate for an IGBT is P
+
as shown
in Figure 3a.
Figure 3a,Cross Section and Equivalent Schematic
of an Insulated Gate Bipolar Transistor (IGBT) Cell
POLYSILICON GATE
N+
P+
N– EPI
N+ BUFFER
P+ SUBSTRATE
P–
R
mod
EMITTER
N+
P+
P–
COLLECTOR
GATE
NPN
MOSFET
PNP
KEY
METAL
SiO
2
R
shorting
Figure 3b,Cross Section and Equivalent
Schematic of an Metal–Oxide–Semiconductor
Field–Effect Transistor (MOSFET) Cell
JFET
channel
Drain–to–Source Body Diode
(Created when NPN
base–emitter is properly
shorted by source metal)
POLYSILICON GATE
N+
P+
N– EPI
N+ SUBSTRATE
SOURCE
N+
P+
DRAIN
NPN
KEY
METAL
SiO
2
GATE
P–
The n– epi resistivity determines the breakdown voltage of
a MOSFET as mentioned earlier using relationship (1).
R
DS(on)
C0084 V
2.7
DSS
(1)
To increase the breakdown voltage of the MOSFET,the
n– epi region thickness (vertical direction in figure) is
increased,As depicted in the classical resistance relationship
(2),reducing the R
DS(on)
of a high voltage device requires
greater silicon area A to make up for the increased n– epi
region.
R C0084
1
A
(2)
Device designers were challenged to overcome the effects
of the high resistive n– epi region,The solution to this came in
the form of conductivity modulation,The n– epi region to this
was placed on the P
+
substrate forming a p–n junction where
conductivity modulation takes place,Because of conductivity
modulation,the IGBT has a much greater current density than
a power MOSFET and the forward voltage drop is reduced.
Now the P
+
substrate,n– epi layer and P
+
,emitter” form a BJT
transistor and the n– epi acts as a wide base region.
The subject of current tailing has been mentioned several
times,Thus far,the device structure as shown in Figure 3
provides insight as to what causes the tailing,Minority carriers
build up to form the basis for conductivity modulation,When
the device turns off,these carriers do not have a current path
to exit the device,Recombination is the only way to eliminate
the stored charge resulting from the build–up of excess
carriers,Additional recombination centers are formed by
placing an N
+
buffer layer between the n– epi and P
+
substrate.
While the N
+
buffer layer may speed up the recombination,
it also increases the forward drop of the device,Hence the
tradeoff between switching speed and conduction loss
becomes a factor in optimizing device performance.
Additional benefits of the N
+
buffer layer include preventing
thermal runaway and punch–through of the depletion region.
This allows a thinner n– epi to be used which somewhat
decreases forward voltage drop.
Figure 4b,MOSFET Schematic Symbol
Figure 4a,IGBT Schematic Symbol
COLLECTOR
EMITTER
GATE
GATE
SOURCE
DRAIN
C0065C0078C0049C0053C0052C0049
4 MOTOROLA
The IGBT has a four layer (P–N–P–N) structure,This
structure resembles that of a thyristor device known as a
Silicon Controlled Rectifier (SCR),Unlike the SCR where the
device latches and gate control is lost,an IGBT is designed
so that it does not latch on,Full control of the device can be
maintained through the gate drive.
To maximize the performance of the IGBT,process steps
are optimized to control the geometry,doping and lifetime.
The possibility of latching is also reduced by strategic
processing of the device,Geometry and doping levels are
optimized to minimize the on–voltage,switching speed and
achieve other key parametric variations,Because the IGBT is
a four–layer structure,it does not have the inverse parallel
diode inherent to power MOSFETs,This is a disadvantage to
motor control designers who use the anti–parallel diode to
recover energy from the motor.
Like a power MOSFET,the gate of the IGBT is electrically
isolated from the rest of the chip by a thin layer of silicon
dioxide,SiO
2
,The IGBT has a high input impedance due to
the isolated gate and it exhibits the accompanying
advantages of modest gate drive requirements and excellent
gate drive efficiency.
Equivalent Circuit of IGBT
Figure 4b shows the terminals of the IGBT as determined by
JEDEC,Notice that the IGBT has a gate like a MOSFET yet
it has an emitter and a collector like a BJT.
The operation of the IGBT is best understood by again
referring to the cross section of the device and its equivalent
circuit as shown in Figure 3a,Current flowing from collector to
emitter must pass through a p–n junction formed by the P
+
substrate and n– epi layer,This drop is similar to that seen in
a forward biased p–n junction diode and results in an offset
voltage in the output characteristic,Current flow contributions
are shown in Figure 3a using varying line thickness with the
thicker lines indicating a high current path,For a fast device,
the N+ buffer layer is highly doped for recombination and
speedy turn off,The additional doping keeps the gain of the
PNP low and allows two–thirds of the current to flow through
the base of the PNP (electron current) while one–third passes
through the collector (hole current).
R
shorting
is the parasitic resistance of the P
+
emitter region.
Current flowing through R
shorting
can result in a voltage
across the base–emitter junction of the NPN,If the
base–emitter voltage is above a certain threshold level,the
NPN will begin to conduct causing the NPN and PNP to
enhance each other’s current flow and both devices can
become saturated,This results in the device latching in a
fashion similar to an SCR,Device processing directs currents
within the device and keeps the voltage across R
shorting
low to
avoid latching,The IGBT can be gated off unlike the SCR
which has to wait for the current to cease allowing
recombination to take place in order to turn off,IGBTs offer an
advantage over the SCR by controlling the current with the
device,not the device with the current,The internal MOSFET
of the IGBT when gated off will stop current flow and at that
point,the stored charges can only be dissipated through
recombination.
The IGBT’s on–voltage is represented by sum of the offset
voltage of the collector to base junction of the PNP transistor,
the voltage drop across the modulated resistance R
mod
and
the channel resistance of the internal MOSFET,Unlike the
MOSFET where increased temperature results in increased
R
DS(on)
and increased forward voltage drop,the forward
drop of an IGBT stays relatively unchanged at increased
temperatures.
Switching Speed
Until recently,the feature that limited the IGBT from
serving a wide variety of applications was its relatively slow
turn–off speed when compared to a power MOSFET,While
turn–on is fairly rapid,initial IGBTs had current fall times of
around three microseconds.
The turn–off time of an IGBT is slow because many minority
carriers are stored in the n– epi region,When the gate is
initially brought below the threshold voltage,the n– epi
contains a very large concentration of electrons and there will
be significant injection into the P
+
substrate and a
corresponding hole injection into the n– epi,As the electron
concentration in the n–region decreases,the electron
injection decreases,leaving the rest of the electrons to
recombine,Therefore,the turn–off of an IGBT has two
phases,an injection phase where the collector current falls
very quickly,and a recombination phase in which the collector
current decrease more slowly,Figure 5 shows the switching
waveform and the tail time contributing factors of a,fast” IGBT
designed for PWM motor control service.
éééé
éééé
éééé
éééé
éééé
Figure 5,IGBT Current Turn–off Waveform
6
5
4
3
2
1
0
–1
10008006004002000
I
C
(AMPS)
TAIL TIME of MOTOROLA GEN,2 IGBT #2 in
1.0 hp MOTOR DRIVE at 1750 RPM
PNP TURN–OFF
PORTION
TAIL TIME
MOSFET TURN–OFF
PORTION
In power MOSFETs,the switching speed can be greatly
affected by the impedance in the gate drive circuit,Efforts to
minimize gate drive impedance for IGBTs are also
recommended,Also,choose an optimal device based on
switching speed or use a slower device with lower forward
drop and employ external circuitry to enhance turn off,A
turn–off mechanism is suggested in a paper by Baliga et al [2].
C0065C0078C0049C0053C0052C0049
5MOTOROLA
A FINAL COMPARISON OF IGBTs,BJTs AND
POWER MOSFETs
The conduction losses of BJTs and IGBTs is related to the
forward voltage drop of the device while MOSFETs determine
conduction loss based on R
DS(on)
,To get a relative
comparison of turn–off time and conduction associated
losses,data is presented in Table 1 where the on–resistances
of a power MOSFET,an IGBT and a BJT at junction
temperatures of 25°C and 150°C are shown.
Note that the devices in Table 1 have approximately the
same ratings,However,to achieve these ratings the chip size
of the devices vary significantly,The bipolar transistor requires
1.2 times more silicon area than the IGBT and the MOSFET
requires 2.2 times the area of the IGBT to achieve the same
ratings,This differences in die area directly impacts the cost
of the product,At higher currents and at elevated
temperatures,the IGBT offers low forward drop and a
switching time similar to the BJT without the drive difficulties.
Table 1 confirms the findings offered earlier in Figure 1a and
elaborates further to include a BJT comparison and
temperature effects,The reduced power conduction losses
offered by the IGBT lower power dissipation and heat sink
size.
Thermal Resistance
An IGBT and power MOSFET produced from the same size
die have similar junction–to–case thermal resistance because
of their similar structures,The thermal resistance of a power
MOSFET can be determined by testing for variations in
temperature sensitive parameters (TSPs),These parameters
are the source–to–drain diode on–voltage,the
gate–to–source threshold voltage,and the drain–to–source
on–resistance,All previous measurements of thermal
resistance of power MOSFETs at Motorola were performed
using the source–to–drain diode as the TSP,Since an IGBT
does not have an inverse parallel diode,another TSP had to
be used to determine the thermal resistance,The
gate–to–emitter threshold voltage was used as the TSP to
measure the junction temperature of an IGBT to determine its
thermal resistance,However before testing IGBTs,a
correlation between the two test methods was established by
comparing the test results of MOSFETs using both TSPs,By
testing for variations in threshold voltage,it was determined
that the thermal resistance of MOSFETs and IGBTs are
essentially the same for devices with equivalent die size,
Short Circuit Rated Devices
Using IGBTs in motor control environments requires the
device to withstand short circuit current for a given period.
Although this period varies with the application,a typical
value of ten microseconds is used for designing these
specialized IGBT’s,Notice that this is only a typical value and
it is suggested that the reader confirm the value given on the
data sheet,IGBTs can be made to withstand short circuit
conditions by altering the device structure to include an
additional resistance (R
e
,in Figure 6) in the main current path.
The benefits associated with the additional series resistance
are twofold.
Figure 6,Cross Section and Equivalent Schematic
of a Short Circuit Rated Insulated Gate
Bipolar Transistor Cell
POLYSILICON GATE
N+
P+
N– EPI
N+ BUFFER
P+ SUBSTRATE
P–
R
mod
EMITTER
N+
P+
P–
COLLECTOR
GATE
NPN
MOSFET
PNP
KEY
METAL
SiO
2
R
shorting
R
e
First,the voltage created across R
e
,by the large current
passing through R
e
,increases the percentage of the gate
voltage across R
e
,by the classic voltage divider equation.
Assuming the drive voltage applied to the gate–to–emitter
remains the same,the voltage actually applied across the
gate–to–source portion of the device is now lower,and the
device is operating in an area of the transconductance curve
that reduces the gain and it will pass less current.
Table 1,Advantages Offered by the IGBT When Comparing the MOSFET,IGBT and Bipolar Transistor On–Resistances
(Over Junction Temperature) and Fall Times (Resistance Values at 10 Amps of Current)
ááááááááááááááá
ááááááááááááááá
Characteristic
áááááááá
áááááááá
TMOS
ááááááá
ááááááá
IGBT
ááááááá
ááááááá
Bipolar
ááááááááááááááá
Current Rating
áááááááá
20 A
ááááááá
20 A
ááááááá
20 A
ááááááááááááááá
Voltage Rating
ááááááá
500 V
áááááá
600 V
áááááá
500 V*
ááááááááááááááá
ááááááááááááááá
ááááááááááááááá
R
(on)
@ T
J
= 25°C
áááááááá
ááááááá
áááááááá
0.2?
ááááááá
áááááá
ááááááá
0.24?
ááááááá
áááááá
ááááááá
0.18?
ááááááááááááááá
ááááááááááááááá
ááááááááááááááá
R
(on)
@ T
J
= 150°C
áááááááá
ááááááá
áááááááá
0.6?
ááááááá
áááááá
ááááááá
0.23?
ááááááá
áááááá
ááááááá
0.24?**
ááááááááááááááá
Fall Time (Typical)
áááááááá
40 ns
ááááááá
200 ns
ááááááá
200 ns
* Indicates V
CEO
Rating
** BJT T
J
= 100°C
C0065C0078C0049C0053C0052C0049
6 MOTOROLA
Second,the voltage developed across R
e
results in a
similar division of voltage across R
shorting
and V
BE
of the
NPN transistor,The NPN will be less likely to attain a V
BE
high enough to turn the device on and cause a latch–up
situation.
The two situations described work together to protect the
device from catastrophic failure,The protection period is
specified with the device ratings,allowing circuit designers
the time needed to detect a fault and shut off the device.
The introduction of the series resistance R
e
also results in
additional power loss in the device by slightly elevating the
forward drop of the device,However,the magnitude of short
circuit current is large enough to require a very low R
e
value.
The additional conduction loss of the device due to the
presence of R
e
is not excessive when comparing a short
circuit rated IGBT to a non–short circuit rated device.
Anti–Parallel Diode
When using IGBT’s for motor control,designers have to
place a diode in anti–parallel across the device in order to
handle the regenerative or inductive currents of the motor,As
discussed earlier,due to structural differences the IGBT does
not have a parasitic diode like that found in a MOSFET.
Designers found that the diode within the MOSFET was,in
fact,a parasitic,i.e.,not optimized in the design process,and
its performance was poor for use as a current recovery device
due to slow switching speed,To overcome the lack of
performance,an optimized anti–parallel diode was used
across the MOSFET source–to–drain,Placing a packaged
diode external to the MOSFET itself created performance
problems due to the switching delays resulting from the
parasitics introduced by the packages,The optimal setup is to
have the diode copackaged with the device,A specific line of
IGBTs has been created by Motorola to address this issue.
These devices work very well in applications where energy is
recovered to the source and are favored by motor control
designers.
Like the switching device itself,the anti–parallel diode
should exhibit low leakage current,low forward voltage drop
and fast switching speed,As shown in Figure 7,the diode
forward drop multiplied by the average current it passes is the
total conduction loss produced,In addition,large reverse
recovery currents can escalate switching losses,A detailed
explanation of reverse recovery can be found in the
Appendix,A secondary effect caused by large reverse
recovery currents is generated EMI at both the switching
frequency and the frequency of the resulting ringing
waveform,This EMI requires additional filtering to be
designed into the circuit,By copackaging parts,the parasitic
inductances that contribute to the ringing are greatly reduced.
Also,copackaged products can be used in designs to reduce
power dissipation and increase design efficiency.
Figure 7,Waveforms Associated with
Anti–Parallel Diode Turn–off
I
RM(rec)
TIME
TIME
TIME
POWER
VOLTAGE
CURRENT
I
IGBT
I
DIODE
V
f
APPLICATION OF IGBTs,
PULSE WIDTH MODULATED INDUCTION
MOTOR DRIVE APPLICATION
Line–operated,pulse–width modulated,variable–speed
motor drives are an application well suited for IGBTs,In this
application,as shown in Figure 8,IGBTs are used as the
power switch to PWM the voltage supplied to a motor to
control its speed.
Depending on the application,the IGBT may be required to
operate from a full–wave rectified line,This can require
devices to have six hundred volt ratings for 230 VAC line
voltage inputs,and twelve hundred volt ratings for 575 VAC
volt line inputs,IGBTs that block high voltage offer fast
switching and low conduction losses,and allow for the design
of efficient,high frequency drives of this type,Devices used in
motor drive applications must be robust and capable of
withstanding faults long enough for a protection scheme to be
activated,Short circuit rated devices offer safe,reliable motor
drive operation.
CONCLUSION
The IGBT is a one of several options for designers to choose
from for power control in switching applications,The features
of the IGBT such as high voltage capability,low on–resistance,
ease of drive and relatively fast switching speeds makes it a
technology of choice for moderate speed,high voltage
applications,New generations of devices will reduce the
on–resistance,increase speed and include levels of
integration that simplify protection schemes and device drive
requirements,The reliability and performance advantages of
IGBTs are value added traits that offer circuit designers energy
efficient options at reduced costs.
C0065C0078C0049C0053C0052C0049
7MOTOROLA
IGBT
1/2 BRIDGE
IGBT
1/2 BRIDGE
Figure 8,Typical Pulse–Width,Modulated,Variable–Speed Induction Motor Drives
Are Where IGBT’s Offer Performance Advantages
C0043
230 VAC
TEMPERATURE
CONTROL
SYSTEM
I/O
LON?
CONTROL IC MCU
OR ASIC
MIXED MODE IC
CUSTOM LINEAR
OR
STANDARD CELL
GATE DRIVE HVIC
OR
OPTO & LVIC
DIODE BRIDGE
FILTER
CAPACITOR
INDUCTION
MOTOR
IGBT
1/2 BRIDGE
PHASE CURRENTS AND VOLTAGES
ACKNOWLEDGEMENTS
The writing of this document was assisted by a number of
internal device designers,Their assistance was greatly
appreciated by the authors,Bill Fragale,Steve Robb and
Vasudev Venkatesan provided device operation insight and
reference materials,Graphic material was provided by Basam
Almesfer and Steve Robb,Finally,C,S,Mitter assisted with
editing and accuracy of the material.
REFERENCES
[1] D,Y,Chen,J,Yang,and J,Lee,Application of the
IGT/COMFET to Zero–Current Switching Resonant
Converters,” PESC,1987.
[2] B,J,Baliga,“Analysis of Insulated Gate Transistor Turn–off
Characteristics,” IEEE Electron Device Lett,EDL–6,(1985),
pp,74–77,
[3] B,J,Baliga,“Switching Speed Enhancement in Insulated
Gate Transistors by Electron Irradiation,” IEEE Transactions
on Electron Devices,ED–31,(1984),pp,1790–1795.
C0065C0078C0049C0053C0052C0049
8 MOTOROLA
APPENDIX
Diode Reverse Recovery Analysis [4]
Figure A–1,Reverse Recovery Waveform
total reverse recovery time
fall time due to stored minority charge
application and device dependent
peak reverse recovery current
ééé
ééé
ééé
ééé
ééé
t
b
t
a
t
rr
I
RM(rec)
I
F
di/dt
Q
a
Q
b
t
rr
=
t
a
=
t
b
=
I
RM(rec)
=
A typical reverse recovery waveform is shown in
Figure A–1,The reverse recovery time t
rr
has been
traditionally defined as the time from diode current
zero–crossing to where the current returns to within 10% of the
peak recovery current I
RM(rec)
,This does not give enough
information to fully characterize the waveform shape,A better
way to characterize the rectifier reverse recovery is to partition
the reverse recovery time into two different regions,t
a
and t
b
,
as shown in Figure A–1,The t
a
time is a function of the forward
current and the applied di/dt,A charge can be assigned to this
region denoted Q
a
,the area under the curve,The t
b
portion of
the reverse recovery current is not very well understood.
Measured t
b
times vary greatly with the switch characteristic,
circuit parasitics,load inductance and the applied reverse
voltage,A relative softness can be defined as the ratio of t
b
to
t
a
,General purpose rectifiers are very soft (softness factor of
about 1.0),fast recovery diodes are fairly soft (softness factor
of about 0.5) and ultrafast rectifiers are very abrupt (softness
factor of about 0.2).
[4] Source:,Motor Controls,” TMOS Power MOSFET
Transistor Data,Q4/92,DL135,Rev 4,(Phoenix,Motorola,
Inc.,1992),pp,2–9–22 to 2–9–23.
Motorola reserves the right to make changes without further notice to any products herein,Motorola makes no warranty,representation or guarantee regarding
the suitability of its products for any particular purpose,nor does Motorola assume any liability arising out of the application or use of any product or circuit,and
specifically disclaims any and all liability,including without limitation consequential or incidental damages.,Typical” parameters can and do vary in different
applications,All operating parameters,including,Typicals” must be validated for each customer application by customer’s technical experts,Motorola does
not convey any license under its patent rights nor the rights of others,Motorola products are not designed,intended,or authorized for use as components in
systems intended for surgical implant into the body,or other applications intended to support or sustain life,or for any other application in which the failure of
the Motorola product could create a situation where personal injury or death may occur,Should Buyer purchase or use Motorola products for any such
unintended or unauthorized application,Buyer shall indemnify and hold Motorola and its officers,employees,subsidiaries,affiliates,and distributors harmless
against all claims,costs,damages,and expenses,and reasonable attorney fees arising out of,directly or indirectly,any claim of personal injury or death
associated with such unintended or unauthorized use,even if such claim alleges that Motorola was negligent regarding the design or manufacture of the part.
Motorola and are registered trademarks of Motorola,Inc,Motorola,Inc,is an Equal Opportunity/Affirmative Action Employer.
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AN1541/D
C0042C0065C0078C0049C0053C0052C0049C0047C0068C0042
C0065C0078C0049C0053C0052C0049
C0073C0110C0116C0114C0111C0100C0117C0099C0116C0105C0111C0110 C0116C0111 C0073C0110C0115C0117C0108C0097C0116C0101C0100 C0071C0097C0116C0101 C0066C0105C0112C0111C0108C0097C0114 C0084C0114C0097C0110C0115C0105C0115C0116C0111C0114C0115
Prepared by,Jack Takesuye and Scott Deuty
Motorola Inc.
INTRODUCTION
As power conversion relies more on switched applications,
semiconductor manufacturers need to create products that
approach the ideal switch,The ideal switch would have:
1) zero resistance or forward voltage drop in the on–state,
2) infinite resistance in the off–state,3) switch with infinite
speed,and 4) would not require any input power to make it
switch.
When using existing solid–state switch technologies,the
designer must deviate from the ideal switch and choose a
device that best suits the application with a minimal loss of
efficiency,The choice involves considerations such as
voltage,current,switching speed,drive circuitry,load,and
temperature effects,There are a variety of solid state switch
technologies available to perform switching functions;
however,all have strong and weak points.
HIGH VOLTAGE POWER MOSFETs
The primary characteristics that are most desirable in a
solid–state switch are fast switching speed,simple drive
requirements and low conduction loss,For low voltage
applications,power MOSFETs offer extremely low
on–resistance,R
DS(on)
,and approach the desired ideal
switch,In high voltage applications,MOSFETs exhibit
increased R
DS(on)
resulting in lower efficiency due to
increased conduction losses,In a power MOSFET,the
on–resistance is proportional to the breakdown voltage raised
to approximately the 2.7 power (1).
MOSFET technology has advanced to a point where cell
densities are limited by manufacturing equipment capabilities
and geometries have been optimized to a point where the
R
DS(on)
is near the predicted theoretical limit,Since the cell
density,geometry and the resistivity of the device structure
play a major role,no significant reduction in the R
DS(on)
is
foreseen,New technologies are needed to circumvent the
problem of increased on–resistance without sacrificing
switching speed.
R
DS(on)
C0084 V
2.7
DSS
(1)
ENTER THE IGBT
By combining the low conduction loss of a BJT with the
switching speed of a power MOSFET an optimal solid state
switch would exist,The Insulated–Gate Bipolar Transistor
(IGBT) technology offers a combination of these attributes.
The IGBT is,in fact,a spin–off from power MOSFET
technology and the structure of an IGBT closely resembles
that of a power MOSFET,The IGBT has high input impedance
and fast turn–on speed like a MOSFET,IGBTs exhibit an
on–voltage and current density comparable to a bipolar
transistor while switching much faster,IGBTs are replacing
MOSFETs in high voltage applications where conduction
losses must be kept low,With zero current switching or
resonant switching techniques,the IGBT can be operated in
the hundreds of kilohertz range [1].
Although turn–on speeds are very fast,turn–off of the IGBT
is slower than a MOSFET,The IGBT exhibits a current fall time
or,tailing.” The tailing restricts the devices to operating at
moderate frequencies (less than 50 kHz) in traditional,square
waveform” PWM,switching applications.
At operating frequencies between 1 and 50 kHz,IGBTs offer
an attractive solution over the traditional bipolar transistors,
MOSFETs and thyristors,Compared to thyristors,the IGBT is
faster,has better dv/dt immunity and,above all,has better gate
turn–off capability,While some thyristors such as GTOs are
capable of being turned off at the gate,substantial reverse
gate current is required,whereas turning off an IGBT only
requires that the gate capacitance be discharged,A thyristor
has a slightly lower forward–on voltage and higher surge
capability than an IGBT.
MOSFETs are often used because of their simple gate drive
requirements,Since the structure of both devices are so
similar,the change to IGBTs can be made without having to
redesign the gate drive circuit,IGBTs,like MOSFETs,are
transconductance devices and can remain fully on by keeping
the gate voltage above a certain threshold.
As shown in Figure 1a,using an IGBT in place of a power
MOSFET dramatically reduces the forward voltage drop at
current levels above 12 amps,By reducing the forward drop,
the conduction loss of the device is decreased,The gradual
rising slope of the MOSFET in Figure 1a can be attributed to
the relationship of V
DS
to R
DS(on)
,The IGBT curve has an
offset due to an internal forward biased p–n junction and a fast
rising slope typical of a minority carrier device.
It is possible to replace the MOSFET with an IGBT and
improve the efficiency and/or reduce the cost,As shown in
Figure 1b,an IGBT has considerably less silicon area than a
similarly rated MOSFET,Device cost is related to silicon area;
therefore,the reduced silicon area makes the IGBT the lower
cost solution,Figure 1c shows the resulting package area
reduction realized by using the IGBT,The IGBT is more space
efficient than an equivalently rated MOSFET which makes it
perfect for space conscious designs.
Order this document
by AN1541/D
C0077C0079C0084C0079C0082C0079C0076C0065
SEMICONDUCTOR APPLICATION NOTE
Motorola,Inc,1995
C0065C0078C0049C0053C0052C0049
2 MOTOROLA
FORWARD DROP (VOLTS)
Figure 1a,Reduced Forward Voltage Drop of
IGBT Realized When Compared to a MOSFET
with Similar Ratings
40
35
30
25
20
15
10
5
0
1086420
PEAK CURRENT
THROUGH DEVICE (AMPS)
V
CE(sat)
MGW20N60D IGBT
V
DS
MTW20N50E
MOSFET
When compared to BJTs,IGBTs have similar ratings in
terms of voltage and current,However,the presence of an
isolated gate in an IGBT makes it simpler to drive than a BJT.
BJTs require that base current be continuously supplied in a
quantity sufficient enough to maintain saturation,Base
currents of one–tenth of the collector current are typical to
keep a BJT in saturation,BJT drive circuits must be sensitive
to variable load conditions,The base current of a BJT must
be kept proportional to the collector current to prevent
desaturation under high–current loads and excessive base
drive under low–load conditions,This additional base current
increases the power dissipation of the drive circuit,BJTs are
minority carrier devices and charge storage effects including
recombination slow the performance when compared to
majority carrier devices such as MOSFETs,IGBTs also
experience recombination that accounts for the current
“tailing” yet IGBTs have been observed to switch faster than
BJTs.
Thus far,the IGBT has demonstrated certain advantages
over power MOSFETs with the exception of switching speed.
Since the initial introduction of IGBTs in the early 1980s,
semiconductor manufacturers have learned how to make the
devices faster,As illustrated in Figure 2,some trade–offs in
conduction loss versus switching speed exist,Lower
frequency applications can tolerate slower switching devices.
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
Figure 1b,Reduced Die Size of IGBT Realized
When Compared to a MOSFET with Similar Ratings
0.10
0.05
0
1
AREA
(SQ,INCHES)
IGBT DIE SIZE
(0.17 X 0.227)
MOSFET DIE SIZE
(0.35 X 0.26)
é
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
éééééé
Figure 1c,Reduced Package Size of IGBT Realized
When Compared to a MOSFET with Similar Ratings
0.60
0.20
0
1
AREA
(SQ,INCHES)
IGBT PACKAGE
SIZE (TO–220)
MOSFET PACKAGE
SIZE (T0–247)
é
0.40
Because the loss period is a small percentage of the total on
time,slower switching is traded for lower conduction loss,In
a higher frequency application,just the opposite would be true
and the device would be made faster and have greater
conduction losses,Notice that the curves in Figure 2 show
reductions in both the forward drop (V
CE(sat)
) and the fall time,
t
f
of newer generation devices,These capabilities make the
IGBT the device of choice for applications such as motor
drives,power supplies and inverters that require devices rated
for 600 to 1200 volts.
Figure 2,Advanced Features Offered by the Latest Motorola IGBT
Technologies for Forward Voltage Drop (V
CE(sat)
) and Fall Time (t
f
)
t
f
(μs)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
1.00.80.60.40.20 0.90.70.50.3
1ST GENERATION COMPETITOR 1985
2ND GENERATION COMPETITOR 1989
1ST GENERATION MOTOROLA 1993
3RD GENERATION COMPETITOR 1993
2ND GENERATION MOTOROLA
DEMONSTRATED
V
CE(sat)
(VOL
TS)
HIGH SPEED
SERIES
LOW SATURATION
SERIES
0.1
C0065C0078C0049C0053C0052C0049
3MOTOROLA
CHARACTERISTICS OF IGBTs,
DEVICE STRUCTURE
The structure of an IGBT is similar to that of a double
diffused (DMOS) power MOSFET,One difference between a
MOSFET and an IGBT is the substrate of the starting material.
By varying the starting material and altering certain process
steps,an IGBT may be produced from a power MOSFET
mask; however,at Motorola mask sets are designed
specifically for IGBTs,In a MOSFET the substrate is N+ as
shown in Figure 3b,The substrate for an IGBT is P
+
as shown
in Figure 3a.
Figure 3a,Cross Section and Equivalent Schematic
of an Insulated Gate Bipolar Transistor (IGBT) Cell
POLYSILICON GATE
N+
P+
N– EPI
N+ BUFFER
P+ SUBSTRATE
P–
R
mod
EMITTER
N+
P+
P–
COLLECTOR
GATE
NPN
MOSFET
PNP
KEY
METAL
SiO
2
R
shorting
Figure 3b,Cross Section and Equivalent
Schematic of an Metal–Oxide–Semiconductor
Field–Effect Transistor (MOSFET) Cell
JFET
channel
Drain–to–Source Body Diode
(Created when NPN
base–emitter is properly
shorted by source metal)
POLYSILICON GATE
N+
P+
N– EPI
N+ SUBSTRATE
SOURCE
N+
P+
DRAIN
NPN
KEY
METAL
SiO
2
GATE
P–
The n– epi resistivity determines the breakdown voltage of
a MOSFET as mentioned earlier using relationship (1).
R
DS(on)
C0084 V
2.7
DSS
(1)
To increase the breakdown voltage of the MOSFET,the
n– epi region thickness (vertical direction in figure) is
increased,As depicted in the classical resistance relationship
(2),reducing the R
DS(on)
of a high voltage device requires
greater silicon area A to make up for the increased n– epi
region.
R C0084
1
A
(2)
Device designers were challenged to overcome the effects
of the high resistive n– epi region,The solution to this came in
the form of conductivity modulation,The n– epi region to this
was placed on the P
+
substrate forming a p–n junction where
conductivity modulation takes place,Because of conductivity
modulation,the IGBT has a much greater current density than
a power MOSFET and the forward voltage drop is reduced.
Now the P
+
substrate,n– epi layer and P
+
,emitter” form a BJT
transistor and the n– epi acts as a wide base region.
The subject of current tailing has been mentioned several
times,Thus far,the device structure as shown in Figure 3
provides insight as to what causes the tailing,Minority carriers
build up to form the basis for conductivity modulation,When
the device turns off,these carriers do not have a current path
to exit the device,Recombination is the only way to eliminate
the stored charge resulting from the build–up of excess
carriers,Additional recombination centers are formed by
placing an N
+
buffer layer between the n– epi and P
+
substrate.
While the N
+
buffer layer may speed up the recombination,
it also increases the forward drop of the device,Hence the
tradeoff between switching speed and conduction loss
becomes a factor in optimizing device performance.
Additional benefits of the N
+
buffer layer include preventing
thermal runaway and punch–through of the depletion region.
This allows a thinner n– epi to be used which somewhat
decreases forward voltage drop.
Figure 4b,MOSFET Schematic Symbol
Figure 4a,IGBT Schematic Symbol
COLLECTOR
EMITTER
GATE
GATE
SOURCE
DRAIN
C0065C0078C0049C0053C0052C0049
4 MOTOROLA
The IGBT has a four layer (P–N–P–N) structure,This
structure resembles that of a thyristor device known as a
Silicon Controlled Rectifier (SCR),Unlike the SCR where the
device latches and gate control is lost,an IGBT is designed
so that it does not latch on,Full control of the device can be
maintained through the gate drive.
To maximize the performance of the IGBT,process steps
are optimized to control the geometry,doping and lifetime.
The possibility of latching is also reduced by strategic
processing of the device,Geometry and doping levels are
optimized to minimize the on–voltage,switching speed and
achieve other key parametric variations,Because the IGBT is
a four–layer structure,it does not have the inverse parallel
diode inherent to power MOSFETs,This is a disadvantage to
motor control designers who use the anti–parallel diode to
recover energy from the motor.
Like a power MOSFET,the gate of the IGBT is electrically
isolated from the rest of the chip by a thin layer of silicon
dioxide,SiO
2
,The IGBT has a high input impedance due to
the isolated gate and it exhibits the accompanying
advantages of modest gate drive requirements and excellent
gate drive efficiency.
Equivalent Circuit of IGBT
Figure 4b shows the terminals of the IGBT as determined by
JEDEC,Notice that the IGBT has a gate like a MOSFET yet
it has an emitter and a collector like a BJT.
The operation of the IGBT is best understood by again
referring to the cross section of the device and its equivalent
circuit as shown in Figure 3a,Current flowing from collector to
emitter must pass through a p–n junction formed by the P
+
substrate and n– epi layer,This drop is similar to that seen in
a forward biased p–n junction diode and results in an offset
voltage in the output characteristic,Current flow contributions
are shown in Figure 3a using varying line thickness with the
thicker lines indicating a high current path,For a fast device,
the N+ buffer layer is highly doped for recombination and
speedy turn off,The additional doping keeps the gain of the
PNP low and allows two–thirds of the current to flow through
the base of the PNP (electron current) while one–third passes
through the collector (hole current).
R
shorting
is the parasitic resistance of the P
+
emitter region.
Current flowing through R
shorting
can result in a voltage
across the base–emitter junction of the NPN,If the
base–emitter voltage is above a certain threshold level,the
NPN will begin to conduct causing the NPN and PNP to
enhance each other’s current flow and both devices can
become saturated,This results in the device latching in a
fashion similar to an SCR,Device processing directs currents
within the device and keeps the voltage across R
shorting
low to
avoid latching,The IGBT can be gated off unlike the SCR
which has to wait for the current to cease allowing
recombination to take place in order to turn off,IGBTs offer an
advantage over the SCR by controlling the current with the
device,not the device with the current,The internal MOSFET
of the IGBT when gated off will stop current flow and at that
point,the stored charges can only be dissipated through
recombination.
The IGBT’s on–voltage is represented by sum of the offset
voltage of the collector to base junction of the PNP transistor,
the voltage drop across the modulated resistance R
mod
and
the channel resistance of the internal MOSFET,Unlike the
MOSFET where increased temperature results in increased
R
DS(on)
and increased forward voltage drop,the forward
drop of an IGBT stays relatively unchanged at increased
temperatures.
Switching Speed
Until recently,the feature that limited the IGBT from
serving a wide variety of applications was its relatively slow
turn–off speed when compared to a power MOSFET,While
turn–on is fairly rapid,initial IGBTs had current fall times of
around three microseconds.
The turn–off time of an IGBT is slow because many minority
carriers are stored in the n– epi region,When the gate is
initially brought below the threshold voltage,the n– epi
contains a very large concentration of electrons and there will
be significant injection into the P
+
substrate and a
corresponding hole injection into the n– epi,As the electron
concentration in the n–region decreases,the electron
injection decreases,leaving the rest of the electrons to
recombine,Therefore,the turn–off of an IGBT has two
phases,an injection phase where the collector current falls
very quickly,and a recombination phase in which the collector
current decrease more slowly,Figure 5 shows the switching
waveform and the tail time contributing factors of a,fast” IGBT
designed for PWM motor control service.
éééé
éééé
éééé
éééé
éééé
Figure 5,IGBT Current Turn–off Waveform
6
5
4
3
2
1
0
–1
10008006004002000
I
C
(AMPS)
TAIL TIME of MOTOROLA GEN,2 IGBT #2 in
1.0 hp MOTOR DRIVE at 1750 RPM
PNP TURN–OFF
PORTION
TAIL TIME
MOSFET TURN–OFF
PORTION
In power MOSFETs,the switching speed can be greatly
affected by the impedance in the gate drive circuit,Efforts to
minimize gate drive impedance for IGBTs are also
recommended,Also,choose an optimal device based on
switching speed or use a slower device with lower forward
drop and employ external circuitry to enhance turn off,A
turn–off mechanism is suggested in a paper by Baliga et al [2].
C0065C0078C0049C0053C0052C0049
5MOTOROLA
A FINAL COMPARISON OF IGBTs,BJTs AND
POWER MOSFETs
The conduction losses of BJTs and IGBTs is related to the
forward voltage drop of the device while MOSFETs determine
conduction loss based on R
DS(on)
,To get a relative
comparison of turn–off time and conduction associated
losses,data is presented in Table 1 where the on–resistances
of a power MOSFET,an IGBT and a BJT at junction
temperatures of 25°C and 150°C are shown.
Note that the devices in Table 1 have approximately the
same ratings,However,to achieve these ratings the chip size
of the devices vary significantly,The bipolar transistor requires
1.2 times more silicon area than the IGBT and the MOSFET
requires 2.2 times the area of the IGBT to achieve the same
ratings,This differences in die area directly impacts the cost
of the product,At higher currents and at elevated
temperatures,the IGBT offers low forward drop and a
switching time similar to the BJT without the drive difficulties.
Table 1 confirms the findings offered earlier in Figure 1a and
elaborates further to include a BJT comparison and
temperature effects,The reduced power conduction losses
offered by the IGBT lower power dissipation and heat sink
size.
Thermal Resistance
An IGBT and power MOSFET produced from the same size
die have similar junction–to–case thermal resistance because
of their similar structures,The thermal resistance of a power
MOSFET can be determined by testing for variations in
temperature sensitive parameters (TSPs),These parameters
are the source–to–drain diode on–voltage,the
gate–to–source threshold voltage,and the drain–to–source
on–resistance,All previous measurements of thermal
resistance of power MOSFETs at Motorola were performed
using the source–to–drain diode as the TSP,Since an IGBT
does not have an inverse parallel diode,another TSP had to
be used to determine the thermal resistance,The
gate–to–emitter threshold voltage was used as the TSP to
measure the junction temperature of an IGBT to determine its
thermal resistance,However before testing IGBTs,a
correlation between the two test methods was established by
comparing the test results of MOSFETs using both TSPs,By
testing for variations in threshold voltage,it was determined
that the thermal resistance of MOSFETs and IGBTs are
essentially the same for devices with equivalent die size,
Short Circuit Rated Devices
Using IGBTs in motor control environments requires the
device to withstand short circuit current for a given period.
Although this period varies with the application,a typical
value of ten microseconds is used for designing these
specialized IGBT’s,Notice that this is only a typical value and
it is suggested that the reader confirm the value given on the
data sheet,IGBTs can be made to withstand short circuit
conditions by altering the device structure to include an
additional resistance (R
e
,in Figure 6) in the main current path.
The benefits associated with the additional series resistance
are twofold.
Figure 6,Cross Section and Equivalent Schematic
of a Short Circuit Rated Insulated Gate
Bipolar Transistor Cell
POLYSILICON GATE
N+
P+
N– EPI
N+ BUFFER
P+ SUBSTRATE
P–
R
mod
EMITTER
N+
P+
P–
COLLECTOR
GATE
NPN
MOSFET
PNP
KEY
METAL
SiO
2
R
shorting
R
e
First,the voltage created across R
e
,by the large current
passing through R
e
,increases the percentage of the gate
voltage across R
e
,by the classic voltage divider equation.
Assuming the drive voltage applied to the gate–to–emitter
remains the same,the voltage actually applied across the
gate–to–source portion of the device is now lower,and the
device is operating in an area of the transconductance curve
that reduces the gain and it will pass less current.
Table 1,Advantages Offered by the IGBT When Comparing the MOSFET,IGBT and Bipolar Transistor On–Resistances
(Over Junction Temperature) and Fall Times (Resistance Values at 10 Amps of Current)
ááááááááááááááá
ááááááááááááááá
Characteristic
áááááááá
áááááááá
TMOS
ááááááá
ááááááá
IGBT
ááááááá
ááááááá
Bipolar
ááááááááááááááá
Current Rating
áááááááá
20 A
ááááááá
20 A
ááááááá
20 A
ááááááááááááááá
Voltage Rating
ááááááá
500 V
áááááá
600 V
áááááá
500 V*
ááááááááááááááá
ááááááááááááááá
ááááááááááááááá
R
(on)
@ T
J
= 25°C
áááááááá
ááááááá
áááááááá
0.2?
ááááááá
áááááá
ááááááá
0.24?
ááááááá
áááááá
ááááááá
0.18?
ááááááááááááááá
ááááááááááááááá
ááááááááááááááá
R
(on)
@ T
J
= 150°C
áááááááá
ááááááá
áááááááá
0.6?
ááááááá
áááááá
ááááááá
0.23?
ááááááá
áááááá
ááááááá
0.24?**
ááááááááááááááá
Fall Time (Typical)
áááááááá
40 ns
ááááááá
200 ns
ááááááá
200 ns
* Indicates V
CEO
Rating
** BJT T
J
= 100°C
C0065C0078C0049C0053C0052C0049
6 MOTOROLA
Second,the voltage developed across R
e
results in a
similar division of voltage across R
shorting
and V
BE
of the
NPN transistor,The NPN will be less likely to attain a V
BE
high enough to turn the device on and cause a latch–up
situation.
The two situations described work together to protect the
device from catastrophic failure,The protection period is
specified with the device ratings,allowing circuit designers
the time needed to detect a fault and shut off the device.
The introduction of the series resistance R
e
also results in
additional power loss in the device by slightly elevating the
forward drop of the device,However,the magnitude of short
circuit current is large enough to require a very low R
e
value.
The additional conduction loss of the device due to the
presence of R
e
is not excessive when comparing a short
circuit rated IGBT to a non–short circuit rated device.
Anti–Parallel Diode
When using IGBT’s for motor control,designers have to
place a diode in anti–parallel across the device in order to
handle the regenerative or inductive currents of the motor,As
discussed earlier,due to structural differences the IGBT does
not have a parasitic diode like that found in a MOSFET.
Designers found that the diode within the MOSFET was,in
fact,a parasitic,i.e.,not optimized in the design process,and
its performance was poor for use as a current recovery device
due to slow switching speed,To overcome the lack of
performance,an optimized anti–parallel diode was used
across the MOSFET source–to–drain,Placing a packaged
diode external to the MOSFET itself created performance
problems due to the switching delays resulting from the
parasitics introduced by the packages,The optimal setup is to
have the diode copackaged with the device,A specific line of
IGBTs has been created by Motorola to address this issue.
These devices work very well in applications where energy is
recovered to the source and are favored by motor control
designers.
Like the switching device itself,the anti–parallel diode
should exhibit low leakage current,low forward voltage drop
and fast switching speed,As shown in Figure 7,the diode
forward drop multiplied by the average current it passes is the
total conduction loss produced,In addition,large reverse
recovery currents can escalate switching losses,A detailed
explanation of reverse recovery can be found in the
Appendix,A secondary effect caused by large reverse
recovery currents is generated EMI at both the switching
frequency and the frequency of the resulting ringing
waveform,This EMI requires additional filtering to be
designed into the circuit,By copackaging parts,the parasitic
inductances that contribute to the ringing are greatly reduced.
Also,copackaged products can be used in designs to reduce
power dissipation and increase design efficiency.
Figure 7,Waveforms Associated with
Anti–Parallel Diode Turn–off
I
RM(rec)
TIME
TIME
TIME
POWER
VOLTAGE
CURRENT
I
IGBT
I
DIODE
V
f
APPLICATION OF IGBTs,
PULSE WIDTH MODULATED INDUCTION
MOTOR DRIVE APPLICATION
Line–operated,pulse–width modulated,variable–speed
motor drives are an application well suited for IGBTs,In this
application,as shown in Figure 8,IGBTs are used as the
power switch to PWM the voltage supplied to a motor to
control its speed.
Depending on the application,the IGBT may be required to
operate from a full–wave rectified line,This can require
devices to have six hundred volt ratings for 230 VAC line
voltage inputs,and twelve hundred volt ratings for 575 VAC
volt line inputs,IGBTs that block high voltage offer fast
switching and low conduction losses,and allow for the design
of efficient,high frequency drives of this type,Devices used in
motor drive applications must be robust and capable of
withstanding faults long enough for a protection scheme to be
activated,Short circuit rated devices offer safe,reliable motor
drive operation.
CONCLUSION
The IGBT is a one of several options for designers to choose
from for power control in switching applications,The features
of the IGBT such as high voltage capability,low on–resistance,
ease of drive and relatively fast switching speeds makes it a
technology of choice for moderate speed,high voltage
applications,New generations of devices will reduce the
on–resistance,increase speed and include levels of
integration that simplify protection schemes and device drive
requirements,The reliability and performance advantages of
IGBTs are value added traits that offer circuit designers energy
efficient options at reduced costs.
C0065C0078C0049C0053C0052C0049
7MOTOROLA
IGBT
1/2 BRIDGE
IGBT
1/2 BRIDGE
Figure 8,Typical Pulse–Width,Modulated,Variable–Speed Induction Motor Drives
Are Where IGBT’s Offer Performance Advantages
C0043
230 VAC
TEMPERATURE
CONTROL
SYSTEM
I/O
LON?
CONTROL IC MCU
OR ASIC
MIXED MODE IC
CUSTOM LINEAR
OR
STANDARD CELL
GATE DRIVE HVIC
OR
OPTO & LVIC
DIODE BRIDGE
FILTER
CAPACITOR
INDUCTION
MOTOR
IGBT
1/2 BRIDGE
PHASE CURRENTS AND VOLTAGES
ACKNOWLEDGEMENTS
The writing of this document was assisted by a number of
internal device designers,Their assistance was greatly
appreciated by the authors,Bill Fragale,Steve Robb and
Vasudev Venkatesan provided device operation insight and
reference materials,Graphic material was provided by Basam
Almesfer and Steve Robb,Finally,C,S,Mitter assisted with
editing and accuracy of the material.
REFERENCES
[1] D,Y,Chen,J,Yang,and J,Lee,Application of the
IGT/COMFET to Zero–Current Switching Resonant
Converters,” PESC,1987.
[2] B,J,Baliga,“Analysis of Insulated Gate Transistor Turn–off
Characteristics,” IEEE Electron Device Lett,EDL–6,(1985),
pp,74–77,
[3] B,J,Baliga,“Switching Speed Enhancement in Insulated
Gate Transistors by Electron Irradiation,” IEEE Transactions
on Electron Devices,ED–31,(1984),pp,1790–1795.
C0065C0078C0049C0053C0052C0049
8 MOTOROLA
APPENDIX
Diode Reverse Recovery Analysis [4]
Figure A–1,Reverse Recovery Waveform
total reverse recovery time
fall time due to stored minority charge
application and device dependent
peak reverse recovery current
ééé
ééé
ééé
ééé
ééé
t
b
t
a
t
rr
I
RM(rec)
I
F
di/dt
Q
a
Q
b
t
rr
=
t
a
=
t
b
=
I
RM(rec)
=
A typical reverse recovery waveform is shown in
Figure A–1,The reverse recovery time t
rr
has been
traditionally defined as the time from diode current
zero–crossing to where the current returns to within 10% of the
peak recovery current I
RM(rec)
,This does not give enough
information to fully characterize the waveform shape,A better
way to characterize the rectifier reverse recovery is to partition
the reverse recovery time into two different regions,t
a
and t
b
,
as shown in Figure A–1,The t
a
time is a function of the forward
current and the applied di/dt,A charge can be assigned to this
region denoted Q
a
,the area under the curve,The t
b
portion of
the reverse recovery current is not very well understood.
Measured t
b
times vary greatly with the switch characteristic,
circuit parasitics,load inductance and the applied reverse
voltage,A relative softness can be defined as the ratio of t
b
to
t
a
,General purpose rectifiers are very soft (softness factor of
about 1.0),fast recovery diodes are fairly soft (softness factor
of about 0.5) and ultrafast rectifiers are very abrupt (softness
factor of about 0.2).
[4] Source:,Motor Controls,” TMOS Power MOSFET
Transistor Data,Q4/92,DL135,Rev 4,(Phoenix,Motorola,
Inc.,1992),pp,2–9–22 to 2–9–23.
Motorola reserves the right to make changes without further notice to any products herein,Motorola makes no warranty,representation or guarantee regarding
the suitability of its products for any particular purpose,nor does Motorola assume any liability arising out of the application or use of any product or circuit,and
specifically disclaims any and all liability,including without limitation consequential or incidental damages.,Typical” parameters can and do vary in different
applications,All operating parameters,including,Typicals” must be validated for each customer application by customer’s technical experts,Motorola does
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systems intended for surgical implant into the body,or other applications intended to support or sustain life,or for any other application in which the failure of
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against all claims,costs,damages,and expenses,and reasonable attorney fees arising out of,directly or indirectly,any claim of personal injury or death
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