18-04-2013, 04:55 PM
High-Power Bidirectional DC–DC Converter for Aerospace Applications
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Abstract
This paper contributes to the steady-state analysis
of the bidirectional dual active bridge (DAB) dc–dc converter by
proposing a new model that produces equations for rms and average
device currents, and rms and peak inductor/transformer currents.
These equations are useful in predicting losses that occur in
the devices and passive components and aid in the converter design.
An analysis of zero-voltage switching (ZVS) boundaries for buck
and boost modes while considering the effect of snubber capacitors
on the DAB converter is also presented. The proposed model
can be used to predict the converter efficiency at any desired operating
point. The new model can serve as an important teachingcum-
research tool for DAB hardware design (devices and passive
components selection), soft-switching-operating range estimation,
and performance prediction at the design stage.
INTRODUCTION
AERONAUTICAL power distribution technology is moving
toward dc due to the increasing proportion of dc electrical
loads. As a result, the aerospace industry is promoting the
use of more electrical technologies to enhance the performance
and increase the reliability of aircraft systems and subsystems.
The power generation capacity of the more electric Boeing 787
and Airbus A380 aeroplanes is about 1.4MW and 850kW, respectively
[1].
NEW STEADY-STATE MODEL OF DAB CONVERTER
Basic Principle of Operation
Future aircraft are likely to employ electrically powered actuators
for adjusting flight control surfaces and other high-power
transient loads. To meet the peak power demands of aircraft electric
loads and to absorb regenerated power, an ultracapacitorbased
energy storage system is examined in which a bidirectional
DAB dc–dc converter is used. The DAB converter shown
in Fig. 1 consists of two full-bridge circuits connected through
an isolation transformer and a coupling inductor L, which may
be provided partly or entirely by the transformer leakage inductance.
The full bridge on the left hand side of Fig. 1 is
connected to the HV dc bus and the full bridge on the righthand
side is connected to the low-voltage (LV) ultracapacitor.
Each bridge is controlled to generate an HF square-wave voltage
at its terminals. By incorporating an appropriate value of coupling
inductance, the two square-waves can be suitably phase
shiftedwith respect to each other to control power flow from one
dc source to another. Thus, bidirectional power flow is enabled
through a small lightweight HF transformer and inductor combination,
and power flows from the bridge generating the leading
square-wave. Although various modes of operation of the DAB
converter have been presented recently [20], [26], [28] for highpower
operation, the square-wave mode is supposedly the best
operatingmode. This is because imposing quasi-square-wave on
the transformer primary and secondary voltages results in trapezoidal,
triangular, and sinusoidal waveforms of inductor current
in the DAB converter ac link.
Steady-State Model
Models for device rms and average currents and peak and
rms currents of the coupling inductor are derived based on
the assumption of lossless components and a piecewise linear
waveform for iL . The difference in voltage between the two
bridges appears across the coupling inductor and the inductor
current changes with an essentially constant slope; this enables
expressions for inductor current peaks corresponding to different
switching instants to be determined.
EXPERIMENTAL VALIDATION
DAB Converter Prototype Design
A DAB converter prototype was designed and constructed
based on the proposed model to transfer 20 kW of power with
a switching frequency of 20 kHz for an input (HV) voltage of
540V and a nominal output (LV) voltage of 125V. These figures
are typical of likely future aerospace systems [32] at the
HV side and the capabilities of ultracapacitor modules at the LV
side. The converter is designed to meet the peak power demands
of aircraft electric loads such as actuators and is based on data
from the Intelligent Electrical PowerNetworksEvaluation Facility
of Rolls-Royce University Technology Centre, University of
Manchester.
CONCLUSION
This paper has presented a new steady-state model for the
DAB converter. The square-wave operating mode of DAB is the
best mode for high-power transfer. The proposed model produced
key design equations for the square-wave mode of the
DAB dc–dc converter. Expressions for average and rms device
currents, along with peak and rms currents of the coupling inductor
were obtained from the model. These equations are useful
in predicting losses that occur in the devices and passive components
and enable a study of the converter characteristics, in
addition to aiding in the practical design of converter prototypes.