The Power Electronics Handbook, Elektronika
[ Pobierz całość w formacie PDF ]
I
Power Electronic
Devices
1
Kaushik Rajashekara, Sohail Anwar, Vrej Barkhordarian,
Alex Q. Huang
Overview • Diodes • Schottky Diodes • Thyristors • Power Bipolar Junction
Transistors • MOSFETs • General Power Semiconductor Switch Requirements • Gate
Turn-Off Thyristors • Insulated Gate Bipolar Transistors • Gate-Commutated Thyristors
and Other Hard-Driven GTOs • Comparison Testing of Switches
© 2002 by CRC Press LLC
Power Electronics
1
Kaushik Rajashekara
1.1
Thyristor and Triac • Gate Turn-Off Thyristor • Reverse-
Conducting Thyristor (RCT) and Asymmetrical Silicon-
Controlled Rectifier (ASCR) • Power Transistor • Power
MOSFET • Insulated-Gate Bipolar Transistor (IGBT) •
MOS-Controlled Thyristor (MCT)
Delphi Automotive Systems
Sohail Anwar
Pennsylvania State University
Vrej Barkhordarian
1.2
International Rectifier
Characteristics • Principal Ratings for Diodes • Rectifier
Circuits • Testing a Power Diode • Protection of Power
Diodes
Alex Q. Huang
Virginia Polytechnic Institute
and State University
1.3
Characteristics • Data Specifications • Testing of Schottky
Diodes
1.4
The Basics of Silicon-Controlled Rectifiers (SCR) •
Characteristics • SCR Turn-Off Circuits • SCR
Ratings • The DIAC • The Triac • The Silicon-Controlled
Switch • The Gate Turn-Off Thyristor • Data Sheet for a
Typical Thyristor
1.5
The Volt-Ampere Characteristics of a BJT • BJT Biasing • BJT
Power Losses • BJT Testing • BJT Protection
1.6
Static Characteristics • Dynamic
Characteristics • Applications
1.7
Requirements
1.8
GTO Forward Conduction • GTO Turn-Off and Forward
Blocking • Practical GTO Turn-Off Operation • Dynamic
Avalanche • Non-Uniform Turn-Off Process among GTO
Cells • Summary
1.9
IGBT Structure and Operation
1.10
Hard-Driven GTOs
Unity Gain Turn-Off Operation • Hard-Driven GTOs
1.11
Pulse Tester Used for Characterization • Devices Used for
Comparison • Unity Gain Verification • Gate Drive
Circuits • Forward Conduction Loss Characterization •
Switching Tests • Discussion • Comparison Conclusions
© 2002 by CRC Press LLC
1.1 Overview
Kaushik Rajashekara
The modern age of power electronics began with the introduction of thyristors in the late 1950s. Now there
are several types of power devices available for high-power and high-frequency applications. The most
notable power devices are gate turn-off thyristors, power Darlington transistors, power MOSFETs, and
insulated-gate bipolar transistors (IGBTs). Power semiconductor devices are the most important functional
elements in all power conversion applications. The power devices are mainly used as switches to convert
power from one form to another. They are used in motor control systems, uninterrupted power supplies,
high-voltage DC transmission, power supplies, induction heating, and in many other power conversion
applications. A review of the basic characteristics of these power devices is presented in this section.
Thyristor and Triac
The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction
pnpn
across the anode and cathode. The thyristor symbol
and its volt–ampere characteristics are shown in
There are basically two classifications of
thyristors: converter grade and inverter grade. The difference between a converter-grade and an inverter-
grade thyristor is the low turn-off time (on the order of a few microseconds) for the latter. The converter-
grade thyristors are slow type and are used in natural commutation (or phase-controlled) applications.
FIGURE 1.1
(a) Thyristor symbol and (b) volt–ampere characteristics. (From Bose, B.K.,
Modern Power Electronics:
Evaluation, Technology, and Applications,
p. 5. © 1992 IEEE. With permission.)
© 2002 by CRC Press LLC
device. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse
across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device.
The turn-off is achieved by applying a reverse voltage
FIGURE 1.2
(a) Triac symbol and (b) volt–ampere characteristics. (From Bose, B.K.,
Modern Power Electronics:
Evaluation, Technology, and Applications,
p. 5. © 1992 IEEE. With permission.)
capability. The
forward voltage drop in thyristors is about 1.5 to 2 V, and even at higher currents of the order of 1000 A,
it seldom exceeds 3 V. While the forward voltage determines the on-state power loss of the device at any
given current, the switching power loss becomes a dominating factor affecting the device junction
temperature at high operating frequencies. Because of this, the maximum switching frequencies possible
using thyristors are limited in comparison with other power devices considered in this section.
Thyristors have
di/dt
, and
dv/dt
withstand capability and can be protected by fuses. The nonrepetitive surge current
capability for thyristors is about 10 times their rated root mean square (rms) current. They must be protected
by snubber networks for
I
2
t
is exceeded, thyristors may start
conducting without applying a gate pulse. In DC-to-AC conversion applications, it is necessary to use an
antiparallel diode of similar rating across each main thyristor. Thyristors are available up to 6000 V, 3500 A.
A triac is functionally a pair of converter-grade thyristors connected in antiparallel. The triac symbol
and volt–ampere characteristics are shown in
Because of the integration, the triac has poor reapplied
dv/dt
and
di/dt
effects. If the specified
dv/dt
, poor gate current sensitivity at turn-on, and longer turn-off time. Triacs are mainly used in phase
control applications such as in AC regulators for lighting and fan control and in solid-state AC relays.
/
dt
Gate Turn-Off Thyristor
The GTO is a power switching device that can be turned on by a short pulse of gate current and turned
off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be
turned off. Hence there is no need for an external commutation circuit to turn it off. Because turn-off
is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more
capability for high-frequency operation than thyristors. The GTO symbol and turn-off characteristics
are shown in
GTOs have the
I
2
t
°
C. GTOs are available up to about 4500 V, 2500 A.
© 2002 by CRC Press LLC
Inverter-grade thyristors are used in forced commutation applications such as DC-DC choppers and
DC-AC inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an
external commutation circuit. This requires additional commutating components, thus resulting in
additional losses in the inverter.
Thyristors are highly rugged devices in terms of transient currents,
dv
withstand capability and hence can be protected by semiconductor fuses. For reliable
operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber
circuit. A GTO has a poor turn-off current gain of the order of 4 to 5. For example, a 2000-A peak current
GTO may require as high as 500 A of reverse gate current. Also, a GTO has the tendency to latch at
temperatures above 125
FIGURE 1.3
(a) GTO symbol and (b) turn-off characteristics. (From
Bose, B.K.,
Modern Power Electronics: Eval-
uation, Technology, and Applications,
p. 5. © 1992 IEEE. With permission.)
Reverse-Conducting Thyristor (RCT) and Asymmetrical
Silicon-Controlled Rectifier (ASCR)
Normally in inverter applications, a diode in antiparallel is connected to the thyristor for commu-
tation/freewheeling purposes. In RCTs, the diode is integrated with a fast switching thyristor in a
single silicon chip. Thus, the number of power devices could be reduced. This integration brings
forth a substantial improvement of the static and dynamic characteristics as well as its overall circuit
performance.
The RCTs are designed mainly for specific applications such as traction drives. The antiparallel
diode limits the reverse voltage across the thyristor to 1 to 2 V. Also, because of the reverse recovery
behavior of the diodes, the thyristor may see very high reapplied
dv/dt
when the diode recovers from its
snubber networks to suppress voltage transients. As the
range of application of thyristors and diodes extends into higher frequencies, their reverse recovery charge
becomes increasingly important. High reverse recovery charge results in high power dissipation during
switching.
The ASCR has similar forward blocking capability to an inverter-grade thyristor, but it has a limited
reverse blocking (about 20 to 30 V) capability. It has an on-state voltage drop of about 25% less than an
inverter-grade thyristor of a similar rating. The ASCR features a fast turn-off time; thus it can work at
a higher frequency than an SCR. Since the turn-off time is down by a factor of nearly 2, the size of the
commutating components can be halved. Because of this, the switching losses will also be low.
Gate-assisted turn-off techniques are used to even further reduce the turn-off time of an ASCR. The
application of a negative voltage to the gate during turn-off helps to evacuate stored charge in the device
and aids the recovery mechanisms. This will, in effect, reduce the turn-off time by a factor of up to 2
over the conventional device.
© 2002 by CRC Press LLC
reverse voltage. This necessitates use of large RC
[ Pobierz całość w formacie PDF ]