IEEE Electrification Magazine - March 2016 - 43

the range of power levels discussed previously, including
capacitor ESR loss, IGBT conduction and switching losses,
and bus bar losses. Conduction losses were, in general,
smaller due to the decreased current levels, and estimated
efficiencies were higher for the HV system. The losses
were nearly proportional to the power level, resulting in
almost constant estimated efficiency across the range of
power levels.

Trade Study Results for Various
Dual-Voltage Architectures
The data for several possible alternative architecture
trade studies are shown in Table 1. Based on this analysis, it was confirmed that a dual-winding ISG system
offers high potential for providing the best size,
weight, and efficiency factors of the concepts studied.
The results of the trade study show a size and weight
benefit to producing 28-Vdc power
from a dual-voltage ISG system for
the 60-70-kW power level studied.
In this study, the dual-winding configurations exhibited significantly
better efficiency than the system
using traditional 28-Vdc dc alternators. This improvement in efficiency
can be traced to the difference in
efficiency between the LV ISG winding and the actual value for a
28-Vdc alternator. The efficiency of
the dual-winding configurations
also surpassed that of both singlevoltage generators with dc-dc converter configurations.
The results of the trade study show
that the dual-voltage ISG architecture
(Figure 12) has significant advantages
over other architectures in terms of
efficiency, size, and weight.

xx
The LV winding efficiency increases with increased LV

generated voltage for all power levels.
xx
The HV winding efficiency increases with increased

LV generated voltage for all power levels.
xx
The total generator size is primarily influenced by the

total power output.
xx
The size of a controller section, both HV and LV, is

directly affected by the power level in that section. The
LV controller (power electronics system) size decreases dramatically with increasing LV generator voltage.

Improved Density and Temperature range
Devices for In-Vehicle Power converters for
Possible Military applications
As noted previously, SiC-based power electronics (Shenai
et al., 2014; Shenai, 2000) are being seriously considered by
the military for certain applications. Although the previous sections described applications
using a relatively low switching frequency for SiC-based devices, a very
high switching frequency was also
investigated for possible use. This section describes partnership between
academia and industry, with funding
from the U.S. Army RDECOM-TARDEC
(Rivas 2013). Its main motivation is to
provide high-temperature power conversion for HEVs and other applications to minimize thermal management or cooling system needs. The
work is expected to lead to significantly increased power density in
power converters and can contribute
to a reduction of weight and good system efficiency.
To achieve these objectives, a multipronged approach was used. This
included an order-of-magnitude
increase in switching frequency (e.g. 13.56 and 27.12 MHz),
with the latter leading to the use of low-valued inductors
without ferrite (i.e., air core), and also low-valued capacitors in the peripheral circuitry. Regular Si devices were
used for switches to study the benefit of using very high
frequency and small inductors and capacitors. This work,
however, served as a precursor to using SiC devices at a
very high switching frequency for future efforts, along
with small inductors and capacitors. All of the aforementioned items open opportunities to introduce a power
converter with high operating temperatures and a
reduced packaging size and weight.
Figure 13 shows the prototype dc-dc converter (using
Si devices). It switches at 13.56 MHz (industrial, scientific, and medical band), i.e., an order higher than a conventional power converter. It converts 170-28 V, delivers
313 W, and achieves ~80 W/in3. In Figure 13(a), the
bunch of copper wires projecting out of the circuit

Toward the process
of greater
electrification in
the military, the use
of an integrated
starter alternator
was considered
important for
achieving better
efficiency and
packaging.

Important Findings and Conclusions on the ISG
xx
The choice of the ISG voltage constant impacts the

total system.
xx
The LV controller (power electronics) efficiency

improves significantly for higher generator output
voltages and, to a small degree, with the output
power level.
xx
The HV controller (power electronics) efficiency is
essentially unaffected by the output power level.
xx
LV winding ohmic losses are strongly influenced by
the LV power output and LV generated voltage. Losses can be minimized by using a higher nominal
generator output voltage.
xx
HV winding ohmic losses are also strongly influenced
by the LV power output and LV generated voltage.
Losses can be minimized by using a higher nominal
generator output voltage.

IEEE Electrific ation Magazine / March 2 0 1 6

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2016