Patent Publication Number: US-2011058396-A1

Title: Flyback-type inverter circuit for network supply or for network-independent operation

Description:
The invention relates to an inverter circuit according to the preamble of claim  1  or  2 . 
     There are various inverter techniques for feeding solar power into the network and for operation independently of the network. This is described briefly in the following: 
     For feeding into the network the input direct voltage is converted into alternating voltage impulses with variable pulse width, so that in a downstream connected filter a sinusoidal current is formed, which flows into the network. Owing to the lower weight and improved efficiency (95-97% instead of 92-94%) more and more inverters are being produced without a network transformer. 
     Inverters with r.f. transformers are also very light but their efficiency is only in the region of 92-94%. A particular feature of all inverters with r.f.-separation is that they are complex in design, have several r.f. stages and temporarily store the energy in the magnetic field of the transformer core, the electric field of storage capacitors and also again in filter stages. 
     Thus the objective of the invention is to create an inverter with galvanic separation, which only has one r.f. converter and temporarily stores the energy in the magnetic field of the r.f. fly-back transformer only once. 
     An inverter is used to achieve this objective which comprises the features of claim  1  or  2 . 
     In this way it is achieved that the efficiency of the inverter according to the invention, as with the inverters without transformers, is at 95 to 97%, and due to the small number of components necessary for the functioning of the inverter the weight is much lower than in known inverters. 
    
    
     
       The invention is explained in more detail in the following with reference to a drawing. 
         FIG. 1  shows an inverter circuit according to the invention for feeding current into the network; 
         FIG. 2  shows a modification of the inverter circuit according to the invention of  FIG. 1  for operation independently of the network; and 
         FIG. 3  shows the voltage curve of the circuits according to  FIGS. 1 and 2  at the output of the primary side and the secondary side. 
     
    
    
     It should be noted that in the figures the same parts are denoted by the same reference numbers. 
       FIG. 1  shows a first embodiment of the invention for feeding electricity into the network, whereby the electricity can be generated for example by the sun or by wind power. 
     At the input of the inverter is the input voltage Ue with its plus terminal connected to a first side of two parallel connected fly-back transformers Tr 1  and Tr 2 , the other sides of which are connected via semiconductor switches T 1  and T 2  to the minus terminal. Between plus and minus there is also connected a capacitor C 1 , which assists with charging the fly-back transformers Tr 1  and Tr 2 . The circuit also consists of a primary side P and a secondary side S. 
     On switching on T 1  a current value is applied to the fly-back transformer Tr 1  and thus magnetic energy is loaded onto the transformer core, which corresponds to 0.5*I 2 *L. In this case I is the supplied electricity and L the inductance of the respective fly-back transformer Tr 1  or Tr 2 . 
     After switching off T 1  the magnetic energy converts into electricity, which flows into the output capacitor C 2  in the secondary side S via a diode D 1  and an active network rectifier, consisting of a T 3 -T 6 . During the discharge phase of Tr 1  the fly-back transformer Tr 2  is charged and discharges via a diode D 2  and the active network rectifier T 3  to T 6  into the output capacitor C 2 . Tr 1  and Tr 2  operate anticyclically and supply an almost continuous current into the output capacitor C 2 . The power received by the fly-back transformers is P in =0.5*I 2 *L*f. Here P in  denotes the input power or the fly-back transformers Tr 1  and Tr 2 , I denotes the current, L the inductance and f the frequency. 
       FIG. 2  shows an inverter circuit for operation independently of the network, in which in a modification of the circuit according to  FIG. 1  in the primary side P the semiconductor switches T 1  and T 2  are bridged by diodes D 3  and D 4  and in the secondary side S the diodes D 1  and D 2  are bridged by semiconductors switches T 7  and T 8 . Furthermore, the capacitor C 2  in the secondary side S is not connected at the output, but between the fly-back transformers Tr 1 ′, Tr 2 ′ and the active network rectifier T 3 -T 6 . However, it should be noted that the capacitor C 2  can also be connected at the output, as indicated in  FIG. 2  by C 2 ′ by dotted lines. 
     At the input of the inverter, on switching on T 1  a current value is applied to the fly-back transformer Tr 1  and thus magnetic energy is charged onto the transformer core, which corresponds to 0.5*I 2 *L. After switching off T 1  the magnetic energy converts into electricity, which flows into the capacitor C 2  and thereby transfers the energy 0.5*U 2 *C. The voltage curve at C 2  can be controlled sinusoidally by the amount of energy portions supplied by the fly-back transformer. Depending on the load conditions at the capacitor C 2  also the energy content charged to the fly-back transformer can be varied by varying the applied current. 
     The fly-back transformer Tr 2  is controlled by T 2  anticyclically in relation to the fly-back transformer Tr 1  and transmits the current via D 2  into the output capacitor C 2 . In order to obtain a sinusoidal curve of the voltage half-waves at C 2  with rising and falling voltage under variable load conditions, the fly-back transformers Tr 1  and Tr 2  are operated forwards for rising voltage and backwards for falling voltage at the capacitor C 2 . 
     During the backwards operation for the primary-side fly-back transformer Tr 1  on the primary side the diode D 3  and on the secondary side the transistor T 7  become active and for the primary-side fly-back transformer Tr 2  the primary-side diode D 4  and the secondary side transistor T 8 . 
     The following active network rectifier or polarity switch, which is formed by the semiconductors switches T 3 -T 6  with bridging diodes D 5 -D 8  for backwards operation, switches every second half-wave to negative voltage and thus produces the complete sinusoidal curve. 
       FIG. 3  shows the curve of the voltage Uc at the capacitor C 2  in the circuit according to  FIG. 2  as a function of time t. Two positive sinusoidal half waves are shown, the second of which is flipped over by the polarity switches T 3 -T 6  to form a complete sinus wave downwards in the direction—Ua.