IEEE Electrification Magazine - March 2017 - 31

capacitance over time. In the event of a localized fault, the
damaged areas can be isolated by opening small fusible
traces in the metallization pattern.
For electromagnetic compatibility (EMC), it is common
practice to add CM filtering to the HVdc bus inside the
inverter. The inverter power semiconductors may have
significant parasitic capacitance to the vehicle chassis.
Similarly, shielded output ac cables and motor winding
may also have large parasitic capacitances to the chassis
(collectively represented by capacitor Cp in Figure 2).
During each PWM switching event, the high rate of
change in voltage on the inverter ac terminals induces
parasitic CM current pulses to the chassis. To mitigate
the adverse effects of the CM current, the designer can
provide a local return path within the inverter as well as
create high-impedance on the possible conduction paths
out of the inverter. One way to achieve this is to add
Y-caps on the input HVdc bus (capacitors Cy in Figure 2).
Additionally, a CM choke can be placed on the internal
bus bars or cabling within the inverter (inductor Lcm in
Figure 2), such that Cy and Lcm work together to form a
second-order CM noise filter. However, requirements
such as the dc charging section of SAE 1772 may limit the
total amount of Y-cap allowed on the HVdc bus. The
achievable CM inductance will be limited as well. For
high-power inverters, the HVdc input current will be on
the order of hundreds of amps. This necessitates large
conductors and makes multiturn chokes impractical. The
equation for inductance is L = uN2Ae /lm. Often, the input
choke consists of a single turn (N = 1) passing through a
toroidal- or rectangular-shaped core with no airgap. The
achievable CM inductance is proportional to the material
permeability (u) and the core cross sectional area (Ae),
and it is inversely proportional to the core mean magnetic path length (lm). The amount of inductance is directly
related to the available space for core volume. Selecting
the appropriate core material is also crucial; two possible
materials are ferrite and nanocrystalline tape. Ferrite
offers several advantages such as low cost and the ability
to be formed into unique shapes that can be easily integrated into high-density inverter packages. The nanocrystalline tape wound solution has the advantages of
much higher permeability and saturation flux density.
The desired frequency range for maximum absorption
and impedance will also help determine the best material for the application. Maximum temperature of the bus
bar should also be considered to prevent heating affects
that cause the core to exceed its curie temperature
(which can be as low as 130 °C for some ferrites).
Using a CM choke on the inverter ac output may be
required in some systems. Parasitic capacitance of the
motor winding to stator iron can be significant, typically
in the range of 5 to 20 nF. If the inverter and motor are
mounted separately in the vehicle, shielded ac cables will
add additional parasitic capacitance to the chassis. The
combined parasitic capacitances and switching dv/dt at

the inverter output will determine the parasitic CM
current flowing out of the inverter. To help minimize this
parasitic current flow, a CM choke is placed on all the ac
output conductors. The necessity of such an output filter
choke can be affected by several factors, such as the method used to connect the inverter to the load motor. If the
inverter is mounted directly to the motor housing (for
example, the second generation Chevy Volt [Anwar et al.,
2015]), then no ac cables are utilized, and the presence of
CM currents flowing to the motor may not be too problematic. There are examples of EV inverters on the road
today with and without the ac CM choke.

Power Semiconductor Device and Package
At the heart of the EV inverter are the power semiconductor devices. Some key characteristics are current rating,
voltage rating, conduction loss, switching loss, and short
circuit capability. Since the introduction of the EV1 in 1996,
the silicon (Si) insulated-gate bipolar transistor (IGBT) has
been, by far, the switch of choice. IGBTs have served the
industry well and have many excellent characteristics as
applied to EV inverters. Figure 2 shows that, because an
IGBT is a unidirectional device, antiparallel diodes are
required across each IGBT switch.
Switching characteristics can be adjusted by manipulating the gate drive, but tradeoffs such as switching
losses, dv/dt, EMI, voltage overshoot, and short circuit protection are produced. Figure 3 shows the typical turn-on
and turn-off switching waveforms.
For systems in the 750-V range, Si carbide (SiC) can present
new opportunities and serious competition to the traditional
IGBT approach. The SiC power devices promise several important advantages over existing Si-based solutions.
xx
Switching loss decreases, and there is increased efficiency for a given switching frequency.
xx
Conduction loss decreases for low- to medium-current
levels due to the resistive channel characteristic. This
can provide an overall drive-cycle efficiency improvement since the heavily weighted operating points are
largely at moderate current levels.
xx
Dead-time period decreases due to fast switching, and
there is less voltage and current distortion.
xx
Power temperature cycling of the die reduces, while
assuming elimination of the discrete diode (the same
die is used for both positive and negative currents
since the channel can conduct in both directions).
These advantages become more significant as dc bus voltage increases.
With the SiC solution, the active device is usually based
on a metal-oxide-semiconductor field-effect transistor
(MOSFET) structure. MOSFETs have several key differences
when compared to an IGBT device. The MOSFET channel
can carry current in both directions, whereas the IGBT is
unidirectional. Additionally, the MOSFET contains an
inherent body diode. For negative current flow, the MOSFET structure introduces a possibility of eliminating the
IEEE Electrific ation Magazine / march 2 0 1 7

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