Modular Approach Simplifies
Power-System Design
By John Mookken, Product Manager, and Kevork Haddad,
Senior Engineer, Semikron, Hudson, N.H.
A new generation of integrated and intelligent
power modules is reducing the time needed to
design, assemble and deliver power-conversion
and drive systems.
T
he introduction of the first ICs in the early 1960s
greatly simplified the design and manufacturability of complex electronic circuits by combining many of the complex functional blocks on
a monolithic semiconductor chip. Similarly, a
new generation of highly integrated and intelligent power
modules can be used as basic building blocks to greatly
simplify the design and reduce the time to market of a new
power-system design.
If we consider the market for power-electronic modules, it becomes evident that motor drives account for a
significant segment (approximately 54% in 2004). A typical
industrial motor drive can be functionally dissected into
four blocks: inverter, front end/converter, filters, electronics
and cooling (Fig. 1).
Generally, the selection of power-electronic devices will
impact the design of all other functional blocks of the system. Most modern low- to medium-power industrial drives
use insulated gate bipolar transistor (IGBT) or metal-oxide
semiconductor field effect transistor (MOSFET) devices—
the choice depends on voltage rating requirements for the
switching devices—for the inverter stage. These drives also
employ a line-commutated diode bridge rectifier front end
and a chopper/resistor grid combination to dissipate energy
during regeneration as heat.
Besides the wasted energy during braking, there also is
the disadvantage of not having any control over the dc link
voltage. Also, the drives have to use large capacitor banks
to “smooth” the dc link voltage. That’s because the ripple
frequency due to the rectifier tends to be relatively low and
depends on the line frequency and number of diode bridge
phases. For example, if the line frequency is three-phase
60 Hz, and if the system uses a three-phase diode bridge
rectifier, then the ripple frequency will be 360 Hz. Some
systems may use 6-, 12- or 18-diode bridges to improve
filter design.
A line-regenerative drive can improve on the diode rectifier front end by employing a 6-pulse SCR/thyristor bridge
for rectification and another antiparallel thyristor bridge
for regenerative feedback through a transformer.
Such a system will allow power to flow back to the
line (full four-quadrant operation), eliminating or
greatly reducing the need for the brake resistors and
improving system efficiency. With the increase in
popularity of distributed generation, such front ends
are especially useful with wind turbines.
This topology provides several advantages when compared to the diode bridge rectifier front end, including four-
Three-phase
ac voltage
source
Coolant in
3
Front end/
converter
DC link filter
Electronics
Cooling
Inverter
3 Three-phase
load
Coolant out
Fig. 1. An industrial motor drive is comprised of five major subcomponents.
Fig. 2. SKAI modules can be configured for air cooling (left) or liquid
cooling (right).
Power Electronics Technology May 2006
46
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POWER MODULES
quadrant operation and control over the
dc link voltage. However, this approach
Integrated controller and driver
Error management, PWM signal generation, control, I/O
has disadvantages such as the addition of
the antiparallel thyristor bridge and the
TA
TC
IA IB IC
requirement for bulky filter components
due to the use of line-commutating deVDC+
vices. Another drawback is the need for
expensive compensation equipment to
maintain power quality of the regenerative feedback. The large size and weight
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of the system can pose a challenge in
�B
some applications.
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Replacing the SCR/thyristor converters with IGBT- or MOSFET-based
active converters, also known as active
front ends (AFEs), will provide all the
advantages of a four-quadrant regenVDC600-V or 1200-V IGBT power stage
erative drive while virtually eliminating
harmonic currents and improving the
power quality without the need for
expensive compensation equipment.
Integrated liquid- or air-cooled heatsink
AFEs also improve system efficiency and
dynamic behavior of the load, as well as Fig. 3. Integrated power modules reduce the complexity of designing and manufacturing
eliminate the need for the antiparallel drives with active front ends.
converter or brake resistors.
size and weight of the system when compared to systems
The active converters also significantly reduce the
based on line-commutated devices. The ability to switch
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47
Power Electronics Technology May 2006
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POWER MODULES
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Fig. 4. A SKAI-integrated power module combines motor-control
electronics with a power stage and cooling.
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Power Electronics Technology May 2006
+
–
+
–
Coolant
in
Intelligent
power
module
(H-bridge half)
Load
Coolant
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Auxiliary power
Load
Intelligent
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Fig. 5. An IPM can drive loads ranging from a transformer primary or three-phase motor
(top) to a switched reluctance motor (bottom).
the devices independent of the line
frequency, typically between 15 kHz
and 20 kHz, makes the filtering
components smaller, lighter and less
expensive.
Although the use of AFEs has numerous advantages, it also adds cost
and complexity to the system. Integrated power modules can offset these
penalties by intelligently integrating
many of the hardware components
needed for both the inverter and the
active converter into a functional
“black box” such that the same module
can be used for either function.
Integrated Power Modules
Power modules took their familiar
plastic package form when the SEMIPACK was first introduced to the market in 1975. Since then, the packaging
48
technology has evolved by integrating
more and more components into the
module. The module package initially
combined discrete switching devices
to form a half bridge, then evolved to
include H-bridge circuits, six-packs
and, more recently, gate drives and
sensors.
Integrated lines of intelligent power
modules (IPMs), such as the Semikron
Advanced Integration (SKAI) modules,
have also recently come on the market (Fig. 2). These modules integrate
all the elements of a drive shown in
Fig. 3. With this approach, dc-link filter
capacitors, current and temperature
sensors, gate drivers, heatsink and the
digital-signaling-processing (DSP)
controller are combined into a single
highly optimized module.
Some novel packaging and assembly
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techniques result in higher reliability
within the SKAI modules. For example,
pressure contact technology eliminates
the large solder interfaces typically
seen between the direct-bonded copper (DBC) and the module base plate.
The pressure system also reduces the
number of wire bonds in the power
module by using spring pins to contact
the gate and sensor connections from
the DBCs to the driver/control board.
Greater reliability also is achieved
by using an aluminum-nitrite (AlN)
substrate rather than the less-expensive
alumina (Al2O3).
Although these modules are highly
integrated, off-the-shelf products, the
underlying technology lends itself to
the development of several standard
configurations. For example, the
modules can be assembled on standard
air-cooled or liquid-cooled heatsinks
(Fig. 2). Other options include additional output current sensors and
a wide selection of switching devices
(75/100/150-V MOSFETs or 600/1200-V
IGBTs) to populate the module.
In both of these applications, the
IPM-based solution is not optimal,
as one-third of the switches are not
used in the H-bridge configuration
and half the switches are not used in
the SRM drive application. However,
the IPMs offer cost savings due to
the commonality of the hardware
and short development time for the
system.
Additionally, the fact that the
module integrates many of the subcomponents means that the user is
getting a fully integrated and tested
package—a package that is already
qualified to meet some of the stringent automotive specifications. This
can significantly simplify the mechanical packaging and qualification
of the target application.
PETech
POWER MODULES
For High Performance Applications
Courtesy Airbus
Building Blocks
New IPMs allow designers to use
a building-block approach when
designing power systems, with software and wiring defining the function of each identical block. We can
conceptually see how the SKAI can
simplify the design of drives with
AFE by placing two identical SKAIs
back to back (Fig. 4). The module
function is configured when the customer downloads the control software
for the AFE and the inverter into the
respective modules. The addition of
filtering components and hook-up
hardware such as plumbing and wiring completes the system.
Other uses of the IPM show the
versatility of the module for different
applications (Fig. 5) . The top diagram
is an application where the source is a
battery and one of the IPMs is being
used as an H-bridge to drive the primary side of a transformer. In the bottom application (Fig. 5), two IPMs are
configured as upper and lower halves
of an H-bridge to drive a three-phase
switched reluctance motor (SRM).
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ADVANCED POWER TECHNOLOGY IS NOW
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Power Electronics Technology May 2006