Mutual Boost Converter for Single Cell Photovoltaic Energy System

Dodi Garinto Indonesian Power Electronics Center

Solo - 57156 Central Java Indonesia

Abstract - Generally, photovoltaic (PV) modules are very sensitive to shading, particularly in urban locations. This

reality is a major barrier in today's photovoltaic technology to

achieve low cost, high efficiency, and high shade-tolerant. Therefore, a new PV module using a single cell area with an

input of 2 - 2.5 V and 100 A per square meter cell is a very promising solution. This paper proposes a power converter

technology to raise 2.3 V, 100 A from the voltage source to 24.6 V at the DC bus using partial segmentation and phase shifting

principles. The proposed converter not only provides low

ripple input current, but naturally also creates a new duty

cycle and increasing duty ratio capability with low conduction

loss. Experimental results show that the converter could raise

from 2.3 V dc to 24.6 V dc with duty cycle under 50%.

I. INTRODUCTION

Basically, photovoltaic energy system is a direct sunlight conversion which generates electricity without moving parts, noise or pollution. Although the main advantage is low operation cost, but photovoltaic is a relative expensive technology because of high manufacturing cost and low efficiency (typically 15 - 25%). If partial shading occurs, this results in significant reduction at the output power generation. It is due to a typical PV module is composed of around 36 or 72 cells connected in series. According to the first law of Kirchoff, these PV cells hold the same current. Because the PV cells must hold the same current and the current is determined by the weakest cell, this means that the photovoltaic module cannot provide the maximum power point. The phenomenon is known as mismatching between PV cells. Although the mismatching difficulty can be diminished using bypass diodes in parallel with PV cells, the parallel connection mitigates the miss-match problem, but the output power generation is relatively low. From the point of view of large-scale PV energy systems, this technical problem is a major barrier in today's photovoltaic technology to achieve low cost, high efficiency, high reliability, and high shade-tolerant [1-5]. For this reason, the paper presents a power converter system for the prospect of single-cell PV module or parallel-connected PV cells, such as ultra triple junction solar cells. [6].

978-1-4577-1250-0/11/$26.00 ©2011 IEEE

II. ApPROACH

A. Single Cell PV Module It is very desirable to eliminate the mismatching problem

between PV cells without introducing harmful functions, such as low reliability, low efficiency, additional components or excessive manufacturing cost. For this reason, a new PV module using parallel-connected PV cells with operating voltage between 2 V to 2.5 V and 100 A per square meter cell is a very interesting solution to remove the technical problem.

m +v, (18 � 35 Vdo'

• +Vs (2 - 2.5 Vdc)

'- fI���� a) b)

Figure I. a) Series-connected PV cells. b) Parallel-connected PV cells or single cell PV module

If 25% of the single cell PV module is shaded, then the output power generation is only reduced by 25%, which is a very important advantage in urban locations.

B. Technical Challenge The technical challenge from the point of view of power

electronics technology is to provide a feasible power converter system. Generally, by using today's circuit technology, non-isolated boost converter is needed, as shown in Fig. 2. Unfortunately, in case of increasing the input voltage from 2.3 V to 24.6 V at the output voltage, today's boost converter suffers high conduction loss and low efficiency due to extreme duty ratio (0 > 90%), that is Vo =

Vin/(1-D). Also, in case of 2.3 V input voltage and 100 A at

the input side, the input current ripple consideration will be a key factor to achieve an optimum converter design.

r-NV\.. L1

+

D1

J6 01 -=- Vin ,.,... Co

a)

DT T Vo = Vin/(l-D) 40A I I I

2:: f---I -----IN 101 � I 40A I I

OA 1.975m.s Time

b)

+Vo

RL

Figure 2. a) Conventional boost converter. b) Duty cycle D > 90%

III. SCIENTIFIC INNOVATION

Although phase shifting principle is not new, as well as partial segmentation principle, but a combination of these principles for boost converter will result in a converter topology which can be referred as a mutual boost converter, illustrated in Fig. 3.

_NY\.. IoJ L1 '"

D1

+ J:; J� - Vin 01 02 '"'"' Co 't-

r +Vo

RL

OO 1800

--L.

a)

DT; To 0 40. 0 0

� 20A � � 101 oJ 0

.0. 0

20.

Ti:.e

b) Figure 3. a). Mutual boost converter. b). Key operation wavefonns.

The main advantage of the mutual boost converter not only increases two times the operating frequency for inductive component Ll, but also it creates a new duty ratio with low conduction loss. Fig. 3 (b) illustrates the key operation waveforms. As depicted in Fig 3(a), the proposed converter circuit is basically composed of boost topology with additional semiconductor switch Q2. In principle, two or more switches can be added to obtain higher switching frequency multiplication effect. In this case, two switches are used for simplicity. Switches Ql and Q2 have a phase shift of 1800 with duty cycle below 50%.

A. The Prospect a/Single Cell PV Module From the point of view of large-scale photovoltaic energy

system, shading effect on photovoltaic modules can have a big negative impact on both the investment and the energy generation. Therefore, this paper attempts to address the shading problem using single cell PV module or parallel­connected PV cells with integrated mutual boost converter. In case of parallel-connected PV cells, the ultra triple junction solar cells are a good alternative from Spectrolab [6], as shown in Fig. 4.

Top Cell: GalnP2

� �------------� bl � Middle Cell: GaAs '0 z ��----------� 0> C .� � o

'NR: Anti-Reflective Coating a)

AMO (135.3 mW/cm2) 28°C, Bare Cell 18 16 14

E 12 u ;a: 10 .S- 8 C 2:' 6 ::;

U 2 0

0.5 1.5

Voltage (V) b)

\ \ \

2.5

Figure 4. a). Ultra Triple Junction Solar Cells (Source: Spectrolab.com). b). Typical IV characteristic.

B. The Mutual Boost Converter Because of the mutual attribute could improve the duty

cycle capacity and create a new duty cycle under 50% for boost topology to provide high voltage gain, the limitation of the proposed converter is not restricted by duty cycle anymore, but this is limited by propagation delay or rise time and fall time of the semiconductor switches Q1 and Q2. Also, as long as the switching frequency allows enough time to transfer energy from voltage source Vin and inductor L 1 to the output capacitor Co through diode D 1, then higher

voltage multiplication effect can be reached. Fig. 5(a) shows the mutual boost converter with extra L1 and C1 as an optional bypass LC filter between input and output to yield ripple-free input current. Fig. 5(b) depicts the key operation waveforms based on simulation results.

�� [ J = Vin 00 J� 01

II �-""' I

II II II II

I Cl Dl

J 1800 J� 02 ,.,.,Co

a)

i----..:.-;;nl---'..--"-- -t f-----' II II I I

--I II

I I

II

TilT.o

b)

+Vo

RL

Figure 5. a). Mutual boost converter with extra bypass LC filter between input and output . b). Key operation wavefonns.

The time interval To - T 1 indicates that switch Q 1 is turned on and the current increases linearly follow the inductor current IL2. At t = T], switch Q1 is turned off. During interval T 1 - T 2, the energy stored in inductor L2 is transferred to the output capacitor Co through diode D1. At this time, the voltage stress V DSQl is equal to the output voltage Vo. During interval T2 - T3, switch Q2 turns on and diode D 1 is reverse biased. During interval T 3 - T 4, switch Q2 is turned off and the energy stored in inductor L 1 is transferred again to the output through diode DI. Due to the switching interaction of switches Q 1 and Q2, the operating

frequency of inductor current III normally two times of the switching frequency QI or Q2. It is clear that to improve the duty ratio capability and create a new duty cycle, the duty cycle of switches QI and Q2 should be designed under 50%.

IV. RESULTS

As illustrated in Fig. 5(b), the switching interaction between switches Q 1 and Q2 relative to diode D 1 creates a new duty cycle. For instance, from the point of view of switch Q2, the duty cycle of switch Q1 is below 50%. However, from the point of view of diode D1, the duty cycle of switch Q 1 or Q2 is higher than 50%. This not only reduces conduction loss, but also raising the operating frequency of inductor currents III and IL2. As a result, smaller passive components can be utilized.

A. Simulation Results The single-phase mutual boost converter operation has

been studied by PSPICE simulation results and also with experimental results accomplished on a fIrst prototype. The preliminary experimental results are presented in the next session. The switching frequency is 350 kHz. The inductance values L 1 and L2 are 320 nH. Due to the switching interaction of switches QI and Q2, the operating frequency of IL2 is normally 720 kHz. Fig. 6 shows additional operation waveform characteristics based on Fig. 5(a).

I To TIIIT2 T3 11T4 I II I I III

II I I II I I II I I

I IL2 I I I

I I I I I I

OA -.J: V:� I I I

I I I

J: q I I Vo .J1-I I I I Vin I I I

I II I I 6.968 ... 6. !l'10m. 6.9'12m. 6.97'1m.

Figure 6. Additional operation wavefonn characteristics based on Fig. 5(a)

The output capacitor Co uses 2 x 47 uF electrolytic plus 5 x 2.2 uF ceramic. The MOSFET Si4154DY is used for switches QI and Q2. The bypass capacitor CI uses 1 uF ceramic. Diode D1 uses PDS1040. To obtain smaller input current ripple Ill, larger capacitance or inductance value for C1 or L1 can be selected. Soft switching techniques for the mutual boost converter can be utilized to remove tum on and tum off switching losses so that higher effIciency can be achieved. As shown in Fig. 5 (b), the extreme duty cycle issue for boost converter topology to attain very high voltage gain is solved by partial segmentation and phase shifting principles.

B. Experimental Results The preliminary experimental results are presented

below. Based on the simulation and experimental results, the proposed circuit technology is very suitable for the prospective of single-cell photovoltaic modules. Moreover, low ripple input current can be obtained without excessive inductance and capacitance values.

Figure 7. Preliminary experimental Results for single-phase converter in light load operation. a). Single phase mutual boost converter prototype. b). The voltage stress characteristic of switch QI due to the switching interaction of switches QI and 02. c) The input and output voltage characteristics

L2 C1 D1

[,. J[)"

CO

1 L3 D2

l)·

Figure 8. Proposed three-phase mutual boost converter for future development of the prospect of single cell photovoltaic modules

15.5 cm x 9 cm

Figure 9. Three-phase mutual boost converter prototype under development

C. Single Cell P V System The sun has a great potential to supply energy for our

society. For that reason, large amount of energy with little

impact on the environment and diminishing global

warming are the most important aspects to harness energy

from the sun. However, experience and study show that

shading of PV module is the largest loss issue. Shading of

one cell limits the current in the entire module. For

example, a partially shaded cell can reduce the yield by 70

% even though only 2% of the module is shaded [3-10].

Due to large-scale PV system needs huge investment, it is

very important to sustain the PV system becomes high

shade-tolerant and more fault-tolerant. The PV modules

should work separately of each other. If one fails, the

others will continue working. Therefore, the single cell PV

system, as shown in Fig. 10, is a very promising solution

to harness energy from the sun with minimum loss.

Low Vohage DC-Ous

24 -48V + 2.3 V, 11O""0

_O

.,..A __ .."

Single-Cell PV Module

Single-Cell PV Module

Single-Cell PV Module

High Vohage DC-Ous

400 - 800 V +

Utility Grid 3 phase

nov. 50 1-1,

Va Vb Vc

100 - 250 kW

Figure 10. Proposed single cell PV configuration for large-scale photovoltaic system to harness energy from the sun with high

shading-tolerant, high fault-tolerant and minimum loss

Several major benefits of the proposed circuit technology are summarized below:

• The combination of partial segmentation and phase shifting principles can be applied for other power converter topology, such as flyback converter.

• The duty ratio capability is improved, as well as low conduction loss and high efficiency can be obtained.

• The miss-match problem between PV cells is solved and the maximum power point can be achieved, particularly in urban locations.

• The main features of the proposed power conversion system are modular extensibility, optimum energy production and relatively low manufacturing cost.

Further investigation of the proposed power conversion system for single cell Photovoltaic module or other application, such as fuel cell energy system in future work is a very interesting research area.

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