Analysis of a bidirectional isolated dc – dc converter for hybrid system

Proceedings of the International Conference on Emerging Trends in Engineering and Management (ICETEM14)
30-31, December, 2014, Ernakulam, India
185
ANALYSIS OF A BIDIRECTIONAL ISOLATED DC – DC
CONVERTER FOR HYBRID SYSTEM
Miss. JAINUJOSEPH1
, Mr. AJITH K A2
1
Student, Department of EEE, Sree Narayana Gurukulam College of Engineering, Kadayiripu, Kerala, India,
2
Assistant professor, Department of EEE,Sree Narayana Gurukulam College of Engineering, Kadayiripu, Kerala, India,
ABSTRACT
In conventional system of hybrid system a unidirectional DC-DC converter is used. Since the power flow is
from source to load only, regenerative is not possible. So here introduce a new bidirectional isolated dc-dc converter.
Now the bidirectional isolated dc-dc converter received a lot of attention due to the increasing needs to the system to
transfer bidirectional power flow between two dc buses. Electric vehicle employing hybrid sources will necessarily draw
energy from the hybrid sources effectively. In this paper a bidirectional isolated dc-dc converter is analyzed for hybrid
system. In this paper a new bidirectional isolated dc-dc converter which is controlled by phase shift modulation and duty
cycle modulation method is introduced. In order to reduce the switching losses the proposed topology minimizes the
number of switches and thereby the associated gate driver components. This is achieved by using two high frequency
transformers, primary side is the combination of a half bridge and a full bridge circuit, secondary side is a voltage
doubler circuit in order to improve the voltage. Here also introduces a quasi-optimal design method in order to reduce
current ripple and ZVS range converter, which is successfully implemented in this project. To verify the performance of
the converter a computer simulation and a prototype model is presented.
Keywords-Bidirectional isolated dc-dc converter, hybrid source, phase shift and duty cycle modulation, quasi – optimal
design method, ZVS
1. INTRODUCTION
In many fields, the hybrid system based on fuel cell or photovoltaic and battery/super capacitors as an
environmentally renewable source is applied. It is mainly applied in the field of uninterruptible power supply (UPS)
hybrid electric vehicle (HEV) and so on.
Compared to other systems, the hybrid electric vehicle (HEV) s possess many advantages such as high fuel
economy, low emissions, and silent operation etc. For different purposes of the vehicle operation, in HEVs two or more
different voltages were used. In order to link different DC voltage and for energy transfer a bidirectional isolated DC-DC
converter is used. For use in electrical equipment the bidirectional converters transfers the high voltage in the main
battery to low voltage and the other converts low voltage to high voltage. The essential requirement for DC-DC
converters used in electric and hybrid vehicles are low switching losses, high efficiency, reversible power flow
capability, compact size, light weight and reliability. In this paper a bidirectional isolated DC-DC converter for hybrid
system is described.
The proposed topology is operated with two sources, a solar panel and a super capacitor as a battery backup.
The block diagram of the proposed topology is illustrated in Fig 1. The major problem associated with solar energy is its
intermittent nature, which means it varies in every time. In day time the availability of solar energy is more whereas in
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night no light energy is available, so that it needs some kind of storage method in order to use the stored energy in the
night time, which is used to drive the motor.
In this paper, the proposed converter characteristics will analyzed in detail. The main input power source, PV
cell or fuel cell bank is connected to the BHB circuit, which can limit the input current ripple and also a super capacitor
as the auxiliary power source which deliver power to the load through the full bridge circuit. From these two different dc
sources, the proposed converter will draw power independently. The bidirectional power flow can be regulated flexibly
using the phase shift modulation and duty cycle modulation methods. The circuit diagram of the proposed topology is
given in Fig 2.
Fig 1 block diagram of photovoltaic system with BDC
Since super capacitors is used as the battery backup, it possess some advantages like higher efficiency, larger
charge/discharge capacity and longer life cycle.
Fig 2 proposed converter
This paper is organized as follows. Section I introduces the research background and the contribution of this
study. Section II gives the operation principles. Section III presents the methods used in the proposed bidirectional dc-dc
converter. Simulation diagram and result is given in Section IV. Conclusion of this paper is given in Section V.
2. OPERATION PRINCIPLES
The proposed bidirectional isolated dc-dc converter topology is shown in the Fig 2. Mainly a combination of
fuel cell and super capacitor is used as the hybrid source in hybrid electric vehicles. To drive hybrid electric vehicles fuel
cells are used, which is an electrochemical device which converts chemical energy of hydrogen directly into electrical
energy. Slow dynamics is the one of the main disadvantage of the fuel cell, so that it is necessary to use an auxiliary
power source along with the battery backup. By using solar panel instead of fuel cell to drive the motor eliminates the
problem associated with the fuel cells. In night time solar energy is not available, this problem is solved by the usage of
battery backup along with the main power source. So during day time PV is used to drive the motor and in the night time
battery is used to drive the motor.
As shown in Fig 2 in the proposed topology a BHB structure is located on the primary side of the transformer
T1, which is associated with the switches S1 and S2 that are operated at 50% duty cycle. Across the dividing capacitors
C1 and C2, the SC banks as an auxiliary power source is connected to the variable low voltage (LV) dc bus. Between the

187
SC bank and the high voltage (HV) dc bus the bidirectional operation can be realized. If the input voltage VFC and VSC
are variable over a wide range, the current stress and ac RMS value is reduced. In order to eliminate the problem the
switches S3 and S4 are controlled by the duty cycle. To realize galvanic isolation and boost a low input to the HV dc bus,
the transformers T1 and T2 with independent primary windings as well as series connected secondary windings are
employed. To avoid transformer saturation caused by asymmetrical operation in full bridge circuit a dc blocking
capacitor Cb is added in series with the primary winding of transformer T2. The voltage doubler circuit is connected on
the secondary side is to increase the voltage conversion ratio further. The power delivering interface element between the
LV side and the HV side is the inductor L2. Based on the direction of power flow there were three modes of operation.
2.1Boost Mode
Boost mode as the name indicates it boost the input voltage. In the boost mode of operation the power is
delivered from both the sources to the dc voltage bus. The timing diagram and the typical waveform of the bidirectional
converter in boost mode is shown in Fig 3.Where n1 and n2 are the turn ratio of the transformer. For simplicity current
flowing through the primary side of the transformer is presented but the voltage and current resonant slopes during the
switching transitions are not shown.When the duty cycle control is utilized together with the phase shift control, the
average power delivered is increased for the same input and output voltages, because the duty cycle control will limit the
required reactive power. When the dc buses charges the battery the direction of power flow is reversed.
Fig 3 Timing Diagram and Typical Waveform of BDC in Boost Mode
Some assumptions are made to analyze the operation principles clearly.1) Switches are ideal with antiparallel
body diodes and parasitic capacitors. 2) Inductance L1 is large enough to be treated as a current source. 2) The output
voltage is controlled well as a constant. 4) Leakage inductance of the transformers, parasitic inductance and extra
inductance can be lumped together as L2 on the secondary side.
2.2 Super Capacitor Recharge Mode
In the proposed converter super capacitor is considered as the battery backup source, so that in this mode the
super capacitor will be charged by the high voltage dc bus, which means that the power flow is from the HV side to the
LV side. In this mode the super capacitor is behaves like load and the motor behaves as source.
2.3 Super Capacitor Power Mode
During the short period of regenerative braking which can be handled by the battery or during the fuel cell
warming up stage the converter will operate under the super capacitor battery mode. At this time the power is taken only
from the battery bank to the dc voltage bus. The waveform is similar to that of boost mode, only difference is that there is
no IL1. The current stress of S1 and S2 are completely same. In case of power failure the battery is used to supply
continuous power. So it is used for high power application and for continuous power supply.

3 METHODS USED IN PROPOSED BIDIRECTIONAL CONVERTER
To control the bidirectional power flow flexibly a phase shift modulation method and
method is utilized. It also makes the converter operate under a quasi
3.1 Phase Shift Modulation Technique
For improving the power flow capability the phase shift modulation technique is used. Many digital systems are
powered by a 5 volt power supply, so filter a signal that has a 50% duty cycle
volts.
3.2 Duty Cycle Modulation Method
The duty cycle is defined as the percentage of digital high to low signals present during a PWM period. In PWM
the duty cycle is modulated, while keeping the period fixed. To increase the conversion efficiency, the phase shift angle
and the duty cycle can be calculated to control the converter and make the total power losses minimal.
3.3 Quasi Optimal Design Method
The quasi optimal design method is proposed here. It includes two design criteria. It minimizes the RMS value
by the phase shift and duty cycle control to reduce the conduction losses and it also it keeps the zero voltage switching
for high voltage side switches to reduce the switching losses.
3.4 Zero Voltage Switching
Zero voltage switching is similar to fixed frequency conversion which uses an adjustable duty cycle for given
unit of time. During the ZVS switch off time, the LC tank resonates, it traverses the voltage across the switch from zero
to its peak and back down again to zero. At this point the switch can be reactivated and lossless zero voltage switching
facilitated.
ZVS benefits
• There is no power loss
• At any frequency there is high efficiency with high voltage
• No higher peak currents
• Reduced EMI / RFI at transition
4 SIMULATION RESULTS
The proposed system is simulated using MATLAB software. Fig 4 shows the simulation diagram of BDC in
boost mode. The simulation results are shown in the Fig 5.
Fig 4
International Conference on Emerging Trends in Engineering and Management (ICETEM14)
188
METHODS USED IN PROPOSED BIDIRECTIONAL CONVERTER
To control the bidirectional power flow flexibly a phase shift modulation method and
method is utilized. It also makes the converter operate under a quasi-optimal condition over a wide input voltage range.
Phase Shift Modulation Technique
powered by a 5 volt power supply, so filter a signal that has a 50% duty cycle it will deliver an average voltage of 2.5
ation Method
while keeping the period fixed. To increase the conversion efficiency, the phase shift angle
ty cycle can be calculated to control the converter and make the total power losses minimal.
Quasi Optimal Design Method
uty cycle control to reduce the conduction losses and it also it keeps the zero voltage switching
side switches to reduce the switching losses.
the ZVS switch off time, the LC tank resonates, it traverses the voltage across the switch from zero
n again to zero. At this point the switch can be reactivated and lossless zero voltage switching
At any frequency there is high efficiency with high voltage
boost mode. The simulation results are shown in the Fig 5.
Fig 4 simulation diagram of BDC in boost mode
, December, 2014, Ernakulam, India
To control the bidirectional power flow flexibly a phase shift modulation method and duty cycle modulation
optimal condition over a wide input voltage range.
it will deliver an average voltage of 2.5
while keeping the period fixed. To increase the conversion efficiency, the phase shift angle
ty cycle can be calculated to control the converter and make the total power losses minimal.
uty cycle control to reduce the conduction losses and it also it keeps the zero voltage switching
the ZVS switch off time, the LC tank resonates, it traverses the voltage across the switch from zero
n again to zero. At this point the switch can be reactivated and lossless zero voltage switching

From the simulation results it is clear that the duty cycle modulation methods and the phase shift modulation
methods effectively reduces the reactive power losses and the zero voltage switching reduces the switching losses.
Thereby improve the efficiency when the input is varied over a wide range. Using the phase shift modulation and duty
cycle modulation schemes the bidirectional power flow can be regulated flexibly and ac current mean square value can
be reduced over a wide input range.
For doing the simulation mainly two power sources are used, a solar panel and a super capacitor as battery
backup as well as power source. Simulation diagram of the proposed BDC consists of a PV panel, gate pulse circuits,
capacitors, inductors, high integrated frequency t
clear that in bidirectional dc-dc converter the power flow can be either in forward direction or in backward direction.
A prototype model of the proposed converter is shown in Fig 6. A dc vo
isolation transformer. Several capacitors are connected with the circuit for the purpose of filtering. In this prototype
model the load is simply a small dc motor.
Fig 5 Simulation Results of
Fig
189
the reactive power losses and the zero voltage switching reduces the switching losses.
when the input is varied over a wide range. Using the phase shift modulation and duty
ulation mainly two power sources are used, a solar panel and a super capacitor as battery
capacitors, inductors, high integrated frequency transformer, power switches like MOSFET. From the simulation it is
dc converter the power flow can be either in forward direction or in backward direction.
A prototype model of the proposed converter is shown in Fig 6. A dc voltage of 24V is supplied to the load through the
model the load is simply a small dc motor.
5 Simulation Results of BDC in Boost Mode
Fig 6 Prototype Model of the Proposed BDC
, December, 2014, Ernakulam, India
the reactive power losses and the zero voltage switching reduces the switching losses.
when the input is varied over a wide range. Using the phase shift modulation and duty
ulation mainly two power sources are used, a solar panel and a super capacitor as battery
ransformer, power switches like MOSFET. From the simulation it is
dc converter the power flow can be either in forward direction or in backward direction.
ltage of 24V is supplied to the load through the

190
5 CONCLUSION
The proposed converter will deliver power from the two different dc sources, solar panel and the super capacitor
independently and simultaneously. By using the solar power as the main power, it is fuel efficient and free of pollution
and also it is ecofriendly. By using the phase shift modulation and the duty cycle modulation methods the ac root mean
square value can be reduced over a wide input range. The limited number of switches and associated gate driver
components are the main advantages of the proposed converter. Zero voltage switching is achieved by the parasitic
capacitance of the switches which reduces the switching losses. Energy management strategies to control the power and
achieve high overall efficiency of the proposed hybrid system will be studied in the future. Recent studies shown that the
combined battery- ultra capacitor bank can provide excellent performance and fuel economy. So the proposed system can
be further modified in the energy requirement and quality basis.
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3, 2012, pp. 236 - 241, ISSN Print : 0976-6545, ISSN Online: 0976-6553

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