Patent Publication Number: US-2013249466-A1

Title: Photovoltaic System Having Burp Charger Performing Concept of Energy Treasuring and Recovery and Charging Method Thereof

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The application claims the benefit of Taiwan Patent Application No. 101109926, filed on Mar. 22, 2012, in the Taiwan Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a photovoltaic system comprising a first charger, a first, a second, and a third batteries, and a charge management controller. In particular, it relates to a photovoltaic burp charger performing concept of energy treasuring and recovery. 
     BACKGROUND OF THE INVENTION 
     Recently, renewable energy has been significantly attractive to our life facing alternative energy sources to replace the fossil fuel. Except for the transferring from the renewable energy directly into such as grid to power utility, green house, and so on, the applications through indirect conversion are also the focus of attention, especially for such as stand-alone system, mobile solar charger, hybrid system etc. For suited equipment using renewable energy, solar and wind energies are advantaged in the mentioned indirect applications. However, the most important buffer for reliably sustaining the conversion between renewable energy and converter is nothing but battery, especially for lead-acid battery (LAB) that is still one of the most popular and widely-used batteries due to high reliability and low cost. Referring to the characteristic of LAB, when the charging of LAB approaches 85-95% of the state-of-charge (SOC), the majority of lead sulfate, PbSO 4  possibly leads the battery voltage to exceed the gassing voltage to cause the evolution of gaseous hydrogen at the negative electrode and oxygen at the positive electrode. This undesired phenomenon may produce heat, increasing the charging time, and shortening the life of the battery. Moreover, if LAB is in multiple discharges, some PbSO 4  may be crystallized on the positive electrode, reducing both the available surface area thereon and its electrochemical reactivity with battery acid, which is associated with the prolongation of battery life. Pulsed-current charging is an effective means of delaying the crystallization process in the active material and minimizing the development of the PbO layer during cycling. Burp charging uses a positive pulse to charge and uses a negative pulse to discharge so as to improve the charging time and prolong the life-cycle of the battery. But, how to give consideration to both the life-cycle of the battery and the concept of energy treasuring and recovery, for example the energy recovery during discharging, is a question deserving of consideration. 
     Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a photovoltaic system having a burp charger performing concept of energy treasuring and recovery. 
     SUMMARY OF THE INVENTION 
     It is a primary objective of the present invention to provide a photovoltaic burp charger and charging method thereof, the photovoltaic burp charger includes a charge management controller used to engage in a photovoltaic burp charge and two pulse charges in a main battery and two auxiliary batteries respectively so as to prolong the life-cycle of the battery and realizing the concept of energy treasuring and recovery. 
     According to the first aspect of the present invention, a photovoltaic system comprises a first charger generating a first pulse train, a first battery receiving the first pulse train at a first time period to engage in a first charge, and engaged in an intense discharge at an initial stage of a second time period so as to generate a second pulse train, a second battery receiving the first pulse train during the second time period to engage in a second charge, a third battery engaged in a third charge via the second pulse train during the initial stage, and a charge management controller controlling the first charge and the intense discharge of the first battery, the second charge of the second battery and the third charge of the third battery. 
     According to the second aspect of the present invention, a photovoltaic system comprises a first battery receiving a first pulse train at a first time period to engage in a first charge, and engaged in an intense discharge at an initial stage of a second time period so as to generate a second pulse train, and a second battery engaged in a second charge via the second pulse train during the initial stage of the second time period. 
     According to the third aspect of the present invention, a charging method for a photovoltaic system comprises steps of: providing a first pulse train; receiving the first pulse train at a first time period to engage in a first charge towards a first battery; and causing the first battery to engage in an intense discharge at an initial stage of a second time period to generate a second pulse train so as to engage in a second charge towards a second battery. 
     The present invention can be best understood through the following descriptions with reference to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(   a ) shows a circuit diagram of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention; 
         FIG. 1(   b ) shows a waveform diagram of a typical burp pulse; 
         FIG. 2(   a ) shows a circuit diagram of a photovoltaic system having photovoltaic burp chargers and guided by a incremental-conductance (INC) MPPT controller according to the preferred embodiment of the present invention; 
         FIG. 2(   b ) shows a circuit diagram of a gassing voltage monitor in the prior art; 
         FIG. 2(   c ) shows a waveform diagram of all the driving signals of the circuit as shown in  FIG. 2(   a ); 
         FIG. 3(   a ) shows a dynamic state diagram of a photovoltaic system having photovoltaic burp chargers when BPP charges to the battery B 1  during a time period of tp 1 , and B 2  and B 3  are at rest; 
         FIG. 3(   b ) shows a dynamic state diagram of a photovoltaic system having photovoltaic burp chargers when the positive pulse (PP) charges to the battery B 2  during a time period of tp 2 , and temporarily stops during a time period of td, and B 1  engages in an intense charge to B 3  during a time period of tp 3 , which equals to a BNP charge; 
         FIG. 3(   c ) shows a dynamic state diagram of a photovoltaic system having photovoltaic burp chargers when B 1  and B 3  are at rest during a time period of td, wherein B 2  engages in the PP charge from t 2  to t 3 , then to tp 2  and until the end of a charging period of Ts 2 ; 
         FIG. 4(   a ) shows a circuit diagram of the equivalent half-circuit of IFC- 1 ; 
         FIG. 4(   b ) shows a waveform diagram of V Fgs1 /V Fgs2 , the primary current i pI,2  and the secondary current i sI,2 ; 
         FIG. 4(   c ) shows a circuit diagram of an equivalent circuit of a model of control to output (PV array to charging current) under maximum power transfer; 
         FIGS. 5(   a )- 5 ( b ) respectively shows simulation and experiment (from design example) results for the control-to-output model as shown in  FIG. 4(   c ), in which the output current and output power versus switching frequency f s1  are measured for various solar insolations; 
         FIG. 6  shows a waveform diagram for predicting dynamic state of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention; 
         FIG. 7(   a ) shows a circuit diagram of an equivalent circuit of a typical solar cell unit; 
         FIG. 7(   b ) shows a waveform diagram presenting I pv -V pv  and P pv -V pv  characteristic curves of the PV array at T=25° C. (solid line) and 55° C. (dotted line) for various solar insolations; 
         FIG. 8  shows a control chart of an INC MPPT controller of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention; 
         FIG. 9(   a ) shows a flow chart of a main program of an INC MPPT controller of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention; 
         FIG. 9(   b ) shows a flow chart of a subroutine of an INC MPPT controller of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention; 
         FIG. 10(   a ) shows a waveform diagram of the gate source voltages V Fgs1  and V Fgs2 , and the primary side currents i p1  and i p2  of the first interleaved flyback converter IFC- 1  according to the preferred embodiment of the present invention; 
         FIG. 10(   b ) shows a waveform diagram of the gate source voltage V Fgs1  and V Fgs2 , and the secondary side currents i s1  and i s2  of the first interleaved flyback converter IFC- 1  according to the preferred embodiment of the present invention; 
         FIG. 11(   a ) shows measured gate drive signals V Tgs1  and V Tgs2  for Q T1 , Q T2 , at a low frequency of 550 Hz, and V Fgs1 , V Fgs2  and V Fgs3  for Q F1 , Q F2 , and Q F3  at a high frequency of 19 kHz according to the preferred embodiment of the present invention; 
         FIG. 11(   b ) shows the measured gate drive signals V Tgs1  and V Tgs2  for Q T1  and Q T2 , the charge current i B1  for B 1  and the charge current i B2  for B 2  according to the preferred embodiment of the present invention; 
         FIG. 11(   c ) shows the measured gate drive signals V Tgs1  and V Tgs2  for Q T1  and Q T2 , and the burp charging currents i B1  and i p3  according to the preferred embodiment of the present invention; 
         FIG. 12(   a ) shows trajectories of the proposed PV burp charger presenting the state-of-charge (SOC) of the three batteries in the process of PV burp charge, in which the charging process excluding the burp charge to B 1  also contributes pulse charges to B 2  and B 3  in the non-burp charge period; and 
         FIG. 12(   b ) compares the charging and temperature trajectories of the proposed PV burp charger with those obtained using the CC/CV charging strategy, at an average charging rate of 0.2 C, where 1 C=45 AH, under solar insolation of 1 kW/m 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Although the following description contains many specifications for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to and without imposing limitations upon, the claimed invention. 
       FIG. 1(   a ) shows a circuit diagram of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention. In  FIG. 1(   a ), the photovoltaic system having the photovoltaic burp charger includes a PV array, an interleaved flyback converter IFC- 1  (a first charger), a main battery (a first battery) B 1 , a first auxiliary battery (a second battery) B 2 , a second auxiliary battery (a third battery) B 3 , an interleaved flyback converter IFC- 2  (a second charger), and three transmission switches Q T1 , Q T2  and Q T3 , wherein the IFC- 1  includes two flyback converters connected to each other in parallel, flyback ( 1 ) and flyback ( 2 ), and outputs an interleaved high-frequency tiny pulse train (a first pulse train). The first battery B 1  receives the first pulse train at a first time period (tp 1 ) to engage in a first charge (burp charge), and B 1  engages in an intense discharge at an initial stage (tp 3 ) of a second time period so as to generate a second pulse train, the second battery B 2  receives the first pulse train during the second time period (tp 2 ) to engage in a second charge, the third battery B 3  engages in a third charge via the second pulse train during the initial stage (tp 3 ). 
       FIG. 1(   b ) shows a waveform diagram of a typical burp pulse. In a charge period T s2  of  FIG. 1(   b ), a positive pulse (BPP) is shown located in the first time period (tp 1 ), a negative pulse (BNP) is shown located in the initial stage (tp 3 ) of the second time period, and then there is a brief break time period T d . 
       FIG. 2(   a ) shows a circuit diagram of a photovoltaic system having photovoltaic burp chargers and guided by an incremental-conductance (INC) MPPT controller according to the preferred embodiment of the present invention. In  FIG. 2(   a ), except for those elements included in  FIG. 1(   a ), the photovoltaic system further includes a PWM-1 controller having variable frequency and constant duty (VFCD) and a charge management controller (CMC), and the second charger IFC- 2  includes a switch Q F3 , wherein the second charger IFC- 2  is electrically connected between the third battery B 3  and the third transmission switch Q T3 , is used to receive the second pulse train and generates a charging pulse train charging the third battery B 3 , the first transmission switch Q T1  is electrically connected between the first charger IFC- 1  and the first battery B 1 , the second transmission switch Q T2  is electrically connected between the first charger IFC- 1  and the second battery B 2 , the third transmission switch Q T3  is electrically connected to the first battery B 1 , and the CMC generates a first, a second, and a third control signals V Tgs1 , V Tgs2  and V Tgs3  controlling a turn-on and a turn-off of the first to the third transmission switches Q T1 , Q T2  and Q T3  respectively, and controlling when the first battery B 1  engages in the first charge and the intense discharge, when the second battery B 2  engages in the second charge, and when the third battery B 3  engages in the third charge. The first battery B 1 , the second battery B 2  and the third battery B 3  generate a first, a second and a third gassing voltage detection values, Vd 1 , Vd 2  and Vd 3  respectively. The first, the second and the third transmission switches Q T1 , Q T2  and Q T3  include respective gates. The charge management controller includes a transmission gate controller (TGC) and a gassing voltage monitor, the transmission gate controller outputs the first, the second and the third control signals V Tgs1 , V Tgs2  and V Tgs3  to the respective gates to control the turn-on and the turn-off of the first, the second and the third transmission switches Q T1 , Q T2  and Q T3 . The gassing voltage monitor receives the first, the second and the third gassing voltage detection values Vd 1 , Vd 2  and Vd 3  and outputs respective enable signals to the transmission gate controller and the INC MPPT controller. The transmission gate controller engages in a normal operation and the INC MPPT controller engages in an MPPT when all the first, the second and the third gassing voltage detection values Vd 1 , Vd 2  and Vd 3  are not reaching a gassing voltage value, and the transmission gate controller ceases the normal operation and the INC MPPT controller ceases the MPPT when at least one of the first, the second and the third gassing voltage detection values Vd 1 , Vd 2  and Vd 3  reaches the gassing voltage value, where a gassing battery should be replaced at this moment. The photovoltaic system further comprises diodes D 1 , DT 1  and DT 2 , and a capacitor C B . The PWM-1 controller receives a control signal from the INC MPPT controller, and outputs a pulse-width modulation (PWM) signal to an inverter A 2  of the IFC- 1 . Each of the two flyback converters, flyback ( 1 ) and flyback ( 2 ), includes a switch Q F1 /Q F2 , a transformer T 1 /T 2 , an inductor L m /L m  and a diode D Fs1 /D Fs2 . 
       FIG. 2(   b ) shows a circuit diagram of a gassing voltage monitor in the prior art. The gassing voltage monitor includes three sub-circuits, each of which includes an operational amplifier (A 1 /A 2 /A 3 ), a transistor (Tr 1 /Tr 2 /Tr 3 ), an LED (LED 1 /LED 2 /LED 3 ) and two resistors. The first, the second and the third gassing voltage detection values Vd 1 , Vd 2  and Vd 3  are respectively inputted to a terminal of each of the three sub-circuits. Once anyone of the batteries is reaching the gassing voltage, the monitor will turn on the LED to indicate the gassing battery, the charge management controller CMC will disable all functions of the IFC- 1  and the transmission switches at this moment, and this is the time to replace the gassing battery. 
       FIG. 2(   c ) shows a waveform diagram of all the driving signals of the circuit as shown in  FIG. 2(   a ). The driving signals include V Tgs1 , V Tgs2  and V Tgs3  applied to respective gates of Q T1 , Q T2  and Q T3  and V Fgs1 , V Fgs2  and V Fgs3  applied to respective gates of Q F1 , Q F2  and Q F3 . 
       FIG. 3(   a ) shows a dynamic state diagram of a photovoltaic system having photovoltaic burp chargers, it is when BPP charges to the battery B 1  during a time period of tp 1 , and B 2  and B 3  are at rest. 
       FIG. 3(   b ) shows a dynamic state diagram of a photovoltaic system having photovoltaic burp chargers, it is when the positive pulse (PP) charges to the battery B 2  during a time period of tp 2 , and is temporarily stopped during a time period of td, and B 1  engages in an intense charge to B 3  during a time period of tp 3 , which equals to a BNP charge. 
       FIG. 3(   c ) shows a dynamic state diagram of a photovoltaic system having photovoltaic burp chargers, it is when B 1  and B 3  are at rest during a time period of td, wherein B 2  engages in the PP charge from t 2  to t 3 , then to tp 2  and until the end of a charging period of Ts 2  (see  FIG. 2(   c )). 
       FIG. 4(   a ) shows a circuit diagram of the equivalent half-circuit of IFC- 1 , in which the inner resistance r Lm  of the transformer is neglected and all parameters of the two flyback converters are presumed the same to facilitate analysis.  FIG. 4(   b ) predicts the primary current i pI,2  drawn from the PV array and the secondary current i sI,2  that charges the battery. The peak current î p1 =î pv1  of the PV array through a single flyback converter of IFC- 1  can be given by 
     
       
         
           
             
               
                 
                   
                     
                       i 
                       ^ 
                     
                     
                       pv 
                        
                       
                           
                       
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                       1 
                     
                   
                   = 
                   
                     
                       
                         V 
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                         L 
                         m 
                       
                     
                      
                     
                       t 
                       on 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The average current I pv1  in the drawing period T s1  is obtained as 
     
       
         
           
             
               
                 
                   
                     I 
                     
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                       1 
                     
                   
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                         d 
                         2 
                       
                     
                     
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                         L 
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                        
                       
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                           s 
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                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where d is duty cycle, L m  is magnetizing inductance, and f s1  is switching frequency. If the characteristics of the two flyback converters of IFC- 1  are presumed the same, I pv1  and I pv2  from PV array are equal and the total average PV current I pv  from (2) will be, 
     
       
         
           
             
               
                 
                   
                     I 
                     pv 
                   
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                         V 
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                        
                       
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                         L 
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                   ( 
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                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Where V o =V B , P o =I s V B  and P pv =I pv V pv . The output current I s  and power P o  can then be obtained from (3) and (4) as 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       s 
                     
                     = 
                     
                       
                         
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                             ( 
                             
                               
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                   ( 
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                     P 
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                   ( 
                   6 
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     From Eq. (5), the control-to-output transfer function between IFC- 1  and PV array can be represented by 
     
       
         
           
             
               
                 
                   
                     
                       I 
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                         s 
                          
                         
                             
                         
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                         1 
                       
                       
                         - 
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                                 pv 
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                              
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                         L 
                         
                           m 
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                   ( 
                   7 
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     The circuit model of the control-to-output (from the PV array to the charging current) is shown in  FIG. 4(   c ), in which the output current I s  is inversely proportional to the control frequency f s1 , which is also designed to suit the tracking chart of  FIG. 8 . The simulation and experiment (from design example) for the control-to-output model are shown in  FIGS. 5(   a )-( b ), in which the output current and output power versus switching frequency f s1  are measured for various solar insolations. However, the impedance of the PV array is inherently capacitive, because of the diffusion and transition capacitances at high frequency. For attaining maximum I s , the conjugate impedance of the PV array should be equal to the inductive impedance of the IFC- 1 , i.e., 
     
       
         
           
             
               
                 
                   
                     
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                   Where 
                 
               
               
                 
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                   ( 
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                        
                       
                         C 
                         pv 
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Since the PV array and the IFC- 1  are frequency-dependent, the INC MPPT using frequency control is feasible for guiding the IFC- 1  in energy pump. If the internal resistances between the PV array and the IFC- 1  are neglected for analysis, the L m C pv  relative to the switching frequency f s1  at maximum power transfer can then be represented by 
     
       
         
           
             
               
                 
                   
                     
                       L 
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                      
                     
                       C 
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                   = 
                   
                     
                       ( 
                       
                         1 
                         
                           2 
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                           π 
                            
                           
                               
                           
                            
                           
                             f 
                             
                               s 
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                       ) 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The period T s2  of the transmission gates Q T1  and Q T2  can be estimated as 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           T 
                           
                             s 
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                          
                         
                           
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                             ( 
                             
                               
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                             T 
                             
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                   ( 
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                   ) 
                 
               
             
           
         
       
     
     Where 
     t p1 =m 1 T s1 , t d1 =m 2 T s1 , t p1 =t d2 , t p2 =t d1 , and t p3 =m 3 T s3 . 
     For ease of analysis and synthesis of the charging currents, the instantaneous current i p1  and i p2  from PV array are redefined as 
     
       
         
           
             
               
                 
                   
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                     { 
                     
                       
                         
                           
                             
                               
                                 
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                                   s 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               &lt; 
                               t 
                               &lt; 
                               
                                 T 
                                 
                                   s 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                             
                           
                         
                       
                        
                       
                         
 
                       
                        
                       and 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
             
               
                 
                   
                     i 
                     
                       p 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               
                                 V 
                                 pv 
                               
                               
                                 L 
                                 m 
                               
                             
                              
                             
                               ( 
                               
                                 t 
                                 - 
                                 
                                   
                                     T 
                                     
                                       s 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                         
                           
                             
                               
                                 T 
                                 
                                   s 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                               2 
                             
                             &lt; 
                             t 
                             &lt; 
                             
                               ( 
                               
                                 
                                   
                                     T 
                                     
                                       s 
                                        
                                       
                                           
                                       
                                        
                                       1 
                                     
                                   
                                   2 
                                 
                                 + 
                                 
                                   dT 
                                   
                                     s 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                       
                         
                           0 
                         
                         
                           Otherwise 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     The two average currents i s1  and i s2  from IFC- 1  before synthesis can be given by, from (5), 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       s 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   = 
                   
                     
                       I 
                       
                         s 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     = 
                     
                       
                         
                           η 
                            
                           
                             ( 
                             
                               
                                 V 
                                 pv 
                               
                                
                               d 
                             
                             ) 
                           
                         
                         2 
                       
                       
                         2 
                          
                         
                           V 
                           B 
                         
                          
                         
                           L 
                           m 
                         
                          
                         
                           f 
                           
                             s 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     The windowed pulse train current i B1  for B 1  flows when Q T1  turns on in the interval t p1 =m 1 T s1 .  FIG. 6  shows a waveform diagram for predicting dynamic state of a photovoltaic system having photovoltaic burp chargers according to the preferred embodiment of the present invention. The average i B1  in the low-frequency charging period, T s2 , as displayed in  FIG. 6 , can be represented by 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       B 
                        
                       
                           
                       
                        
                       1 
                     
                   
                   = 
                   
                     
                       
                         m 
                         1 
                       
                       
                         
                           m 
                           1 
                         
                         + 
                         
                           m 
                           2 
                         
                       
                     
                     · 
                     
                       
                         
                           η 
                            
                           
                             ( 
                             
                               
                                 V 
                                 pv 
                               
                                
                               d 
                             
                             ) 
                           
                         
                         2 
                       
                       
                         
                           V 
                           
                             B 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                          
                         
                           L 
                           m 
                         
                          
                         
                           f 
                           
                             
                               s 
                                
                               
                                   
                               
                                
                               1 
                             
                              
                             
                                 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     When Q T2 , complementary to Q T1 , turns on in the interval t p2 =m 2 T s1 , the average current i B2  that charges B 2  is obtained as, 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       B 
                        
                       
                           
                       
                        
                       2 
                     
                   
                   = 
                   
                     
                       
                         m 
                         2 
                       
                       
                         
                           m 
                           1 
                         
                         + 
                         
                           m 
                           2 
                         
                       
                     
                     · 
                     
                       
                         
                           η 
                            
                           
                             ( 
                             
                               
                                 V 
                                 pv 
                               
                                
                               d 
                             
                             ) 
                           
                         
                         2 
                       
                       
                         
                           V 
                           
                             B 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                          
                         
                           L 
                           m 
                         
                          
                         
                           f 
                           
                             
                               s 
                                
                               
                                   
                               
                                
                               1 
                             
                              
                             
                                 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     Then, the average intense discharging current i B1,d  from the B 1  through IFC- 2  in the interval t p3 =m 3 T s1 , equivalent to the average charging current of the B 3 , is given by 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       
                         B 
                          
                         
                             
                         
                          
                         1 
                       
                       , 
                       d 
                     
                   
                   = 
                   
                     
                       
                         V 
                         
                           B 
                            
                           
                               
                           
                            
                           1 
                         
                       
                        
                       
                         d 
                         2 
                       
                     
                     
                       
                         L 
                         m 
                       
                        
                       
                         f 
                         
                           s 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
           
         
       
     
     where the peak discharging current from B 1  is given by 
     
       
         
           
             
               
                 
                   
                     
                       i 
                       ^ 
                     
                     
                       
                         B 
                          
                         
                             
                         
                          
                         1 
                       
                       , 
                       d 
                     
                   
                   = 
                   
                     
                       
                         V 
                         
                           B 
                            
                           
                               
                           
                            
                           1 
                         
                       
                       
                         L 
                         m 
                       
                     
                      
                     
                       dT 
                       
                         s 
                          
                         
                             
                         
                          
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     For ease of analysis, all parameters of IFC- 2  are presumed to be identical to those of IFC- 1 ; the dynamic states of IFC- 2  are the same as those in  FIG. 4(   b ). From Eq. (18), the average discharging current I p3  is 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       p 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   = 
                   
                     
                       
                         m 
                         3 
                       
                       
                         
                           m 
                           1 
                         
                         + 
                         
                           m 
                           2 
                         
                       
                     
                     · 
                     
                       
                         
                           V 
                           
                             B 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                          
                         
                           d 
                           2 
                         
                       
                       
                         
                           L 
                           m 
                         
                          
                         
                           f 
                           
                             s 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
           
         
       
     
     Accordingly, the average current I s3  that charges the B 3  through IFC- 2  in each charging period T s  is then given by 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       s 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   = 
                   
                     
                       
                         η 
                          
                         
                           ( 
                           
                             
                               V 
                               
                                 B 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                              
                             d 
                           
                           ) 
                         
                       
                       2 
                     
                     
                       
                         V 
                         
                           B 
                            
                           
                               
                           
                            
                           3 
                         
                       
                        
                       
                         L 
                         m 
                       
                        
                       
                         f 
                         
                           s 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
           
         
       
     
     and the average charging current that charges the B 3  is to be 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       B 
                        
                       
                           
                       
                        
                       3 
                     
                   
                   = 
                   
                     
                       
                         m 
                         3 
                       
                       
                         
                           m 
                           1 
                         
                         + 
                         
                           m 
                           2 
                         
                       
                     
                     · 
                     
                       
                         
                           η 
                            
                           
                             ( 
                             
                               
                                 V 
                                 
                                   B 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                               
                                
                               d 
                             
                             ) 
                           
                         
                         2 
                       
                       
                         
                           V 
                           
                             B 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                          
                         
                           L 
                           m 
                         
                          
                         
                           f 
                           
                             
                               s 
                                
                               
                                   
                               
                                
                               1 
                             
                              
                             
                                 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
           
         
       
     
       FIG. 7(   a ) shows a circuit diagram of an equivalent circuit of a typical solar cell unit, wherein D is an LED, Rsh is a parallel-connected inner resistor, Rs is a series-connected inner resistor, and Iph is an output current of the solar cell unit. 
     Via a principle that a rate of change of an output power with respect to a voltage of a solar panel is zero at an MPPT, and at a place corresponding to dP/dV=0 on the current-voltage characteristic curve, e.g. as shown in  FIG. 7(   b ), the incremental conductance method directly finds out 
     
       
         
           
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         I 
                       
                       
                         Δ 
                          
                         
                             
                         
                          
                         V 
                       
                     
                     = 
                     
                       - 
                       
                         I 
                         V 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   23 
                   ) 
                 
               
             
           
         
       
     
     where I is a solar cell current, V is a solar cell voltage, ΔV is a voltage increment, and ΔI is a current increment. Via measuring a conductance value of ΔI/ΔV and compared it with an instantaneous conductance of −I/V of the solar panel to judge whether ΔI/ΔV is larger than, smaller than, or equivalent to −I/V so as to determine whether the next incremental change should be continued. When the incremental conductance conforms to formula (23), the solar panel is for sure to be operated at a maximum power point (MPP), and there will be no more next increment. This method engages in a tracking via the modification of the logic expression, there is not any oscillation around the MPP such that it is more suitable to the constantly changing conditions of the atmosphere. The incremental conductance method can accomplish the MPPT more accurately and decrease the oscillation problem as in the perturbation and observation method. 
     According to  FIG. 7(   a ), the current-voltage characteristic of the solar cell unit can be indicated as 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       pv 
                     
                     = 
                     
                       
                         I 
                         
                           p 
                            
                           
                               
                           
                            
                           h 
                         
                       
                       - 
                       
                         
                           I 
                           pvo 
                         
                          
                         
                           { 
                           
                             
                               exp 
                                
                               
                                 [ 
                                 
                                   
                                     q 
                                     AkT 
                                   
                                    
                                   
                                     ( 
                                     
                                       
                                         V 
                                         pv 
                                       
                                       + 
                                       
                                         
                                           I 
                                           pv 
                                         
                                          
                                         
                                           R 
                                           s 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                                 ] 
                               
                             
                             - 
                             1 
                           
                           } 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   ( 
                   24 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     pv 
                   
                   = 
                   
                     
                       
                         AkT 
                         q 
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           
                             
                               
                                 I 
                                 
                                   p 
                                    
                                   
                                       
                                   
                                    
                                   h 
                                 
                               
                               - 
                               
                                 I 
                                 pv 
                               
                               + 
                               
                                 I 
                                 pvo 
                               
                             
                             
                               I 
                               
                                 pvo 
                                  
                                 
                                     
                                 
                               
                             
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         I 
                         pv 
                       
                        
                       
                         R 
                         s 
                       
                     
                   
                 
               
               
                 
                   ( 
                   25 
                   ) 
                 
               
             
           
         
       
     
     where I ph  denotes light-generated current; I pvo  is dark saturation current; I pv  is PV electric current; V pv  is PV voltage; R s  is series resistance; A is the non-ideality factor; k is Boltzmann&#39;s constant; T is temperature, and q is the electronic charge. The output power from the PV cell can then be given by 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           P 
                           pv 
                         
                         = 
                           
                          
                         
                           
                             V 
                             pv 
                           
                            
                           
                             I 
                             pv 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             I 
                             pv 
                           
                            
                           
                             { 
                             
                               
                                 
                                   AkT 
                                   q 
                                 
                                  
                                 
                                   ln 
                                    
                                   
                                     ( 
                                     
                                       
                                         
                                           I 
                                           
                                             p 
                                              
                                             
                                                 
                                             
                                              
                                             h 
                                           
                                         
                                         - 
                                         
                                           I 
                                           pv 
                                         
                                         + 
                                         
                                           I 
                                           pvo 
                                         
                                       
                                       
                                         I 
                                         pvo 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               - 
                               
                                 
                                   I 
                                   pv 
                                 
                                  
                                 
                                   R 
                                   s 
                                 
                               
                             
                             } 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   26 
                   ) 
                 
               
             
           
         
       
     
     The PV array operating at MPP is when 
     
       
         
           
             
               
                 
                   
                     
                       
                          
                         
                           P 
                           pv 
                         
                       
                       
                          
                         
                           V 
                           pv 
                         
                       
                     
                     = 
                     0 
                   
                    
                   
                     
 
                   
                    
                   or 
                 
               
               
                 
                   ( 
                   27 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                              
                             
                               P 
                               pv 
                             
                           
                           
                              
                             
                               V 
                               pv 
                             
                           
                         
                         = 
                           
                          
                         
                           
                             
                               V 
                               pv 
                             
                              
                             
                               
                                  
                                 
                                   I 
                                   pv 
                                 
                               
                               
                                  
                                 
                                   V 
                                   pv 
                                 
                               
                             
                           
                           + 
                           
                             I 
                             pv 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   28 
                   ) 
                 
               
             
           
         
       
     
     As for the INC MPPT, the criterion can then be given by, from (28), 
     
       
         
           
             
               
                 
                   
                     
                        
                       
                         I 
                         pv 
                       
                     
                     
                        
                       
                         V 
                         pv 
                       
                     
                   
                   = 
                   
                     - 
                     
                       
                         I 
                         pv 
                       
                       
                         
                           V 
                           pv 
                         
                          
                         
                             
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   29 
                   ) 
                 
               
             
           
         
       
     
     In reality, an alternative expression to replace the derivative in (29) is frequently used for ease of calculation in the algorithm, i.e. 
     
       
         
           
             
               
                 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           I 
                           pv 
                         
                       
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           V 
                           pv 
                         
                       
                     
                     ≈ 
                     
                       
                          
                         
                           I 
                           pv 
                         
                       
                       
                          
                         
                           V 
                           pv 
                         
                       
                     
                   
                   = 
                   
                     - 
                     
                       
                         I 
                         pv 
                       
                       
                         V 
                         pv 
                       
                     
                   
                 
               
               
                 
                   ( 
                   30 
                   ) 
                 
               
             
           
         
       
     
     Design Considerations 
     1. Charge Management 
     As presented in  FIG. 1(   b ), two complementary transmission gates Q T1  and Q T2  involved in charging the B 1  and B 2  are for energy-treasuring. A third transmission gate Q T3  introduced to intensely discharge the B 1  through IFC- 2  to charge B 3 , equivalent to BNP charging, is for energy recovery. In this design of the present invention, the BPP charge to B 1  via Q T1  is programmed using such that 80% of the burp period; 10% is associated with a BNP for intensely discharging B 1  to B 3  via Q T3 , and 10% is the relaxation period. B 2  accepts a windowed PP for charging during 20% of the burp period via Q T2  excluding the 80% for Q T1 , which ensures the continuity of INC MPPT and increases the utilization of the PV array. Three gassing voltage detections are always on-line monitoring the instant charging behaviors of the batteries (KAWASAKI NF50B24LS 45-AH battery) with a gassing reference of 13.8V. 
     2. Interleaved IFC 
     IFC- 1  is designed according to the tracking of INC MPPT with VFCD control and IFC- 2  for B 1  executing BNP discharging to B 3  can be either single or interleaved flyback converter using constant-frequency control. Moreover, the two IFCs are designed to operate in discontinuous-conduction mode (DCM) to avoid overlap between adjacent tiny pulses, to reduce the sulfating crystallization on the positive electrode of LAB. 
     3. Algorithm of INC MPPT 
     The algorithm of INC MPPT is executed by Microchip dsPIC33FJ06GS202 according to the flowchart in  FIG. 9 . The tracking chart in  FIG. 8  is the primary reference for the algorithm. 
     Design and Experiment 
     An experimental setup of a PV burp charger system is established with the circuit structure in  FIG. 2(   a ), which equips with a 260-W two-PV-in-series module and three 45-AH LABs of KAWASAKI NF50B24LS. The charger provides maximum peak current of 24 A for the tiny pulse train @ 1 kW/m 2  and a 38-A peak current for the intense-discharge pulse, where the IFC- 1  and IFC- 2  with a duty ratio of 0.26 operate at 14.6 kHz, and the transmission gates Q T1 , Q T2 , and Q T3  operate as suggested 550 Hz. The interval of BPP, t p1 , for the B 1  is designed to be 80% of T s2 (=1/f s2 ) and that of PP, t p2 , for B 2  is 20% of T s2 . For the BNP equivalent to intense discharge from the B 1  is 10% of T s2 . Each PV module of Kyocera KC130T has an open voltage of 21.9V and a short current of 8.2 A @ 1 kW/m 2 .  FIG. 7(   b ) presents I pv -V pv  and P pv -V pv  characteristic curves of the PV array at T=25° C. (solid line) and 55° C. (dotted line) for various solar insolations. 
       FIG. 10(   a ) shows a waveform diagram of the gate source voltages V Fgs1  and V Fgs2  and the primary side currents i p1  and i p2  of the first interleaved flyback converter IFC- 1  according to the preferred embodiment of the present invention.  FIG. 10(   b ) shows a waveform diagram of the gate source voltage V Fgs1  and V Fgs2 , and the secondary side currents i s1  and i s2  of the first interleaved flyback converter IFC- 1  according to the preferred embodiment of the present invention. 
       FIG. 11(   a ) shows measured gate drive signals V Tgs1  and V Tgs2  for Q T1  and Q T2  at a low frequency of 550 Hz, and V Fgs1 , V Fgs2  and V Fgs3  for Q F1 , Q F2 , and Q F3  at a high frequency of 19 kHz according to the preferred embodiment of the present invention.  FIG. 11(   b ) shows the measured gate drive signals V Tgs1  and V Tgs2  for Q T1  and Q T2 , the charge current i B1  for B 1  and the charge current i B2  for B 2  according to the preferred embodiment of the present invention.  FIG. 11(   c ) shows the measured gate drive signals V Tgs1  and V Tgs2  for Q T1  and Q T2 , and the burp charging currents i B1  and i p3  according to the preferred embodiment of the present invention. 
       FIG. 12(   a ) shows trajectories of the proposed PV burp charger presenting the state-of-charge (SOC) of the three batteries in the process of PV burp charge, in which the charging process excluding the burp charge to B 1  also contributes pulse charges to B 2  and B 3  in the non-burp charge period. 
       FIG. 12(   b ) compares the charging and temperature trajectories of the proposed PV burp charger with those obtained using the CC/CV charging strategy, at an average charging rate of 0.2 C, where 1 C=45AH, under solar insolation of 1 kW/m 2 . To reach 85% SOC, it takes 85 minutes for burp charge and 105 minutes for CC/CV charge to the same battery, which clearly reveals shorter charging time of burp charge than that of CC/CV charge about 20%. Besides, there causes low heating for the B 1  using burp charge in comparison to that using the CC/CV charge around 2° C., measured at environmental temperature of 20° C. The experiment successfully validates the performance of the PV system having the burp chargers that can provide rapid charging and low heating to the battery for benefiting the prolongation of the battery life. 
     EMBODIMENTS 
     1. A photovoltaic system, comprising: 
     a first charger generating a first pulse train; 
     a first battery receiving the first pulse train at a first time period to engage in a first charge, and engaged in an intense discharge at an initial stage of a second time period so as to generate a second pulse train; 
     a second battery receiving the first pulse train during the second time period to engage in a second charge; 
     a third battery engaged in a third charge via the second pulse train during the initial stage; and 
     a charge management controller controlling the first charge and the intense discharge of the first battery, the second charge of the second battery and the third charge of the third battery. 
     2. A system according to Embodiment 1 further comprising a maximum power point tracking (MPPT) controller and a photovoltaic (PV) array, wherein the MPPT controller is electrically connected to the PV array to cause the PV array to engage in an MPPT, the PV array is electrically connected to the first charger, the first charger is an interleaved flyback converter and includes a first flyback converter and a second flyback converter, the first flyback converter is electrically connected to the second flyback converter in parallel, the first and the second flyback converters generate a third pulse train and a fourth pulse train respectively, and the third and the fourth pulse trains are synthesized to generate the first pulse train. 
     3. A system according to Embodiment 2 or 3 further comprising a first, a second and a third transmission switches and a second charger, wherein the second charger is electrically connected between the third battery and the third transmission switch, is used to receive the second pulse train and generates a charging pulse train charging the third battery, the first transmission switch is electrically connected between the first charger and the first battery, the second transmission switch is electrically connected between the first charger and the second battery, the third transmission switch is electrically connected to the first battery, and the charge management controller generates a first, a second, and a third control signals controlling a turn-on and a turn-off of the first to the third transmission switches respectively, and controlling when the first battery engages in the first charge and the intense discharge, when the second battery engages in the second charge, and when the third battery engages in the third charge. 
     4. A system according to anyone of the above-mentioned Embodiments, wherein the first, the second and the third batteries generate a first, a second and a third gassing voltage detection values respectively, the first, the second and the third transmission switches include respective gates, the charge management controller includes a transmission gate controller and a gassing voltage monitor, the transmission gate controller outputs the first, the second and the third control signals to the respective gates to control the turn-on and the turn-off of the first, the second and the third transmission switches, the gassing voltage monitor receives the first, the second and the third gassing voltage detection values and outputs respective enable signals to the transmission gate controller and the MPPT controller. 
     5. A system according to anyone of the above-mentioned embodiments, wherein the transmission gate controller engages in a normal operation and the MPPT controller engages in an MPPT when all the first, the second and the third gassing voltage detection values are not reaching a gassing voltage value, the transmission gate controller ceases the normal operation and the MPPT controller ceases the MPPT when at least one of the first, the second and the third gassing voltage detection values reaches the gassing voltage value, and the MPPT controller is an incremental-conductance (INC) MPPT controller. 
     6. A system according to anyone of the above-mentioned embodiments, further comprising a pulse-width modulation (PWM) controller, wherein the PWM controller uses a variable frequency constant duty control and outputs a PWM signal to the first charger, the first charger has a charging period including the first time period, the second time period and a brief break time period, and the first charge is a burp charge. 
     7. A photovoltaic system, comprising: 
     a first battery receiving a first pulse train at a first time period to engage in a first charge, and engaged in an intense discharge at an initial stage of a second time period so as to generate a second pulse train; and 
     a second battery engaged in a second charge via the second pulse train during the initial stage of the second time period. 
     8. A photovoltaic system according to Embodiment 7 further comprising: 
     a first charger generating the first pulse train; 
     a third battery receiving the first pulse train during the second time period to engage in a third charge; and 
     a charge management controller controlling the first charge and the intense discharge of the first battery, the second charge of the second battery and the third charge of the third battery. 
     9. A system according to Embodiment 7 or 8 further comprising a maximum power point tracking (MPPT) controller and a photovoltaic (PV) array, wherein the MPPT controller is electrically connected to the PV array to cause the PV array to engage in an MPPT, the PV array is electrically connected to the first charger, the first charger is an interleaved flyback converter and includes a first flyback converter and a second flyback converter, the first flyback converter is electrically connected to the second flyback converter in parallel, the first and the second flyback converters generate a third pulse train and a fourth pulse train respectively, and the third and the fourth pulse trains are synthesized to generate the first pulse train. 
     10. A system according to anyone of the above-mentioned embodiments, further comprising a first, a second and a third transmission switches and a second charger, wherein the second charger is electrically connected between the second battery and the second transmission switch, is used to receive the second pulse train and generates a charging pulse train charging the second battery, the first transmission switch is electrically connected between the first charger and the first battery, the third transmission switch is electrically connected between the first charger and the third battery, the second transmission switch is electrically connected to the first battery, and the charge management controller generates a first, a second, and a third control signals controlling a turn-on and a turn-off of the first to the third transmission switches, and controlling when the first battery engages in the first charge and the intense discharge, when the second battery engages in the second charge, and when the third battery engages in the third charge. 
     11. A system according to anyone of the above-mentioned embodiments, wherein the first, the second and the third batteries generate a first, a second and a third gassing voltage detection values respectively, the first, the second and the third transmission switches include respective gates, the charge management controller includes a transmission gate controller and a gassing voltage monitor, the transmission gate controller outputs the first, the second and the third control signals to the respective gates to control the turn-on and the turn-off of the first, the second and the third transmission switches, the gassing voltage monitor receives the first, the second and the third gassing voltage detection values and outputs respective enable signals to the transmission gate controller and the MPPT controller. 
     12. A system according to anyone of the above-mentioned embodiments, wherein the transmission gate controller engages in a normal operation and the MPPT controller engages in an MPPT when the first, the second and the third gassing voltage detection values are all not reaching a gassing voltage value, the transmission gate controller ceases the normal operation and the MPPT controller ceases the MPPT when at least one of the first, the second and the third gassing voltage detection values reaches the gassing voltage value, and the MPPT controller is an incremental-conductance (INC) MPPT controller. 
     13. A system according to anyone of the above-mentioned embodiments, further comprising a pulse-width modulation (PWM) controller, wherein the PWM controller uses a variable frequency constant duty control and outputs a PWM signal to the first charger, the first charger has a charging period including the first time period, the second time period and a brief break time period, and the first charge is a burp charge. 
     14. A charging method for a photovoltaic system, comprising steps of: 
     providing a first pulse train; 
     receiving the first pulse train at a first time period to engage in a first charge towards a first battery; and 
     causing the first battery to engage in an intense discharge at an initial stage of a second time period to generate a second pulse train so as to engage in a second charge towards a second battery. 
     15. A method according to Embodiment 14, further comprising steps of: 
     providing a third battery and a first charger generating the first pulse train; and 
     causing the third battery to engage in a third charge via the second pulse train during the initial stage of the second time period. 
     16. A method according to Embodiment 14 or 15, wherein the photovoltaic system comprises a controller and a second charger, the method further comprising steps of: 
     controlling the first charge and the intense discharge of the first battery, the second charge of the second battery and the third charge of the third battery; and 
     causing the second charger to receive the second pulse train and to output a third pulse train so as to charge the third battery. 
     According to the aforementioned descriptions, the present invention provides a photovoltaic burp charger and charging method thereof, the photovoltaic burp charger includes a charge management controller used to engage in a photovoltaic burp charge and two pulse charges in a main battery and two auxiliary batteries respectively so as to prolong the life-cycle of the battery and realizing the concept of energy treasuring and recovery so as to possess the non-obviousness and the novelty. 
     While the present invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention need not be restricted to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. Therefore, the above description and illustration should not be taken as limiting the scope of the present invention which is defined by the appended claims.