high step-up coupled-inductor-based converter using bi-direction energy transmission

7/28/2019 High Step-Up Coupled-Inductor-based Converter Using Bi-Direction Energy Transmission

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High Step-up Coupled-inductor-based Converter Using Bi-direction Energy Transmission

Rong-Jong Wai, Member,IEEE

Department of Electrical Engineering, Yuan Ze University,Chung Li 32026, Taiwan, R.O.C.

E-mail: [email protected]

Rou-Yong Duan

Department of Industrial Safety & Health, Hung KuangUniversity, Tai Chung 433, Taiwan, R.O.C.

E-mail: [email protected]

AbstractIn this study, a high step-up converter with coupled-inductor by way of bi-direction energy transmission is

investigated. In the proposed strategy, a coupled inductor with a

lower-voltage-rated switch is used for raising the voltage gain

whether the switch is turned on or turned off. Moreover, a

passive regenerative snubber is utilized for absorbing the energy

of stray inductance so that the switch duty cycle can be operated

under a wide range, and the related voltage gain is higher than

other coupled-inductor-based converters. The capacity of the

magnetic core can be utilized completely by way of bi-direction

energy transmission. In addition, all devices in this scheme also

have voltage-clamped properties and their voltage stresses are

only related to the output voltage. Thus, it can select low-voltage

low-conduction-loss devices, and there are no reverse-recoverycurrents within the diodes in this circuit. Some experimental

results via an example of a proton exchange membrane fuel cell

(PEMFC) power source are given to demonstrate the

effectiveness of the proposed power conversion strategy.

I. INTRODUCTION

In recent, dc-dc converters with steep voltage ratio are

usually required in many industrial applications. Forexamples, the front-end stage for clean-energy sources, the dc

back-up energy system for an uninterruptible power supply

(UPS), high-intensity discharge lamps for automobile

headlamps, and telecommunication industry [1][3]. The

conventional boost converters cannot provide such a high dcvoltage gain, even for an extreme duty cycle. It also may

result in serious reverse-recovery problem and increase therating of all devices. As a result, the conversion efficiency is

degraded and the electromagnetic interference (EMI)

problem is severe under this situation [4]. In order to increase

the conversion efficiency and voltage gain, many modifiedboost converter topologies have been investigated in the past

decade [5][12].Although voltage-clamped techniques are manipulated in

the converter design to overcome the severe reverse-recovery

problem of the output diode in high-level voltageapplications, there still exists overlarge switch voltage

stresses and the voltage gain is limited by the turn-on time of

the auxiliary switch [5], [6]. Silva et al. [7] presented a boostsoft-single-switch converter, which has only one single active

switch. It is able to operate with soft switching in a pulse-

width-modulation (PWM) way without high voltage andcurrent stresses. Unfortunately, the voltage gain is limited

below four in order to achieve the function of soft switching.

In [8] and [9], coupled inductors were employed to provide ahigh step-up ratio and to reduce the switch voltage stress

substantially, and the reverse-recovery problem of the output

diode was also alleviated efficiently. In this case, the leakageenergy of the coupled inductor is another problem as the

switch was turned off. It will result in the high-voltage ripple

across the switch due to the resonant phenomenon induced bythe leakage current. In order to protect the switch devices,

either a high-voltage-rated device with higher )(onDSR or a

snubber circuit is usually adopted to deplete the leakage

energy. By these ways, the power conversion efficiency willbe degraded. Zhao and Lee [10] introduced a family of high-

efficiency, high step-up dc-dc converters by only adding oneaddition diode and a small capacitor. It can recycle the

leakage energy and alleviate the reverse-recovery problem. In

this scheme, the magnetic core can be regarded as a flyback

transformer and most of the energy was stored in themagnetic inductor. However, the leakage inductor of .the

coupled inductor and the parasitic capacitor of the outputdiode resonated after the switch was turned on, a proper

snubber is necessary to reduce the output rectifier peak

voltage. Moreover, the capacity of the magnetic core should

be increased substantially when the demand of high outputpower is required. The aim of this study is to design a high-

efficiency, high step-up converter with coupled-inductor byway of bi-direction energy transmission to regulate a stable

constant dc voltage.

II. CONVERTERDESIGN AND ANALYSES

The system configuration of the proposed converter

topology is depicted in Fig. 1, where it contains seven parts

including a dc input circuit, a primary-side circuit, a

secondary-side circuit, a passive regenerative snubber circuit,a filter circuit, a dc output circuit and a feedback control

mechanism. The major symbol representations are

summarized as follows. INV and II denote dc input voltage

and current, and INC is an input filter capacitor in the dc

input circuit. 1L and 2L represent individual inductors in

the primary and secondary sides of the coupled inductor ( rT ),

respectively. Q is a switch in the primary-side circuit and QTis a trigger signal in the feedback control mechanism. 1C ,

1D and 2D denote a clamped capacitor, a clamped diode

and a rectifier diode in the passive regenerative snubber

circuit. 2C is a high-voltage capacitor in the secondary-side

circuit. OD and OC are output diode and filter capacitor in

the filter circuit. OV and OI describe output voltage and

current; OR is an output load.

4060-7803-9033-4/05/$20.00 2005 IEEE.


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2L1L

1D

+

OC

2C

OV

rT

2DQ

1C

OD

OR

DC Input

Circuit

Primary-side

Circuit

Secondary-side

Circuit

Passive Regenerative

Snubber Circuit

Filter

Circuit

DC Output

Circuit

+

INV INC

2Li

II

O

I

Feedback Control Mechanism

Voltage Feedback

Proportional-

Integral Control

& PWM

Voltage CommandDriving Circuit

& Trigger Signal

QT

QT

Fig. 1. System configuration of high step-up converter.

The characteristic waveforms of the proposed high step-up converter are depicted in Fig. 2. Moreover, Fig. 3

illustrates the topological modes in one switching cycle andthe detailed operation stages are described in Section II-A.The coupled inductor in Fig. 1 is modeled as an ideal

transformer, a magnetizing inductor ( mL ), and a leakage

inductor ( kL ) in Fig. 3. The turn ratio (n) and coupling

coefficient (k) of this ideal transformer are defined as

12/ NNn = (1)

)(mkm

LLLk += (2)

where 1N and 2N are the winding turns in the primary and

secondary sides, respectively. For simplicity, the dc input

circuit in Fig. 1 is denoted as a constant voltage source, SV .

The voltages across the switch, the primary and secondary

winding of the ideal transformer, and the leakage inductor are

denoted as DSv , Lmv , 2Lv and Lkv , respectively. Moreover,

the primary current ( 1Li ) of the coupled inductor is composed

of the magnetizing current ( Lmi ) and the primary induced

current ( 1i ). The secondary current ( 2Li ) is formed by the

primary induced current ( 1i ) through the ideal transformer,

and its value is related to the turns ratio (n). In addition, the

conductive voltage drops of the switch (Q) and all diodes

( OD , 1D and 2D ) are neglected to simplify circuit analyses.

A. Operation Stages

Mode 1 (t0t1) [Fig. 3(a)]:In this mode, the switch (Q) was turned on for a span.

Because the magnetizing inductor ( mL ) is charged by the

input voltage source ( SV ), the magnetizing current ( Lmi )

increases gradually in an approximately linear way. The

secondary voltage ( 2Lv ) and the clamped capacitor voltage

( 1Cv ) are connected in series to charge the high-voltage

capacitor ( 2C ) through the switch (Q) and the rectifier diode

( 2D ). This behavior is the key path of bi-direction energy

transmission. Thus, the magnitude of the secondary current

( 2Li ) is decreased since the high-voltage capacitor voltage

( 2Cv ) is increased gradually. Since the primary current ( 1Li )

is the summation of the complementary currents ( Lmi and 1i ),the current curve of 1Li is similar to a square wave. At the

same reason, the switch current ( DSi ) is also close to a square

curve because the switch current ( DSi ) is equal to the current

summation of 1i , Lmi and 2Li . The square primary current

( 1Li ) will result in lower copper and core losses in the

coupled inductor, and the conduction loss of the switch also

can be alleviated by the square switch current ( DSi ).

Lmi

1Li

2Li

1Li

2Li

DSiDSv

DSi

DSv

2Di

1Dv

1Di

1Dv

Lmi1i

400V

2Dv

DOiDOv

400V

0t 1t 2t 3t 4t 5t 0t

GSv

1i

2Di

2Dv

DOi

DOv

Mode 1

1Di

Mode 4

Mode2

Mode3

Mode5

Mode6

Fig. 2. Characteristic waveforms.

Mode 2 (t1t2) [Fig. 3(b)]:At time

1tt= , the switch (Q) is turned off. At this time,

the primary and secondary currents ( 1Li and 2Li ) of the

coupled inductor starts to charge the parasitic capacitor of the

switch. After the switch voltage ( DSv ) is higher than the

clamped capacitor voltage ( 1Cv ), the clamped diode ( 1D )

conducts to transmit the energy of the primary-side leakage

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output diode ( OD ) decays to zero and starts to conduct, and

the rectifier diode ( 2D ) is cut off. At this time, the series

voltages of SV , Lkv , Lmv , 2Cv and 2Lv charges the output

capacitor ( OC ) and supplies the output load ( OR ) by way of

low current type. According to the conservation law of

magnetic energy, it still supplies currents in the primary andsecondary sides of the coupled inductor persistently after the

entire consumption of the leakage inductor energy. The

primary current ( 1Li ) charges the clamped capacitor ( 1C ) and

passes through the secondary side of the coupled inductor,

and the secondary current ( 2Li ) delivers to the output

terminal. In the middle stage of this mode, the high-voltage

capacitor ( 2C ) is discharged and its voltage ( 2Cv ) is

descended sustainability. Moreover, the clamped capacitor

voltage ( 1Cv ) is increased by electrifying for a long time, and

the primary current ( 1Li ) is equal to the secondary current

( 2Li ) when the clamped diode ( 1D ) is reverse-biased.

Mode 5 (t4t5) [Fig. 3(e)]:Since the clamped diode ( 1D ) is a low-voltage Schottky

diode, it will be cut off promptly without reverse-recovery

current when the switch (Q) is turned on at time 4tt= .

Because the raising rate of the primary current ( 1Li ) is limited

by the primary-side leakage inductor ( kL ), and the secondary

current ( 2Li ) needs time to decay to zero, these two currents

depends on each other. Because it can not derive any current

from these three paths including the primary-side circuit,

secondary-side circuit and passive regenerative snubber

circuit, the switch (Q) is turned on under zero-current-switching (ZCS) and this soft switching property is helpful

for alleviating the switching loss. In this mode, the circuitcurrent flow still directs to the output terminal, but its

magnitude decreases gradually.

Mode 6 (t5t0) [Fig. 3(f)]:After releasing the leakage energy, the secondary current

( 2Li ) decays to zero at time 5tt= and starts to pass through

the switch (Q) inversely. At the same time, the output currentprovides the reverse recovery current for the output diode

( OD ) to build its reverse-biased voltage ( DOv ), and the

secondary current ( 2Li ) leads the rectifier diode ( 2D ) to be

forward-biased. When the rectifier diode ( 2D ) is conducted

and the output diode ( OD ) is cut off ( 0tt= ), it begins the

next switching cycle and repeats the operation in mode 1.

B. Formula Derivation

When the switch (Q) is turned on, the voltages across the

magnetizing inductor ( mL ) can be denoted via (2) as

SLmkVv = (3)

Moreover, the voltage across the secondary winding of the

ideal transformer can be represented via (3) as

SLmLkVnnvv ==

2(4)

Because the series voltages of2L

v and1C

v charge the high-

voltage capacitor (2

C ), the voltage across2

C can be

described via (4) as

12 CSCvkVnv += (5)

When the switch (Q) is turned off, the current of the

leakage inductor (k

L ) in the primary side of the coupled

inductor flows persistently through the clamped capacitor

(1

C ) until the secondary current (2L

i ) reacts upon the energy

from the magnetizing inductor (m

L ). Due to the concept of

the zero average voltage across the leakage inductor (k

L )

over one period [10], the required cycle to release the energy

of the leakage inductor (k

L ) can be denoted as

13),1/()1(2 tttnDTtD

LSLL=+== (6)

whereS

T is the switching period, D is the duty cycle of the

switch (Q), andLt is the time from mode 2 to mode 3.

Moreover, the voltages ofLk

v andLm

v are given as

SLkV

D

knDv

)1(2

)1)(1(

+= (7)

)1/( DVkDvSLm

= (8)

Therefore, the clamped capacitor voltage ( 1Cv ) can be

represented via (7) and (8) as

DSS

S

SLmLkCvV

D

nkD

D

VVvvv =

+

=++=

)1(2

)1)(1(

11

(9)

Note that, the voltage of1C

v is equal to the switch voltage

( DSv ). According to (8) and (9), the voltages of 2Cv and 2Lvcan be rewritten as

SCV

D

nkDnkv ]

)1(2

)1)(1(2[

2

++= (10)

)1/(2

DnVkDvnvSLmL

== (11)

In the meantime, the voltages of1C

v ,2C

v and2L

v charge

the output capacitor (O

C ) and output load (O

R ); therefore,

the output voltage (O

V ) can be calculated as

SSLCCOV

D

nkDV

D

nkvvvV

+

+=++=

1

)1)(1(

1

2221

(12)

As a result, the voltage gain of the proposed high step-up

converter can be represented as

D

nkD

D

nk

V

VG

S

O

V

+

+==

1

)1)(1(

1

2(13)

Substituting 1=k and n=1,2,4,6,8 into (13), the curve of

the voltage gain (V

G ) with respect to the duty cycle (D) is

depicted in Fig. 4(a), where the line labeled with star denotes

the voltage gain curve of the newly designed converter and

the real line represents the one in [10]. As can be seen from

this figure, the voltage gain of the proposed high step-up

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converter is higher than a coupled-inductor-based converter

in [10], especially in the smaller duty cycle. For example, one

can obtain 40=V

G if the values of 1=k , 6=n and

8.0=D are selected. It can verify that the switch duty cycle

in the proposed converter can be operated under a wide range.

Moreover, the voltage gain curve by substituting 1~9.0=k

and 6=n into (13) is depicted in Fig. 4(b). By observingthis figure, the voltage gain (

VG ) is less sensitive to the

coupling coefficient, k. For simplicity, the coupling

coefficient (k) is set at one, then (9) and (13) can be rewrittenas

DSSCvDVv == )1/(

1(14)

D

n

V

VG

S

O

V

+==

1

2(15)

If the value of 5.0=D is selected, the voltage gain in (15) is

two times the ones in [10], [11]. According to (14) and (15),one can obtain

)2/( += nVvODS

(16)

By analyzing (16), the switch voltage ( DSv ) is not related to

the input power source ( SV ) and the switch duty cycle (D) if

the values of the output voltage ( OV ) and the turns ratio (n)

are fixed. Thus, it can ensure that the maximum sustainable

voltage of the switch (Q) is constant. As long as the inputvoltage is not higher than the switch voltage-rated, the

proposed high step-up converter can be applied well to low-

voltage power sources even with large voltage variations, e.g.photovoltaic cells, wind generator, fuel cells, batteries, etc.

Duty Cycle (D)

(a)

VoltageGain(Gv)

Coupling Coefficient k= 1

1* =n

2*=n

4

*

=n

6* =n

8* =n

1=n

8=n

6=n

2=n4=n

Coupled-inductor-based converter in [10]

* Proposed high step-up converter

9.0=k

1=k

VoltageGain(Gv)

Duty Cycle (D)

(b)

Turn Ratio n = 6

Fig. 4. Voltage gain curve: (a) Coupling coefficient k=1; (b) Turn ratio n=6.

III. EXPERIMENTAL RESULTS

In order to verify the effectiveness of the designed

topology, a PEMFC system is utilized for a low-voltagepower source in the proposed high step-up converter. The

PEMFC system used in this study is the PowerPEMTM-

PS250 manufactured by the Hpower Company. It is a dc

power source with 250 watts dc nominal power rating. The

system operates on ambient air and clean pressurizedhydrogen fuel. The fuel cell system consists of a (40) cell

stack of the PEM type, mechanical auxiliaries, and electroniccontrol module.

In experimentation, the high step-up converter is designedinitially to operate from the fuel cell variability dc input,

V3825 =IN

V , to deliver a constant dc output, V400=OV .

Assume that the maximum value of the switch voltage is

clamped at 50V, the turn ratio 62)/( (max) == DSO vVn

according to (16). From (15), the related duty cycle,

8.0=D , is reasonable in practical applications if theminimum input voltage is assumed to be 10V. In order to

solve the problem of the fuel cell output voltage varied with

the load variations, the proposed converter with dc voltage

feedback control is utilized to ensure the system stability, anda PWM control IC TL494 is adopted to achieve this goal of

feedback control. The prototype with the followingspecifications is designed in this section to illustrate thedesign procedure given in Section II.

Switching frequency: kHz100=Sf ;

Coupled-inductor: H131 =L ; H4702 =L ; 18:3: 21 =NN ;

98.0=k ; EE-55 core;

Capacitor: 2*V50/F3300=INC ; V100/F51 =C ;

V250/F8.62 =C ; V450/F47=OC ;

Switch Q: FQI90N08 (80V/71A, = m16)(onDS

R );

Diode: 1D : Schottky diode STPS20H100CT (100V/2*10A);

2D ,

OD : SFA1606G, TO-220AB (400V/16A).

The experimental voltage and current responses of theproposed high step-up converter operating at 300W-output

power is depicted in Fig. 5. From Fig. 5(a), the switch

voltage ( DSv ) is clamped at 50V that is much smaller than the

output voltage, V400=OV , and the curve of the switch

current ( DSi ) is similar to a square wave so that it can further

reduce the conduction loss of the switch (Q). By observing

Fig. 5(b) and (c), the primary current ( 1Li ) keeps about 20A,

thus only a smaller core capacity is necessary for H131

=L .

According to Fig. 5(d)(j), the reverse-recovery currents in

all diodes ( OD , 1D and 2D ) can be alleviated effectively,

and the voltages of the clamped capacitor ( 1C ) and the high-voltage capacitor ( 2C ) are close to constant values. Therefore,

it can alleviate the reverse-recovery problem and exhibit the

voltage-clamped effect for further raising the conversionefficiency. In order to examine the robust performance of the

proposed converter scheme, the experimental result of output

voltage ( OV ) and output current ( OI ) under the step load

variation between light-load (20W) and heavy-load (300W)

is depicted Fig. 5(k). As can be seen from this figure, the

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converter output voltage, V400=OV , is insensitive to the

load variations due to the utilization a small coupled-inductor

and a closed-loop control, and the output voltage ripple isalso slight extremely as a result of high switching frequency.

(c) (d)

(e) (f)

(b)

0V

(20V/div)

DSv

0A

DSi (10A/div)

0V

(50V/div)DSv

0A

1L

i(10A/div)

0A

2Li

(a)

0V

(20V/div)

INV

0A

1Li(10A/div)

0A

II

(2us/div) (2us/div)

(2us/div) 0V

0A

0A

1Di

2Di

1Cv

(5A/div)

(5A/div)

(50V/div)

(2us/div)

1Ci

0V

0A

0A

1Cv

(5A/div)

(10A/div)

(2us/div)(2us/div)

DSi

1Di

(50V/div) (50V/div)

(2A/div)

0V

0A

DSv

DSv

(10A/div)

(10A/div)

(g) (h)

(50V/div)

(5A/div)

(2us/div)

1Di

1Dv

(50V/div)DSv

(2A/div)

(2us/div)

2Di

2Dv (200V/div)

0V 0V

0A 0A

0A

(j)

0V

(200V/div)

0A

DOi(2A/div)

0V

(200V/div)2Cv

0A

22 , LC ii (2A/div)

(i)

(2us/div) (2us/div)

DOv210V

(k)

0

(200mA/div)

OV

(100V/div)

20W300W

(200ms/div)

OI

Fig. 5. Experimental voltage and current responses of high step-up converter

for PEMFC with W300=O

P and V400=O

V .

(b)(a)

0V

(20V/div)

0A

DSi (10A/div)

(2us/div)

DSv

0V

(20V/div)

0A

DSi (10A/div)

(2us/div)

DSv

WPO

32= WPO 120=

(c) (d)

0V

(20V/div)

0A

DSi (10A/div)

(2us/div)

DSv

0V

(20V/div)

0A

DSi(10A/div)

(2us/div)

DSv

WPO

210= WPO 272=

(e) (f)

0V

(20V/div)

0A

DSi(10A/div)

(2us/div)

DSv

0V

(20V/div)

0A

DSi(10A/div)

(2us/div)

DSv

WPO

332=

WPO 372=

Fig. 6. Experimental switch voltage and current curves of high step-up

converter for PEMFC with V400=O

V under different output powers.

Output Power (W)

ConversionEffici

ency(%)

InputVoltage(V)

* Conversion Efficiency-Output Power

oInput Voltage-Output Power

Fig. 7. Conversion efficiency and fuel cell voltage for PEMFC with

V400=O

V under different output powers.

For the sake of verifying the effectiveness of the proposed

converter for different output powers, the experimentalswitch voltage and current responses at 32W, 120W, 210W,272W, 332W and 372W-output powers are given in Fig. 6.

As can be seen from these results, it needs to raise the duty

cycle (D) to keep a constant output voltage ( V400=OV )

since the fuel cell voltage drops when the output power

increases. Moreover, the switch voltage ( DSv ) is still clamped

at 50V, and the curve of the switch current ( DSi ) is also close

to a square wave with low ripple. Note that, the oscillated

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switch voltage in Fig. 6(a) is caused by the resonance of the

leakage inductance ( kL ) and the switch parasitic capacitor at

low power output [12]. It is helpful to alleviate the switchingloss in mode 5. Fig. 7 summarizes the experimental

conversion efficiency of the proposed converter and fuel cell

voltage under different output powers. From the experimental

results, the output voltage of the fuel cell decreases as theoutput power increases, and it is varied easily with respect to

the load variations. In order to solve this phenomenon, theproposed high step-up converter with dc voltage feedback

control is utilized in this study to ensure the system stability.

In addition, the conversion efficiency at 40W-output power isover 94.5% and the maximum efficiency is over 97% at

210W-output power, which is comparatively higher than

conventional converters.

IV. CONCLUSIONS

This study has successfully developed a high step-up

converter with coupled-inductor by way of bi-direction

energy transmission, and this converter has been applied wellfor a PEMFC system. According to the experimental results,the maximum efficiency was measured to be over 97%,

which is comparatively higher than conventional converters

with the same voltage gain. This high-efficiency converter

topology provides designers with an alternative choice toconvert renewable energy efficiently, and it also can be

extended easily to other power conversion systems for

satisfying high-voltage demands.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of theNational Science Council of Taiwan, R.O.C. through grant

number NSC 92-2623-7-155-014 and the Ministry of

Economic Affairs of Taiwan, R.O.C. through grant number

92-EC-17-A-05-S1-0012.

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high step-up coupled-inductor-based converter using bi-direction energy transmission

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