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B. Stalin et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945

Int J Adv Engg Tech/Vol. VII/Issue III/July-Sept.,2016/42-53

Research Paper PERFORMANCE EVALUATION OF BIDIRECTIONAL SOFT-SWITCHING CONVERTER WITH IMPLEMENTATION OF PI

AND FUZZY LOGIC CONTROLLERS 1B. Stalin, 2T.S. Sivakumaran

Address for Correspondence 1 Research Scholar, Faculty of Electrical Engineering, Anna University, Chennai. 600025, India

2Professor & Dean PG Studies, Arunai College of Engineering, Tiruvannamalai, India

ABSTRACT Power converters are needed to convert one kind of electrical energy to another. A DC–DC converter is a power supply that changes a DC input voltage into a desired DC output voltage. Nowadays the bi-directional PWM DC-DC converter connected to the memory system receives a prominent option of the electrical conveniences. The proposed circuit topology will opt for keeping up the conversion efficiency during static and dynamic working condition. Besides, the operating mode of both buck and boost conversion will be studied by the equivalent circuit model. The auxiliary circuit will keep the soft switching condition of the proposed circuit topology. Hence, the auxiliary circuit will be improved the conversion efficiency, and the conduction loss will be scaled down. The objective of this work is to develop PI and Fuzzy Logic controls for bi-directional soft-switching converters to ensure constant load voltage under supply and load disturbances using MATLAB software. The simulation results are presented and evaluated. They validate the superiority of Fuzzy Logic control developed. KEYWORDS: DC-DC converter, Buck converter, Boost Converter, PI controller, Fuzzy Logic Controller.

1. INTRODUCTION Power electronic converters may be found in a plethora of applications such as electrical machine motion control, switched-mode power supplies (SMPS), lighting drives, energy storage, distributed power generation, active power filters, flexible AC transmission systems (FACTS), renewable energy conversion and vehicular and embedded technology. The design of a switching converter is a nontrivial task due to multiple objectives that must be fulfilled. Cost, gauge, power efficiency, power quality and reliability of the overall structure must be considered when one is defining the objective to be optimized in the design process. DC-DC conversion technique has developed in recent years since it is required to reach a high power density, high-voltage transfer gain, and high power efficiency [1-6]. To regulate the output voltage of DC-DC converters irrespective of load variations and supply disturbances, it is necessary to operate the DC-DC converters as closed loop systems. With pulse-width modulation control, the regulation of output voltage of DC-DC converters is achieved by varying the duty cycle of the electronic switch keeping the frequency of operation constant. These converters, in general, have complex non-linear models with parameter variation problems. One of the major movements in power electronics is increasing the switching frequencies [7]. Usually power converters use a diode rectifier followed by a bulk capacitor to change AC voltage to DC voltage [8]. An increase in efficiency can be achieved by combining a smaller battery with an ultra-capacitor (UC) [5]. High switching power losses doesn't only restrict switching frequency, but as well bring down the system efficiency and produces heat within the inverter [9-10]. Increasing the switching frequency can reduce the weight and size of the passive element but the switching losses in power shifts will increase accordingly [3]. To resolve this problem, in that respect are several soft-switching PWM converters presented [11]. The ZVS technique and ZCS technique are using soft switching methods [12]. The conventional hard switching converters posses increased size, weight, increased switching losses [13]. The soft-switching power conversion technologies in order to overcome the limitations of the hard-switching technologies [14]. The full-bridge (FB) ZVS converter is the most popular topology for

DC-DC converters due to fixed switching frequency [15]. The control strategy of converter allows a zero voltage transient on the power switches of the inverter [16]. Moreover, all switches of the converter are turned on at zero voltage, the conversion efficiency is relatively high [17]. The main target of the analysis is the evaluation of a DC-DC converter topology that supplies the electrolyser in order to obtain the optimization of the converter efficiency [16] . Various methods exist to achieve DC-DC voltage conversion. Each of these methods has its specific benefits and disadvantages, depending on some operating conditions and specifications. Examples of such specifications are the voltage conversion ratio range, the maximal output power, power conversion efficiency, number of components, power density, galvanic separation of in- and output, etc., [18]. PI control and Fuzzy Logic Controllers are used in this work for regulating the output voltage of bi-directional soft-switching converters under supply and load disturbances. Tests for load regulation and line regulation are carried out to evaluate the controllers' performances. The results are presented and evaluated. 2. BI-DIRECTIONAL SOFT-SWITCHING DC-DC CONVERTER The soft switching PWM converter is specified as the combination of converter topologies and switching strategies that result in zero voltage and zero current switching. On soft switching based storage system, the efficiency of the system depends on the size and price; it can be increased by a combination of batteries and ultracapacitors. The required system energy is supplied by a battery while an ultra-capacitor is applied at high load power pulses. The ultra-capacitor voltage changes during the charges and release modes. The leakage current of the ultracapacitor and the inductor provides the imbalance condition. Here we suggested an enhanced ultra-capacitor interface circuit based bidirectional soft switching converter, which improves the conversion ratio of the converter and to scale down the leakage current across the capacitance and the inductor. The proposed bidirectional circuit efficiency increased due to soft switchings like (Zero Voltage Switching) ZVS and Zero Current Switching (ZCS). The converter acts as the Zero Current



Transient (ZCT) buck to shoot down an ultra-capacitor and Zero Voltage Transient (ZVT) boost to discharge an ultra-capacitor. The leakage flow is controlled by the series connection of the resistor, and the switched inductor has amended the voltage gain of the converter. The switched inductor will reduce the leakage current of the circuit. Fig.1 shows the proposed bi-directional soft-switching converter.

Fig.1. Bi-directional soft switching DC-DC

converter The proposed circuit is operated as two modes such as buck mode and the boost mode. During the buck

mode of operation the switch 1M and the diode 2D

is ON and the circuit act as ZCT buck to charge an ultracapacitor [20]. Similarly for the boost mode of

operation the switch 2M and the diode 1D is ON

and the circuit act as ZVT boost to discharge an ultracapacitor. The proposed circuit also consists of

the two auxiliary switches 1aM and 2aM , which

uses the switched inductor that includes inductors

( 1sL and 2sL ) and diodes ( 54 , DD and 6D ).

The switched inductor improves the voltage gain of

the proposed circuit. The auxiliary inductor yL and

resonant capacitors rsC are also included in the

proposed circuit. The leakage current of the ultracapacitor is limited by the series connection of

the resistances ( nRRR 21 , ). Its time constant

can be defined depending on the branch range, i.e., the equivalent circuit of the proposed circuit have single branch and has time constant is given by the following equation (1)

1RrR jj

and ji

r

CC

21

(1)

Where, r is a positive constant, iR and iC is the

branch resistor and capacitor, nj 2,1 of the

branch 1. The proposed circuit buck mode of operation is as discussed below. 2.1. BUCK MODE OF OPERATION The proposed circuit is the bidirectional operation circuit, i.e., buck mode and boost mode. The buck operation consists of six modes, during this mode the circuit act as a ZCT buck to charge an ultracapacitor.

The buck mode of operation diode 2D and the

switch 1M is turned ON. In this mode of operation

circuit input voltage and the inductor fL current are

assumed as constant. Similarly all the components present in the circuit considered to be an ideal condition [19]. The one is switching cycle of the proposed circuit consists of six modes of operation as shown in Fig.2. 2.1.1. SIX MODES OF OPERATION The proposed circuit buck mode of operation have six ways of operation, i.e., one switching cycle has six modes. The six modes equivalent circuit is given in the Fig.2, and the modes of operation are details given below.

(a) Mode 1 (b) Mode 2

(c) Mode 3 (d) Mode 4



(e) Mode 5 (f) Mode 6

Fig.2. Buck mode of operation of the proposed circuit

Mode 1: ( 10 tt ):At the starting time of the mode one operation is 0t , during this time the diode 2D and the

switch 1M are turned ON. The output current outI is flowing through the diode 2D ; the switch is turned ON

with the ZCS to charge an ultracapacitor. At the end of the mode, the time is 1t , in this period the switched

inductor current is reached to the output current outI . The voltage equation of the mode 1 is given by the

following formula (2).

n

jjjout

Ds

outT

f

f

n

ji

n

j

jj

sjlow CRII

InV

dt

dIL

q

kT

dt

dILE

121

1 1

)(ln (2)

The output current across the diode 2D is provided by the following equation (3)

SL

EI s

out (3)

The following defines the current of the switched inductor

)1()(

1 )(5

21

Td nVV

s

ss

sSL eILL

EI (4)

Where, K is the Boltzmann constant, T absolute temperature, q magnitude of the charge, is the ideality

factor, 5sI is the saturation current of the diode 5, dV is the voltage across the diode, 1sL and 2sL are the

switched inductors, TV is the thermal voltage of the diode, sE is the supply voltage, outI is the output current

and SL is the switched inductor.

Mode 2 ( 21 tt ): During this mode of operation current of the main switch reaches to zero at 1tt and the

diode 2D is turned OFF, i.e., soft switching turn OFF ZCZVS. The resonance of the switched inductors ( 1sL

and 2sL ) and capacitors rsC start flows through the diode 1aD . The resonant condition components current

and the voltage of the mode 2 are given in the following equations.

n

jjjoutrs

rsDas

outT

f

f

n

ji

j

sjlow CRIdttiCI

InV

dt

dIL

q

kT

dt

dILE

1111

1 )()(1

ln

(5)

The output current and the capacitor voltage of this mode are given in the following equations (6) and (7).

))(sin(

1)( 22 t

ZEItI SLC

SLC

sSLout (6)

))(cos(1)( 22 tEtE SLCsCrs (7)

Where,

rsss

SLCCLL )(

1

21 and

rs

ssSLC

C

LLZ 21

Mode 3 ( 32 tt ): The third mode of operation resonant capacitor current reaches to zero at 2tt . The

auxiliary switch 1aM is turned off, so the resonance of the inductor and capacitor is stopped. This mode of

operation the resonant capacitor rsC voltage reaches sE2 and the switched inductor current reaches to the

output current outI . The power flows from the source input to the output enhanced ultracapacitor [19]. The

voltage of the mode is given by the following equation (8).



n

jjjout

f

f

n

ji

j

sjlow CRIdt

dIL

q

kT

dt

dILE

11

1

1 )( (8)

Mode 4 ( 43 tt ): During this mode of operation the auxiliary switch 1aM is turned off under ZCS condition

at time 3t . In this mode the resonance between the resonant capacitor and the switched inductor starts, so that

the switched inductor fully discharge i.e., it reaches zero. At the end of the mode operation 4, the switch 1M

softly turned OFF at ZCS. The voltage equation of this mode is given in the following equation (9).

n

jjjoutrs

rs

ff

n

ji

n

j

jjsjlow CRIdttI

Cdt

dIL

q

kT

dt

dILE

11

1 1

)()(1

(9)

The output current of this mode can be described by the following formula (10)

))(sin(

1)( 44 t

ZEItI SLC

SLC

sSLout (10)

))(cos(1)( 44 tEtE SLCsCrs (11)

SLC

sout

Z

EI (12)

When the switched inductor current SLI reaches zero, the ZCS condition occurs for switch 1M . The equation

11 shows the resonant capacitor rsC voltage at the mode 4 operation. The above equation 12 is the constraint

for ZVS, if the output current is less than the condition, i.e.,SLC

sout

Z

EI , the additional one mode is added

to the buck operation, and also the body diode of the main switch is conducting.

Mode 5 ( 54 tt ): This mode started at 4t , in which the switched inductor current SLI fully decreases to zero

and the output current flows through the resonant capacitor rsC . So the resonance voltage decreases, at the end

of this mode, the resonant capacitor voltage reaches zero. The voltage of this mode is given in the following equation (12) [19].

n

jjjout

outf

ars

rs

low CRIdt

dIL

q

kTdttI

CE

1

1 )()(1

(12)

Mode 6 ( 65 tt ): At the starting of this mode 5t the resonant capacitor rsC voltage reduced to zero, so the

diode 2D is turned ON under the soft switching ZVS condition. In this mode, the auxiliary switch 1aM is

turned OFF under the ZVS condition, and the energy flows from the inductor fL to the

ultracapacitor uciE where ni 2,1 . In this condition, the voltage of this mode is given by the following

equation (13) [20].

n

jjjout

outf

Ds

outTlow CRI

dt

dIL

I

InVE

12

)(ln (13)

The above six modes are completed in one switching cycle, i.e., one switching cycle have six modes of operation. The boost mode operation of the proposed circuit has eight modes of operation [19]. 2.2. BOOST MODE OF OPERATION The boost operation consists of six modes, during this mode the circuit act as a ZVT boost to discharge an

ultracapacitor. The boost mode of operation diode 1D and the switch 2M is turned ON. In this mode of

operation circuit input voltage and the inductor fL current are assumed as constant. Similarly all the

components present in the circuit considered to be an ideal condition. The one switching cycle of the proposed circuit consists of eight modes of operation as shown in Fig.3. 2.2.1. EIGHT MODES OF OPERATION The proposed circuit boost mode of operation have eight modes of operation, i.e., one switching cycle has eight modes. The eight modes equivalent circuit is shown in Fig. 3.



(a) Mode 1 (b) Mode 2

(c) Mode 3 (d) Mode 4

(e) Mode 5 (f) Mode 6

(g) Mode 7 (h) Mode 8

Fig.3. Boost mode of operation of the proposed circuit

Mode 1 ( 10 tt ): At this mode of operation starting at 0t , during this time the diode 1D is turned ON, the

current flows from the ultracapacitor uciE to the inductor fL . In this time the auxiliary switches 1aM and

2aM is turned ON, due to this the auxiliary inductor yL voltage is ucis EE and causes the current of the

inductor to increase. At the end of this mode, the auxiliary inductor current LyI reaches the output

current outI . The voltage equation of this mode can be described in Equation (14).



n

j Djs

outT

outy

n

ji

n

j

j

rs

rs

j

fsj

n

jjjouthigh

I

InV

dt

dIL

q

kTdttI

Cdt

dILLCRIE

111 11

ln)(1

)()(

(14)

The output current of the circuit is given by the following equation (15)

Ly

suciout

I

EEI

, ni 2,1 (15)

Mode 2 ( 21 tt ): This mode of operation starts at the time 1t , in this time the auxiliary inductor yL current

decreases to zero. During this condition the resonant capacitor rsC voltage is outSLCs IZE .The auxiliary

switches 1aM and 2aM turned ON under the soft switching ZCS and resonance between the resonant

capacitor rsC , auxiliary inductor yL and switched inductor SL flow through these auxiliary switches. At the

end of this mode, the resonant capacitor rsC voltage reaches zero. The voltage equation of this mode is given

by the following equation (16).

n

j Djs

outT

n

ji

n

j

j

rs

rs

j

yfsj

n

jjjouthigh

I

InV

q

kTdttI

Cdt

dILLLCRIE

111 11

ln)(1

)()(

(16) The output current and the voltage across the resonant capacitor are given in the following equations (17) and (18).

))(sin()()( 21

22 tZ

EIZE

tItI SLY

SLY

n

iucioutSLCs

SLout

(17)

n

iuciSLY

n

iuciSLCscrs EtEZEE

12

1

))(cos( (18)

Where, )(

1

21 yssrs

SLYLLLC

and rs

yss

SLYC

LLLZ

21

Mode 3 ( 32 tt ): During this mode of operation starts at 2t , the resonant capacitor rsC is completely

discharged due to the resonance cycle. At this time diode 2D conducts and the switched inductor SL current

reaches the output current outI . At the end of this mode of operation, the switch 2M is turned ON under soft

switched ZCS condition. The voltage equation of this mode is given by the following equation (19).

n

j Djs

outT

n

ji

aj

yfsj

n

jjjouthigh

I

InV

q

kT

dt

dILLLCRIE

11

1

2

1

ln)()(

(19) The output current of this mode can be described by the following equation (20)

)(1)()( 2

21

1

21

1

1133 t

LLL

E

EIZE

E

Z

EIZE

tItIyss

n

iuci

n

iucioutSLCs

n

iuci

SLY

n

iucioutSLCs

SLout

(20)

Mode 4 ( 43 tt ): This mode of operation starts at 3t , during this time the switched inductor SL current

decreases from the output current outI . The switch current 2MI increases linearly and at the end of this mode

the switch current 2MI reaches to the output current outI at 4t . The voltage equation of this mode equation is

given by the following equation (21) and output current is given by the equation (22)

n

j Djs

outT

n

ji

n

j

jj

yfsj

n

jjjouthigh

I

InV

q

kT

dt

dILLLCRIE

11

1 11

ln)()( (21)



)()()( 4

21

144 t

LLL

E

tItIyss

n

iuci

SLout

(22)

Mode 5 ( 54 tt ): During this mode of operation the auxiliary switches are turned ON, and the energy flows

from the Ultracapacitor uciE , where ni 2,1

to the inductor fL . The voltage of this mode of operation is

given by the following equation (23).

n

j

jff

n

jjjouthigh

q

kT

dt

dILCRIE

11

)( (23)

Mode 6 ( 65 tt ): This mode of operation the switch is turned OFF, the inductor fL current flows through the

resonant capacitor rsC , the resonant capacitor is charged by the current and voltage sE . Due to this condition

the switch is turned OFF under the ZVS condition. The equivalent circuit of this mode is shown in figure 3. The voltage of the equivalent circuit is given by the following equation (24).

11

ln)(1

)(Das

outTrs

rs

ff

n

jjjouthigh

I

InVdttI

Cdt

dILCRIE

(24)

The output current of this mode of operation can be described by the following equation (25).

)(*)()( 666 tCtEtI rscrsout (25)

Mode 7 ( 76 tt ): At the starting time of this mode is 6t , the capacitor voltage reaches to sE . During this

time, the diode 1D starts to conduct and the diode current increases due to the switched inductor SL . At the

end of this mode of operation, the capacitor voltage reaches outSLCs IZE . The voltage equation of this

mode of operation is given by the following equation (26).

n

ji

n

j Dajs

outTrs

rs

j

fsj

n

jjjouthigh

I

InVdttI

Cdt

dILLCRIE

11 11

ln)(1

)()(

(26)

Mode 8 ( 87 tt ): During this mode of operation the energy flows from the ultracapacitor uciE ,

where ni 2,1 because the resonant capacitor is overcharged and the auxiliary switch 1aM is turned off

under ZCS. At this condition, the switched inductor current reaches to the output current outI . The voltage

equation of this mode is given by the following equation (27).

n

ji Ds

outT

j

fsj

n

jjjouthigh

I

InV

dt

dILLCRIE

11 11

ln)()( (27)

These above eight modes are completed in one switching cycle, i.e., one switching cycle has eight modes. The proposed enhanced circuit performance has been compared to the existing model. The proposed method is implemented on the MATLAB platform; the corresponding results of the model is discussed in the following section 4. 3. DESIGN OF CONVENTIONAL CONTROLLER In engineering applications, the controllers appear in different forms as a standalone controller, distributed control systems, or built into embedded components. PI controllers are commonly used in industry, instruments and laboratory equipment. These controllers have proven to be robust and extremely beneficial in the control applications. Despite the abundance of sophisticated tools including advanced controller the proportional (P) and integral (I) is still widely used in modern industry. PI controllers have been found possible to set satisfactory controller parameters from less plant information than a complete mathematical model a continuous development of new control algorithms ensure that the PI controller has not become obsolete and that this basic control algorithm will have its part to play in process control in foreseeable future. It can be expected that it will be a backbone of many complex control systems. While proportional and integrative modes are also used as single control modes, a derivative mode is rarely used in control systems. PI controller forms the control signal U (t) from error e (t) in the following way [21]:

t

p deT

teKtU0

1 (28)

Where KP and Ti are PI controller settings. The tuning of this controller is done using the reaction curve method. The Simulink representation of proposed bi-directional soft-switching converter implemented with PI controller is shown in Fig. 4.



Fig. 4 Bi-directional soft switching DC-DC converter with PI controller

Table. 1 Specifications of proposed circuit parameters

Parameters Specifications Vin 12V Fsw 80kHz Crs 1000 µF

Ls1=Ls2 1mH Ly 10µH Lf 47µH Cf 1000µF

Load1 - R1= R2= R3 1kΩ L2 0.1mH C3 1000µF Es2 15V Es3 5V

PI controller settings KP and Ki are designed using Ziegler-Nichols tuning technique based on the converter’s open loop step responses with high loads [22]. Controller tuning involves the selection of the best values of KP and Ki. This is often a subjective procedure and is certainly process dependent. KP = 10.793, Ki =1.284 in this work. Table 1 shows the specification of circuit parameters.

Figs. 5 to 9 indicates the performance evaluation of bi-directional soft switching DC-DC converter implemented with PI controller. The operation of the proposed converter has been studied for various disturbances created in the supply and load side of the converter. Fig. 5 demonstrates the simulated voltage and current start-up response of PI controller. From the figure 5, it is observed that the PI controller response of the proposed converter is determined at 0.0024 sec, but the controller response could not have gone down with a fine response due to its oscillations. Fig. 6 demonstrates the simulated voltage and current servo response of PI controller. From figure 6, it is observed that the response of the controller set-point change by 10V at t=0.02-0.04s and 6V at t=0.06-0.08s. Besides the controller response settled at 0.0023 sec for increment in voltage and 0.0017 sec for the decrement in voltage

without any overshoot of 3.6% and undershoot of 8.6% for servo responses. Fig. 7 demonstrates the simulated voltage and current regulatory response of PI controller. From figure 7, it is observed that the regulatory disturbance occurred at t=0.02-0.04s with 2V and t=0.06-0.08s with -2V. During the regulatory increment and decrement responses, the converter settled with less settling time of 0.0008 sec for the increment to load and 0.0014 sec for the decrement in load response without overshoot and 24% undershoot with the implementation of PI controller. Likewise, the input and load disturbances of the proposed DC-DC converter with PI controller is shown in Fig.8 and Fig. 9 respectively. From figure 8, it is observed that the simulated voltage and current response of the PI controller for disturbance at input (t=0.-0.02s, Vin=12V; t=0.02-0.04s, Vin=14V; t=0.04-0.06s, Vin=12V; t=0.06-0.08s, Vin=10V; t=0.08-0.10s, Vin=12V). From figure 9, it is observed that the simulated voltage and current response of the PI controller for disturbance in Load (t=0.-0.02s, R=1kΩ; t=0.02-0.04s, R=1.5kΩ; t=0.04-0.06s, R=1kΩ; t=0.06-0.08s, R=0.5kΩ; t=0.08-0.10s, R=1kΩ).

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

4

8

12

I 0 (

mA

)

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

4

8

12

V0 (

V)

Time (s) Fig. 5 Simulated voltage and current start-up response

of PI controller



0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (m

A)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s) Fig. 6 Simulated voltage and current servo response of

combined PI controller for a set-point change at t=0.02-0.04s, V=10V; t=0.06-0.08s, V=6V.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s) Fig. 7 Simulated voltage and current regulatory

response of PI controller for a disturbance at t=0.02-0.04s, V=+2V; t=0.06-0.08s, V=-2V.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s)

0.02 0.04 0.06 0.08 0.1

8

Fig. 8 Simulated voltage and current response of PI

controller for disturbance at input (t=0.-0.02s, Vin=12V; t=0.02-0.04s, Vin=14V; t=0.04-0.06s, Vin=12V; t=0.06-0.08s, Vin=10V; t=0.08-0.10s,

Vin=12V).

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s)

0 0.02 0.04 0.06 0.08 0.1

8

Fig. 9 Simulated voltage and current response of PI

controller for disturbance in Load (t=0.-0.02s, R=1kΩ; t=0.02-0.04s, R=1.5kΩ; t=0.04-0.06s, R=1kΩ; t=0.06-

0.08s, R=0.5kΩ; t=0.08-0.10s, R=1kΩ). 4. DESIGN OF FUZZY CONTROLLER The fuzzy control provides a formal methodology for representing, manipulating, and implementing a

human’s heuristic knowledge about how to control a system. The fuzzy controller has the following main components

The “rule-base” holds the knowledge, in the form of a set of rules, of how best to control the system.

The inference mechanism evaluates which control rules are relevant at the current time and then decides what the input to the plant should be.

The fuzzification interface simply modifies the inputs so that they can be interpreted and compared to the rules in the rule-base.

The defuzzification interface converts the conclusions reached by the inference mechanism into the inputs to the plant.

For the design of fuzzy logic controller, the following design steps are to be followed:

Choice of state and control variables Select inference method. Fuzzification. Process of making a crisp

quantity fuzzy Design the knowledge base. Select defuzzification method. Test and tuning. Produce a Lookup table.

The derivation of fuzzy control rules is based on the following criteria:

When the output of the converter is far from the set point, the change of duty cycle must be large so as to bring the output to the set point quickly.

When the output of the converter is approaching the set point, a small change of duty cycle is necessary.

When the output of the converter is near the set point and is approaching it rapidly, the duty cycle must be kept constant so as to prevent overshoot.

When the set point is reached, and the output is still changing, the duty cycle must be modified a little bit to avoid the output from moving away.

When the set point is reached, and the output is steady, the duty cycle remains unchanged.

When the output is above the set point, the sign of a change of duty cycle must be negative and vice versa.

Fuzzy memberships NL, NM, NS, ZE, PS, PM, PL are defined as negative large, negative medium, negative small, zero, positive small, positive medium and positive large [23].

Fig.5 Block diagram of fuzzy logic controller



The “fuzzy inference system” is a popular computing framework based on the concepts of fuzzy set theory, fuzzy if-then rules, and fuzzy reasoning [24]. It has found successful applications in a wide variety of fields, such as automatic control, data classification, decision analysis, expert systems, time series prediction, robotics, and pattern recognition [25]. Because of its multidisciplinary nature, the fuzzy inference system is known by numerous other names, such as “fuzzy expert system” [26], “fuzzy model” [27], “fuzzy associative memory” [28], and simply “fuzzy system.” The basic structure of a type-1 fuzzy inference system consists of three conceptual components: a “rule base”, which contains a selection of fuzzy rules; a “database”, which defines the membership functions used in the fuzzy rules; and a “reasoning mechanism”, which performs the inference procedure upon the rules and given facts to derive a reasonable output or conclusion. A possible cause of the better performance of the type-1 controllers over the type- 2 controllers, in the inertia change case, is that the level of noise applied was too low.

Table. 2 Fuzzy rule base ce e

NL NM

NS ZE PS PM

PL

NL NL NL NL NL NM NS Z NM NL NL NL NM NS Z PS NS NL NL NM NS Z PS PM ZE NL NM NS Z PS PM PL PS NM NS Z PS PM PL PL PM NS Z PS PM PL PL PL PL Z PS PM PL PL PL PL

Table 2 shows the fuzzy rule base created in the present work based on intuitive reasoning and experience [29]. The block diagram of a fuzzy logic controller for chosen converter is shown in Fig.10.

Figs. 11 to 15 shows the performance evaluation of bi-directional soft switching DC-DC converter implemented with Fuzzy Logic Controller. The operation of the proposed converter has been studied for various disturbances created in the supply and load side of the converter. Fig. 11 demonstrates the simulated voltage and current start-up response of Fuzzy Logic Controller. From figure 11, it is observed that the Fuzzy Logic Controller response of the proposed converter is determined at 0.001 sec, the controller response is made up with a fine response without oscillations compared with the PI controller response. Fig. 12 demonstrates the simulated voltage and current servo response of Fuzzy Logic Controller. From figure 12, it is observed that the response of the controller set-point change by 10V at t=0.02-0.04s and 6V at t=0.06-0.08s. Also, the controller response settled at 0.0009 sec for voltage increment and 0.001 sec for voltage decrement without overshoot and 21% with undershoot for servo responses. Fig. 13 shows the simulated voltage and current regulatory response of Fuzzy Logic Controller. From figure 13, it is observed that the regulatory disturbance occurred at t=0.02-0.04s with 2V and t=0.06-0.08s with -2V. During the regulatory increment and decrement responses, the converter settled with a less settling time of 0.0006 sec with an overshoot of 1.1% and settled 0.0002 sec without undershoot with the implementation of Fuzzy Logic Controller. Similarly, the input and load disturbances

of the proposed DC-DC converter with Fuzzy Logic Controller is shown in Fig.14 and Fig. 15 respectively. From figure 14, it is observed that the simulated voltage and current response of Fuzzy Logic Controller for disturbance at input (t=0.-0.02s, Vin=12V; t=0.02-0.04s, Vin=14V; t=0.04-0.06s, Vin=12V; t=0.06-0.08s, Vin=10V; t=0.08-0.10s, Vin=12V). From figure 15, it is observed that the simulated voltage and current response of Fuzzy Logic Controller for disturbance in Load (t=0.-0.02s, R=1kΩ; t=0.02-0.04s, R=1.5kΩ; t=0.04-0.06s, R=1kΩ; t=0.06-0.08s, R=0.5kΩ; t=0.08-0.10s, R=1kΩ).

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

4

8

12

I 0 (

mA

)

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

4

8

12

V0 (

V)

Time (s) Fig. 11 Simulated voltage and current start-up response

of Fuzzy Controller

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s) Fig. 12 Simulated voltage and current servo response of Fuzzy controller for a set-point change at t=0.02-0.04s,

V=10V; t=0.06-0.08s, V=6V.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (m

A)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s) Fig. 13 Simulated voltage and current regulatory response of Fuzzy controller for a disturbance at

t=0.02-0.04s, V=+2V; t=0.06-0.08s, V=-2V.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (m

A)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s)

0.02 0.04 0.06 0.08 0.1

8

Fig. 14 Simulated voltage and current response of Fuzzy controller for disturbance in input (t=0.-0.02s, Vin=12V;

t=0.02-0.04s, Vin=14V; t=0.04-0.06s, Vin=12V; t=0.06-0.08s, Vin=10V; t=0.08-0.10s, Vin=12V).



0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s)

0 0.02 0.04 0.06 0.08 0.1

8

Fig. 15 Simulated voltage and current response of Fuzzy controller for disturbance in Load (t=0.-0.02s, R=1kΩ; t=0.02-0.04s, R=1.5kΩ; t=0.04-0.06s, R=1kΩ; t=0.06-

0.08s, R=0.5kΩ; t=0.08-0.10s, R=1kΩ).

The comparison of both PI and Fuzzy Logic Controller responses of the start-up transient, servo, and regulatory responses are shown in Fig. 16, 17 and 18 respectively. The overall comparison of the controllers are listed in Table.3

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

4

8

12

I 0 (

mA

)

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.020

4

8

12

V0 (

V)

Time (s)

PI

FUZZY

Fig. 16 Simulated voltage and current start-up response

of combined PI and Fuzzy controller

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s)

PI

FUZZY

Fig. 17 Simulated voltage and current servo response of

combined PI and a Fuzzy controller for a set-point change at t=0.02-0.04s, V=10V; t=0.06-0.08s, V=6V.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

I 0 (

mA

)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

4

8

12

V0 (

V)

Time (s)

PI

FUZZY

Fig. 18 Simulated voltage and current regulatory

response of combined PI and a Fuzzy controller for a disturbance at t=0.02-0.04s, V=+2V; t=0.06-0.08s, V=-

2V.

Table. 3 Performance evaluation of controller for bi-directional soft switching DC-DC converter using MATLAB

Controller

Start-up transient Servo Response Regulatory Response

Increment of 33%

Decrement of 33%

Increment of 15%

Decrement of 15%

Peak over shoot

Under shoot

Settling time

Peak overs hoot

Settling time

under shoot

Settling time

Peak over shoot

Settling time

under shoot

Settling time

PI - - 0.0024 3.6 0.0023 8.6 0.0017 - 0.0008 24 0.0014 Fuzzy - - 0.0010 - 0.0009 21 0.001 1.1 0.0006 - 0.0002

5. CONCLUSION The Simulation results show that the proposed fuzzy logic controller regulates satisfactorily the output voltage of bi-directional soft switching DC-DC converter irrespective of line and load disturbances. It also overcomes the effects of parasitic elements and greatly increases the output voltage of the DC-DC converters, introducing the characteristics of high efficiency, high power density, and cheap topology and near zero output voltage and current ripples. Carefully selecting the parameters, we can obtain mirror-symmetrical output voltages from the input source. These converters can be used in computer periphery circuits, medical equipment, and industrial applications with high output voltages. REFERENCES

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