With the combination of an easily driven MOS gate
and low conduction loss, IGBTs quickly displaced
power bipolar transistors as the device of choice for
high current and high voltage applications. The
balance in tradeoffs between switching speed,
conduction loss, and ruggedness is now being ever
finely tuned so that IGBTs are encroaching upon the
high frequency, high efficiency domain of power
MOSFETs. In fact, the industry trend is for IGBTs to
replace power MOSFETs except in very low current
applications. To help understand the tradeoffs and to
help circuit designers with IGBT device selection and
application, this application note provides a relatively
painless overview of IGBT technology and a
walkthrough of Advanced Power Technology IGBT
datasheet information.
How to Select an IGBT
This section is intentionally placed before the technical
discourse. Answers to the following set of burning
questions will help determine which IGBT is
appropriate for a particular application. The
differences between Non Punch-Through (NPT) and
Punch-Through (PT) devices as well as terms and
graphs will be explained later.
1. What is the operating voltage? The highest
voltage the IGBT has to block should be no more
than 80% of the VCES rating.
2. Is it hard or soft switched? A PT device is better
suited for soft switching due to reduced tail
current, however a NPT device will also work.
3. What is the current that will flow through the
device? The first two numbers in the part number
give a rough indication of the usable current. For
hard switching applications, the usable frequency
versus current graph is helpful in determining
whether a device will fit the application.
Differences between datasheet test conditions and
the application should be taken into account, and
an example of how to do this will be given later.
For soft switching applications, the IC2 rating could
be used as a starting point.
4. What is the desired switching speed? If the
answer is “the higher, the better”, then a PT device
is the best choice. Again, the usable frequency
versus current graph can help answer this question
for hard switching applications.
5. Is short circuit withstand capability required? For
applications such as motor drives, the answer is
yes, and the switching frequency also tends to be
relatively low. An NPT device would be required.
Switch mode power supplies often don’t require
short circuit capability.
IGBT Overview
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MOSFET constructed on a p-type substrate, as
illustrated by the generic IGBT cross section in Figure
1. (PT IGBTs have an additional n+ layer as well as
will be explained.) Consequently, operation of an
IGBT is very similar to a power MOSFET. A positive
voltage applied from the emitter to gate terminals
causes electrons to be drawn toward the gate terminal
in the body region. If the gate-emitter voltage is at or
above what is called the threshold voltage, enough
electrons are drawn toward the gate to form a
conductive channel across the body region, allowing
current to flow from the collector to the emitter. (To
be precise, it allows electrons to flow from the emitter
to the collector.) This flow of electrons draws positive
ions, or holes, from the p-type substrate into the drift
region toward the emitter. This leads to a couple of
simplified equivalent circuits for an IGBT as shown in
Figure 2.
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driving a wide base PNP bipolar transistor in a
Darlington configuration. The second circuit simply
shows a diode in series with the drain of an N-channel
power MOSFET. At first glance, it would seem that
the on state voltage across the IGBT would be one
diode drop higher than for the N-channel power
MOSFET by itself. It is true in fact that the on state
voltage across an IGBT is always at least one diode
drop. However, compared to a power MOSFET of the
same die size and operating at the same temperature
and current, an IGBT can have significantly lower on
state voltage. The reason for this is that a MOSFET is
a majority carrier device only. In other words, in an Nchannel
MOSFET only electrons flow. As mentioned
before, the p-type substrate in an N-channel IGBT
injects holes into the drift region. Therefore, current
flow in an IGBT is composed of both electrons and
holes. This injection of holes (minority carriers)
significantly reduces the effective resistance to current
flow in the drift region. Stated otherwise, hole
injection significantly increases the conductivity, or the
conductivity is modulated. The resulting reduction in
on state voltage is the main advantage of IGBTs over
power MOSFETs.
Nothing comes for free of course, and the price for
lower on state voltage is slower switching speed,
especially at turn-off. The reason for this is that during
turn-off the electron flow can be stopped rather
abruptly, just as in a power MOSFET, by reducing the
gate-emitter voltage below the threshold voltage.
However, holes are left in the drift region, and there is
no way to remove them except by voltage gradient and
recombination. The IGBT exhibits a tail current during
turn-off until all the holes are swept out or recombined.
The rate of recombination can be controlled, which is
the purpose of the n+ buffer layer shown in Figure 1.
This buffer layer quickly absorbs trapped holes during
turn-off. Not all IGBTs incorporate an n+ buffer layer;
those that do are called punch-through (PT), those that
do not are called non punch-through (NPT). PT IGBTs
are sometimes referred to as asymmetrical, and NPT as
symmetrical.
The other price for lower on state voltage is the
possibility of latchup if the IGBT is operated well
outside the datasheet ratings. Latchup is a failure mode
where the IGBT can no longer be turned off by the
gate. Latchup can be induced in any IGBT through
misuse. Thus the latchup failure mechanism in IGBTs
warrants some explanation.
The basic structure of an IGBT resembles a thyristor,
namely a series of PNPN junctions. This can be
explained by analyzing a more detailed equivalent
circuit model for an IGBT shown in Figure 3.
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power MOSFETS and consequently all N channel IGBTs. The base of this transistor is the body
region, which is shorted to the emitter to prevent it
from turning on. Note however that the body region
has some resistance, called body region spreading
resistance, as shown in Figure 3. The P-type substrate
and drift and body regions form the PNP portion of the
IGBT. The PNPN structure forms a parasitic thyristor.
If the parasitic NPN transistor ever turns on and the
sum of the gains of the NPN and PNP transistors are
greater than one, latchup occurs. Latchup is avoided
through design of the IGBT by optimizing the doping
levels and geometries of the various regions shown in
Figure 1.
The gains of the PNP and NPN transistors are set so
that their sum is less than one. As temperature
increases, the PNP and NPN gains increase, as well as
the body region spreading resistance. Very high
collector current can cause sufficient voltage drop
across the body region to turn on the parasitic NPN
transistor, and excessive localized heating of the die
increases the parasitic transistor gains so their sum
exceeds one. If this happens, the parasitic thyristor
latches on, and the IGBT cannot be turned off by the
gate and may be destroyed due to over-current heating.
This is static latchup. High dv/dt during turn-off
combined with excessive collector current can also
effectively increase gains and turn on the parasitic
NPN transistor. This is dynamic latchup, which is
actually what limits the safe operating area since it can
happen at a much lower collector current than static
latchup, and it depends on the turn-off dv/dt. By
staying within the maximum current and safe operating
area ratings, static and dynamic latchup are avoided
regardless of turn-off dv/dt. Note that turn-on and
turn-off dv/dt, overshoot, and ringing can be set by an
external gate resistor (as well as by stray inductance in
the circuit layout).
PT versus NPT Technology
Conduction Loss
For a given switching speed, NPT technology generally
has a higher VCE(on) than PT technology. This
difference is magnified further by fact that VCE(on)
increases with temperature for NPT (positive
temperature coefficient), whereas VCE(on) decreases
with temperature for PT (negative temperature
coefficient). However, for any IGBT, whether PT or
NPT, switching loss is traded off against VCE(on).
Higher speed IGBTs have a higher VCE(on); lower speed
IGBTs have a lower VCE(on). In fact, it is possible that a
very fast PT device can have a higher VCE(on) than a
NPT device of slower switching speed.
Switching Loss
For a given VCE(on), PT IGBTs have a higher speed
switching capability with lower total switching energy.
This is due to higher gain and minority carrier lifetime
reduction, which quenches the tail current.
Ruggedness
NPT IGBTs are typically short circuit rated while PT
devices often are not, and NPT IGBTs can absorb more
avalanche energy than PT IGBTs. NPT technology is
more rugged due to the wider base and lower gain of
the PNP bipolar transistor. This is the main advantage
gained by trading off switching speed with NPT
technology. It is difficult to make a PT IGBT with
greater than 600 Volt VCES whereas it is easily done
with NPT technology. Advanced Power Technology
does offer a series of very fast 1200 Volt PT IGBTs,
the Power MOS 7? IGBT series.
Temperature Effects
For both PT and NPT IGBTs, turn-on switching speed
and loss are practically unaffected by temperature.
Reverse recovery current in a diode however increases
with temperature, so temperature effects of an external
diode in the power circuit affect IGBT turn-on loss.
For NPT IGBTs, turn-off speed and switching loss
remain relatively constant over the operating
temperature range. For PT IGBTs, turn-off speed
degrades and switching loss consequently increases
with temperature. However, switching loss is low to
begin with due to tail current quenching.
As mentioned previously, NPT IGBTs typically have a
positive temperature coefficient, which makes them
well suited for paralleling. A positive temperature
coefficient is desirable for paralleling devices because
a hot device will conduct less current than a cooler
device, so all the parallel devices tend to naturally
share current. It is a misconception however that PT
IGBTs cannot be paralleled because of their negative
temperature coefficient. PT IGBTs can be paralleled
because of the following:
• Their temperature coefficients tend to be almost
zero and are sometimes positive at higher current.
• Heat sharing through the heat sink tends to force
devices to share current because a hot device will
heat its neighbors, thus lowering their on voltage.
• Parameters that affect the temperature coefficient
tend to be well matched between devices.
IGBTs from Advanced Power Technology
Advanced Power Technology offers three series of
IGBTs to cover a broad range of applications:
• Power MOS 7® Series – 600V and 1200V PT
technology IGBTs designated by ‘GP’ in the part
number, one of the fastest IGBTs on the market,
designed for operation at high frequencies and/or
for tail current sensitive applications such as soft
switching.
• Thunderbolt® Series – 600V only NPT technology
IGBTs designated by ‘GT’ in the part number, fast
IGBTs capable of 150kHz in hard switching
applications, short circuit rated rugged devices
suitable for switch-mode power supplies as well as
motor drives.
• Fast Series – 600V and 1200V NPT technology
IGBTs designated by ‘GF’ in the part number,
short circuit rated rugged devices with low on
voltage suitable for hard switching operation
below 100kHz such as in motor drives.
Power MOS 7® IGBTs from APT are unique in that
they are designed to switch extremely fast, and they
incorporate a proprietary metal gate and open cell
structure. The result is extremely low internal
equivalent gate resistance (EGR), typically a fraction
of an Ohm; one to two orders of magnitude lower than
for poly-silicon gate devices. Low EGR enables faster
switching and consequently lower switching loss. The
metal gate and open cell structure also result in
extremely uniform and fast excitation of the gate,
minimizing hot spots during switching transients and
improving reliability. An open cell structure is also
more tolerant of defects induced during the
manufacturing process.
Datasheet Walkthrough
The intent of datasheets provided by APT is to include
relevant information that is useful and convenient for
the power circuit designer, both for selection of the
appropriate device as well as predicting its
performance in an application. Graphs are provided to
enable the designer to extrapolate from one set of
operating conditions to another. It should be noted
though that test results are very strongly circuit
dependent, especially on stray emitter inductance but
also on stray collector inductance and gate drive circuit
design and layout. Different test circuits yield different
results.
The following walkthrough provides definition of
terms in APT datasheets as well as further details on
IGBT characteristics.
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