# Patent

## Title
Transformer-isolated switching converter

## Abstract

Exemplary systems and methods provide a transformer-isolated current-fed quadratic full-bridge converter topology. The optimal interfacing of a current source, such as a solar panel, can be implemented by using current-fed converters. The current-fed converter can operate within the whole range of a UI curve from short-circuit to open-circuit condition and its input voltage can be readily controlled. The quadratic behaviour between input and output in regard of a duty cycle allows large conversion ratios.

## Background
RELATED APPLICATION
This application claims priority under 35 U.S.C. §119 to European Patent Application No. 10173167.7 filed in Europe on Aug. 18, 2010, the entire content of which is hereby incorporated by reference in its entirety.
FIELD
The present invention relates to switching converters, such as converters interfacing solar generators.
BACKGROUND INFORMATION
New forms of power generation and storage have introduced a need for ways to interface with them. One interfacing method, such as solar power generation, converts solar power into a more useable form.
A solar panel can include an array of series/parallel-connected solar cells. The voltage produced by the solar panel can be too low to be used directly in an application for supplying electricity to the grid. On the other hand, the voltage can be too high to be fed directly to a battery or array of batteries. In addition, the power output of a solar panel can depend greatly on the lighting conditions. As a result, the output power of a solar panel can fluctuate rapidly.
Therefore, implementation of modular solar energy systems where solar panels or a group of panels are directly connected to a low voltage or a high voltage bus usually specifies the use of a maximum-power-point-tracking (MPPT) converter with a large conversion ratio. The converter converts the voltage produced by the solar panels into another voltage which is more suitable for the application. A conventional way to interface with solar panels is to use a voltage-fed switching converter.
A disadvantage associated with the above arrangement is due to its limited operational range. The application may have to operate under conditions specifying a greater operational range. The solar systems are also known to be sensitive to common mode currents, which can reduce the useful life of the solar cells and cause human safety issues when using conventional converters.
SUMMARY
An exemplary switching converter is disclosed. The switching converter comprising, a first input terminal and a second input terminal, both for connecting to a current source; a first output terminal and a second output terminal, that are both connected to a load; a transformer with a primary winding, a secondary winding and a galvanic isolation between the windings, wherein the primary winding and the secondary winding include two terminals; a primary side first capacitor connected between the first input terminal and the second input terminal; a primary side first inductor with two terminals, connected from its first terminal to the first input terminal; a primary side second inductor with two terminals; a primary side first rectifying device connected between the primary side first inductor second terminal and the primary side second inductor second terminal, thus allowing a flow of current from the primary side first inductor second terminal to the primary side second inductor second terminal; a primary side second rectifying device connected between the primary side first inductor second terminal and the primary side second inductor first terminal, thus allowing a flow of current from the primary side first inductor second terminal to the primary side second inductor first terminal; a primary side second capacitor connected between the second inductor first terminal and the second input terminal; a first switching device connected between the primary side second inductor second terminal and the transformer primary winding first terminal and adapted to be able to set the connection between the primary side second inductor second terminal and the transformer primary winding first terminal to a conducting and a non-conducting state; a second switching device connected between the primary side second inductor second terminal and the transformer primary winding second terminal and adapted to be able to set the connection between the primary side second inductor second terminal and the transformer primary winding second terminal to a conducting and a non-conducting state; a third switching device connected between the transformer primary winding first terminal and second input terminal and adapted to be able to set the connection between the transformer primary winding first terminal and the second input terminal to a conducting and a non-conducting state; a fourth switching device connected between the transformer primary winding second terminal and the second input terminal and adapted to be able to set the connection between the transformer primary winding second terminal and the second input terminal to a conducting and a non-conducting state; a secondary side rectifying means for rectifying a current of the transformer secondary winding; a secondary side filtering means for filtering a current between the rectifier means and a converter output; and means for controlling a voltage between the input terminals.
An exemplary switching converter is disclosed. The switching converter comprising: a first input terminal and a second input terminal, both for connecting to a current source; a first output terminal and a second output terminal, that are both connected to a load; a transformer with a primary winding, a secondary winding and a galvanic isolation between the windings, wherein each of the primary and secondary windings includes two terminals; a primary side first capacitor connected between the first input terminal and the second input terminal; a primary side first inductor connected to the first input terminal; a primary side second inductor; a primary side first rectifying device connected between a second terminal of the primary side first inductor and a second terminal of the primary side second inductor; a primary side second rectifying device connected between the second terminal of primary side first inductor and a first terminal of the primary side second inductor; a primary side second capacitor connected between a first terminal of the second inductor and the second input terminal; a first switching device connected between the second terminal of the primary side second inductor and a first terminal of the transformer primary winding; a second switching device connected between the second terminal of the primary side second inductor and a second terminal of the transformer primary winding; a third switching device connected between the first and second terminals of the transformer primary winding; a fourth switching device connected between the second terminal of the transformer primary winding and the second input terminal; a secondary side rectifying means for rectifying a current of the transformer secondary winding; and a secondary side filtering means for filtering a current between the rectifier means and a converter output.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which:
FIG. 1 illustrates a power stage of a switching converter in accordance with an exemplary embodiment;
FIG. 2 illustrates polarities of relevant currents and voltages of a power stage of a switching converter in accordance with an exemplary embodiment;
FIG. 3 illustrates a first on-time state circuit structure of a switching converter in accordance with an exemplary embodiment;
FIG. 4 illustrates an off-time state circuit structure of a switching converter in accordance with an exemplary embodiment;
FIG. 5 illustrates a second on-time state circuit structure of a switching converter in accordance with an exemplary embodiment;
FIG. 6 illustrates approximate waveforms of capacitor voltages and inductor currents in accordance with an exemplary embodiment, and
FIG. 7 illustrates a converter and solar generator arrangement in accordance with an exemplary embodiment.



DETAILED DESCRIPTION
The exemplary embodiments of the present disclosure are directed to providing a method and an apparatus to overcome the above problems of known systems.
These embodiments are based on the idea of using a transformer-isolated current-fed quadratic full-bridge converter topology. The optimal interfacing of a current source, such as a solar panel, can be implemented by using current-fed converters.
A current-fed converter can, in principle, operate from short-circuit to open-circuit condition. Its input voltage can also be controlled. The quadratic behaviour allows conversion ratios which are larger than those with a conventional converter.
High-frequency transformer isolation can also be used to solve the problems associated with the common mode currents.
FIG. 1 illustrates a power stage of a switching converter in accordance with an exemplary embodiment. The exemplary embodiment uses a transformer-isolated current-fed quadratic full-bridge converter topology.
A current-fed converter can, in principle, operate within the whole range of a UI curve of a current source from a short-circuit to an open-circuit condition, and its input voltage can be readily controlled. The quadratic behaviour between input and output in regard of a duty cycle can allow conversion ratios that are larger than those with a conventional converter.
The switching converter 1 includes a first input terminal X1 and a second input terminal X2, both of which are adapted to be connected to a current source 2. The current source 2 can be, for example, a solar panel or a group of solar panels. The current source can also be a Superconducting Magnetic Energy Storage (SMES).
The switching converter 1 also includes a first output terminal Y1, and a second output terminal Y2, both of which can be connected to a load 3. The load can be, for example, a high voltage or a low voltage DC bus.
The switching converter 1 can include a transformer T with a primary winding, a secondary winding, and a galvanic isolation between the windings, wherein the primary winding includes two terminals and the secondary winding includes two terminals. By modifying the turns ratio n between the windings of the transformer, different conversion ratios between the input and the output can be achieved. High-frequency transformer isolation can also solve the problems associated with the common mode currents.
A primary side first capacitor C2 can be connected between the first input terminal X1 and the second input terminal X2. A primary side first inductor L1 with two terminals is connected from its first terminal to the first input terminal X1. Together the first capacitor C2 and the first inductor L1 form an input EMI filter.
The switching converter 1 can also include a primary side second inductor L3 with two terminals. A primary side first rectifying device D5 is connected between the primary side first inductor L1 second terminal and the primary side second inductor L3 second terminal, thus allowing a flow of current from the primary side first inductor L1 second terminal to the primary side second inductor L3 second terminal. The primary side first and second rectifying device can be, for example, diodes or any other suitable rectifying as desired.
A primary side second rectifying device D6 is connected between the primary side first inductor L1 second terminal and the primary side second inductor L3 first terminal, thus allowing a flow of current from the primary side first inductor L1 second terminal to the primary side second inductor L3 first terminal.
A primary side second capacitor C3 is connected between the second inductor L3 first terminal and the second input terminal X2.
The converter 1 can also include four switching devices S1, S2, S3, and S4, which form a full bridge. The switching can be, for example, power MOSFETs or IGBTs or any other suitable switching device as desired. The first switching device S1 is connected between the primary side second inductor L3 second terminal and the transformer T primary winding first terminal, and can set the connection between the primary side second inductor L3 second terminal and the transformer T primary winding first terminal to a conducting and a non-conducting state.
In a similar manner, the second switching device S2 is connected between the primary side second inductor L3 second terminal and the transformer T primary winding second terminal, and can set the connection between the primary side second inductor L3 second terminal and the transformer T primary winding second terminal to a conducting and a non-conducting state.
The third switching device S3 is connected between the transformer T primary winding first terminal and the second input terminal X2 and can set the connection between the transformer T primary winding first terminal and the second input terminal X2 to a conducting and a non-conducting state.
The fourth switching device S4 is connected between the transformer T primary winding second terminal and the second input terminal X2 and can set the connection between the transformer T primary winding second terminal and second input terminal X2 to a conducting and a non-conducting state.
The transformer T secondary winding side includes a secondary side rectifying means for rectifying the current of the transformer T secondary winding, and a secondary side filtering means for filtering the rectified current.
The transformer T has two secondary winding terminals. In another exemplary embodiment, other secondary windings, for example, a secondary winding with a center tap, can also be used.
As shown in FIG. 1, a secondary side first inductor Lo with two terminals is connected from its second terminal to the first output terminal Y1. A secondary side first capacitor Co is connected between the secondary side first inductor Lo first terminal and the second output terminal Y2. Together the secondary side first inductor Lo and the secondary side first capacitor Co form an EMI filter in the output. Other filter topologies can be used in a converter in accordance with various other exemplary embodiments of the present disclosure.
The topology of the rectifier means depends on the type of the transformer T secondary winding. The converter 1 can also include four secondary side rectifying devices D1, D2, D3, and D4 as the rectifying means. The rectifying devices form a rectifying bridge, which rectifies the current from the transformer T. In FIG. 1, the rectifying devices are illustrated as diodes, and can be implemented as any other suitable rectifying device as desired.
The secondary side first rectifying device D1 is connected between the transformer T secondary winding first terminal and the secondary side first inductor Lo first terminal, thus allowing a flow of current from the transformer T secondary winding first terminal to the secondary side first inductor Lo first terminal.
The secondary side second rectifying device D2 is connected between the transformer T secondary winding second terminal and the secondary side first inductor Lo first terminal, thus allowing a flow of current from the transformer T secondary winding second terminal to the secondary side first inductor Lo first terminal.
The secondary side third rectifying device D3 is connected between the second output terminal Y2 and the transformer T secondary winding first terminal, thus allowing a flow of current from the second output terminal Y2 to the transformer T secondary winding first terminal.
The secondary side fourth rectifying device D4 is connected between the second output terminal Y2 and the transformer T secondary winding second terminal, thus allowing a flow of current from the second output terminal Y2 to the transformer T secondary winding second terminal.
The connection between the transformer T secondary winding second terminal and the secondary side second and fourth rectifying device D2 and D4 can also include a secondary side second capacitor C1. The secondary side second capacitor C1 can be connected from its first terminal to the transformer T secondary winding second terminal, which provides voltage balancing for the transformer T.
The converter 1 also includes means for controlling the voltage between the input terminals.
FIG. 2 illustrates polarities of relevant currents and voltages of a power stage of a switching converter in accordance with an exemplary embodiment. As shown in FIG. 2, the transformer T is represented in FIG. 2, and as later discussed in FIGS. 3, 4, and 5, by an ideal transformer with a magnetizing inductor LM between the terminals of the ideal transformer secondary winding.
The exemplary converters according to the present disclosure can have three operational states: a first on-time state, an off-time state, and a second on-time state.
FIG. 3 illustrates a first on-time state circuit structure of a switching converter in accordance with an exemplary embodiment. As shown in FIG. 3, during the on-time 1, the controlled switching devices S2, S3 and the rectifying devices D2, D3, and D5 are non-conducting and the switching devices S1, S4 as well as the rectifying devices D1, D4 and D6 are conducting, resulting in a first on-time state circuit structure.
FIG. 4 illustrates an off-time state circuit structure of a switching converter in accordance with an exemplary embodiment. As shown in FIG. 4, during the off-time state, all of the controlled switching devices and the rectifying device D5 are conducting while the rest of the rectifying devices are off, resulting in an off-time circuit structure.
FIG. 5 illustrates a second on-time state circuit structure of a switching converter in accordance with an exemplary embodiment. As shown in FIG. 5, the off-time state is followed by the second on-time state during which the circuit structure is otherwise identical to that shown in FIG. 3 except that the other switching device pair S1, S4 is conducting. A complete operation cycle comprises (e.g., consists of) two off-time state cycles and of one first on-time state and one second on-time 2 state cycle.
The secondary side second capacitor C1 provides Vs-balance of a transformer magnetizing inductor LM. Assuming that the average DC voltage of the secondary side second capacitor C1 is negligible, the converter has two sub-circuits, since the on-time state sub-circuits are identical. Hence, by applying Kirchhoff's laws, the following can be calculated for the on-time states:

uL1=uC2−uC3 

uL3=uC3−nuCo 

uLo=uC3−uo 

iC2=iin−iL1 

iC3=iL1−iL3 

iCo=niL3−iLo 

uin=uC2 

io=iLo,  (1)

where the main transformer turns ratio n is defined as





  
    
      
        n
        =
        
          
            
              N
              1
            
            
              N
              2
            
          
          .
        
      
    
    
      
        (
        2
        )
      
    
  




Similarly, the following can be calculated for the off-time state:

uL1=uC2 

uL3=uC3 

uLo=uC3−uo 

iC2=iin−iL1 

iC3=−iL3 

iCo=−iLo 

uin=uC1 

io=iLo.  (3)

According to (1) and (3), average voltages across the inductors, average currents through the capacitors as well as an average output current and an average input voltage become as

uL1=uC2−duC3

uL3=uC3−dnuCo

uL3=uC3−uo

iC2=iin−iL1

iC3=diL1−iL2

iCo=dniL3−iLo

uin=uC2

io=iLo.  (4)

where angle brackets  denote an average over time and d denotes a duty ratio.

In order to maintain flux linkage and charge balances, the average voltages across the inductors and the average currents through the capacitors should equal zero. According to these principles, operating-point-related steady-state variables are given as

UC2=nD2Uo 

UC3=nDUo 

UCo=Uo 

IL1=Iin 

IL3=DIin 

ILo=nD2Iin 

Io=nD2Iin 

Uin=nD2Uo,  (5)

which indicate that the input-to-output and output-to-input relations are functions of the quadratic duty ratio D2 as well as the transformer turns ratio n. The switching frequency of the converter is assumed to be fs and consequently, the switching period T=1/fs. The duty cycle D=Ton/Ts and therefore, the length of on-time Ton=DTs and the length of off-time Toff=(1−D)Ts because Ts=Ton+Toff. An operational cycle has two switching cycles, since it takes two complete switching cycles to obtain magnetizing flux balance. A switching cycle includes an off-time state cycle and an on-time state cycle. The on-time state cycle is either a first or a second on-time state cycle.

FIG. 6 illustrates approximate waveforms of capacitor voltages and inductor currents in accordance with an exemplary embodiment. FIG. 6 shows approximate waveforms of capacitor voltages and inductor currents. Average voltages Ux and currents Ix are represented by dashed lines, and instantaneous voltages ux and currents ix are represented by solid lines. The vertical dashed lines divide the waveforms into repeating operation cycle sections Toff, Ton, and Ton2, which represent the off-time state cycle and the first and second on-time state cycles, respectively. The approximate waveform of the currents of the inductors L1 and L3 is shown as a curve (a) where the on-time and off-time state slopes can be given as

mL1,ON=−nD(D−1)Uo/L1 

mL1,OFF=−nD2Uo/L1 

mL3,ON=−n(D−1)Uo/L3 

mL3,OFF=−nDUo/L3.  (6)

The corresponding peak-to-peak ripple currents can be given as

ΔiL1-pp=mL1,ONDTs,

ΔiL3-pp=mL3,ONDTs.  (7)

The corresponding average currents IL1 and IL3 are defined in (5). The selection of the inductors L1, L3 can be carried out based on the defined ripple and average currents.
The approximate waveforms of the capacitor voltages uC2 and uC3 are shown in FIG. 6 as a curve (b) exhibiting a triangle shape. The on-time and off-time state slopes can be given for the capacitor C2 as

mC2,ON=(1−D)Iin/C2,

mC2,OFF=−DIin/C2  (8)

as well as for the capacitor C3 as

mC3,ON=DIin/C3,

mC3,OFF=−DIin/C3.  (9)

According to the defined slopes (9), the peak-to-peak ripples associated with the capacitors can be determined by

ΔuCi-pp=mCi,ONDTs.  (10)

The average voltages UCi are defined in (5). The selection of the capacitors can be carried out according to the defined peak-to-peak ripple and average voltages.
The approximate waveform of the capacitor Co voltage uCo is shown in FIG. 6 as a curve (c). The on-time and off-time state slopes can be given for the capacitor Co as

mCo,ON=nD(1−D)Iin/Co,

mCo,OFF=D2Iin/Co.  (11)

According to the defined slope in (11), the peak-to-peak ripple associated with the capacitor Co can be determined by

ΔuCo-pp=mCo,ONDTs.  (12)

The average voltage UCo is defined in (5). The selection of the capacitor can be carried out according to the defined peak-to-peak ripple and average voltage.
In FIG. 6, the approximate waveform of the current of inductor Lo is a curve (d) based on the voltage appearing across it which is the ripple voltage of the capacitor Co specified previously. The corresponding average current ILo is defined in (5). The ripple current of the inductor Lo is small and can be usually neglected when sizing the inductor. The peak-to-peak ripple current can be given by

ΔiLo-pp=mCo,ONDTs2/(8Lo)  (13)

based on the change in the flux linkage of the inductor. The selection of the inductor can be based on the ripple and average currents.

As shown in FIG. 6, the approximate waveform of the main transformer magnetizing inductor LM current iLM is a curve (e) having a distinctive shape. Since the magnetizing inductor is placed on the secondary side of the transformer T, the rising and falling slopes of the current during the on-time states can be defined as

mLM,UP=Uo/LM,

mLM,DOWN=−Uo/LM  (14)

whereas during the off-time state the current slope is zero. Therefore, the peak-to-peak magnetizing current can be defined as

ΔiLM-pp=mLM,UPDTs.  (15)

The magnetizing current iLM adds to the transformer winding currents ipri and isec, which are shown in FIG. 6 as a curve (f).
The input voltage of the converter is pulsating and therefore, an EMI filter can be used for producing the average voltage (≈nD2Uo) at its input. The capacitor C2 and the inductor L1 effectively form this kind of filter and no additional filter should be specified. Moreover, the primary side rectifying devices D5, D6 can be replaced with controlled switches in order to minimize conduction losses.
FIG. 7 illustrates a converter and solar generator arrangement in accordance with an exemplary embodiment of the present invention. As shown in FIG. 7, the inputs of the converter are connected to a solar generator 2. Owing to the constant-current nature of the solar generator, the current-fed converter of the present invention is highly suitable for interfacing with the generator. A high voltage DC bus 3 acts as a load. In FIG. 7, all rectifying devices can be diodes and all switching devices can be power MOSFETs or other suitable rectifying devices as desired.
As stated in (5), a transformer-isolated current-fed quadratic full-bridge converter as illustrated in FIG. 7 has such steady-state conversion ratios that the steady-state input current Iin is reflected to the steady-state output current Io multiplied by the turns ratio n=N1/N2 and the square of the duty ratio D as follows:




  
    
      
        
          I
          o
        
        =
        
          
            
              N
              1
            
            
              N
              2
            
          
          ⁢
          
            D
            2
          
          ⁢
          
            
              I
              in
            
            .
          
        
      
    
    
      
        (
        16
        )
      
    
  




The output voltage Uo is reflected to the input Uin with the same conversion ratio:




  
    
      
        
          U
          in
        
        =
        
          
            
              N
              1
            
            
              N
              2
            
          
          ⁢
          
            D
            2
          
          ⁢
          
            
              U
              o
            
            .
          
        
      
    
    
      
        (
        17
        )
      
    
  




By selecting properly the turns ratio n=N1/N2, a high overall conversion ratio as well as controlling range can be obtained.
The inductor L1 and the capacitor C2 form an input-side EMI filter, which produces the average of the input voltage reflected from the output voltage.
The converter 1 includes means for measuring the input voltage uin, means for measuring the input current iin, means 1.1 for measuring the output voltage uo, and a control means 1.2. The means 1.1 for measuring the output voltage uo can also provide a galvanic isolation between the output voltage and the controller 1.2. The converter includes means 1.3 for controlling the switching devices.
The control means 1.2 outputs a control signal, and a value of the control signal is determined based on the measured voltages and currents. Means 1.3 for controlling the switching devices then control the switching devices based on the control signal.
The control means 1.2 are arranged such that the input voltage uin is kept at a level corresponding to maximum power by a feedback controller and a maximum power tracker.
The maximum power tracker may, for instance, determine the maximum power point based on the measured input current iin. As a result, the tracker can determine a reference for the input voltage uin. The feedback controller adjusts the input voltage uin to correspond to the voltage reference.
If the output voltage uo exceeds a predetermined level, an output-voltage feedback loop using the means 1.2 automatically reduces the output current to keep the output voltage uo constant.
Thus, it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

## Claims

1. A switching converter comprising:
a first input terminal and a second input terminal, both for connecting to a current source;
a first output terminal and a second output terminal, that are both connected to a load;
a transformer with a primary winding, a secondary winding and a galvanic isolation between the windings, wherein the primary winding and the secondary winding include two terminals;
a primary side first capacitor connected between the first input terminal and the second input terminal;
a primary side first inductor with two terminals, connected from its first terminal to the first input terminal;
a primary side second inductor with two terminals;
a primary side first rectifying device connected between the primary side first inductor second terminal and the primary side second inductor second terminal, thus allowing a flow of current from the primary side first inductor second terminal to the primary side second inductor second terminal;
a primary side second rectifying device connected between the primary side first inductor second terminal and the primary side second inductor first terminal, thus allowing a flow of current from the primary side first inductor second terminal to the primary side second inductor first terminal;
a primary side second capacitor connected between the second inductor first terminal and the second input terminal;
a first switching device connected between the primary side second inductor second terminal and the transformer primary winding first terminal and adapted to be able to set the connection between the primary side second inductor second terminal and the transformer primary winding first terminal to a conducting and a non-conducting state;
a second switching device connected between the primary side second inductor second terminal and the transformer primary winding second terminal and adapted to be able to set the connection between the primary side second inductor second terminal and the transformer primary winding second terminal to a conducting and a non-conducting state;
a third switching device connected between the transformer primary winding first terminal and second input terminal and adapted to be able to set the connection between the transformer primary winding first terminal and the second input terminal to a conducting and a non-conducting state;
a fourth switching device connected between the transformer primary winding second terminal and the second input terminal and adapted to be able to set the connection between the transformer primary winding second terminal and the second input terminal to a conducting and a non-conducting state;
a secondary side rectifying means for rectifying a current of the transformer secondary winding;
a secondary side filtering means for filtering a current between the rectifier means and a converter output; and
means for controlling a voltage between the input terminals.

2. The switching converter according to claim 1, wherein the secondary side filtering means comprises:
a secondary side first inductor with two terminals, connected from its second terminal to the first output terminal;
a secondary side first capacitor connected between the secondary side first inductor first terminal and the second output terminal, and in that the secondary side rectifying means comprises:
a secondary side first rectifying device connected between the transformer secondary winding first terminal and the secondary side first inductor first terminal, thus allowing a flow of current from the transformer secondary winding first terminal to the secondary side first inductor first terminal;
a secondary side second rectifying device connected between the transformer secondary winding second terminal and the secondary side first inductor first terminal, thus allowing a flow of current from the transformer secondary winding second terminal to the secondary side first inductor first terminal;
a secondary side third rectifying device connected between the second output terminal and the transformer secondary winding first terminal, thus allowing a flow of current from the second output terminal to the transformer secondary winding first terminal; and

a secondary side fourth rectifying device connected between the second output terminal and the transformer secondary winding second terminal, thus allowing a flow of current from the second output terminal to the transformer secondary winding second terminal.

3. The switching converter according to claim 2, wherein the connection between the transformer secondary winding second terminal and the secondary side second and fourth rectifying device comprises a secondary side second capacitor.

4. The switching converter according to claim 1, wherein the current source is a solar generator.

5. The switching converter according to claim 1, wherein the load is essentially a DC bus.

6. The switching converter as claimed in claim 1, wherein the means for controlling the voltage between the input terminals comprises:
means for measuring an input voltage;
means for measuring an input current;
means for measuring an output voltage;
control means for determining a value of a control signal on the basis of the measured voltages and currents; and
means for controlling the switching devices on the basis of the control signal.

7. A method for controlling a voltage between input terminals of a switching converter as claimed in claim 1, the method comprising:
measuring an input voltage;
measuring an input current;
measuring a output voltage;
determining a value of a control signal on the basis of the measured voltages and currents; and
controlling the switching devices on the basis of the control signal.

8. The method according to claim 7, wherein the method comprises:
determining a maximum power point;
keeping the input voltage at a level corresponding to the maximum power point and, if the output voltage exceeds a predetermined level; and
reducing an output current to keep the output voltage constant.

9. A switching converter comprising:
a first input terminal and a second input terminal, both for connecting to a current source;
a first output terminal and a second output terminal, that are both connected to a load;
a transformer with a primary winding, a secondary winding and a galvanic isolation between the windings, wherein each of the primary and secondary windings includes two terminals;
a primary side first capacitor connected between the first input terminal and the second input terminal;
a primary side first inductor connected to the first input terminal;
a primary side second inductor;
a primary side first rectifying device connected between a second terminal of the primary side first inductor and a second terminal of the primary side second inductor;
a primary side second rectifying device connected between the second terminal of primary side first inductor and a first terminal of the primary side second inductor;
a primary side second capacitor connected between a first terminal of the second inductor and the second input terminal;
a first switching device connected between the second terminal of the primary side second inductor and a first terminal of the transformer primary winding;
a second switching device connected between the second terminal of the primary side second inductor and a second terminal of the transformer primary winding;
a third switching device connected between the first and second terminals of the transformer primary winding;
a fourth switching device connected between the second terminal of the transformer primary winding and the second input terminal;
a secondary side rectifying means for rectifying a current of the transformer secondary winding; and
a secondary side filtering means for filtering a current between the rectifier means and a converter output.

10. The switching converter of claim 9, wherein primary side rectifying device allows a flow of current from the second terminal of the primary side first inductor to the second terminal of the primary side second inductor.

11. The switching converter of claim 9, wherein the primary side second rectifying device allows a flow of current from the second terminal of the primary side first inductor to the first terminal of the primary side second inductor.

12. The switching converter of claim 9, wherein first switching device is connected to set a connection between the second terminal of the primary side second inductor and the first terminal of the transformer primary winding to a conducting and a non-conducting state.

13. The switching converter of claim 9, wherein the second switching device is connected to set a connection between the second terminal of the primary side second inductor and the second terminal of the transformer primary winding to a conducting and a non-conducting state.

14. The switching converter of claim 9, wherein the third switching device is connected to set the connection between the first terminal of the transformer primary winding and the second input terminal to a conducting and a non-conducting state.

15. The switching converter of claim 9, wherein the fourth switching device is connected to set the connection between the second terminal of the transformer primary winding and the second input terminal to a conducting and a non-conducting state.

16. The switching converter of claim 9, comprising:
means for controlling a voltage between the input terminals.