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

1. B. Bai, C. Mi, and S. Gargies, “The short-time-scale transient processes in high-

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.