Presenting a new switching method in the Shepard-Taylor power factor correction converter

Dissertation

Master's thesis

Persian abstract:

Nowadays, with the expansion of DC consumers and non-linear loads connected to the grid, the design and construction of power factor correction circuits using power electronic converters has gained special importance. With the advancement of technology, integrated circuits have come to help in this field. Due to the complexity and non-linear performance of these converters and the need for instantaneous control of the input current and output voltage in them, the use of integrated circuits and electronic processors is inevitable. The Shepard-Taylor converter is one of the new types of power factor correction converters, which has attracted the attention of researchers in recent years due to its optimal performance in this field and its unusual and unique characteristics in maintaining output voltage regulation with very low input voltage values.

Several control methods have been presented to control the switching pulse and correct the power factor for the Sheppard-Taylor power factor correction converter. In this thesis, we try to improve the circuit performance and reduce the switching losses by modifying the switching method and providing a new control method in order to reduce the switching losses of the Shepard-Taylor power factor correction converter, while maintaining the waveform of the input current in the sinusoidal state (in phase with the input voltage).  In order to implement this converter in practice, the algorithm of the proposed control method is designed and introduced using a simple and efficient control circuit using AVR microcontroller. At the end, to ensure the accuracy of the circuit performance, the results obtained from the simulation are compared with the values ??obtained from the practical tests. In order to achieve the final goal of building a modified Shepard-Taylor converter with a new keying method based on the hysteresis control method, initially, based on the research done by other researchers in the past, the Shepard-Taylor power factor correction converter was simulated in MATLAB and Proteus software, and after obtaining acceptable results, the modified converter in the proposed control method was also simulated with these two software. Then, according to the simulation results, the circuit was implemented in laboratory dimensions. The results of simulation and practical tests are given in the fourth chapter, which clearly shows that the converter has a good performance in the proposed control method.

Key words: correction of power factor, Shepard-Taylor converter, hysteresis control, AVR microcontroller, implementation and construction, reduction of switching losses.

1-1-     Introduction

Power electronics is a combination of power, electronics and control. Control examines dynamic and steady state characteristics of closed loop systems. Power examines static and rotating power devices used in the generation, transmission, and distribution of electrical power. Electronics examines circuits and signal processing devices that are used to achieve desired control objectives. Power electronics can also be defined as applications of solid state electronics in the control and conversion of electric power. Power electronics is based on the switching properties of power semiconductor elements. With the advancement of power semiconductor technology, the ability to work with high power and high switching speed in power electronic devices has improved significantly. The progress in microcontroller technology [1] has had a great impact on the control and creation of control methods for power semiconductor elements [1].

As mentioned earlier, power electronics is based on the switching of power elements. Using these elements also brings disadvantages. The non-linearity of these elements causes distortion in the waveform of the line current, which itself causes many disadvantages, including the reduction of the power factor[2] (P.F) as one of the most important effects. Power factor correction converters [3] (PFC) bring the input closer to a sinusoidal state and in phase with the voltage. The problem of input current distortions has been known for a long time. Recently, paying attention to the harmful effects of harmonics has led to the creation of a strategic formulation as well as standards that have caused more attention to ways to limit current distortions [3]. Generally, PFC is the capacity to produce or absorb reactive power in a load connected to the network without using a source.The power factor can be defined as the ratio of the real power [4] to the apparent power [5] and in the form of the equation (1-1):

(1-1)

where the real power is the average value of the product of the instantaneous voltage and the instantaneous current in one cycle, and the apparent power is the product of the effective value of the current and the effective value of the voltage. If the sinusoidal voltage and current are in phase, the power factor will be equal to one. If the voltage and current are sinusoidal and out of phase, the cosine power factor will be their phase difference. This definition of the power factor is only valid when the voltage and current are sinusoidal, in other words, the above definition of the power factor is used only under the condition that the load is a combination of linear elements such as resistors, capacitors, and inductors, while there is usually a half-wave or full-wave rectifier at the input of the power factor correction converters, which means that the use of non-linear elements in these converters makes us unable to use the above definition for the power factor.

1-2-     Thesis topic

In a general classification, the power factor correction methods can be divided into two general categories: passive[6] and active[7]. Passive power factor correction methods generally control the reactive power absorbed from the source. In the passive power factor correction method, to eliminate the phase difference between the voltage and the current drawn from the source, a capacitor with a suitable capacity is paralleled with the load so that the current and the input voltage are in phase.  In the active power factor correction method, which is used in power electronic power factor correction converters and switching power supplies, the power factor correction is done by controlling the input current waveform. In the active power factor correction method, the pulse will be applied to the converter keys so that the input current is sinusoidal and in phase with the source voltage.

In this thesis, in chapter 2, we will review the definitions provided for the power factor and also the power factor correction methods, then the circuit performance of the DC converters used in the power factor correction converters will be examined, and finally, the control methods of these converters will be compared and how they work. One of the control methods will be fully described. In chapter 3, we will have an overview of how the Shepard-Taylor converter [8] works, then in the next chapter, we will briefly review the articles that have been published in this field. At the end, how the proposed new keying method works will be explained. In chapter 4, the results of simulations and the results of practical tests using the proposed keying method will be presented. 

In this thesis, the performance of the Shepard-Taylor converter will be investigated as one of the new types of power factor correction converters. Then, by changing the structure of this circuit, a new switching method based on the hysteresis control method has been presented to reduce the losses of the said circuit. In the proposed method, the switching losses of the converter have been reduced compared to the conventional switching method. In order to achieve the final goal of building a modified Shepard-Taylor converter with a new keying method based on the hysteresis control method, initially, based on the research done by other researchers in the past, the Shepard-Taylor power factor correction converter was simulated in MATLAB and Proteus software, and after obtaining acceptable results, the modified converter in the proposed control method was also simulated with these two software. Then, according to the circuit simulation results, it was implemented in the laboratory dimension. The simulation results and practical tests are presented in the fourth chapter, which clearly shows that the converter has a good performance in the proposed control method. 1-3- Research Innovation Today, with the expansion of DC consumers and non-linear loads connected to the grid, the design and construction of power factor correction circuits using power electronic converters has gained special importance. With the advancement of technology, integrated circuits have come to help in this field, due to the complexity and non-linear performance of these types of converters and the need for instant control of the input current and output voltage in them, the use of integrated circuits and electronic processors is inevitable.  The Shepard-Taylor converter is one of the new types of power factor correction converters, which has attracted the attention of researchers in recent years due to its favorable performance in this field. Several control methods have been presented to control the switching pulse and power factor correction for the Shepard-Taylor power factor correction converter.