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Quasi Z-source converter is a single stage soft switched power converter derived from Z-source converter topology, employing an impedance network coupling the source with the converter. The quasi Z-source source converter can buck or boost the voltage and current flow is bidirectional. The duty cycle of the switch can be adjusted to maintain constant voltage during load change. To obtain constant output voltage, proper controller design is a must. This paper presents closed loop control of quasi Z-source converter using PI controller where controller parameters are estimated using the small signal model of the entire system. The transfer function of the system with AC sweep is used to obtain appropriate proportional and integral gain constants to reduce transient dynamics and to reduce steady state error.

Many DC-DC converters can either buck or boost the voltages. The voltage fed converters can only buck the voltage and current fed converters can boost the voltages. There are applications which demands both buck and boost operation, for example battery charging and discharging. During batter charging, the voltage has to be stepped down whereas during battery discharge, the voltage has to be stepped up. The converters which can perform both the operations are Z-source converters which were first proposed by F. Z. Peng [

By controlling the shoot through period, the duty cycle of Z-source converter can be controlled. The Z-source converter can produce any desired output ac voltage, even greater than the line voltage by controlling the shoot through period. Therefore Z-source inverters can be used to compensate the voltages when voltage sag occurs in power systems.

The concept of the Z-source network can be applied to DC-DC power conversion. Z-source dc-dc converter (ZSC) is proposed in [

Z-source network has both the current-fed topology and the voltage-fed topology. This paper focuses on voltage-fed power conversion circuit which is represented in _{in} and the diode D_{0}. The Z-source network part is composed of the inductors L_{1}, L_{2} and the capacitors C_{1}, C_{2}. The inductors L_{1}, L_{2} have the same inductance value. Also, the capacitors C_{1}, C_{2} have the same capacitance value. In the case of ZSC, the output part is composed of the switch S, the low pass filter L_{f}-C_{f}, and the load resistance R_{0}.

ZSC has two operation modes; state 0 and state 1. During the term of state 0, the switch S is on. The Z-source inductors L_{1}, L_{2} are magnetized, and the capacitors C_{1}, C_{2} of Z-source are discharged. During the state 1 cycle,

the switch S is off. The Z-source inductors L_{1}, L_{2} and input voltage source V_{in} provide the energy to the output part and the Z-source capacitors C_{1}, C_{2}. By repeating these two operation modes, ZSC can output positive polar boost voltage. The equivalent circuits and the associated expressions corresponding to different stages of operation of the PWM Z-source dc-dc converter in CCM, the dc input-to-output voltage conversion factor and minimum inductance required to ensure CCM operation for power losses in the components of the PWM Z-source dc-dc converter and the overall efficiency, output voltage ripple across the filter capacitor and experimental results to validate the theoretical analysis [

In common with Z-source network, quasi-Z-source network has both the current-fed topology and the voltage- fed topology. This paper focuses on voltage-fed power conversion circuit which is shown in _{f}-C_{f} output filter is used to smoothen the output current and load voltage respectively.

To overcome the above problems of ZSC, quasi-Z-source converter is proposed in [

qZSC has two operation modes, state 0 and state 1. During the term of state 0, the switch s is on and the diode D_{1} is off. The inductor L_{1} is magnetized by the input voltage source V_{in} and the capacitor C_{2}. Also, the inductor L_{2} is magnetized by the capacitor C_{1}. During the term of State 1, the switch is off and the diode D_{1} is on. The input voltage source V_{in} and the inductor L_{1}, L_{2} provide the energy to the load resistance R_{0}. Moreover, the capacitor C_{1} is charged by the input voltage source V_{in} and the inductor L_{1}, and the capacitor C_{2} is charged by the inductor L_{2}. By repeating these two operation modes, qZSC can boost the voltage.

1) State 0(t_{o} ≤ t ≤ t_{on}) when the switch is ON.

During the term of state 0, the switch s is on and the diode D_{1} is off. The inductor L_{1} is magnetized by the input voltage source V_{in} and the capacitor C_{2}. Also, the inductor L_{2} is magnetized by the capacitor C_{1}.

As shown in

2) State 1(t_{on} ≤ t ≤ t_{off}) when the is switch OFF.

During the term of State 1, the switch is off and the diode D_{1} is on. The input voltage source V_{in} and the inductor L_{1}, L_{2} provide the energy to the load resistance R_{0}. Moreover, the capacitor C_{1} is charged by the input voltage source V_{in} and the inductor L_{1}, and the capacitor C_{2} is charged by the inductor L_{2}.

As shown in

D―duty ratio.

T_{s}―total time period.

T_{on}―switch ON period.

T_{off}―switch OFF period.

Where

_{1} voltage.

_{2} voltage.

_{f} voltage.

_{1}._{ }

_{2} with reference to

where D―duty ratio.

T_{s}―total time period.

T_{on}―switch ON period.

T_{off}―switch OFF period.

In steady-state, the averaged voltages of L_{1}, L_{2} are zero for one switching cycle T_{S}. Therefore, the following equations are satisfied.

Substitute (1) and (4) in (9)

Substitute (2) and (5) in (10)

Since the voltages across C_{1}, C_{2}, increase and decrease linearly in two operation modes, the averaged voltages_{1}, C_{2}, are expressed as follows in steady state.

Substitute (7), (8), (13) and (14) in (11) we will get

Substitute (7), (8), (13) and (14) in (12) we will get

Equating Equations (15) and (16) we will get

In steady-state, the averaged voltages of L_{f} are zero for one switching cycle T_{S}. Therefore, the following equations are satisfied.

Substitute (3), (6) in (18) we will get

In steady-state, the input power is equal to output power

The ripple Current allowed to the inductance

δ―allowed Ripple.

Smart Control is a general?purpose controller design software specifically for power electronics applications. To design the controller of a dc-dc converter with a single control loop using the Smart Control software. Before going to Smart control find bode plot of the Plant. The converter selected in this example is a quasi Z-source converter with voltage model control, as shown in

Note that if there is not enough attenuation at the switching frequency, the system will likely oscillate in the high frequency region. Also, if a design is not proper, the edit boxes will be change to the red color, warning users to re?select the design. After the design is completed, Smart Control provides the component values for the sensor and the regulator.

In this paper the simulation model is developed with Psim software. The simulation is carried out for closed loop control of converter shown in _{0} = 200 Ω. _{1} and C_{2} of qZSC. Current through inductor of L_{1} and L_{2} of qZSC is shown in

As observed from

Component | Values |
---|---|

f_{sw} | 20 kHz |

R_{a} | 23.5 KΩ |

R_{b} | 500 Ω |

V_{ref} | 2.5 V |

R_{2} | 87.752 KΩ |

R_{11} | 10 KΩ |

C_{2} | 24.5036 nF |

G_{mod} | 0.2 |

Quasi Z-source network | Component | Values |
---|---|---|

Inductor L_{1} | 6.8717 mH | |

Inductor L_{2} | 6.8717 mH | |

Capacitor C_{1} | 33.763 µF | |

Capacitor C_{2} | 33.763 µF | |

Low passfilter | Inductor L_{f} | 6.87 mH |

Capacitor C_{f} | 0.4815 µF | |

Switching frequency | 20 kHz | |

Load resistance | 200 Ω | |

Duty cycle | 0.47 | |

Output power | 392 W |

In this paper, the PI controller is designed by using smart control and the closed loop control performance of quasi Z-source dc-dc converter was analyzed for step change in load. By PWM duty ratio control, it can boost the input voltage. It can reduce cost and improve reliability. Quasi-Z-source dc-dc converter has been proposed with low pass filter. The quasi Z-source converter draws continuous current from supply and the input current ripple is also less compared to Z-source converter. By the circuit analysis and experiment, the operation of the proposed circuit has been confirmed.

A. Suresh,M. R. Rashmi,V. Madusuthanan,P. Vinoth Kumar, (2016) Closed Loop Control of Bi-Directional Soft Switched Quasi Z-Source DC-DC Converter. Circuits and Systems,07,574-584. doi: 10.4236/cs.2016.75049