Three-Port DC-DC Converter for Stand-Alone Photovoltaic Systems

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPEL.2014.2331343, IEEE Transactions on Power Electronics

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Three-Port DC-DC Converter for Stand-Alone Photovoltaic Systems Yihua Hu, Member, IEEE, Weidong Xiao, Member Wenping Cao, Senor Member, IEEE, Bing Ji, Member, IEEE, D. John Morrow, Member, IEEE 

popularity for both grid-connected and stand-alone systems Abstract—System efficiency and cost effectiveness are of

critical importance for photovoltaic (PV) systems. This paper

[1]-[5]. Currently, the global installation is over 40 GW and increases at an annual rate of 50% since 2005 [6].

addresses the two issues by developing a novel three-port DC-DC

Stand-alone systems are independent of utility grids and

converter for stand-alone PV systems, based on an improved

commonly employed for satellites, space stations, unmanned

Flyback-Forward topology. It provides a compact single-unit

aerial vehicles (UAV) and domestic applications [7]-[10]. Such

solution with a combined feature of optimized maximum power

systems require storage elements to accommodate the

point tracking (MPPT), high step-up ratio, galvanic isolation and

intermittent generation of solar energy [11]-[15]. Over the

multiple

aerospace

years, research effort has been directed toward improving the

applications. A theoretical analysis is conducted to analyze the

power conversion efficiency as well as the power density by

operating modes followed by simulation and experimental work.

weight (PDW) and the power density by volume (PDV)

The paper is focused on a comprehensive modulation strategy

[16][17].

operating

modes

for

domestic

and

utilizing both PWM and phase-shifted control that satisfies the

Traditionally, the two-port topology utilizes the dual active

requirement of PV power systems to achieve MPPT and output

bridges (DAB) [18]-[21] and the half or full bridges can support

voltage regulation. A 250 W converter was designed and

the multiport structure to some extent [22]-[25]. A combination

prototyped to provide experimental verification in term of system

of Flyback-Forward converter with full bridge has shown some

integration and high conversion efficiency.

advantages in zero voltage switching (ZVS) and high

Index Terms— DC-DC power conversion, maximum power

conversion ratio for fuel cell applications [26]. A modified half

point tracking, phase shift, photovoltaic power system, voltage

bridge converter is reported in [27] which consists of one PV

control.

input port, one bidirectional battery port, and an isolated output for satellite applications. However, in these converters, a I. INTRODUCTION

S

OLAR

energy is a primary and renewable source of

multi-input-multi-output (MIMO) solution is generally difficult to achieve for power electronic applications.

energy. As the cost of photovoltaic (PV) panels is seen to

In theory, multiple-input converters (e.g. three-port

reduce continuously, PV-based power generation is gaining in

converters) can provide a single-unit solution interfacing multiple energy sources and common loads [28]-[30]. They

__________________________________________________ Manuscript received February 7, 2014; revised March 19, 2014; accepted May 30, 2014. Y. Hu is with the Department of Electronic and Electrical Engineering, University of Strathclyde, Glasgow, G1 1XQ, U.K. W. Xiao is with Faculty of Science and Technology, Masdar Institute, Abu Dhabi, UAE. W. Cao and J. Morrow are with the School of Electronics, Electrical Engineering and Computer Science, Queen’s University Belfast, Belfast, BT9 5AH, U.K. (e-mail: [email protected]). B. Ji is with the School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

perform better than traditional two-port converters due to their lower part count and smaller converter size. In particular, the isolated three-port converter (ITPC) has become an attractive topology for various applications owing to their multiple energy source connection, compact structure and low cost [31]-[33]. In this topology, a simple power flow management scheme can be used since the control function is centralized. A high-frequency transformer can provide galvanic isolation and flexible voltage conversion ratio. The ITPC is usually 1

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2 integrated into an individual converter such as forward,

the daytime operation of the PV system. Two 180°out-of-phase

push-pull, full bridge, and Flyback converters [34][35].

gate signals with the same duty ratio (D) are applied to S 1

The ITPC utilizes the triple active bridges (TAB) with

and S 2 while S3 and S4 remain in a synchronous rectification

inherent features of power controllability and ZVS. Their

state. When in the steady-state operation, there are four states in

soft-switching

two

one switching period, of which the equivalent circuits are

series-resonant tanks are implemented [36]. An advanced

performance

can

be

improved

if

shown in Fig. 3. The steady-state waveforms of the four states

modulation strategy is reported in [37] which incorporates a

are depicted in

phase shift (PS) and a PWM to extend the operating range of

Fig. 4, where VGS1, VGS2, VGS3 and VGS4 are the gate drive

ZVS. Nonetheless, the TAB topology suffers from the circuit

signals, Vds1 and Vds2 are the voltage stresses of S1 and S2, iL1a

complexity using three active full bridges or half bridges and

and iL2a are the currents through L1a and L2a, respectively. iB is

the power loss caused by reactive power circulation. Therefore

the current through the battery, is1 is the current through S1, vDo1

a Buck-Boost converter is proposed [38] to integrate a

is the voltage stress of the output diode Do1, and iDo1 is the

three-port topology in the half bridge and to decompose the

current through Do1.

multivariable control problem into a series of independent

components of the PV array, battery, and loads. However, in

Cs2

S2

Cs3

Do

loop can be independently controlled. The system is suitable for

L Lk

S4

Do1

Co1

n2

+

+ Cc

Vpv

PV-battery applications since one converter interfaces the three

Cs1

S1

single-loop subsystems. By doing so, the power flow in each

-

S3

-

n1

each energy transfer state, current passes through at least five

Vo

Ro +

-

Cs4

VB

n2

n1

*

Do2

•

* L1

L2

Co2

inductor windings, especially under high switching frequency conditions, giving rise to power loss; its peak efficiency is less

Fig. 1. The proposed converter topology.

than 90% and its power capability is limited by the transformer

Mode 1

Mode 3

Mode 2

x

x

design, making it impossible for current sharing.

Load

Load

Load

Based on these topologies, a new three-port DC-DC converter is developed in this paper to combine a new ITPC topology and an improved control strategy, and to achieve

Fig. 2. Three operation modes of the proposed converter.

decoupled port control, flexible power flow and high power

i in

capability while still making the system simple and cheap. S1

Cs1

II. TOPOLOGY AND OPERATION The proposed converter topology is illustrated in Fig. 1. The

S2 Cs3

Do

L Lk

Cs2

S4

Do1

n2

+ Cc

Vpv -

VB

n1

main switches S1 and S2 transfer the energy from the PV to the

Ro

Vab

S3

Co1 Vo

Cs4 n1

n

*

n2

•

*

Do2

Co2

battery or load, and can work in either interleaved or (a)

synchronous mode. The switches S3 and S4 are operated in the interleaved mode to transfer energy from source to load. L1 and

i in

L2 are two coupled inductors whose primary winding (n1) is Do

employed as a filter and the secondary windings (n2) are connected in series to achieve a high output voltage gain. LLK is the leakage inductance of the two coupled inductors and N is the turns ratio from n2/n1. CS1, CS2, CS3 and CS4 are the parasitic

S1

Cs1

S2 Cs3

Co1

iLK Cc

-

Do1

n2

+ Vpv

L Lk

Cs2

S4

n1 *

Ro

Vab

S3 VB iB

Vo

Cs4 n1

n

*

n2

•

Do2

Co2

capacitors of the main switches S1, S2, S3 and S4, respectively. There are three operational modes for the converter, as

(b)

illustrated in Fig. 2 [39]. In mode 1, the PV array supplies power to load and possibly also to the battery, corresponding to 2 0885-8993 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.


3

Vab  N

i in

S1

Cs1

Do

L Lk

S2 Cs3

Cs2

S4

n2

+ Cc

Vpv

VB

n1

coupled inductors work in the flyback state to store energy and

Vo

Cs4 n1

n *

•

*

(2)

State 3 [t2-t3]: At t2, S2 turns ON, which forces the two

Co1

Ro

Vab

S3

-

Do1

NVB 1 D VB  N (VB )  D D

Do2 is reverse-biased. The energy stored in Co1 and Co2 transfers

n2 Do2

to the load. At t3, the leakage inductor current decreases to zero

Co2

and the diode Do1 turns OFF. State 4 [t3-t4]: At t3, S1 turns OFF and S3 turns ON, which

(c)

turns Do2 ON. The primary side of coupled inductor L1 charges

i in

S1

Do

Cs1

S2 Cs3

the battery through S3. During this state, L2 operates in the

L Lk

Cs2

S4

Do1

n2

iLK + Cc

Vpv -

Ro

Vab

S3 VB

n1

forward mode and L1 operates in the flyback mode to transfer

Co1

energy to the load. When S1 turns ON and Do2 turns OFF,

Vo

Cs4 n1

n

*

followed by a new switching period.

n2

•

*

Do2

iB

In mode 2, the battery supplies power to the load, as shown in

Co2

Fig. 5(a), indicating the nighttime operation of the stand-alone system. The circuit works as the Flyback-Forward converter,

(d)

where S3 and S4 are the main switches, Cc, S1 and S2 form an

Fig. 3. Four operating states of the proposed converter in mode 1. Vgs1

Vgs3

Vgs1

Vgs2

Vgs4

Vgs2

active clamp circuit. When the load is disconnected, the

Vgs1

stand-alone system enters into mode 3. The PV array charges

Vgs2

battery without energy transferred to the load due to the

iL2a

iL1a iL1a iL2a

opposite series connected structure of the coupled inductor (see

ib

Fig. 5b). S1 and S2 work simultaneously and the topology is iS1

equivalent to two paralleled Buck-Boost converters.

vds1

vds1 iS1

Cs1

S1 vDo1 vDo1 iDo1

iDo1 t2

t1

t3

L Lk

S4

Do1

Co1

n2

+

+

t4

Cc

Vpv

t0

Do

Cs2

S2

Cs3

Fig. 4. Waveforms of the proposed converter under mode 1.

S3

-

-

n1

State 1 [t0-t1]: The main switches S1 and S2 are both in turn-on

Vo

Ro +

VB

-

Cs4 n2

n1

*

Do2

•

* L1

L2

Co2

state before t0. The two coupled inductors work in the flyback state to store energy from the PV array. The output rectifier

(a) Mode 2

diodes Do1 and Do2 are both reverse-biased. The energy stored Cs1

S1

in the secondary output capacitors Co1 and Co2 transfers to the

Cs3

Do

load. diodes Do1 is ON. The primary side of the coupled inductor L2 charges the battery through S4. During this state, L1 operates in

S4

L Lk

Do1

Co1

n2

+

+

Vpv

State 2 [t1-t2]: At t1, S2 turns OFF, S4 turns ON, while the

Cs2

S2

Cc

-

S3 n1 *

L1

Ro +

VB

Vo -

Cs4 n1

n2

*

•

L2

Do2

Co2

the forward mode and L2 operates in the flyback mode to transfer energy to the load. When S1 turns on and S2 turns off,

(b) Mode 3

the primary voltage of the coupled inductor L1 is Vpv and the

Fig. 5. Converter operating modes 2 and 3.

voltage on L2 is –VB. III. PERFORMANCE ANALYSIS AND FEEDBACK LOOP DESIGN

According to the voltage balance law, DVPV  (1  D)VB

In order to realize flexible energy flow control, the (1)

modulation strategy is proposed to combine PWM with PS 3

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4 schemes. Firstly, the relationship of voltage gains with duty

L Lk

ratio and PS needs to be derived. In the following analysis, S1 and S2 have the same duty ratio D, whilst S3 and S4 share another duty ratio. The gate signals for S1 and S3 are

S1

D S1 S4 φ

S1

S3 S2S2

S2

S4

B

A

When the duty cycle D≥0.5, there are five operating cases

D S1S1 S4 φ

SS33

SS22

S2

SS33

S4 φ

NVB D

Do2

Co2

Ro /2

S2

Ro /2

LLk

t

(a) Signal waveforms (b) Equivalent circuit at φ=φcrit1 Fig. 7. System operation in Case 1.

D S1S1

S1 SS44

Co1

NVB/D

iLk

which need to be analyzed, as shown in Fig. 6.

Do1

Vab

complementary, and so are S2 and S4. A. Analysis of Circuit Performance for D≥0.5

NVB D

NVB/D

V  NVB crit1   Ts  o (1  D  crit1 )Ts D 2 2 2

S1

SS22

SS44

(3)

NVB/D

Vab

Vab

iLk

crit1    D  (1  D) 

iLk

t

Vout N  VB

(4)

t

The secondary side of the coupled inductor is equivalent to (a) Case 1 D S1S1

D S1S1

S1

SS33

SS22

S4 φ

(b) Case 2

S4S4

S2

φ

NVB/D

two Buck converters connected in parallel at the DCM

S S33 SS44

operational condition. The corresponding equivalent duty ratio

S1 SS22

of the Buck converter is φ/2π. Provided the voltage gain of the

NVB/D

Vab

Vab

iLk

iLk

t

Vo  2  t

(c) Case 3

(d) Case 4 D S1S1

S2

Buck converter in DCM, the output voltage is given by:

φ

S S33

S1

Vab

t

(5)

voltage equations at φcrit1 and φcrit2 are derived by Eqs. 7 and 8. 4  (1  D) N  VB crit 2   D Vout (6) V  NVB (1  D)  o crit 2 DLk 2 Lk 2

Fig. 6. Five operational cases for D ≥ 0.5.

In case 1, the phase shift angle is between 0 and φcrit1. From the waveform of the leakage inductor current, the secondary side of the coupled inductor is equivalent to a discontinuous conduction mode (DCM) of a Buck converter. When φ=φcrit1, the current pulses A and B is in a boundary conduction mode, as shown in Fig. 7. For pulse A, the current decreases to the negative peak value and increases to zero at the time of (1-D)Ts. The decrement time

Tscrit1 / 2

NVB D

In case 2, the phase shift angle is between φcrit1 and φcrit2. φcrit2

(e) Case 5

is equal to



(CCM) to a DCM, which can be determined by Eq. 6. The

NVB/D

iLk

4  2 Lk 1 1 Ro  Ts  ( / 2 ) 2 / 2

is the transition point from a continuous conduction mode

S2S2

SS 4 4

2

and the increment time is

(1  D  crit1 / 2 )Ts . Following the voltage-second balance (Eq. 3), the critical phase angle can be determined by Eq. 4.

V NVB crit1  o (1  D) DLk 2 2 Lk

(7)

(8) In case 3, the angle shifts from φcrit2 to φcrit3. The duty ratio of the secondary side of the Buck converter stays constant, and the voltage gain reaches the highest. Therefore, the critical point, φcrit3, and the corresponding voltage can be calculated by Eqs. 9 and 10. φcrit3 is the boundary point between DCM and CCM. With the increase in the PS angle, the voltage declines. In this case, the output voltage cannot be controlled by PS, as suggested by Eq. 11.

crit 3  2  crit 2 V  NVB / D (1  D)  o (1  crit 3 ) Lk 2 Lk 2

(9) (10)

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5

Vo  2 

2 4  2 Lk 1 1 Ro  Ts  (1  D) 2 / 2



In Case 1 (0