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1
ISOLATED BIDIRECTIONAL FULL-BRIDGE DC–DC
CONVERTER WITH A FLYBACK SNUBBER
A PROJECT REPORT
Submitted by
NIJITH BABU (Reg.No.92208121037)
PRAISE JOSEPH (Reg.No.92208121042)
SABIN.S (Reg.No.92208121048)
SAJITH BABU (Reg.No.92208121049)
in partial fulfillment for the award of the degree
of
BACHELOR OF ENGINEERING
IN
ELECTRICAL AND ELECTRONICS ENGINEERING
VICKRAM COLLEGE OF ENGINEERING, ENATHI
APRIL 2012
2
ANNA UNIVERSITY : CHENNAI 600 025
BONAFIDE CERTIFICATE
Certified that this project report “ISOLATED BIDIRECTIONAL FULL
BRIDGE DC-DC CONVERTER WITH A FLYBACK SNUBBER” is the
bonafied work of NIJITH BABU (92208121037), PRAISE JOSEPH
(92208121042), SABIN.S (92208121048), SAJITH BAU(92208121049) who
carried out the project work under my supervision.
SIGNATURE SIGNATURE
Prof.P.Sivachandran, M.E, (PhD) Mr.Harikrishnan M.E
HEAD OF THE DEPARTMENT SUPERVISOR
Lecturer
Electrical and Electronics Electrical and Electronics
Engineering Engineering
VICKRAM COLLEGE OF ENGINEERING VICKRAM COLLEGE OF ENGINEERING
Sreenivasa gardens, Sreenivasa gardens,
Madurai-Sivagangai road, Madurai-Sivagangai road,
Enathi, Enathi,
Tamilnadu-630561 Tamilnadu-630561
Submitted for the project work (EE1452) viva held at Vickram College of Engineering
on......................
Internal Examiner External Examiner
3
TABLE OF CONTENTS
CHAPTER NO. TITLE PAGE NO.
ABSTRACT i
LIST OF TABLE ii
LIST OF FIGURES iii
LIST OF SYMBOLS iv
I INTRODUCTION v
1.1 Renewable DC system 1
1.2 Dc-Dc converter 1
1.2.1 Bidirectional operation 2
1.2.2 Need of snubber circuit 2
1.2.2.1 Snubber circuit 3
1.2.2.2 RCD snubber circuit 4
1.2.2.3 Flyback snubber circuit 4
1.2.3 Current fed and voltage fed switches 5
1.3 Buck -Boost operation 5
II LITERATURE REVIEW vi
2.1 Converter using Snubber 7
2.2 Existing system limitation 8
III PROPOSED SYSTEM vii
3.1 Proposed system merit 9
3.2 Converter using flyback snubber 9
5.3.8.3 Features 37
4
5.3.8.4 Absolute Maximum Ratings 38
5.3.8.5 Recommended Operating Conditions 38
5.3.8.6Applications 38
5.3.9 Transistor 38
5.3.9.1 Transistor as an Amplifier 38
5.3.9.2 General Description 40
5.3.9.3 Darlington Pair Circuit 40
5.3.9.4 Transistor-2n2222 41
5.3.9.5 Features 41
5.3.9.6 Pinning 41
VI APPENDICES x
1 PIC16F877A Microcontroller 42
6.1.1 Features 42
6.1.2 Block diagram 44
2 Memory organization 45
6.2.1 Program memory organization 45
6.2.2 Data memory organization 45
3 Interrupts 48
4 MOSFET switch-IRFP250N 50
VII CONCLUSION xi
VIII REFERENCES xii
5
LIST OF ABBREVIATIONS
SL NO. ABBREVIATIONS EXPANSION
1 PIC PERIPHERAL INTERFACE
CONTROL
2 RCD RESISTOR, CAPACITOR
AND DIODE
3 RISC REDUCED INSTRUCTION
SET COMPUTING
4 Vhv VOLTAGE IN HIGH
VOLTAGE SIDE
5 Vlv VOLTAGE IN LOW
VOLTAGE SIDE
6 CC CLAMPING CAPACITOR
7 LED LIGHT EMITING DIODE
8 Vcc SUPPLY VOLTAGE
6
ABSTRACT
An isolated bidirectional full-bridge dc–dc converter with high conversion
ratio, high output power, and soft start-up capability is proposed in this paper. The
use of a capacitor, a diode, and a flyback converter can clamp the voltage spike
caused by the current difference between the current-fed inductor and leakage
inductance of the isolation transformer, and can reduce the current flowing through
the active switches at the current-fed side. Operational principle of the proposed
converter is first described, and then, the design equation is derived. A 1.5-kW
prototype with low-side voltage of 48 V and high-side voltage of 360 V has been
implemented, from which experimental results have verified its feasibility.
7
LIST OF TABLES
TABLE NO. NAME OF TABLES PAGE NO.
1 Status register 46
2 Pin details 47
3 Opcode field descriptions 48
4 PIC 16F877A instruction set 48
8
LIST OF FIGURES
FIGURE NO. NAME OF FIGURE PAGENO.
1.1 RCD snubber circuit 4
2.1 Existing system limitation 8
3.1 Converter using flyback snubber 9
3.2 Circuit diagram 10
3.3(a) Operating modes of step up conversion 14
3.3(b) MODE 2 23
3.3(c) MODE3 24
3.3(d) MODE 4 24
3.3(e) MODE 5 25
3.4(a) Operating modes of step down conversion 26
3.4(b) MODE2 27
3.4(C) M0DE3 28
3.4(D) MODE4 28
3.4(E) MODE5 28
4.1 SCHEMATIC DIAGRAM OF FORWARD DIRECTION 31
4.2 SCHEMATIC DIAGRAM OF BACKWARD DIRECTION 32
4.3 SIMULATION OF INPUT SUPPLY 33
4.4 SIMULATION OF INPUT PULSE 33
4.5 SIMULATION OF OUTPUT 34
4.6 OUTPUT WAVEFORM 34
5.1 PROPOSED SYSTEM BLOCKDIAGRAM 35
5.2 OPERATION OF PROPOSED SYSTEM 38
5.3 POWER SUPPLY DIAGRAM 39
5.4 DIODE BRIDGE RECTIFIER 40
9
5.5 VOLTAGE REGULATOR 41
5.6 DRIVER CIRCUIT 42
5.7 OPTO COUPLER 43
5.8 PINN DIAGRAM 45
5.9 SCHEMATIC DIAGRAM 45
5.10 DARLIGTON PAIR 48
6.1 PIC BLOCK DIAGRAM 52
6.2 MOSFET switch-IRFP250N 59
10
CHAPTER I
1.INTRODUCTION
This paper introduces a flyback snubber to recycle the absorbed energy in
the clamping capacitor. The flyback snubber can be operated independently to
regulate the voltage of the clamping capacitor; therefore, it can clamp the voltage
to a desired level just slightly higher than the voltage across the low-side
transformer winding. Since the current does not circulate through the full-bridge
switches, their current stresses can be reduced dramatically under heavy-load
condition, thus improving system reliability significantly. Additionally, during
start-up, the flyback snubber can be controlled to precharge the high-side capacitor,
improving feasibility significantly. A bidirectional converter with low-side voltage
of 48 V, high-side voltage of 360 V, and power rating of 1.5 kW has been designed
and implemented, from which experimental results have verified the discussed
performance.
1.1 RENEWABLE DC SYSTEM
In Renewable dc-supply systems, batteries are usually required to back-up
power for electronic equipment. Their voltage levels are typically much lower than
the dc-bus voltage. Bidirectional converters for charging/discharging the batteries
are therefore required. For high-power applications, bridge-type bidirectional
converters have become an important research topic over the past decade . For
raising power level, a dual full-bridge configuration is usually adopted, and its low
side and high side are typically configured with boost type and buck-type
topologies, respectively.
1.2 DC-DC CONVERTERS
Dc-Dc converters are used to convert a fixed voltage DC source into a
variable DC source. They are widely used for traction motor control in electric
11
automobiles,tolly,cars,marine hoists, forklift trucks and mine haulers. They provide
smooth acceleration control, high efficiency and fast dynamic response .DC
converters can be used in regenerative braking of dc motors to return energy back
into the supply, and this feature results in energy savings for transportation system
with frequent stops. Dc converters are used in dc voltage regulators and also used
in conjunction with an inductor to generate a dc current source especially for the
current source inverter.
1.2.1 BI-DIRECTIONAL OPERATION
The system can be operated bi-directionally with respect to the direction of
power flow. In forward mode of operation it converts low voltage dc to high
voltage dc. This time it acts as a boost converter. In reverse mode of operation it
converts high voltage dc to low voltage dc and store in the renewable dc systems.
This time it acts as a buck converter.
1.2.2 NEED FOR SNUBBER CIRCUITS
The major concerns of these studies include reducing switching loss,
reducing voltage and current stresses, and reducing conduction loss due to
circulation current. A more severe issue is due to leakage inductance of the
isolation transformer, which will result in high voltage spike during switching
transition. Additionally, the current freewheeling due to the leakage inductance
will increase conduction loss and reduce effective duty cycle. An alternative
approach is to precharge the leakage inductance to raise its current level up to that
of the current-fed inductor, which can reduce their current difference and, in turn,
reduce voltage spike. However, since the current level varies with load condition, it
is hard to tune the switching timing diagram to match these two currents. Thus, a
passive or an active clamp circuit is still needed. An active commutation principle
was published to control the current of leakage inductance; however, clamping
circuits are additionally required. Passive and active clamping circuits have been
12
proposed to suppress the voltage spikes due to the current difference between the
current-fed inductor and leakage inductance of the isolation transformer.
1.2.2.1 SNUBBER CIRCUITS
Switching devices and circuit components may fail due to the following
reasons.
1. Overheating – thermal failure
2. Over current
3. Overvoltage – usually happens during turn-off
4. Switching loss –excessive switching loss is a major contributing factor of
Overheating.
Power electronic circuits and their switching devices and components can be
protected from over current by placing fuses at suitable locations. Heat sinks, fins
andfans are used to take the excess heat away from switching devices and other
components.Snubber circuits are required to limit dv/dt,di/dt and overvoltage
during turn-on and turnoff.
It consists of a fast recovery diode in series with a parallel combination of
snubber capacitor and a resistor. The leakage inductance current of primary
winding finds a low impedance path through the snubber diode to the snubber
capacitor. It can be seen that the diode end of snubber capacitor will be height
potential. To check the excessive voltage across the snubber capac]]itor a resistor
is putting across it. Under steady state operation the resistor dissipate the leakage
flux energy
1.2.2.2 RCD SNUBBER CIRCUITS
13
Figure1.1 RCD snubber circuits
RCD passive snubber to clamp the voltage, and the energy absorbed in the
clamping capacitor is dissipated on the resistor, thus resulting in lower efficiency.
A buck converter was employed to replace an RCD passive snubber, but it still
needs complex clamping circuits. A simple active clamping circuit was proposed,
which suits for bidirectional converters. However, its resonant current increases the
current stress on switches significantly. This simplest approach is employing an
proposed a topology to achieve soft-starting capability, but it is not suitable for
step-down operation.
1.2.2.3 FLYBACK SNUBBER CIRCUIT
The flyback snubbed to recycle the absorbed energy in the clamping
capacitor. The flyback snubber can be operated independently to regulate the
voltage of the clamping capacitor; therefore, it can clamp the voltage to a desired
level just slightly higher than the voltage across the low-side transformer winding.
Since the current does not circulate through the full-bridge switches, their current
stresses can be reduced dramatically under heavy-load condition, thus improving
system reliability significantly. Additionally, during start-up, the flyback snubber
can be controlled to precharge the high-side capacitor, improving feasibility
significantly.
14
1.2.3 CURRENT FED AND VOLTAGE FED SWITCHES
The power MOSFETS are used as the switching devices. 9 MOSFETS are
used in this particular unit. The low voltage side switches are called as current fed
switches and the high voltage side switches are called as voltage fed switches .the
snubber unit is located at the low voltage side, because of limiting the inrush
current flows through the circuits.
1.3 BUCK-BOOST OPERATION
The boost conversion takes place during the forward mode of operation of
device. It converts the low voltage dc to high voltage dc in accordance with the
demand at load. Similarly the buck conversion takes place during the reverse mode
of operation. When demand at the load is low, it reduces the excess current and
stores the low current at the renewable supply systems.
15
CHAPTR II
2. LITERATURE REVIEW
In renewable dc-supply systems, batteries are usually required to back-up
power for electronic equipment. Their voltage levels are typically much lower than
the dc-bus voltage. Bidirectional converters for charging/discharging the batteries
are therefore required. For high-power applications, bridge-type bidirectional
converters have become an important research topic over the past decade . For
raising power level, a dual full-bridge configuration is usually adopted, and its low
side and high side are typically configured with boost type and buck-type
topologies, respectively. The major concerns of these studies include reducing
switching loss, reducing voltage and current stresses, and reducing conduction loss
due to circulation current. A more severe issue is due to leakage inductance of the
isolation transformer, which will result in high voltage spike during switching
transition. Additionally, the current freewheeling due to the leakage inductance
will increase conduction loss and reduce effective duty cycle. An alternative
approach is to precharge the leakage inductance to raise its current level up to that
of the current-fed inductor, which can reduce their current difference and, in turn,
reduce voltage spike. However, since the current level varies with load condition, it
is hard to tune the switching timing diagram to match these two currents. Thus, a
passive or an active clamp circuit is still needed.
An active commutation principle was published to control the current of
leakage inductance; however, clamping circuits are additionally required. Passive
and active clamping circuits have been proposed to suppress the voltage spikes due
to the current difference between the current-fed inductor and leakage inductance
of the isolation transformer. The simplest approach is employing an RCD passive
snubber to clamp the voltage, and the energy absorbed in the clamping capacitor is
16
dissipated on the resistor, thus resulting in lower efficiency. A buck converter was
employed to replace an RCD passive snubber, but it still needs complex clamping
circuits. A simple active clamping circuit was proposed which suits for
bidirectional converters. However, its resonant current increases the current stress
on switches significantly. The proposed a topology to achieve soft-starting
capability, but it is not suitable for step-down operation.
This paper introduces a flyback snubber to recycle the absorbed energy in
the clamping capacitor. The flyback snubber can be operated independently to
regulate the voltage of the clamping capacitor; therefore, it can clamp the voltage
to a desired level just slightly higher than the voltage across the low-side
transformer winding. Since the current does not circulate through the full-bridge
switches, their current stresses can be reduced dramatically under heavy-load
condition, thus improving system reliability significantly. Additionally, during
start-up, the flyback snubber can be controlled to precharge the high-side capacitor,
improving feasibility significantly. A bidirectional converter with low-side voltage
of 48 V, high-side voltage of 360 V, and power rating of 1.5 kW has been designed
and implemented, from which experimental results have verified the discussed
performance.
2.1 CONVERTER USING SNUBBER
DC-DC Converters are being increasingly needed for applications such as
battery charge/discharge systems uninterruptable power supplies (UPS) hybrid
electric vehicles, aero power systems, a and etc. Converter consists of many
switching circuits according to the needs. So there will be the chance for voltage
drops and voltage spikes during the switching process. In order to avoid these
17
switching losses we are using snubber circuits. Snubber circuits are required to
limit dv/dt,di/dt and overvoltage during turn-on and turnoff.
2.3 EXISTING SYSTEM LIMITATION
In earlier literature we are using an RCD passive snubber to clamp the
voltage, and the energy absorbed in the clamping capacitor is dissipated on the
resistor, thus resulting in lower efficiency.
Figure 2.1 RCD Snubber
A buck converter was employed to replace an RCD passive snubber, but it
still needs complex clamping circuits. A simple active clamping circuit was
proposed which suits for bidirectional converters. However, its resonant current
increases the current stress on switches significantly.
18
CHAPTER III
3.1 PROPOSED SYSTEM MERITS
There is no dissipation of energy as that of RCD circuits.
Circuit complexity is reduced.
Switching losses are reduced.
The energy losses are flyback to the system directly, so the efficiency is
improved.
It can work in forward and reverse direction.
3.2 CONVERTER USING FLYBACK SNUBBER
Here we are replacing the RCD snubber with a flyback snubber as shown in
the figure. The flyback snubber to recycle the absorbed energy in the clamping
capacitor. The flyback snubber can be operated independently to regulate the
voltage of the clamping capacitor; therefore, it can clamp the voltage to a desired
level just slightly higher than the voltage across the low-side transformer winding.
Since the current does not circulate through the full-bridge switches, their current
stresses can be reduced dramatically under heavy-load condition, thus improving
system reliability significantly.
Figure 3.1converter using flyback snubber
19
3.2.1 INTRODUCTION
Converter is the most commonly used SMPS circuit for low output power
applications where the output voltage needs to be isolated from the input main
supply. The output power of fly-back type SMPS circuits may vary from few watts
to less than 100 watts. The overall circuit topology of this converter is considerably
simpler than other SMPS circuits. Input to the circuit is generally unregulated dc
voltage obtained by rectifying the utility ac voltage followed by a simple capacitor
filter. The circuit can offer single or multiple isolated output voltages and can
operate over wide range of input voltage variation. In respect of energy-efficiency,
fly-back power supplies are inferior to many other SMPS circuits but its simple
topology and low cost makes it popular in low output power range.
Here we are using a flyback snubber circuit to avoid the switching losses and
work on both boost and buck operation.While using this converter the losses which
are present in RCD passive snubber circuit is reduced .Hence we can use this type
of dc-dc converters in high power application.
3.2.2 DESCRIPTION
CIRCUIT DIAGRAM:
Figure 3.2 operation of isolated bidirectional full bridge dc-dc converter with
flyback snubber
20
The proposed isolated bidirectional full-bridge dc–dc converter with a
flyback snubber is shown above. The converter is operated with two modes: buck
mode and boost mode. Fig. 1 consists of a current-fed switch bridge, a flyback
snubber at the low-voltage side, and a voltage-fed bridge at the high-voltage side.
Inductor Lm performs output filtering when power flows from the high-voltage
side to the batteries, which is denoted as a buck mode. On the other hand, it works
in boost mode when power is transferred from the batteries to the high-voltage
side. Furthermore, clamp branch capacitor CC and diode DC are used to absorb the
current difference between current-fed inductor Lm and leakage inductance Lll and
Llh of isolation transformer Tx during switching commutation.
The flyback snubber can be independently controlled to regulate VC to the
desired value, which is just slightly higher than VAB. Thus, the voltage stress of
switchesM1–M4 can be limited to a low level. The major merits of the proposed
converter configuration include no spike current circulating through the power
switches and clamping the voltage across switches M1–M4, improving system
reliability significantly. Note that high spike current can result in charge migration,
over current density, and extra magnetic force, which will deteriorate in MOSFET
carrier density, channel width, and wire bonding and, in turn, increase its
conduction resistance.
A bidirectional dc–dc converter has two types of conversions: step-up
conversion (boost mode) and step-down conversion (buck mode). In boost mode,
switches M1–M4 are controlled, and the body diodes of switches M5–M8 are used
as a rectifier. In buck mode, switches M5–M8 are controlled, and the body diodes
of switches M1–M4 operate as a rectifier. To simplify the steady-state analysis,
several assumptions are made, which are as follows.
21
1) All components are ideal. The transformer is treated as an ideal transformer
associated with leakage inductance.
2) Inductor Lm is large enough to keep current iL constant over a switching period.
3) Clamping capacitor CC is much larger than parasitic capacitance of switches
M1–M8 .
A. Step-Up Conversion
In boost mode, switches M1–M4 are operated like a boost converter, where
switch pairs (M1, M2) and (M3 , M4 ) are turned ON to store energy in Lm. At the
high-voltage side, the body diodes of switches M5–M8 will conduct to transfer
power to VHV. When switch pair (M1 , M2) or (M3 , M4 ) is switched to (M1 , M4)
or (M2 , M3 ), the current difference iC (= iL − ip ) will charge capacitor CC , and
then, raise ip up to iL . The clamp branch is mainly used to limit the transient
voltage imposed on the current-fed side switches. Moreover, the flyback converter
can be controlled to charge the high-voltage-side capacitor to avoid over current.
The clamp branch and the flyback snubber are activated during both start-up and
regular boost operation modes. A non phase-shift PWM is used to control the
circuit to achieve smooth transition from start-up to regular boost operation mode.
Referring to Fig. 1, the average power PC transferred to CC can be determined as
follows:
PC = 1/2CC [(iLZo) 2 + 2iLZoVC(R)] fs ………….. (3.1)
Where
Zo = Г (Leq/CC) ………….. (3.2)
Leq = Lll + Llh (N2P/N2S) .…………. (3.3)
VC (R) stands for a regulated VC voltage, which is close to (VHV (NP /NS)), fs is
the switching frequency, and Lm _Leq. Power PC will be transferred to the high-
side voltage source through the flyback snubber, and the snubber will regulate
clamping capacitor voltage VC to VC (R) within one switching cycle Ts (=1/fs ).
22
Note that the flyback snubber does not operate over the interval of inductance
current ip increasing toward iL. The processed power PC by the flyback snubber is
typically around 5% of the full-load power for low-voltage applications. With the
flyback snubber, the energy absorbed in CC will not flow through switches M1–
M4, which can reduce their current stress dramatically when Leq is significant.
Theoretically, it can reduce the current stress from 2iL to iL .
The peak voltage VC (P) of VC will impose on M1–M4 and it can be
determined as follows:
VC (P) = iL (M) Zo + VHV (Np/Ns) ..……… (3.4)
Where IL (M) is the maximum inductor current of iL, which is related to the
maximum load condition. Additionally, for reducing conduction loss, the high-side
switchesM5–M8 are operated with synchronous switching. Reliable operation and
high efficiency of the proposed converter are verified on a prototype designed for
alternative energy applications. The operation waveforms of step-up conversion
are shown in Fig. 2.Adetailed description of a half-switching cycle operation is
shown as follows.
OPERATION MODES OF STEP-UP CONVERSION
Mode 1 [t0 ≤ t < t1 ]:
Figure 3.3(a) Operation mode of step-up conversion mode 1
23
In this mode, all of the four switches M1–M4 are turned ON. Inductor Lm is
charged by VLV. inductor current iL increases linearly at a slope of VLV /Lm, and
the primary winding of the transformer is short-circuited. The equivalent circuit is
shown in Fig.3..3(a).
Mode 2 [t1 ≤ t < t2 ]
At t1 , M1 and M4 remain conducting, while M2 and M3 are turned OFF.
Clamping diode Dc conducts until the current difference (iL (t2) − ip (t2)) drops to
zero at t = t2 . Moreover, the body diodes of switch pair (M5,M8) are conducting to
transfer power. During this interval, the current difference (IL (t) − ip (t)) flows
into clamping capacitor CC. The equivalent circuit is shown in Fig. 3.3(b).
Figure 3.3(b) Operation mode of step-up conversion mode 2
Mode 3 [t2 ≤ t < t3]:
At t2, clamping diode Dc stops conducting, and the flyback snubber starts to
operate. At this time, clamping capacitor Cc is discharging, and flyback inductor is
storing energy. Switches M1 and M4 still stay in the ON state, while M2 and M3
remain OFF. The body diodes of switch pair (M5,M8) remain ON to transfer
power. The equivalent circuit is shown in Fig. 3.3(c).
24
Figure 3.3(c) Operation mode of step-up conversion mode 3
Mode 4 [t3 ≤t<t4]
At t3, the energy stored in flyback inductor is transferred to the high-voltage
side. Over this interval, the flyback snubber will operate independently to regulate
VC to VC (R) . On the other hand, switches M1 and M4 and diodes D5 and D8 are
still conducting to transfer power from VLV to VHV.
The equivalent circuit is shown in Fig. (d).
Figure 3.3(d) Operation mode of step-up conversion mode 4
25
Mode 5 [t4 ≤ t < t5]
At t4, capacitor voltage VC has been regulated to VC (R), and the snubber is idle.
Over this interval, the main power stage is still transferring power from VLV to
VHV. It stops at t5 and completes a half-switching cycle operation.
Figure 3.3(e) Operation mode of step-up conversion mode 5
OPERATION MODES OF STEP-DOWN CONVERSION
In the analysis, leakage inductance of the transformer at the low voltage side
is reflected to the high-voltage side, as shown in Fig. 3.3(d), in which equivalent
inductance Leq equals (Llh + Lll (N2 P/N2 S )). This circuit is known as a phase-
shift full-bridge converter. In the step-down conversion, switches M5–M8 are
operated like a buck converter, in which switch pairs (M5 , M8 ) and (M6 , M7 )
are alternately turned ON to transfer power from VHV to VLV . SwitchesM1–M4 is
operated with synchronous switching to reduce conduction loss. For alleviating
leakage inductance effect on voltage spike, switchesM5–M8 are operated with
phase-shift manner. Although, there is no need to absorb the current difference
between iL and ip , capacitor CC can help to clamp the voltage ringing due to Leq
equals (Llh + Lll (N2 P/N2 S )) and parasitic capacitance of M1–M4 .The operation
26
waveforms of step-down conversion are shown in Fig. 3.3(e). A detailed
description of a half-switching cycle operation is shown as follows.
Mode 1 [t0 ≤ t < t1 ]:
In this mode, M5 and M8 are turned ON, while M6 and M7 are in the OFF
state. The high-side voltage VHV is immediately exerted on the transformer, and
the whole voltage, in fact, is exerted on the equivalent inductance Leq and causes
the current to rise with the slope of VHV /Leq . With the transformer current
increasing linearly toward the load current level at t1 , the switch pair (M1 , M4 )
are conducting to transfer power, and the voltage across the transformer terminals
on the current-fed side changes immediately to reflect the voltage from the
voltage-fed side, i.e., (VHV (Np /Ns )). The equivalent circuit is shown in Fig.3.4
(a).
Figure 3.4(a) The equivalent circuit of step down conversion
Mode 2 [t1 ≤t<t2 ]
At t1 ,M8 remains conducting, whileM5 is turned OFF. The body diode of
M6 then starts to conduct the freewheeling leakage current. The transformer
27
current reaches the load-current level at t1 , and VAB rise to the reflected voltage
(VHV (Np /Ns )). Clamping diode Dc starts to conduct the resonant current of Leq
and the clamp capacitor CC . This process ends at t2 when the resonance goes
through a half resonant cycle and is blocked by the clamping diode Dc . The
equivalent circuit is shown in Fig. 3.4(b).
Figure 3.4(b) The equivalent circuit of step down conversion mode 2
Mode 3 [t2 ≤ t < t3]:
At t2, with the body diode of switchM6 conducting, M6 can be turned ON
Figure 3.4(c) The equivalent circuit of step down conversion mode 3
28
with zero-voltage switching (ZVS). The equivalent circuit is shown in Fig 3.4(c).
Mode 4 [t3 ≤t<t4]:
At t3 ,M6 remains conducting, whileM8 is turned OFF. The body diode of
M7 then starts to conduct the freewheeling leakage current. The equivalent circuit
is shown in Fig. (d).
Figure 3.4(d) The equivalent circuit of step down conversion mode 4
Mode 5 [t4 ≤ t < t5 ]
At t4 , with the body diode of switch M7 conducting, M7 can be turned ON
Figure 3.4(e) The equivalent circuit of step down conversion mode 5
29
with ZVS. Over this interval,the active switches change to the other pair of
diagonal switches, and the voltage on the transformer reverses its polarity.
Figure 3.5 graphical representation of step down transformer out put
30
Figure3.6 Graphical representation of step up transformer out put
31
CHAPTER IV
4. MATLAB SIMULATION OUTPUT OF PROPOSED SYSTEM
4.1 SCHEMATIC DIAGRAM
FORWARD DIRECTION:
Figure 4.1 schematic diagram at forward direction
32
REVERSE DIRECTION
Figure 4.2 schematic diagram at backward direction
33
4.2 SIMULATION OUTPUT
INPUT VOLTAGE:
Figure 4.3 simulation output of input voltage
PULSE
Figure 4.4 simulation output of pulse
34
OUTPUT VOLTAGE
Figure 4.5 simulation output of output voltage
OUTPUT WAVEFORM
Figure 4.6 simulation output waveform
35
CHAPTER V
IMPLEMENTATION OF PROPOSED SYSTEM
5.1 BLOCK DIAGRAM
Figure 5.1 block diagram of proposed system
DC SOURCE
(BATTERY)
DC-DC CONVERTER
WITH FLYBACK
SNUBBER
DC BUS
SYSTEM
DRIVER UNIT
PULSE
GENERATING
UNIT
SUPPLY
UNIT
36
5.2 CONTENT OF BLOCK DIAGRAM
5.2.1. DC SUPPLY SOURCE
The source is a renewable dc source, usually a rechargable battery. Initially
230 V AC supply is reduced to 50V with the help of a step down transformer
having a capacity of 1A.This AC supply is converted to a DC voltage by using a
diode bridge rectifier. Output from the rectifier unit having harmonic contents, so
we provided the filter circuit, filter circuit is used to reduce the harmonics. Here we
can use the electrolytic capacitor. This eliminates the harmonics from both voltage
and current signals.
5.2.2. DC-DC CONVERTER WITH A FLYBACK SNUBBER
It includes flyback snubber circuit, inverter unit, isolation transformer and
rectifier unit. Here in our project for full wave rectification we use bridge rectifier.
Two diodes(say D2 & D3) are conducting while the other two diodes (D1 & D4)
are in off state during the period t = 0 to T/2.Accordingly for the negative cycle of
the input the conducting diodes are D1 & D4 .Thus the polarity across the load
is the same. DC to AC converters are known as inverters. The function of an
inverter is to change a dc input voltage to a symmetrical ac output voltage of
desired magnitude and frequency. The output voltage could be fixed or variable at
a fixed or variable frequency. A variable output voltage can be obtained by varying
the input dc voltage and maintaining the gain of the inverter constant.
5.2.3. DC BUS
It is the high voltage side of the system. The voltage in the bus system
determines the mode of operation of the circuit, that is the step up and step down
37
convertion.The dc voltage level in the bus system may be around 360V and the
input voltage level is 48V.
5.2.4. DRIVER UNIT
The driver circuit forms the most important part of the hardware unit because
it acts as the backbone of the inverter as it gives the triggering pulse to the
switches in the proper sequence. This driver circuit, which produces amplified,
pulses for the Mosfet‟s power circuit, which uses emitter coupled amplifier circuit
to boost the triggering pulse low voltage to the high voltage. It is used to provide
9 to 20 volts to switch the MOSFET Switches of the inverter. Driver amplifies
the voltage from microcontroller which is 5volts. Also it has an optocoupler for
isolating purpose. So damage to MOSFET is prevented.
5.2.5.PULSE GENERATOR UNIT
Here we have used PIC microcontroller(PIC 16F877A) to make a switching
signal.The Pic microcontroller are driven via the driver circuit so as to boost the
voltage triggering signal to 9V.To avoid any damage to micro controller due to
direct passing of 230V supply to it we provide an isolator in the form of
optocoupler in the same driver
5.2.6. AC SUPPLY UNIT
Here we are using a stepdown transformer 230V-12V in order to provide
proper supply to the driver and the pulse generating unit. In order to obtain a dc
voltage of 0 Hz, we have to use a low pass filter. So that a capacitive filter circuit
is used where a capacitor is connected at the rectifier output& a dc is obtained
across it. The filtered waveform is essentially a dc voltage with negligible ripples
& it is ultimately fed to the load.
38
5.3 DESCRIPTION
CIRCUIT DIAGRAM
Figure 5.2operation of isolated bidirectional full bridge dc-dc converter with
flyback snubber
The proposed isolated bidirectional full-bridge dc–dc converter with a
flyback snubber is shown in Fig.1. The converter is operated with two modes: buck
mode and boost mode. Fig.1 consists of a current-fed switch bridge, a flyback
snubber at the low-voltage side, and a voltage-fed bridge at the high-voltage side.
Inductor Lm performs output filtering when power flows from the high-voltage
side to the batteries, which is denoted as a buck mode. On the other hand, it works
in boost mode when power is transferred from the batteries to the high-voltage
side. Furthermore, clamp branch capacitor CC and diode DC are used to absorb the
current difference between current-fed inductor Lm and leakage inductance Lll and
Llh of isolation transformer Tx during switching commutation.
The flyback snubber can be independently controlled to regulate VC to the
desired value, which is just slightly higher than VAB . Thus, the voltage stress of
switchesM1–M4 can be limited to a low level. The major merits of the proposed
39
converter configuration include no spike current circulating through the power
switches and clamping the voltage across switches M1–M4, improving system
reliability significantly. Note that high spike current can result in charge migration,
over
5.3.1POWER SUPPLY DIAGRAM
Figure 5.3 power supply diagram
5.3.2 TRANSFORMER
Mainly there are two types of transformers
1) 230v-12v
2) Isolation transformer 12v-12v
+5vJP1
POWER PIN
123
C30.1 uF
R1220 ohmC1
470 uFC2100 uF
- +
D1
U17805
1
2
3VIN
GN
D
VOUT
D2
POWER SUPPLY
40
5.3.3 DIODE BRIDGE RECTIFIER
Figure 5.4 diode bridge rectifier
FEATURES
High reverse voltage to 1000V
Surge overload ratings to 50 amperes peak
Good for printed circuit board assembly
Mounting position: Any
Weight: 1.20 grams
Voltage Range 50 to 1000 Volts CURRENT 1.5 Amperes
5.3.4 FILTERING UNIT
Passive filters
Passive implementations of linear filters are based on combinations
of resistors (R), inductors (L) and capacitors (C). These types are collectively
known as passive filters, because they do not depend upon an external power
supply and/or they do not contain active components such as transistors.
Active filters
Active filters are implemented using a combination of passive and active
(amplifying) components, and require an outside power source. Operational
41
amplifiers are frequently used in active filter designs. These can have high Q
factor, and can achieve resonance without the use of inductors. However, their
upper frequency limit is limited by the bandwidth of the amplifiers used.
5.3.5 Voltage Regulators
Figure 5.5 Voltage regulator
Description
The KA78XX/KA78XXA series of three-terminal positive regulator are
available in the TO-220/D-PAK package and with several fixed output voltages,
making them useful in a wide range of applications. Each type employs internal
current limiting, thermal shut down and safe operating area protection, making it
essentially indestructible. If adequate heat sinking is provided, they can deliver
over 1A output current. Although designed primarily as fixed voltage regulators,
these devices can be used with external components to obtain adjustable voltages
and currents.
Features
Output Current up to 1A
Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24V
Thermal Overload Protection
Short Circuit Protection
Output Transistor Safe Operating Area Protection
42
5.3.6 DRIVER CIRCUIT
Figure 5.6 Driver circuit unit
It is used to provide 9 to 20 volts to switch the MOSFET Switches of
the inverter. Driver amplifies the voltage from microcontroller which is 5volts.
Also it has an optocoupler for isolating purpose. So damage to MOSFET is
prevented.
The driver circuit forms the most important part of the hardware unit
because it acts as the backbone of the inverter because it gives the triggering pulse
to the switches in the proper sequence. The diagram given above the circuit
operation of the driver unit.
5.3.6.1 COMPONENTS
A. Optocoupler
B. Capacitor
C. Supply
D. Diode
E. Resistor
43
5.3.7 OPTOCOUPLER
Optocoupler is also termed as optoisolator. Optoisolator a device which
contains a optical emitter, such as an LED, neon bulb, or incandescent bulb, and an
optical receiving element, such as a resistor that changes resistance with variations
in light intensity, or a transistor, diode, or other device that conducts differently
when in the presence of light. These devices are used to isolate the control voltage
from the controlled circuit.
Uses of optocoupler
Basically the simplest way to visualize an optocoupler is in terms of its two
main components: the input LED and the output transistor or diac. As the two are
electrically isolated, this gives a fair amount of flexibility when it comes to
connecting them into circuit.
Figure 5.7 Optocoupler
All we really have to do is work out a convenient way of turning the input
LED on and off, and using the resulting switching of the phototransistor/ diac to
44
generate an output waveform or logic signal that is compatible with our output
circuitry.
For example: just like a discrete LED, you can drive an optocoupler‟s input
LED from a transistor or logic gate/buffer. All that‟s needed is a series resistor to
set the current level when the LED is turned on. And regardless of whether you use
a transistor or logic buffer to drive the LED, you still have the option of driving it
in pull down or pull up mode.
5.3.8 BUFFER IC
The CD4049UB and CD4050B devices are inverting and non-inverting hex
buffers, respectively, and feature logiclevel conversion using only one supply
voltage (VCC). The input-signal high level (VIH) can exceed the VCC supply
voltage when these devices are used for logic-level conversions. These devices are
intended for use as CMOS to DTL/TTL converters and can drive directly two
DTL/TTL loads. (VCC =
45
5.3.8.1 PIN DIAGRAM
Figure 5.8 Pin diagram of buffer IC
5.3.8.2 SCHEMATIC DIAGRAMS
CD4049UB CD4050B
Figure 5.9 Schematic diagram
46
5.3.8.3 Features
CD4049UB Inverting
CD4050B Non-Inverting
High Sink Current for Driving 2 TTL Loads
High-To-Low Level Logic Conversion
100% Tested for Quiescent Current at 20V
Temperature Range; 100nA at 18V and 25oC
5V, 10V and 15V Parametric Ratings
5.3.8.4 ABSOLUTE MAXIMUM RATINGS
Supply voltage (v+ to v-)…………………………. -0.5V to 20V
DC input current, any one input……………………. ±10mA
5.3.8.5 RECOMMENDED OPERATING CONDITIONS
Temparature range…………………………….. -55 ℃ to 125 ℃
5.3.8.6 APPLICATIONS
CMOS to DTL/TTL Hex Converter
CMOS Current “Sink” or “Source” Driver
CMOS High-To-Low Logic Level Converter.
5.3.9 TRANSISTOR
The transistor is the fundamental building block of modern electronic
devices, and is ubiquitous in modern electronic systems. Following its
development in the early 1950s the transistor revolutionized the field of
47
electronics, and paved the way for smaller and cheaper radios, calculators,
and computers, among other things.
5.3.9.1 TRANSISTOR AS AN AMPLIFIER
First we must connect it appropriately to the supply voltages, input signal,
and load, so it can be used.
A useful mode of operation is the common emitter configuration.
The emitter voltage is always 0.7V below Vb, so if Vb changes by ∆Vin, so
does Ve.
Thus the emitter current increases by ∆Vin/Re.
But Ic=-𝛼Ie≅-Ie, so it also increases by ∆Vin/Re.
Thus the voltage at the collector will increase by -∆Vin RL/Re. (that is, it will
decrease).
In this case is RL/Re10, so the circuit amplifies the input voltage signal by a
factor of -10.
In general the gain is –RL/Re. the negative sign indicates that a increase in
input voltage leads to a decrease in output voltage.
This is an example of an inverting amplifier.
5.3.9.2 GENERAL DESCRIPTION
A transistor is a semiconductor device used to amplify and switch electronic
signals and power. It is composed of a semiconductor material with at least three
terminals for connection to an external circuit. A voltage or current applied to one
pair of the transistor's terminals changes the current flowing through another pair
of terminals. Because the controlled (output) power can be higher than the
48
controlling (input) power, a transistor can amplify a signal. Today, some
transistors are packaged individually, but many more are found embedded
in integrated circuits.
5.3.9.3 DARLINGTON PAIR CIRCUIT
Transistors are an essential component in a sensor circuit. Usually transistors
are arranged as a pair, known as a „DARLINGTON PAIR‟.A Darlington pair is
used to amplify weak signals so that they can be clearly detected by another circuit
or a computer/microprocessor.
Figure 5.10 Darlington pair circuit
5.3.9.4 TRANSISTOR-2N2222
NPN switching transistors 2N2222; 2N2222A
5.3.9.5 FEATURES
High current (max. 800 mA)
Low voltage (max. 40 V).
49
DESCRIPTION
NPN switching transistor in a TO-18 metal package.
PNP complement: 2N2907A.
5.3.9.6 PIN Description
1 Emitter
2 Base
3 Collector, connected to case
5.3.9.7 APPLICATIONS
Linear amplification and switching
Used as amplifier
High speed applications
Low power application
Linear amplification
Switching
Fabrication of ICs
50
APPENDIX I
6.1 PIC16F877A MICROCONTROLLER
We are using PIC 16F877A for producing switching pulses to multilevel
inverter. So as to use those vectors which do not generate any common
mode voltage at the inverter poles. .This eliminates common mode voltage
Also it is used to eliminate capacitor voltage unbalancing. The microcontroller are
driven via the driver circuit so as to boost the voltage triggering signal to 9V.To
avoid any damage to micro controller due to direct passing of 230V supply to it
we provide an isolator in the form of optocoupler in the same driver circuit.
6.1.1 FEATURES OF PIC MICROCONTROLLER
The microcontroller has the following features:
1. High-Performance RISC CPU:
Only 35 single- word instructions to learn .Hence it is user friendly.
Easy to use
All single - cycle instructions except for program branches, which are
two-cycle
Operating speed: DC – 20 MHz clock input DC – 200 ns
instruction cycle
Up to 8K x 14 words of Flash Program Memory, Up to 368 x 8 bytes of
Data Memory (RAM), Up to 256 x 8 bytes of EEPROM Data
Memory. It is huge one
2. Peripheral Features:
Timer0: 8-bit timer/counter with 8 – bit presaler. It is used for
synchronization
51
Timer1:16-bit timer/counter with prescaler, can be incremented during Sleep
Timer2:8-bit timer/counter with 8-bit period register, prescaler and
postscaler
Two Capture, Compare and some PWM modules, having following features
Capture is 16-bit, max. Resolution is 12.5 ns
Compare is 16-bit, max. Resolution is 200 ns
PWM maximum resolution that is 10-bit
Synchronous Serial Port (SSP) with SPI (Master mode) and
I2C(Master/Slave)
Universal Synchronous Asynchronous Receiver Transmitter with 9 bit
address
Parallel Slave Port (PSP) 8 bits wide with external RD, WR and CS
controls
3 .Analog features:
It has an analog Comparator module with:
(1)Two analog comparators
(2) Programmable on-chip voltage reference (VREF) module (3) Programmable
input multiplexing from device inputs and internal voltage reference
thus 3 parts
52
6.1.2 BLOCK DIAGRAM
Figure 6.1 block diagram of PIC
53
APPENDIX II
6.2 MEMORY ORGANIZATION:
There are three memory blocks in each of these PICmicro MCUs. The
Program Memory and Data Memory have separate buses so that concurrent access
can occur
6.2.1PROGRAM MEMORY ORGANIZATION
The PIC16F87X devices have a 13-bit program counter capable of
addressing an 8K x 14 program memory space. The PIC16F877/876 devices have
8K x 14 words of FLASH program memory. Accessing a location above the
physically implemented address will cause a wraparound. The reset vector is at
0000h and the interrupt vector is at 0004h.
6.2.2 DATA MEMORY ORGANIZATION:
The data memory is partitioned into multiple banks which contain the
General Purpose Registers and the Special Function Registers. Bits RP1
(STATUS<6>) and RP0 (STATUS<5>) are the bank select bits. Each bank extends
up to 7Fh (128 bytes). The lower locations of each bank are reserved for the
Special Function Registers. Above the Special Function Registers are General
Purpose Registers, implemented as static RAM. All implemented banks contain
Special Function Registers. Some “high use” Special Function Registers from one
bank may be mirrored in another bank for code reduction and quicker access.
GENERAL PURPOSE REGISTER FILE
The register file can be accessed either directly, or indirectly through the File
Select Register FSR.
54
SPECIAL FUNCTION REGISTERS
The Special Function Registers are registers used by the CPU and peripheral
modules for controlling the desired operation of the device. These registers are
implemented as static RAM. The Special Function Registers can be classified into
two sets; core (CPU) and peripheral.
STATUS REGISTER:
The STATUS register contains the arithmetic status of the ALU, the RESET
status and the bank select bits for data memory. Bits RP1 (STATUS<6>) and RP0
(STATUS<5>) are the bank select bits.
TABLE 1
I/O PORTS:
PIC16F877 has 5 PORTS, named PORTA through PORTE. All are bi-
directional ports. Each ports are having corresponding data direction registers i.e.
TRISA for PORTA etc. Setting a TRISx bit (=1) will make the corresponding
PORT pin an input (i.e., put the corresponding output driver in a hi-impedance
mode). Clearing a TRISx bit (=0) will make the corresponding PORTx pin an
output (i.e., put the contents of the output latch on the selected pin). Reading the
55
PORTx register reads the status of the pins, whereas writing to it will write to the
port latch. All write operations are read-modify-write operations.
Therefore, a write to a port implies that the port pins are read, the value is
modified and then written to the port data latch. Port Pins are multiplexed with
peripherals Eg.PORTA pins are multiplexed with analog inputs and analog VREF
input. All PORT pins have TTL input levels and full CMOS output drivers.
PORTA and PORTE requires additional register configuration to select the ports as
ANALOG or DIGITAL. The operation of each pin is selected by clearing/setting
the control bits in the ADCON1 register (A/D Control Register1).
Pin Details:
PORT PINS
PORTA 6
PORTB 8
PORTC 8
PORTD 8
PORTE 3
TABLE 2
56
APPENDIX III
6.3 INTERRUPTS
The PIC16F87X family has up to 14 sources of interrupt. The interrupt
control register (INTCON) records individual interrupt requests in flag bits. It also
has individual and global interrupt enable bits. A global interrupt enable bit, GIE
(INTCON<7>) enables (if set) all un-masked interrupts or disables (if cleared) all
interrupts. When bit GIE is enabled, and an interrupt‟s flag bit and mask bit are set,
the interrupt will vector immediately.
Individual interrupts can be disabled through their corresponding enable bits
in various registers. Individual interrupt bits are set regardless of the status of the
GIE bit. The GIE bit is cleared on reset. The “return from interrupt” instruction,
RETFIE, exits the interrupt routine, as well as sets the GIE bit, which re-enables
interrupts. The RB0/INT pin interrupt, the RB port change interrupt and the TMR0
overflow interrupt flags are contained in the INTCON register. The peripheral
interrupt flags are contained in the special function registers, PIR1 and PIR2. The
corresponding interrupt enable bits are contained in special function registers, PIE1
and PIE2, and the peripheral interrupt enable bit is contained in special function
register INTCON.
When an interrupt is responded to, the GIE bit is cleared to disable any
further interrupt, the return address is pushed onto the stack and the PC is loaded
with 0004h. The interrupt flag bit(s) must be cleared in software before re-enabling
interrupts to avoid recursive interrupts. For external interrupt events, such as the
INT pin or PORTB change interrupt, the interrupt latency will be three or four
instruction cycles. The exact latency depends when the interrupt event occurs. The
57
latency is the same for one or two cycle instructions. Individual interrupt flag bits
are set regardless of the status of their corresponding mask bit or the GIE bit.
TABLE 3 OPCODE FIELD DESCRIPTIONS
58
TABLE 4 PIC 16F877A INSTRUCTION SET
59
APPENDIX IV
6.4 MOSFET SWITCH-IRFP250N
Figure 6.2 MOSFET switch-IRFP250N
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-
FET,or MOSFET)isa transistor used for amplifying or switching electronic signals.
Although the MOSFET is a four-terminal device with source (S), gate (G), drain
(D), and body (B) terminals, the body (or substrate) of the MOSFET often is
connected to the source terminal, making it a three-terminal device like other field-
effect transistors. The MOSFET is by far the most common transistor in
both digital and analog circuits, though the bipolar junction transistor was at one
time much more common.
60
In enhancement mode MOSFETs, a voltage drop across the oxide induces
a conducting channel between the source and drain contacts via the field effect.
The term "enhancement mode" refers to the increase of conductivity with increase
in oxide field that adds carriers to the channel, also referred to as the inversion
layer. The channel can contain electrons (called an nMOSFET or nMOS), or holes
(called a pMOSFET or pMOS), opposite in type to the substrate, so nMOS is made
with a p-type substrate, and pMOS with an n-type substrate (see article
on semiconductor devices). In the less common depletion mode MOSFET,
described further later on, the channel consists of carriers in a surface impurity
layer of opposite type to the substrate, and conductivity is decreased by application
of a field that depletes carriers from this surface layer.
61
CHAPTER VII
7.1 CONCLUSION
This paper has presented an isolated bidirectional full-bridge dc–dc
converter with a flyback snubber for high-power applications. The flyback snubber
can alleviate the voltage spike caused by the current difference between the
current-fed inductor and leakage inductance of the isolation transformer, and can
reduce the current flowing through the active switches at the current fed side by
50%. Since the current does not circulate through the full-bridge switches, their
current stresses can be reduced dramatically under heavy-load condition, thus
improving system reliability significantly. The flyback snubber can be also
controlled to achieve a soft start-up feature. It has been successful in suppressing
inrush current which is usually found in a boost-mode start-up transition. A 1.5-
kW isolated full-bridge bidirectional dc–dc converter with a flyback snubber has
been implemented to verify its feasibility.
62
CHAPTER VIII
8.1 REFERENCES
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voltage and high-power isolated bidirectional DC-DC converters,” IEEE Trans.
Power Electron., vol. 23, no. 6, pp. 2648–2656, Nov. 2008.
2. H.Bai andC.Mi, “Eliminate reactive power and increase system efficiency of
isolated bidirectional dual-active-bridge DC-DC converters using novel dual-
phase-shift control,” IEEE Trans. Power Electron.,
3. R. Huang and S. K. Mazumder, “A soft-switching scheme for an isolated
DC/DC converter with pulsating DC output for a three-phase highfrequency- link
PWM converter,” IEEE Trans. Power Electron., vol. 24, no. 10, pp. 2276–2288,
Oct. 2009.vol. 23, no. 6, pp. 2905–2914, Dec. 2008.
4. F. Krismer and J. W. Kolar, “Accurate small-signal model for the digital control
of an automotive bidirectional dual active bridge,” IEEE Trans. Power Electron.,
vol. 24, no. 12, pp. 2756–2768, Dec. 2009.
5. G. Ma, W. Qu, and Y. Liu, “A zero-voltage-switching bidirectional DC-DC
converter with state analysis and soft-switching-oriented design consideration,”
IEEE Trans. Ind. Electron., vol. 56, no. 6, pp. 2174–2184, Jun. 2009.
6. H. Xiao and S. Xie, “A ZVS bidirectional dc-dc converter with phased shift
plus PWM control scheme,” IEEE Trans. Power Electron., vol. 23, no. 2, pp. 813–
823, Mar. 2008.