IEEE Electrification Magazine - March 2017 - 32

(a)

(b)

(c)
Figure 4. Several package options for inverter power semiconductors: (a) a discrete SuperTO247 (photo courtesy of Renesas), (b) hex
pack module with integrated pin fins (photo courtesy of Mitsubishi
Electric), and (c) double sided cooled half-bridge module (photo courtesy of Infineon).

traditional antiparallel diode and using the FET channel
for synchronous rectification. In addition, the inherent
body diode can conduct the short dead-time period, which
can be reduced in comparison to the IGBT solution due to
faster switching times of the SiC device. While the body
diode does have a very high forward drop, its conduction
time would be a very small fraction of the overall PWM
period. Eliminating the discrete antiparallel diode frees up
significant package footprint that can be used to maximize the active switch area. Robustness of the body diode
has been the subject of discussion, but it appears that
semiconductor manufacturers have made good progress
toward addressing this concern. Cost is currently the biggest hurdle to widespread use of SiC.
There are several common methods of packaging
and cooling the power devices. On one end of the spectrum are discrete devices. In this case, multiple devices
in TO-247- like packages are operated in parallel to
achieve the desired inverter output current rating. Tesla
is one OEM that has chosen to employ this approach
in some of its inverters. These packages have been
optimized over time to lose the mounting hole, which
allows for larger die and lower thermal impedance in
the same package footprint [see Figure 4(a)]. Each package contains a single IGBT plus antiparallel diode. In

32

I E E E E l e c t r i f i c ati o n M agaz ine / march 2017

some cases, the internal construction has been highly
modified as well, such as eliminating wire bonds. Most
of these packages are nonisolated; for example, the
cooling interface on the back side of the device package
is electrically connected to the IGBT collector. This
necessitates an additional layer of electrical insulation
between the package and the heatsink that adds to the
overall thermal impedance.
On the other end of the power semiconductor packaging spectrum are large power modules (such as duals
and hex packs). They come in two common cooling
styles: flat baseplate or integrated pin fin. Modules with
flat baseplates are less expensive. However, they usually
require a thermal interface material between the module
and the water-cooled chassis. Some of the cost savings at
the module level will be passed on to the more complex
chassis casting and thermal interface material. Other
modules include integrated pin fins on the module baseplate, such as the module shown in Figure 4(b). A coolant
manifold is used to direct water flow over the pin fin area
of the module. The module is mounted into the manifold
using an O-ring to seal the interface and prevent coolant
entry into the sensitive power electronics area. Thorough
mechanical design is required to engineer the O-ring and
groove to insure a quality and robust seal. Pressure decay
test is often used to verify the integrity of the seal during
manufacturing end-of-line test of each inverter. In some
cases, the coolant manifold is included by the supplier as
part of the power module assembly. Internal construction of the power modules can vary widely. In general,
module suppliers have spent a great deal of engineering
resources to optimize their product for the automotive
environment. Over the last decade, there have been significant improvements in construction methods that
increase module lifetime under extreme power and temperature cycling conditions.
While traditional devices extract heat largely from one
surface of the package, recently, there has been the emergence of double-sided cooled devices [see Figure 4(c)]. The
concept is to extract heat from both sides of the power
semiconductor die, thus creating much lower overall
thermal impedance. While offering significant improvements in cooling capability, the necessity of extracting
heat in two directions complicates the mechanical
design. Very dense mechanical packages are possible, but
a higher level of packaging sophistication and manufacturing engineering is required to execute the design.

current Sensing
Most EVs use the current-regulated voltage source inverter shown in Figure 2. To regulate the output ac current,
some form of current sensing feedback is required. Some
of the key considerations are cost, size, weight, measurement range, voltage isolation, noise immunity, accuracy,
robustness, temperature range, and bandwidth. There are
several competing technologies available today, and a



Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2017

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