# High-Temperature SOI/SiC-Based DC-DC Converter Suite

Authors: Bradley A. Reese; Brice McPherson; Robert Shaw; Jared Hornberger; Roberto M. Schupbach; Alexander B. Lostetter
Journal: International Journal of Power Management Electronics
 (2008)

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## Abstract

A complete design strategy (mechanical and electrical) for a 25 W 28 V/5 V dc-dc converter utilizing SiC and SOI electronics is presented. The converter includes a high-temperature SOI-based PWM controller featuring 150 kHz operation, a PID feedback loop, maximum duty cycle limit, complementary or symmetrical outputs, and a bootstrapped high-side gate driver. Several passive technologies were investigated for both control and power sections. Capacitor technologies were characterized over temperature and over time at 300C∘, power inductors designed and tested up to 350C∘, and power transformers designed and tested up to 500C∘. Northrop Grumman normally-off SiC JFETs were used as power switches and were characterized up to 250C∘. Efficiency and mass optimization routines were developed with the data gained from the first prototype. The effects of radiation on SiC and SOI electronics are then discussed. The results of the first prototype module are presented, with operation from 25C∘ up to an ambient temperature of 240C∘ .


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## Body

## 1. Introduction

The weight of a power electronic
converter can be significantly reduced by allowing the active power devices to
operate at elevated temperatures, minimizing the requirements of the bulky
cooling system. High-temperature power converters would find new applications
in space vehicles (i.e., satellites, shuttles, etc.), where increased power
density results in important launch cost reductions or increased payload.Other applications of this
technology are electronics needed for extreme environments, such as the
downhole orbital vibrator (DHOV) [1]. High-temperature
power electronics could increase mapping resolution by an order of magnitude,
significantly reducing operation times for geological exploration activities
while improving the capability to discover hidden petroleum reserves.This paper presents a suite of high-temperature (300°C) dc-dc converters developed
using silicon carbide (SiC) power devices (i.e., power switches and diodes) and high-temperature
HTMOS devices. Power electronic
converters have been previously demonstrated at 250°C operation in [1]. The
work presented in this paper is a stepping stone to higher temperatures (300°C)
and lighter weight due to converter optimization. The paper first outlines
the dc-dc converter electrical design approach in Section 2. Next, the
efficiency and mass optimization method is described in Section 3. Section 4
focuses on the mechanical design of the dc-dc converter. Section 5 discusses
potential advantages of the design in terms of radiation tolerance due to the
use of silicon on insulator (SOI) and SiC components. Finally, Section 6
presents 25 W 28 V/5 V dc-dc
converter prototype and testing results.


## 2. MCPM Electrical Design

The prototype converter is rated
for 28 V input voltage and 5 V output voltage at 25 W. These low-power and low-voltage
levels were chosen to ease the design of the power stage and to provide a safe
environment for testing control and gate driver circuitries. The half-bridge converter topology was
selected due to its versatility and ability to demonstrate a variety of
components used in power processing (i.e., transistors, diodes, capacitors,
inductors, and transformers).The PWM controller was built utilizing high-temperature SOI (HTSOI) components (i.e., HT1104 SOI quad op-amps
and HT1204 SOI quad analog switches) manufactured by Honeywell, Plymouth, Minn, USA. This suite of HTSOI components operates
at temperatures as high as 300°C. In
this design, an op-amp stage serves as a relaxation oscillator, which feeds
into two analog switches configured as inverters. The inverters drive RC
networks, which produce two near-linear triangle waves. The triangle waves are
compared to the output of the error amplifier with two op-amp stages configured
as differential amplifiers. The outputs of the differential amplifiers are
passed through analog switches to complete the comparison process and produce
pulse-width-modulated (PWM) signals.The power switches used in the
power stage of the converter are normally off 4H-SiC JFETs manufactured by Northrop Grumman, Los Angeles, Calif, USA. SiC
JFETs are most commonly available in the normally on type, which requires
special gate driver circuitry. These normally off devices can enable the use of
more conventional gate drive techniques. These devices were initially characterized
by a temperature up to 250°C. Figure 1 shows the on-state
characteristics of a single cell in the device (there are 7 cells on a single
die) with VGS=2.5 V. The on-resistance of the device increases
with increasing temperature from 400 mΩ at 25°C to 1 Ω at 250°C. Figure 2
shows its blocking voltage for several values of
gate voltage. The blocking capability of the devices decreases with increasing
temperature, which allows about 90 V at 250°C with VGS=−2 V.Figure 1
On-state characteristics of Northrop Grumman SiC JFETs.Figure 2
Blocking voltage of Northrop Grumman SiC JFETs.Once the devices were
characterized, a set of specifications was determined for the design of the
gate driver circuitry. The gate drivers were built utilizing high-temperature
SOI power MOSFETs (i.e., HTNFETs) manufactured by Honeywell and SiC Schottky diodes of Cree, Inc., Durham, NC,
USA. The outputs of
the drivers are ac-coupled into the gates of the power transistors and clamped
with diode networks to provide +/− drive capability. The high-side driver is a
unique variation of a bootstrapped floating driver in which the pull-up and
pull-down networks float independently, each with its own bootstrap capacitor.High-value capacitors are a
particular problem in high-temperature power electronics. Arkansas Power Electronics International, Inc., Fayetteville, Ark, USA, is working with TRS Technologies, Inc., State College, Pa, USA,
to develop high-temperature capacitor solutions; however, it was necessary to
determine the high-temperature capability of currently available technologies
for demonstration purposes. Several technologies were tested at 300°C for a period of two weeks (330 hours).
The NP0 and X8R ceramic types performed well for the duration of the test. NP0s
varied little over the temperature range and over time. X8Rs decreased to
approximately one third of their nominal capacitance at 300°C, and varied little over time. Although
the capacitance of the X8Rs decreased greatly at high temperature, the
capacitors obtained higher energy storage density than NP0s; therefore, X8R
technology was used for the filter capacitors in the demonstration prototype.For the power transformer and the output inductor, several core and winding materials
were considered [2]. Using these
materials, APEI engineers developed a transformer that operates at an ambient
temperature of 500°C. The transformer was tested for two months (1500 hours)
at 500°C with no observable degradation in characteristics.


## 3. MCPM Efficiency and Mass Optimization

Optimization software was also
developed, which calculates all of the necessary components for the dc-dc converter
and analyzes the impact of changing parameters on other systems. This software
selects the optimal operating conditions (i.e., switching frequency, baseplate
temperature, etc.) and component technologies (i.e., magnetic cores, power
switches, etc.) which will provide the most lightweight power management and
distribution system available. Additionally, the software can evaluate the
effects that the chosen operating conditions have on the power generation and
thermal management systems.The DIRECT optimization algorithm
[3] was utilized to arrive at the optimum operating point quickly. DIRECT
identifies potentially optimal hyper-rectangles in the given design space, and
iteratively divides these into deeper levels of hyper-rectangles. The main
advantage of DIRECT is that the global minimum can be found in a highly
discontinuous function, which is usually not the case in less exhaustive
algorithms.The DIRECT algorithm iteratively
passes design parameters to a hierarchical function tree, which begins with the
topology selection. The topology functions calculate the necessary parameters
for the components in a given topology and pass those parameters to the
component design functions. For instance, the full-bridge topology function
finds the required capacitance for the output capacitor, turns ratio and power
requirements for the transformer, output inductor inductance, and dc bias, and
so forth. These values are then passed to the respective component design
functions, which return the designs to the topology function (e.g., size,
weight, part number). Based on these results, the topology selection function
chooses the optimal topology (and its corresponding components) for the given operating
conditions. Many types of topologies can easily be implemented in the future by
entering the required design equations into the program.One of the most important factors
in increasing power density is the highly optimized power transformer design
included in the developed software. Figure3 shows thermal images of two
transformer designs rated for 1 kW (operating at 500 W as pictured). These
transformers were designed to operate at an ambient temperature up to 300°C, although the thermal images show
operation at room temperature. Figure 3(a) shows the initial transformer
design. The transformer operates at 75°C (from 25°C ambient temperature) and
has 20 W of losses. Figure 3(b) shows the optimized transformer, which
operates at 135°C (from 25°C ambient temperature) and
has 15 W of losses. Although the optimized design has lower losses, the temperature rise is greater
because the power is being dissipated in a much smaller volume. The weight of
the transformer was reduced from 171 grams to an impressive weight of 13 grams
and the volume from 51 cm3 to 9 cm3. Additionally, the
transformer design routine was able to calculate the temperature rise of the
transformer to within 5°C and the weight to within 1 gram.Thermal images of high-power transformers.
(a)(b)Using the results from hardware prototypes,
the design processes for several functions were confirmed to be operational and
accurate. Other functions produced slightly inaccurate results due to the lack
of experimental results, but these were corrected with the information gained
in the prototypes. With the validation of the program, an optimal design can be
reached. Figure4 shows a plot of the weight of a 1 kW power system (including resulting
weights of the power generation system, the thermal management system, and the dc-dc converter) over
a range of switching frequencies and baseplate temperatures.
For this case, the minimum weight for this system was found to be at 200°C
baseplate temperature and 30 kHz switching frequency. The thermal management
system was assumed to be a heatsink utilizing only natural convection cooling. The
power generation system was assumed to be a solar array, which has a low power
density (i.e., 100 W/kg) compared to other power generation systems. This
greatly affects the sensitivity of changes in weight due to efficiency;
therefore, this curve will look quite different for a higher power density generation unit.Figure 4
Mass of a power system over frequency and temperature.


## 4. MCPM Mechanical Design

In order to make the converter compact and lightweight, a high-temperature multichip power module (MCPM)
packaging approach was taken combining power devices and control electronics into
a single substrate. The packaging design focused on overcoming a few main
issues to enable the converter to operate at high temperatures. First, the
packaging materials were carefully selected to have closely matched
coefficients of thermal expansion (CTE) and a high thermal conductivity.
Secondly, interconnects such as wire bonds and die attachments were chosen to be compatible
with the substrate metallization and the converter components.A thick-film approach was chosen for the MCPM
substrate due to its ease of processing, inherent high-temperature resilience,
and excellent CTE match with the bare die devices. Thick-film metallizations
are glass-metal structures that provide extremely high film strength and high
bond strength with the substrate (the glass frit adheres strongly to the
ceramic base). Single-layer substrates are formed on either alumina (Al2O3)
or aluminum nitride (AlN). Alumina is a much stronger material than aluminum
nitride; however, AlN has higher thermal conductivity. For this converter,
alumina was chosen, as strength and reliability were traded for thermal
conductivity.The researchers have performed a number of
experiments with wire bonds in extreme environments, and found that, given the
proper metallization and wire bond interface, wire bonding can be a reliable
interconnect at high temperature. Recent
extensive testing in other projects has shown 3-mil Au-Au bonding to be highly
reliable, even withΔTs in excess of 600°C (−20°C to +600°C), exceeding
military destructive pull-force specifications by 4-5 times on
average [4]. For this converter, 3-mil Au wire was utilized for the SiC power
JFETs, and 1-mil Al wire was used for connecting the SOI devices.


## 5. Radiation Tolerance of SiC and SOI

Radiation hardness is a major concern for any project designed for space
application. SiC has been considered a
promising technology in the arena of rad-hard technology for many years. This is largely due in part to the wide bandgap
present in SiC (3.26 eV in SiC compared to 1.12 eV in Si). Due to the wide bandgap, SiC has an innate
hardness to displacement damage and ionizing radiation [5]. 6H-SiC also has a carrier removal rate that
is about three times lower than that of Si; carrier concentration actually
increases by two to three times as the temperature is increased from 25°C to
300°C [6]. These two factors together
greatly enhance the attractiveness of SiC for extreme environments when
radiation is a factor. Most failures due
to displacement damage or ionizing radiation are caused by carrier removal or
entrapment. Not only does SiC's greater
carrier concentration prove to be advantageous over silicon at lower
temperatures, but also SiC has the added benefit of being able to operate at
temperatures well above the threshold of silicon while providing no noticeable
degradation of performance.SiC has another unique advantage
over silicon in the fact that it is inherently rad-hard to most single-event
effects (SEEs); however, SiC SBDs have shown susceptibility to single-event
burnout (SEB) failures [7]. The most significant result of the study in [7] was
that the bias voltage of the diodes could be derated to satisfy radiation
needs. For example, an SiC SBD rated for 600 V reverse bias breakdown shows no SEB failure while
operating at 450 V reverse bias with fluences up to 1×108 p/cm2 (70 MeV).The control portion of the
converter is composed of high-temperature SOI devices, which, much like SiC,
are considered to have inherent rad-hard characteristics. SOI devices are much harder to single-event
upset (SEU) failures when compared to bulk-silicon devices; this is attributed
to SOI having a much smaller sensitive volume for charge collection. Previous radiation testing of Honeywell
devices has received recognition for the manufacturing process as truly
rad-hard, showing no signs of SEU with linear energy transfer (LET) of up to
120 MeV·cm2/mg,
with statistically zero upsets per day (no orbit dependence) [8].


## 6. 25-W 28-V/5-V Prototype DC-DC Converter

Figure5 shows the completed SOI/SiC-based dc-dc converter. The control and
power circuitries were
both implemented on the same ceramic substrate. The converter is enclosed by a
gold-plated Kovar power package and mounted to an aluminum heatsink. The
converter was tested up to a maximum temperature of 250°C and continuously at
240°C for a period of approximately 24 hours.Figure 5
Completed high-temperature SOI/SiC-based dc-dc converter.Figure6 shows the switching
waveforms for the converter at 240°C. Channel 1 (dark blue) and channel 2
(light blue) are the gate-to-source signals seen at the power switches, and
channel 3 (pink) is the drain-to-source voltage of the low-side power
transistor. Slight distortion can be seen in the drain-to-source signal due to
distortion in the gate-to-source signals. This ultimately limits the maximum
temperature of operation to about 250°C, though each component in the
converter can be operated up to 300°C. With correct temperature compensation
of the controller's op-amp stages, the converter will operate at 300°C ambient
temperature. This, however, is the topic of another paper.Figure 6
Switching waveforms at 240°C.Figure7 shows the power
waveforms for the converter at 240°C. Channel 1 (dark blue) is the output
current, channel 2 (light blue) is the output voltage, channel 3 (pink) is the
input current, and channel 4 (green) is the input voltage. The ripple
components are small with respect to the dc components of the output waveforms,
indicating a clean supply. The input current has a high ripple component, which
is most likely caused by the voltage source used to power the dc-dc converter.Figure 7
Power waveforms at 240°C.Figures8 and 9 show the transient waveforms for the
converter. Figure 8 shows the behavior of the output voltage when the load
transitions from high current to low current. Similarly, Figure 9 shows the sag
in output voltage when the load transitions from low current to high current.
In both cases, the voltage transients are less than the requirement of 1 V, and
the settling time of each is about 200 microseconds.Figure 8
High-to-low current transient.Figure 9
Low-to-high current transient.Figure10 shows the efficiency of
the converter over temperature for several levels of input voltage and output
power. All of the curves are at a low efficiency level since the forward
voltage drop (about 1.4 V) of the output diodes is a significant portion of the
output voltage (5 V). For this reason, the main sources of losses in the
converter are the output diodes; therefore, efficiency could be increased with
synchronous rectification. Generally, converters with higher output voltages
will be much more efficient.Figure 10
Efficiency over temperature.The efficiency of the converter is
lower for low output power and higher input voltage. This indicates that a significant
portion of losses is associated with switching loss in the power switches and
core loss in the transformer. At the nominal input voltage (28 V), the medium
and high power levels are very close together. The highest efficiency is
achieved with the lowest input voltage, further indicating that switching
losses are significant. Depending on the power and voltage levels, the
efficiency may increase or decrease with increasing temperature up to 200°C. However,
all of the curves tend to increase within 200–225°C range.


## 7. Conclusion

The complete design philosophy for a high-temperature dc-dc converter utilizing
SiC and SOI electronics was presented. The prototype dc-dc converter presented
in this paper was successfully tested up to 240°C.
This work has demonstrated the feasibility of using active and passive
technologies in extreme environments, and provided a stepping stone to
ultra-lightweight power electronic system design.Future developments include a
120 V/28 V 1 kW dc-dc converter that operates at high junction temperature to
reduce heatsink size (ambient temperature will be within the normal industrial
range). The new converter will utilize the technology presented in this paper
and optimized design methods to create a lightweight power electronic system.
Additionally, initial results show an increase in efficiency up to 93%. This
change is mostly due to the increased output voltage and partly due to
optimized design.



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