A Purely Ultracapacitor Energy Storage System for
Hybrid Electric Vehicles Utilizing a Microcontroller-
Based dc-dc Boost Converter
Erik J. Cegnar, Herb L. Hess, and Brian K. Johnson
Department of Electrical and Computer Engineering
University of Idaho
Abstract— The design and testing of a purely ultracapacitor The hybrid design requires the use of an energy storage
energy storage system for the improvement of hybrid electric system that provides a constant output voltage regardless of
vehicles is presented. The system utilizes two large the stored electrical energy and load requirements. The
ultracapacitor banks for energy storage and a dc-dc boost output then appears as a stiff voltage source to the electric
converter that is capable of supplying 8kW for voltage motor controller. This behavior is similar to that of a battery.
regulation. The system provides greater roundtrip efficiency Voltage regulation is accomplished with use of a dc-dc boost
over batteries, improves a vehicle’s ability to recapture energy converter, which is designed to allow for a widely varying
from regenerative braking, and is controlled and protected by input voltage from one ultracapacitor bank and a virtually
a microcontroller. The paper presents design considerations,
unchanging output voltage.
simulation, and hardware results of the system.
A custom, reprogrammable microcontroller board was
Keywords-ultracapacitor; hybrid electric vehicle; boost fabricated to measure system parameters and control the
converter; microcontroller; snubber circuit pulse width modulation (PWM) signals to the dc-dc boost
converter and the regenerative braking system of the vehicle.
I. INTRODUCTION The board utilizes control that can tightly regulate the energy
Despite improvements in battery technologies, the entering the low-voltage and high-voltage ultracapacitor
greatest limiting factor of the hybrid electric or electric banks shown in Fig. 1. This control ensures that neither bank
vehicle is still the energy storage system. The poor will ever exceed its maximum voltage specification.
efficiencies, short life cycles, and low current capabilities of
batteries are some of the more significant problems faced by II. DESIGN
hybrid-electric vehicle designers today. These unwanted The specifications of the energy storage system are as
characteristics limit the performance and desirability of the follows:
hybrid electric vehicle. An alternative energy storage
technology to the battery is the ultracapacitor . • 42 volts output
Ultracapacitor technology is superior to batteries in many
• 200 A continuous; 600 A max output current
aspects, but also provides some new challenges for the
designer. • 600 A continuous; 1000 A max regenerative braking
The advantages of ultracapacitors over batteries include current
the following: long life cycles (>500,000), high efficiencies • 200 kJ of usable energy storage
(>90%), and large current/power capabilities over a wide
range of operating temperatures . The two notable • >500,000 cycles
drawbacks to the ultracapacitor are a low specific energy, • 70-80% total system efficiency
and wide voltage variations as energy is taken out of or put
into the device. The ultracapacitor energy storage system (shown in Fig.
1) was designed specifically for a vehicle quipped with a
The energy storage system presented here is designed for hybrid electric power train. The electric portion of the power
a mild-parallel electric hybrid 2002 model sport utility train consists of a series wound dc electric motor that
vehicle (SUV). The design focuses on regenerative braking provides assist during acceleration and three large series
as the sole source of energy. The stored energy is then used connected alternators that recapture energy during braking.
only in acceleration events. This type of use requires a Although the energy storage system was designed for this
relatively small amount of energy storage capability and particular vehicle, it could be easily scaled to operate in other
large current capabilities for maximizing energy recaptured types of hybrid electric vehicles and in other situations where
acceleration. energy storage is required. The schematic of the system,
University of Idaho National Institute for Advanced Transportation
Figure 1. Ultracapacitor energy storage system schematic
illustrating the ultracapacitor banks and boost converter, is process slowly causes the output voltage to drop until the
shown in Fig. 1. The design of the boost converter has current drawn from the system falls below 200 A and the
several unique specifications. First is the previously voltage increases until it returns to its set value.
mentioned large voltage swing of the low-voltage
ultracapacitor bank. Equation (1) represents the energy The output voltage is regulated by first determining a set
stored in a capacitor. In order to maximize the energy point voltage, which is based on the voltage of the low-bank
storage capabilities of the system, the low-voltage, 540 F and expressed by
bank must be allowed to swing from a low voltage (ideally 0
V) to its maximum voltage of 25 V. It was also designed to
be able to output 200 A at 40 V to 45 V throughout this large
V lowbank 3
V setpo int = + 38 . (2)
input voltage swing. The significance of this criterion is that: 4 4
1) the converter must have low resistance in the source (the
input inductor, the power semiconductor device, and the The set point output voltage is limited to the range of 40 V to
wires that connect them) and 2) the microcontroller that 45 V. The output voltage is then related to the amount of
controls it must have imbedded information that describes energy in the low-bank. This relationship improves energy
the behavior of the converter in both continuous and balance as naturally more energy will be transferred to the
discontinuous conduction modes. drive train at a higher output voltage, and less energy will be
transferred at a lower voltage. The result is that when the
system has more available energy, more energy is used and
1 when the system has less available energy, less energy is
E = CV 2 (1)
Second, the actual high-bank voltage is compared to the
The ultracapacitor banks are composed of 2700 F, 2.5 V set point voltage and a desired PWM duty cycle is calculated
cells. The capacitors were connected in series and parallel to that is proportional to the error of the two. The desired
acquire the appropriate specifications for the two banks. PWM duty cycle is then compared to a calculated duty cycle
These ultracapacitors have an effective series resistance of 1 limit, which is based on the low-bank and high-bank
mΩ. The resistance was low enough to provide high currents voltages. If the desired duty cycle exceeds the calculated
at low input voltages, which gave the dc-dc converter the duty cycle limit, the output will be set to the calculated limit.
capability to boost a very low voltage (5-10 V) to high Otherwise, the output duty cycle will be the desired duty
voltage (40-45 V). Detailed converter simulation was cycle. A graph describing the limits of the converter is
performed using PSPICE to map its behavior over a wide shown in Fig. 5.
range of input voltages, various PWM duty cycles, and
operation in continuous and discontinuous conduction The controller board utilizes a microcontroller and
modes. The simulation yielded information that was used in oversees the operation of the boost converter mentioned
the control of the converter over a wide range of operating above and ensures that the ultracapacitors are not
conditions. overcharged. It also regulates the temperature of the
semiconductor devices by controlling independent fans
The second unique specification is that the system must attached to the heat sinks of each device and can limit the
be able to handle output current transients of up to 400 A for PWM duty cycle in the event that one of the devices is
several seconds without a significant output voltage drop. overheated. Fig. 2 is a block diagram of the controller board.
This criterion is met by having a large output capacitance,
which was chosen to be 150 F. The boost converter is The controller board is interfaced to a 4x20 backlit LCD
capable of providing 200 A at the output. In the event that display mounted in the dash of the vehicle. It provides the
more than 200 A is drawn from the system, the net current is driver with system information including: total available
provided by both the low-voltage, 540 F, ultracapacitor bank energy, boost converter status, PWM duty cycles, and
and the high-voltage, 150 F, ultracapacitor bank. This semiconductor device temperatures.
Figure 2. Block diagram of the boost converter controller board
III. DEVICE PROTECTION
Power electronic circuits often require a means to protect
the switching semiconductor devices against over voltage. In
the case of the dc-dc boost converter, the physical
dimensions are large and the inductor currents are high. This
combination along with a fast turn off time of the IGBT
results in inductive voltage spikes at device turn off .
Voltage spikes with high dv/dt are undesirable because they
cause EMI and could exceed the voltage rating of the device.
The stray inductance in Fig. 1 is due to the large layout
and more specifically the length of the copper bar connecting
the power diode to the large filter capacitor located near the
Figure 4. Oscillogram of the voltage waveform over the IGBT
output terminals of the converter and labeled Cfilter2 in Fig. 1.
This stray inductance causes a large voltage spike at turn off.
This is shown in Fig. 3, which is an oscillogram of voltage IV. SIMULATION
over the IGBT during typical operation with no protection. In
order to absorb the energy in the voltage spike, a small filter Simulation was used to acquire much of the information
capacitor was placed directly between the collector of the describing the behavior of the dc-dc boost converter.
IGBT and the cathode of the power diode, which is labeled Although analysis provided some data on the general
Cfilter1 in Fig. 1. behavior of the converter, simulation was utilized to locate
the limitations of the converter over the range of input
voltages from 5 V to 25 V and over the range of output
voltages from 10 V to 45 V. This process aided in creating a
control algorithm that would ensure safe operation of the
electronic components of the boost converter. Fig. 5 shows
the maximum PWM duty cycle that will allow for safe
operation of the boost converter, which was acquired from
Figure 3. Oscillogram of the voltage waveform over the IGBT
In addition to the filter capacitor, a turn-off snubber
circuit was designed to further ensure that the IGBT was kept
well within its safe operating area . The snubber circuit
of Fig. 1 utilizes a fast diode along with a low-loss
metallized polypropylene capacitor. The result of adding the
filter capacitor and snubber circuit is illustrated by Fig. 4,
which shows the voltage over the IGBT after installation. It
is clear that the while the addition of these components
helped to reduce the inductive voltage spike and slowed
down the rate of change of the voltage over the IGBT, the Figure 5. Boost converter limit characterization
addition of the filter capacitor has the added side effect of
V. TESTING AND VERIFICATION amps, which for graphical purposes has been scaled down by
The entire system (including the ultracapacitor banks, a factor of four.
the dc-dc boost converter and the controller board) was next The energy storage system was then integrated into a
tested in a power lab to acquire data that was compared to hybrid electric 2002 model SUV for testing with the
the simulation results for verification of the model. The vehicle’s drive train. The truck successfully participated in
testing was performed so that the system would experience competition and the system proved to be rugged, reliable and
conditions similar to those in a hybrid electric vehicle. stable. Figure 7 shows a graph of telemetry data taken from
Energy supplied to the system from a large dc machine the vehicle’s on-board computer system during a city cycle
mimicked the operation of the vehicle regenerative braking event. It shows the voltage in the low-bank increasing as the
system, while a resistive load mimicked the vehicle’s electric vehicle recovers energy during deceleration. As the vehicle
motor. Fig. 6 is data acquired from testing that illustrates begins to accelerate current is drawn from the output of the
how the system voltages behave as current from a resistive system. Energy is taken from the low-voltage bank of
load of approximately 0.36 Ω draws it from a nearly full ultracapacitors and transferred through the dc-dc boost
state of charge to a nearly depleted state. The figure shows converter to the output.
the voltages in volts of each bank and the output current in
The graph shows the vehicle speed in miles per hour, the
voltages of the high and low ultracapacitor banks in volts,
and the current of the electric motor and regenerative braking
system in amps. For graphing purposes, the current shown is
scaled down by a factor of four. The voltage of the high
ultracapacitor bank remains near constant throughout the
operation. The information shown in Fig. 7 is data acquired
while the system is being used at only a fraction of its
potential. The amount of energy put into and taken out of the
system is limited by the vehicle’s electric drive train. Future
plans for the vehicle involve implementing an electric drive
train capable of higher currents to utilize more of the stored
As a primary storage device, the ultracapacitor has key
advantages over batteries with the drawback of a lower
Figure 6. Voltage of the high and low ultracapacitor banks and output relative specific energy. A system of two large
current as energy is drawn from the system ultracapacitor banks and a dc-dc converter can behave like a
battery with a stiff voltage source with no degradation during
Figure 7. Vehicle telemetry data of the system during deceleration and acceleration
high output currents. A microcontroller-based control http://www.maxwell.com/ultracapacitors/support/papers/Ultracapacit
system ensures that the ultracapacitors and boost converter ors_and_HEVs.html
operate safely. Simulation provides useful information on  E. J. Cowgiallo and J. E. Hardin, “Perspective on ultracapacitors for
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hybrid electric vehicle proved the system to be rugged and Applied Power Electronics Conference and Exposition, 2001 (APEC
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 N. Mohan, T.M. Undeland, and W.P. Robbins, Power Electronics:
Converters, Applications, and Design, 2th ed. New York: John Wiley
ACKNOWLEDGMENT & Sons, Inc., 1995.
This project was made possible by funding and support  L. Zubieta and R. Bonert, “Characterization of double-layer
from the University of Idaho National Institute for Advanced capacitors for power electronics applications,” IEEE Trans. On
Industry Applications, Vol. 36, No. 1, Jan./Feb. 2000, pp. 199-205.
Transportation Technology and Maxwell Technologies.
 R. M. Schupbach, J. C Balda, M. Zolot, B. Kramer, “Design
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