Abstract:
Disclosed is a bi-directional battery power inverter ( 1 ) comprising a DC-DC converter circuit element ( 3 ) to which the battery ( 2 ) can be connected in order to generate an AC output voltage from a battery ( 2 ) voltage in a discharging mode while charging the battery ( 2 ) in a charging mode. The inverter ( 1 ) further comprises an HF transformer which forms a resonant circuit along with a resonant capacitor ( 6 ). In order to increase the efficiency of said battery power inverter, the transformer is provided with two windings ( 11,12 ) with a center tap ( 20 ) on the primary side, said center tap ( 20 ) being connected to a power electronic center-tap connection with semiconductor switches ( 21,31 ) while a winding ( 13 ) to which the resonant capacitor ( 6 ) is serially connected provided on the secondary side.

Description:
[0001]     This application claims Priority from German Application No. DE 10 2005 023 290.6 filed on 20, Mai 2005  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a bidirectional battery power inverter as set forth in the preamble of claim  1  and claim  2 .  
         [0004]     2. Description of the Prior Art  
         [0005]     Bidirectional power inverters serve on the one side to convert a DC battery voltage from 12 volt into a 50 Hz or 60 Hz AC voltage of e.g., 230 volt, on the other side however also to charge the battery from the AC voltage. The energy flows both from the battery to a DC-AC converter and from the DC-AC converter to the battery. The problem arising here is that the conduction losses are quite high at such small DC voltages.  
         [0006]     A power inverter with a half-bridge array on the primary side and a full bridge array on the secondary side is known from the document EP 0 820 893 A2.  
         [0007]     A bidirectional DC-DC converter is known from EP 1 458 084 A2. It includes a bridge circuit on the input side, a bridge circuit on the output side and a transformer connected between said two circuits, said transformer having a resonant capacitor and a resonant inductor.  
         [0008]     A DC-AC converter having a transformer, which comprises two windings with center tap on the primary side, is connected to a power electronic midpoint circuit with semiconductor switches and comprises a winding on the secondary side, is shown and described in U.S. Pat. No. 6,507,503 B2.  
         [0009]     Further converter circuits have been disclosed in the document DE 40 13 506 A1 and in U.S. 2003/0142513 A1.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     It is the object of the invention to provide a high efficiency power inverter.  
         [0011]     This object is achieved by a power inverter having the characterizing features of claim  1  in connection with the features of the preamble.  
         [0012]     The invention allows benefiting from all the advantages of resonant switching without the tradeoff of high conduction losses on the undervoltage side. A high switch frequency can be utilized hereby. As a result, a planar transformer, which is provided in accordance with a preferred embodiment, may be utilized.  
         [0013]     The present invention combines the advantages of low conduction losses and the possibility of utilizing a planar transformer without the trade-off of an unfavorable high apparent power of the transformer and switch overvoltages on the primary side, a restricted operating range and high conduction and switch losses on the low voltage side. Put another way, this means that with the topology of the invention, as contrasted to prior art, a very low-loss, fully resonant switching is made possible on the high current side by the fact that the adjustment stage (boost-buck chopper) is activated in operation ranges in which the voltage needs to be adapted.  
         [0014]     In accordance with the invention, there is provided that a DC-AC converter circuit element is provided, said converter circuit element lying on the output side of the inverter and comprising a boost-buck chopper that is connected between the DC-DC converter and the DC-AC converter circuit element. As a result, switching losses due to non-optimal operation in which the DC-DC converter cannot be switched resonantly and an unfavorable current/voltage configuration in the optimal point of operation are avoided.  
         [0015]     By virtue of the fact that a resonant switching element is distributed in circuit parts with staggered clocking, the capacitor load is low and the HF ripple current in the DC source is low as well.  
         [0016]     Appropriately, the clock rate of the half-bridge circuit is less than the resonant frequency that is determined by the transformer leakage inductance and the resonant capacitor, depending on whether there is provided one such capacitor or two capacitors connected in series. The resonant frequency is obtained from the leakage inductance of the transformer and the series resonant capacitance or from the leakage inductance of the transformer and the series connected capacitors. Selecting this clock rate, the semiconductors are switched both on and off with zero current.  
         [0017]     If the clock rate of the half-bridge circuit is less than the resonant frequency obtained from the leakage inductance of the transformer and the series resonant capacitance, switching on should always occur at zero current since in this case the switching losses are low or inexistent. In accordance with another advantageous developed implementation of the invention, there is provided a synchronous actuation of the boost-buck chopper and of the resonant converter. The synchronous actuation has the advantage that the effective current load is minimized in the capacitors of the power inverter.  
         [0018]     A particular effect of benefit is achieved if the transformer is configured to be a planar transformer, said planar transformer being provided with two printed circuit boards. Both the core of the transformer and the printed circuit boards are housed in a housing. A cast housing is manufacturable at lower cost since projections needed in the cast housing of a planar transformer are easier to manufacture. A planar transformer with two printed circuit boards is moreover manufacturable at a lower cost than a planar transformer having a large printed circuit board. A large printed circuit board is more expensive than two small ones.  
         [0019]     Other advantageous implementations of the invention are described in the subordinate claims.  
         [0020]     An exemplary embodiment is explained in closer detail with reference to the drawings, other advantageous developed implementations of the invention and advantages thereof being described.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0021]      FIG. 1  shows a circuit diagram of a DC-DC converter of the invention,  
         [0022]      FIG. 2  shows an equivalent circuit diagram of the transformer HFT,  
         [0023]      FIG. 3  shows a circuit diagram of a DC-DC converter of the invention with two HF transformers,  
         [0024]      FIG. 4  shows a sectional view of a planar transformer and  
         [0025]      FIG. 5  shows another view of the planar transformer.  
         [0026]      FIG. 6  shows a circuit diagram of a DC-DC converter of the invention with half-bridge capacitors as the resonant elements;  
         [0027]      FIG. 7  shows transformer current and transformer voltage on the secondary side of the transformer. 
     
    
       [0028]     In the Figs., like elements bear the same reference numerals.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0029]      FIG. 1  shows a power inverter  1  of the invention, for example for supplying AC consumers in island networks. Said power inverter is connected to a battery  2 . The power inverter  1  includes one DC-DC converter circuit element  3  with a capacitor  4  connected in parallel to the battery  2  and one HF transformer HFT.  
         [0030]     The power inverter  1  further includes a DC-AC converter circuit element  5  that lies on the output side of the power inverter  1  and a boost-buck chopper  8  that is connected between the DC-DC converter circuit element  3  and the DC-AC converter circuit element  5 . The DC-AC converter circuit element  5  is configured to be monophase.  
         [0031]     The inverter  1  is configured to be a bidirectional battery power inverter and serves for generating an AC output voltage from the battery voltage in a discharge mode of operation and for charging the battery  2  in the charge mode. Together with a resonant capacitor  6 , the HF transformer HFT forms a resonant circuit.  
         [0032]      FIG. 2  shows an equivalent circuit diagram of the transformer HFT. In accordance with the invention, the transformer HFT has, on the primary side, two windings  11 ,  12  with a center tap  20 , said center tap being connected to a power electronic midpoint circuit with semiconductor switches  21 ,  31 , a winding  13  to which the resonant capacitor  6  is connected in series being provided on the secondary side. The semiconductor switches  21 ,  31  form a midpoint circuit.  
         [0033]      FIG. 2  shows the leakage inductances  15 ,  16  on the primary side and the leakage inductance  17  on the secondary side. The core of the transformer bears the reference numeral  14 . The center tap is indicated at  20 . The reference numeral  10  designates an additional inductance.  
         [0034]     The semiconductor switches  21 ,  31  switch alternately and generate the alternating voltage needed for voltage transformation, said alternating voltage being converted back into a direct voltage on the secondary side by the bridge-connected semiconductor elements  41 ,  51 ,  61 ,  71  and being smoothed by a filter capacitor  7 . The DC-DC converter element works in a constant point of operation so that its input and output voltage are in a fixed relation dictated by the transformation ratio of the transformer.  
         [0035]     Due to the resonant mode of operation, high switching frequencies of more than 50 kHz are appropriate so that a planar transformer may be utilized. It only makes sense to utilize planar transformers at frequencies in excess of 50 kHz.  
         [0036]     Preferably, the resonant frequency of the alternating voltage circuit, which is composed of leakage inductances of the transformer HFT and a series resonant capacitance  6  ( FIG. 1 ), is higher than the clock frequency of the semiconductor switch. As a result, the semiconductor switches  21 ,  31  can be switched both on and off with zero current.  
         [0037]     There is provided the use of the boost-buck chopper  8  for allowing using the circuit for strongly fluctuating battery voltages. Thanks to the boost-buck chopper  8 , the intermediate voltage is prevented from dropping to such an extent at small battery voltages that no output nominal voltage is achieved. If the pulse width in the DC-DC converter were reduced in the charging mode at low battery voltage, the resonant mode of operation could no longer be ensured. As a result, non-optimal operation would cause switch losses to occur. A boost-buck chopper  8  is utilized in particular, said boost-buck chopper adapting the variable battery voltage to a constant voltage at the capacitor  19 . The boost-buck chopper  8  consists of a choke  18 , the switch elements  141  and  151  and the capacitor  19 . If the input voltage (battery voltage) exceeds e.g., 12 V, the voltage at the capacitor  19  increases proportionally with the input voltage. In this case, the boost-buck chopper needs no longer be clocked.  
         [0038]     As shown in  FIG. 3 , the transformer circuit can be made from two circuit parts with two HF transformers HFT 1  and HFT  21 . The arrays are thereby provided with staggered clocking so that the capacitor load and the HF ripple current in the DC source are lower. The two circuit parts shown in  FIG. 3  are configured to be half-bridge arrays. In a configuration with half-bridges, the transformation ratio of the transformer is only half as high. A smaller transformer transformation ratio is advantageous because then the leakage inductance  15  and  16  transformed from the overvoltage side by the transformation ratio will not become too high.  
         [0039]     A half-bridge array needs less semiconductor switches and involves less cost as a result thereof.  
         [0040]     The planar transformer  29  shown in  FIG. 4  is embedded in an aluminium cast housing  24  and comprises two printed circuit boards  22 ,  23 . The power semiconductors can be configured to be built in SMD construction. As shown in  FIG. 5 , the transformer comprises a primary winding  26  and a secondary winding  27  that are arranged on a printed circuit board  25 . The choke core  30  is provided with an air gap and is also integrated in the printed circuit board  25 . It is only magnetized by the secondary winding  27 , not by the primary winding  26 .  
         [0041]     In  FIG. 6 , there is shown a design of the circuit in which the capacitors  34 ,  35  form a resonant circuit together with the leakage inductance of HFT. With such a design, the parasite capacitors of the semiconductor switches  21 ,  31  are discharged to particularly low values by the currents in the main inductance of the transformer prior to switching them on. Smaller switch-on losses occur as a result thereof. The transformer currents and the voltages occurring on the secondary side in this implementation correspond to the illustration shown in  FIG. 7 .  FIG. 7  shows the current  60  and the voltage  50  on the secondary side of the transformer.  
         [0042]     The DC-AC converter  5  can be configured to be a H-bridge for converting a one-phase AC voltage or a three-phase bridge for converting a three-phase AC voltage.  
         [0043]     An arrangement in which the DC-AC converter  5  is always operated in such a manner that the voltage at the capacitor  19  is always higher than the peak value of the nominal voltage may also be advantageous. As a result, in case the battery is being charged, the DC-AC converter can be operated in a PFC (Power Factor Correction) mode of operation and sinusoidal current in phase with the mains voltage can be tapped from the mains. In case the battery is being discharged, a sinusoidal AC voltage can be generated at any form of current.  
         [0044]     In order to be capable of supplying any load (including asymmetric load and unipolar loads) in the case of three-phase converters, a connected N can be added as the fourth phase and the entire circuit can be implemented so that the capacitor  19  consists of two capacitors with a grounded midpoint, each capacitor being charged or discharged separately by a resonant converter having a boost-buck chopper connected downstream thereof.  
         [0045]     The semiconductors  21 ,  31 ,  41 ,  51 ,  61 ,  71 ,  141 ,  151  may be configured to be MOSFETs, IGBTs, bipolar transistors or GTOs. The parallel diodes can be separate components or consist of parasite diodes of the MOSFETs utilized. The DC source  2  can be a battery, a fuel cell, a generator-supplied DC intermediate circuit or a double-layer capacitor (Ultracap).  
         [0046]     The resonant converter can be advantageously utilized in automobiles in order to allow for energy exchange between different DC sources such as traction battery, double-layer capacitor, auxiliary operating battery and so on. The direction of the energy flow may thereby be reversed while accelerating and braking.  
         [0047]     The planar transformer may be advantageously implemented in such a manner that the secondary leakage inductance is increased by integrating an additional choke into the secondary transformer winding. This may occur in the manner shown in  FIG. 5 .  
         [0048]     As contrasted with the half- and full bridge circuits, the semiconductor voltages of the semiconductors  21 ,  31 ,  221  and  231  are not limited to the capacitor voltage of the capacitor  4 . As contrasted with normal midpoint circuits, the capacitor voltage is not limited to the double of the capacitor voltage of capacitor  4 ; the voltage drop above the primary side portion of the resonant inductance of the transformers HFT  1  and HFT  21  is added thereto. This effect can only be controlled if the substantial portion of the resonant inductance is displaced for its major part onto the secondary side. This is achieved by the transformer structure described and/or by additional resonant inductances  10  on the secondary side.  
       LIST OF NUMERALS  
       [0049]      1  power inverter  
         [0050]      2  battery  
         [0051]      3  DC-DC converter circuit element  
         [0052]      4  capacitor  
         [0053]      5  DC-AC converter circuit element  
         [0054]      6  resonant capacitor  
         [0055]      7  filter capacitor  
         [0056]      8  boost-buck chopper  
         [0057]      10  additional leakage inductance  
         [0058]      11 ,  12  primary side windings  
         [0059]      13  secondary side winding  
         [0060]      14  transformer core  
         [0061]      15 ,  16  primary side leakage inductances  
         [0062]      17  leakage inductance  
         [0063]      18  choke  
         [0064]      19  capacitor  
         [0065]      20  center tap  
         [0066]      21  semiconductor switch  
         [0067]      22 ,  23  printed circuit board  
         [0068]      24  aluminium cast housing  
         [0069]      25  printed circuit board  
         [0070]      26  primary winding  
         [0071]      27  secondary winding  
         [0072]      28  resonant capacitor  
         [0073]      29  planar transformer  
         [0074]      30  choke core  
         [0075]      31  semiconductor switch  
         [0076]      32 ,  33  half-bridge capacitors  
         [0077]      34 ,  35  half-bridge capacitors as resonant elements  
         [0078]      41 ,  51 ,  61 ,  71  semiconductor elements  
         [0079]      50  secondary side transformer voltage  
         [0080]      60  secondary side transformer current  
         [0081]      141 ,  151  semiconductor switch  
         [0082]     HFT HF transformer  
         [0083]     HFT  1 , HFT  21  HF transformer  
         [0084]     LSP air gap  
         [0085]      221 ,  231  switch element