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An introduction to
IGBTs
When it comes to high power switching applications
circuit designers generally choose between bipolar
transistors or Mosfets. But there is an alternative
which combines the best of both devices – the insulated
gate bipolar transistor or IGBT. It can be thought as a
bipolar transistor with a high impedance gate instead
of a low impedance base.
More and more we are seeing heavy duty switchmode power circuits – inverters, power supplies, induction motor control and so on. As the applications
continue to become more stringent, 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 on
and off with infinite speed; and (4) would not require
any input power to make it switch.
Fig.1: reduced forward voltage drop of an IGBT compared
to a Mosfet with similar ratings.
76 Silicon Chip
Since we don’t yet have the ideal switch, designers
must choose a device that best suits the application.
The choice involves considerations such as voltage,
current, switching speed, drive circuitry, load and temperature effects. There are a variety of solid state switch
types available and they all have their strong and weak
points.
High voltage power Mosfets
The 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
very low on-resistance [RDS(on)] and approach the
desired ideal switch. But in high voltage applications,
Mosfets exhibit increased RDS(on) which results in
increased conduction losses. In a power Mosfet, the
on-resistance is proportional to the breakdown voltage
raised to approximately 2.7:
RDS(on) = (VDS)2.7
Mosfet technology has now advanced to a point
where RDS(on) is near the theoretical limit. A new
approach is needed to obtain very low on-resistance
without sacrificing switching speed. This is where the
IGBT comes in.
By combining the low conduction loss of a BJT
(bipolar junction transistor) with the switching speed
of a power Mosfet an optimal solid state switch would
be obtained. In fact, the IGBT is a spin-off from power
Mosfet technology and its structure closely resembles
Fig.2: reduced die size of an IGBT compared to a Mosfet
with similar ratings.
Fig.3: reduced package size of an IGBT compared to a
Mosfet with similar ratings.
that of a power Mosfet. The IGBT has a high input
impedance and fast turn-on like a Mosfet. And they
have an on-voltage and current density comparable to a
bipolar transistor.
Compared to SCRs, the IGBT is faster, has better dv/dt
immunity and above all, has better gate turn-off capability. While GTOs (gate turn-off SCRs) are capable of
being turned off at the gate, substantial reverse gate
current is required, whereas turning off an IGBT only
requires the gate capacitance to be discharged. Against
that, SCRs have a slightly lower forward voltage and a
higher surge current capability than IGBTs.
Many of today’s switching circuits use Mosfets
because of their simple gate drive. Since the structure
of both devices is similar, the change to IGBTs can be
made without having to redesign the gate drive circuit.
Like Mosfets, IGBTs are transconductance devices and
can remain fully on if the gate voltage is held above a
certain threshold.
As shown in Fig.1, using an IGBT in place of a
power Mosfet dramatically reduces the forward voltage drop at currents above 12 amps. By reducing the
forward drop, the conduction loss is decreased. The
gradual rising slope of the Mosfet in Fig.1 can be
attributed to the relationship of VDS to RDS(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.
Replacing a Mosfet with an IGBT can improve the
efficiency and/or reduce the cost. As shown in Fig.2, an
IGBT has considerably less silicon area than a similarly
rated Mosfet. The reduced silicon area makes the IGBT
the lower cost solution. Fig.3 shows the package area
reduction by using an IGBT. This suits it for designs
where space is restricted.
Speaking IGBT
Before we go any further, perhaps we should tell
you how to say IGBT. Instead of referring to them as
“Iggbets” most designers call them by the initials,
“eye gee bee tees” – more of a mouthful perhaps but
that’s the way it is.
IGBTs are replacing Mosfets in high voltage applications where conduction losses must be kept low. In
fact, SILICON CHIP featured a 2kW sinewave inverter
with IGBTs in the October 1992 to February 1993
issues. Four 1kV IGBTs were used in the high voltage
H-pack section where 365V DC is converted to a 50Hz
sinewave using pulse width modulation at around
4kHz. In this instance, we were forced to use IGBTs
because no combination of currently available power
Mosfets was sufficiently rugged for the job.
With zero current switching or resonant switching
techniques, IGBTs can be operated in the hundreds of
kilohertz range. Typically though, although turn-on
speeds are very fast, turn-off of the IGBT is slower than
a Mosfet. It exhibits a significant current fall time or
“tailing”. This tailing restricts IGBTs to operating at less
than 50kHz in traditional “square wave” PWM switching applications.
Up to 50kHz then, IGBTs are often a better solution
than bipolar transistors, Mosfets or thyristors (SCRs).
Fig.4: forward voltage
drop (VCE(sat)) and fall
time (tf) has improved
since IGBTs were
introduced.
August 1996 77
Introduction to IGBTs – continued
Fig.5: cross-section and equivalent schematic of an
insulated gate bipolar transistor (IGBT) cell.
When compared to BJTs, IGBTs have similar ratings
in terms of voltage and current but the isolated gate in
an IGBT makes it simpler to drive. BJTs used as switches require sufficient base current to maintain saturation.
Typically, the base current needs to be at least 1/10th of
the collector current. BJT drive circuits must therefore
be sensitive to variable load conditions.
In other words, base current for a BJT must be kept
proportional to the collector current; otherwise the device will come out of saturation with high-current loads
and will have excessive base drive under low-load
conditions. Either way, it can lead to increased power
dissipation.
BJTs are minority carrier devices and charge storage
Fig.6: cross-section and equivalent schematic of a metaloxide-semiconductor field-effect transistor (Mosfet) cell.
78 Silicon Chip
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.
Since the introduction of IGBTs in the early 1980s,
semiconductor manufacturers have learned how to
make the devices faster. As illustrated in Fig.4, some
trade-offs in conduction loss versus switching speed
exist. Lower frequency applications can tolerate slower
switching devices. 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 Fig.4 show reductions in
both the forward drop VCE(sat) and the fall time tf of
newer generation devices. These capabilities suit the
IGBT for applications such as motor control, power
supplies and inverters which require devices rated at
600-1200V.
IGBT 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 substance
is P+ as shown in Fig.5.
The n- epi resistivity determines
the breakdown voltage of a Mosfet as
mentioned earlier using the relationship: RDS(on) = (VDS)2.7
To increase the breakdown voltFig.7: the
age of the Mosfet, the n- epi region
symbols for
thickness (vertical direction in the
IGBTs (a) and
diagram) is increased. Reducing
Mosfets (b).
the RDS(on) of a high voltage device
requires a greater silicon area to make up for the increased n- epi region.
The effects of the high resistive n-epi region were
overcome by conductivity modulation. The n-epi was
placed on the P+ substrate, forming a pn 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.
Current tailing has been mentioned above. The device structure shown in Fig.5 provides an insight into
tailing. Minority carriers build up to form the basis for
conductivity modulation. When the IGBT 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 centres are formed
Fig.8: IGBT current turn-off waveform.
by placing an N+ buffer layer between the n-epi and P+
substrate.
While the N+ buffer layer may speed up recombination, it also increases the forward voltage drop. Hence
the tradeoff between switching speed and conduction
loss becomes a factor in optimising performance. The
N+ buffer layer also prevents thermal runaway and
punch-through of the depletion region. This allows a
thinner n- epi to be used which somewhat decreases
forward voltage drop.
Four layers
The IGBT has a four layer (PNPN) structure, resembling that of an SCR. But unlike the SCR where the
device latches on and gate control is lost, an IGBT is
designed so that it does not latch on. Full gate control is
available at all times.
Because the IGBT is a four-layer structure, it does not
have the inverse parallel diode inherent in power Mosfets. This is a disadvantage to motor control designers
who use the anti-parallel diode to recover energy from
the motor.
Like a Mosfet, the gate of an IGBT is electrically isolated from the rest of the chip by a thin layer of silicon
dioxide, SiO2. This gives it a high input impedance and
excellent drive efficiency.
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 on in a fashion similar to an SCR. Device pro
cessing directs currents within the device and keeps the
voltage across Rshorting 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 the sum of
the offset voltage of the collector base junction of the
PNP transistor, the voltage drop across the modulated resistance Rmod and the channel resistance of the
internal Mosfet. Unlike the Mosfet where increased
temperature results in increased RDS(on) and increased
forward voltage drop, the forward drop of an IGBT stays
relatively unchanged at increased temperatures.
Switching speed
Until recently, slow turn-off speed limited IGBTs
from serving a wide variety of applications. 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, 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
Equivalent circuit
IGBT operation is best understood by again referring
to the cross section of the device and its equivalent
circuit shown in Fig.5. Current flowing from collector
to emitter must pass through a pn junction formed by
the P+ substrate and n- epi layer. This drop is similar
to that seen in a forward biased pn junction diode and
results in an offset voltage in the output characteristic.
Current flow contributions are shown in Fig.5 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).
Rshorting is the parasitic resistance of the P+ emitter
region. Current flowing through Rshorting can result in
Fig.9: cross-section and equivalent schematic of a short
circuit rated IGBT cell.
August 1996 79
Introduction to IGBTs – continued
quickly and a recombination phase in which the collector current decreases more slowly.
Fig.8 shows the switching waveform and the contributing factors to tail time of a “fast” IGBT designed for
PWM motor control. In power Mosfets, the switching
speed can be greatly affected by the impedance in the
gate drive circuit and the same rules apply to IGBTs.
Comparing IGBTs, BJTs & Mosfets
The conduction loss of BJTs and IGBTs is related to
the forward voltage drop of the device while a Mosfet’s
conduction loss is based on RDS(on). Table 1 gives a comparison of turn-off and conduction losses at 10 amps for
a power Mosfet, an IGBT and a BJT at junction temperatures of 25°C and 150°C.
Note that while the devices in Table 1 have approximately the same ratings, their chip sizes vary significantly. The bipolar transistor requires 1.2 times more
silicon area than the IGBT while the Mosfet requires 2.2
times the area of the IGBT. This difference in die area
has a direct effect on the cost of the devices.
At higher currents and high temperatures, the IGBT
offers low forward drop and a switching time similar to
the BJT without the drive difficulties. The lower conduction losses of the IGBT reduce power dissipation
and heatsink size.
Thermal resistance
An IGBT and power Mosfet produced from the same
size die have similar junction-to-case thermal resistance
Fig.11: IGBTs offer performance advantages in PWM
variable-speed induction motor drives. They can directly
control 3-phase motors from a rectified mains supply.
80 Silicon Chip
Fig.10: the waveforms associated with anti-parallel diode
turn-off.
because of their similar structures.
Short circuit rated devices
Using IGBTs in motor control circuit requires them to
withstand short circuit current for a given period. Although this varies with the application, a typical value
of ten microseconds is used for designing these specialised IGBTs. Notice that this is only a typical 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 (Re, in Fig.9) in the main
current path.
The benefits associated with the additional series
resistance are twofold.
First, the voltage created across
Characteristic
Re, by the large current passing
through Re, increases the percentCurrent Rating
age of the gate voltage across Re, by
Voltage Rating
the classic voltage divider equation.
R(on) <at> TJ = 25°C
Assuming the drive voltage applied
to the gate-to-emitter remains the
R(on) <at> TJ = 150°C
same, the voltage actually applied
Fall Time (typical)
across the gate-to-source portion of
* Indicates VCEO rating
the device is now lower. This causes the device to operate in an area
of the transconductance curve that
reduces the gain and it will pass less current.
Second, the voltage developed across Re results in a
similar division of voltage across Rshorting and VBE of
the NPN transistor. The NPN will be less likely to attain
a VBE high enough to turn the device on and cause a
latch-up situation.
These two situations work together to protect the device from catastrophic failure. The protection period is
specified in the ratings, giving the circuit time to detect
a fault and shut off the device.
The introduction of the series resistance Re also
results in additional power loss by slightly elevating the
forward drop of the device. However, the magnitude of
short circuit current is large enough to require a very
low Re value. The additional conduction loss of the
device due to the presence of Re is not excessive when
comparing a short-circuit rated IGBT to a non-short
circuit rated device.
Anti-parallel diode
When using IGBTs for motor control, designers have
to place a diode in anti-parallel across each device in
order to handle the regenerative or inductive currents
of the motor. The optimal setup is to have the diode
co-packaged 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 favoured by
Table 1: Device Characteristics
TMOS
IGBT
Bipolar
20A
20A
20A
500V
600V
500V*
0.2 ohms
0.24 ohms
0.18 ohms
0.6 ohms
0.23 ohms
0.24 ohms**
40ns
200ns
200ns
** BJT TJ = 100°C
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
Fig.10, 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 secondary effect caused by large reverse recovery
currents is EMI at the switching frequency and the
frequency of the resulting ringing waveform. This EMI
requires additional filtering in the circuit. By co-packaging the IGBT with its anti-parallel diode, the parasitic inductances that contribute to ringing are greatly
reduced.
Induction motor drive
Mains operated, PWM variable speed motor drives
are an application well suited for IGBTs. As shown
in Fig.11, IGBTs may be used to directly control the
voltage supplied to a 3-phase motor to control its
speed. Depending on the application, the IGBT may be
required to operate from the full-wave rectified mains
supply.
Acknowledgement
This article reproduced by arrangement from Motorola Semiconductor Application Note AN1541.
SC
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August 1996 81
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