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In this paper, the modified LCC type of series-parallel Resonant Converter (RC) was designed and state-space modeling analysis was implemented. In this proposed converter, one leg of full bridge diode rectifier is replaced with Synchronous Rectifier (SR) switches. The proposed LCC converter is controlled using frequency modulation in the nominal state. During hold-up time, the SRswitches control is changed from in-phase to phase-shifted gate signal to obtain high DC voltage conversion ratio. Furthermore, the closed loop PI and fuzzy provide control on the output side without decreasing the switching frequency. The parameter such as conduction loss on primary and secondary side, switching loss, core and copper also reduced. Simultaneously, the efficiency is increased about 94.79 is realized by this scheme. The proposed converter with an input of 40 V is built to produce an output of 235 V with the help of ZVS boost converter [1] even under line and load disturbances. As a comparison, the closed loop fuzzy controller performance is feasible and less sensitive than PI controller.

Today, researches have been made for DC-DC converter to improve the efficiency, power density with reduced switching losses. Due to these increased power semiconductor switching losses, the soft-switching converters has been developed. The objective of the proposed converter is to provide remarkable efficiency with regulated responses even under fluctuations in line and load sides by modified LCC Resonant Converter and different control techniques. There are various types of resonant converters [

Generally, there are two stages in power supply that has a power factor correction (PFC) stage, a front-end DC-DC converter stage [_{link}-nom to the DC-DC conversion stage [

The LCC resonant converter is now becoming more popular for its easy design and high efficiency, because of zero voltage switching (ZVS) and zero current switching (ZCS) switching conditions. There are many possible combinations of the resonant tank circuit depends upon the inductor and capacitor connections [

In this paper, an SR method is proposed for a new modified LCC resonant converter. It consists of two power MOSFET semiconductor switches for single phase half bridge inverter on the inverter side which gets input from the ZVS boost converter [

The ZVS boost converter is similar to DC-DC boost converter which works under ZVS switching condition thus reduces the switching losses. The capacitive turn-on loss is eliminated by zero voltage switching (ZVS) technique. So, ZVS is more beneficial than ZCS. In this proposed system, the ZVS boost converter is boost up the input voltage of 40 V to 150 V which acts as the input to inverter circuit on the primary side of transformer.

The output parameters can be controlled by changing the duty cycle. The ratio of output voltage to input voltage is given in Equation (1).

In this paper, the state-space mathematical modeling technique is adopted for LCC resonant converter based on different operating modes [

Some of the following assumptions are made while doing the state-space modeling [

a) The elements such as switches, inductors, capacitors and diodes used are ideal.

b) The effect of snubber and losses includes tank circuit, semiconductor switches and filter losses are neglected.

c) The filter capacitor C_{f} should be large enough than the parallel capacitor CP to produce constant output voltage V_{o}.

d) The high frequency transformer is ideal with turns ratio n = 1.

e) The input voltage V_{dc} and output voltage V_{o} kept constant without ripples in steady state.

In mode A, the M_{1} and SR_{2} switches are turned on. The tank circuit voltage and resonant current are in-phase. The resonant capacitor Cs boosts inductor current i_{L}. So both i_{L}(t) and V_{CS}(t) are positive. The voltage of CP is equal to the output voltage V_{o}, because C_{P} is connected in parallel to filter capacitor C_{f}. The state-space input and output matrix is given below as Equations (2) and (3).

And the output equation is,

With initial condition i_{L}(0) and V_{cs}(0).

In this mode of operation, M_{2} and SR_{1} pairs are turned on. The power delivered to both tank circuit and load. The C_{p} is connected anti parallel to the filter capacitor. The resonant capacitor V_{Cs}(t) and i_{L}(t) are negative. The state-space input and output matrix is given below as Equations (4) and (5).

And the output equation is,

With initial condition i_{L}(0) and V_{cs}(0).

All switches are turned off instead of diode pairs. In this mode, the inductor current and tank circuit voltages are out of phase. So the i_{L}(t) decreases faster. So the energy stored in LCC tank is returned to the source. Here, the resonant current may be positive or negative. The state-space input and output matrix is given below as Equations (6) and (7).

And the output equation is,

The Stability analysis refers to the stable working condition of a control system [

The transfer function of LCC Resonant Converter is written in Equation (8),

Substituting the values of L_{s}, C_{s} and C_{P}, the equation obtained is,

The state-space input matrix for the equivalent circuit of LCC converter is written as:

And the output equation is,

The Nyquist Plot for the closed loop control of LCC Resonant Converter is shown in

The values are considered for design the LCC resonant tank is m = C_{s}/C_{Pratio} = 1, Q = 5, y = 1. The Load Resistance

Resonant frequency f_{0} is written as f_{0} = f/y = 127.27 KHz. But,

The Value of resonant component L and C are L_{s} = 1.99 μH and C = 959.6 Nf.

The transformer of high frequency causes loss on both primary and secondary side. The secondary side is calculated by summing the loss of diode rectifier and SR switches. The transformer size will be reduced because of its high frequency. There is various loss parameters such as conduction loss, core loss, copper loss, switching loss can be calculated as following equations.

where, I_{on} is the drain current of the power MOSFET switch, R_{ds} is the drain to source resistance of the switch.

But, the loss occur at SR switch is very less than diode rectifier.

The Switching loss is calculated as follows,

where, C_{o} is the output capacitance of the MOSFET, C_{P} is the Parasitic winding capacitance of the MOSFET switch. f_{sw} is the switching frequency of the Resonant Converter. The total losses are calculated by the sum of conduction loss and switching loss. _{s} is blocking voltage. When switch becomes turned ON, the value of u_{s} is zero. I_{s} is switching current and T_{J} is junction temperature of the switch [_{cond} = 20.2W and P_{sw} = 10.3W. The loss chart for the LCC Resonant converter is shown in

The proposed LCC Resonant converter has different voltage conversion ratios depend on its operation. In the nominal state, the LCC converter operates near Resonant Frequency f_{r} to attain optimum efficiency. The Voltage Conversion Ratio and operating frequency also of the LCC Resonant Converter is analyzed and the corresponding graph can be obtained by using the equation given below.

where V_{0} is the output Voltage, V_{in} is the input Voltage, Gain K = C_{P}/C_{S} ratio which is equal to 1. The Resonant frequency and Quality factor of LCC Resonant Converter are given as,

The load impedance_{s}/ω_{r}. Where ω_{s} = 2πf_{s} is the angular switching frequency of LCC converter. ω_{r} = 2πf_{r} is the angular resonant frequency [

Efficiency calculation is done for different loading conditions.

The closed loop PI controller is provided on the output side to provide controlled gate signal to switches under different load conditions. Both proportional and integral term is to increase the speed of the response and also to eliminate steady state error. The controller gains K_{P} and K_{I} are tuned by trial and error according to the system error signal.

The fuzzy controller is a problem solving control system that operates in a closed-loop system in real time. The limitations of conventional (PI) controller are overcome by fuzzy controller. The fuzzy controller is beneficial because it is very robust, cheap, and simple to design and has the multiple inputs and multiple outputs.

Fuzzy logic system is an artificial decision maker based that operates on combinations of Linguistic variable and Boolean logic. Usually fuzzy logic control structure is created from four major elements presented on

This first phase of fuzzy logic is to deliver the crisp input variables for given fuzzy with the help of membership functions. The inference mechanism formulating the mapping for given input to output. It is done by using mamdani or sugeno type toolbox.

It consist of a database and linguistic control rules are framed by if-then conditional statement with AND/OR logic operation. The fuzzy rules based on the error E, the rate of change of error ∆E and the change in the control signal is the output obtained. Here, 49 rules are used to form rule base table.

Defuzzification phase converts the fuzzy output from the crisp output. The crisp output is the pulse signal generated to the power switches on the SR switches and half bridge inverter. There are many methods of defuzzification that have been proposed. The center of gravity is physically appealing among those methods. It is found by calculating the area bounded by the membership function curve. The general structure of fuzzy controller in block diagram representation is shown in

In this proposed work, the seven triangular membership functions are used. The fuzzy rule base table is shown in

The simulation results shows the responses of closed loop PI and Fuzzy controller of LCC Resonant Converter with set point of 40 V for nominal load of 100 Ω. The

E CE | NB | NM | NS | Z | PB | PM | PS |
---|---|---|---|---|---|---|---|

NB | NB | NB | NM | NM | NS | NS | Z |

NM | NB | NM | NM | NS | NS | Z | PB |

NS | NM | NM | NS | NS | Z | PB | PB |

Z | NM | NS | NS | Z | PB | PB | PM |

PB | NS | NS | Z | PB | PB | PM | PM |

PM | NS | Z | PB | PB | PM | PM | PS |

PS | Z | PB | PB | PM | PM | PS | PS |

and current under sudden load disturbance (100 Ω - 90 Ω - 100 Ω) at 0.8sec.The

LCC Resonant Converter | Supply Voltage Increase by 10 V (Line disturbance) & Load Resistance decrease by 10 Ω (Load disturbance) | ||
---|---|---|---|

Delay time (msec) | Rise Time (msec) | Settling time (msec) | |

FUZZY | 20.1 | 70.1 | 60.2 |

PI | 90.01 | 95.01 | 100.02 |

PARAMETERS | VALUES |
---|---|

Input Voltage, V_{dc} | 40 V |

Resonant Inductor, L_{r} | 1.99 μH |

Resonant Series Capacitor, C_{S} | 470 nF |

Resonant Parallel Capacitor, C_{P} | 960 nF |

Load Resistance, R_{L} | 100 Ω |

Output Voltage, V_{o} | 235 V |

Output Current, I_{o} | 2.35 A |

the results of PI and Fuzzy controlled converter, we could evident that Fuzzy controlled converter had less distraction under sudden load and line disturbances.

In this paper, the modified LCC type of Series-Parallel Resonant Converter was proposed. During hold-up time, the SR switches operate with phase-shifted gate signal to obtain high voltage conversion ratio without reduction in switching frequency. Thus, the closed loop control of LCC resonant converter performance was obtained using fuzzy and PI controller. The simulation results show that the fuzzy controller yields better dynamic performance even under sudden load and line disturbances. The stability of the system was also analyzed with the help of Nyquist plot. Furthermore, state-space analysis modeling technique is adapted. Likewise, the reduction of loss will lead to attain 94.79% of efficiency. From the simulation results, it is found that the closed loop fuzzy control system is less susceptible to the disturbances than PI.

N. Madhanakkumar,T. S. Sivakumaran, (2016) Performance Analysis of PI and Fuzzy Control for Modified LCC Resonant Converter Incorporating Boost Converter. Circuits and Systems,07,835-848. doi: 10.4236/cs.2016.76072