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| Número de pieza | AN1541 | |
| Descripción | Introduction to Insulated Gate Bipolar Transistors | |
| Fabricantes | Motorola Semiconductors | |
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MOTOROLA
SEMICONDUCTOR APPLICATION NOTE
Order this document
by AN1541/D
AN1541
Introduction to Insulated Gate Bipolar Transistors
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 of f–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 MOSFET s offer extremely low
on–resistance, R DS(on), and approach the desired ideal
switch. In high voltage applications, MOSFET s exhibit
increased R DS(on) resulting in lower ef ficiency 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
RDS(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.
TRDS(on) VD2S.7S
(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–of f from power MOSFET
technology and the structure of an IGBT closely resembles
that of a power MOSFET. The IGBT has high inpuitmpedance
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, IGBsToffer
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 GT Os are
capable of being turned of f at the gate, substantial reverse
gate current is required, whereas turning of f 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. IGBT s, 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 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.
It is possible to replace the MOSFET with an IGBT and
improve the ef ficiency 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 siliconarea;
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.
© MMoOtoTroOlaR, IOncL. 1A995
1
http://www.Datasheet4U.com
1 page  
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 MOSFEsTdetermine
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 T able 1 have approximately the
same ratings. However, to achieve these ratings the chip size
of the devices vary significantl.yThe 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 dificulties.
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). Theseparameters
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 MOSFET s 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 IGBT
s, 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 MOSFET s and IGBT s are
essentially the same for devices with equivalent die size .
AN1541
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. IGBT s can be made to withstand short circuit
conditions by altering the device structure to include an
additional resistance (Re, in Figure 6) in the main current path.
The benefits associated with the additional series resistance
are twofold.
EMITTER
GATE
POLYSILICON GATE
N+ P–
Re
P+ Rshorting
NPN
MOSFET
Rmod
P–
KEY
METAL
SiO2
N+
P+
PNP N– EPI
N+ BUFFER
P+ SUBSTRATE
COLLECTOR
Figure 6. Cross Section and Equivalent Schematic
of a Short Circuit Rated Insulated Gate
Bipolar Transistor Cell
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)
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁCharacteristic
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁCurrent Rating
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁVoltage Rating
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁR(on) @ TJ = 25°C
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁR(on) @ TJ = 150°C
Fall Time (Typical)
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ* Indicates VCEO Rating
TMOS
20 A
500 V
0.2 Ω
0.6 Ω
40 ns
IGBT
20 A
600 V
0.24 Ω
0.23 Ω
200 ns
Bipolar
20 A
500 V*
0.18 Ω
0.24 Ω**
200 ns
** BJT TJ = 100°C
MOTOROLA
5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet AN1541.PDF ] | |
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