Patent Publication Number: US-2015070942-A1

Title: Dc-dc converter

Description:
TECHNICAL FIELD 
     The present invention relates to a DC-DC converter for carrying out a step-up operation, and particularly, to the shape of a core used for a transformer. 
     BACKGROUND ART 
       FIG. 1  is a circuit diagram illustrating a DC-DC converter according to a related art.  FIG. 2  is an equivalent circuit diagram illustrating a coupling transformer  20  in the DC-DC converter of the related art illustrated in  FIG. 1 . The DC-DC converter illustrated in  FIG. 1  has a DC power source Vi, the coupling transformer  20 , switches Tr 1  and Tr 2 , diodes D 1  and D 2 , a smoothing capacitor Co, a load resistance Ro, and a controller  100 . 
     The coupling transformer  20  has, as illustrated in  FIG. 2 , a transformer T 3 , a transformer T 4 , and a reactor L 3 . The transformer T 3  has a primary winding  105   a  (having the number of turns of np), a coiled winding  105   b  (having the number of turns of np1) connected in series with the primary winding  105   a,  and a secondary winding  105   c  (having the number of turns of ns) electromagnetically coupled with the primary winding  105   a  and coiled winding  105   b.  The transformer T 4  is configured same as the transformer T 3  and has a primary winding  106   a  (having the number of turns of np), a coiled winding  106   b  (having the number of turns of np1) connected in series with the primary winding  106   a,  and a secondary winding  106   c  (having the number of turns of ns) electromagnetically coupled with the primary winding  106   a  and coiled winding  106   b.    
     Both ends of the DC power source Vi are connected through the primary winding  105   a  of the transformer T 3  to the collector and emitter of the switch Tr 1  of an IGBT (Insulated Gate Bipolar Transistor). The both ends of the DC power source Vi are connected through the primary winding  106   a  of the transformer T 4  to the collector and emitter of the switch Tr 2  made of an IGBT. A connection point between the primary winding  105   a  of the transformer T 3  and the collector of the switch Tr 1 , as well as the emitter of the switch Tr 1  are connected to a series circuit that includes the coiled winding  105   b  of the transformer T 3 , the diode D 1 , and the smoothing capacitor Co. A connection point between the primary winding  106   a  of the transformer T 4  and the collector of the switch Tr 2 , as well as the emitter of the switch Tr 2  are connected to a series circuit that includes the coiled winding  106   b  of the transformer T 4 , the diode D 2 , and the smoothing capacitor Co. 
     Both ends of a series circuit that includes the secondary winding  105   c  of the transformer T 3  and the secondary winding  106   c  of the transformer T 4  are connected to the reactor L 3 . The controller  100  controls according to an output voltage Vo of the smoothing capacitor Co so that the switch Tr 2  turns on after the switch Tr 1  turns on and before the switch Tr 1  turns off and so that the switch Tr 1  turns on before the switch Tr 2  turns off. Namely, it alternately turns on the switches Tr 1  and Tr 2  and makes the switches Tr 1  and Tr 2  simultaneously ON for a predetermined overlapping period on every half cycle. 
     According to the DC-DC converter of the related art having such a configuration, the controller  100  issues a control signal Tr 1   g  to turn on the switch Tr 1 , and after the predetermined overlapping period, issues a control signal Tr 2   g  to turn off the switch Tr 2 , so that a current passes through a path extending along Vi (plus (+) side),  105   a,  Tr 1 , and Vi (minus (−) side) to linearly increase the current of the switch Tr 1 . At the same time, the secondary winding  105   c  of the transformer T 3  generates a voltage to pass a current L 3   i  clockwise through a path extending along  105   c,  L 3 ,  106   c,  and  105   c.    
     The current L 3   i  causes according to the law of equal ampere-turns of the transformer, to accumulate energy in the reactor L 3  and the same current passes through the secondary winding  106   c  of the transformer T 4 . As a result, the primary winding  106   a  and coiled winding  106   b  of the transformer T 4  induce voltages depending on the numbers of turns thereof. 
     When the transformer T 4  has a turn ratio A as expressed by A=(np+np1)/np, a current of “1/A” of the current to the switch Tr 1  passes to the diode D 2  through a route extending along Vi+,  106   a,    106   b,  D 2 , Co, and Vi−. The current passes through the diode D 2  until the switch Tr 2  turns on. The output voltage Vo of the smoothing capacitor Co is the sum of a voltage generated by the primary winding  106   a  of the transformer T 4  and a voltage generated by the coiled winding  106   b  of the transformer T 4 . 
     A voltage generated on the transformer T 4  is expressed by a relationship of A×Vi×D, where D is an ON-duty of the switch Tr 1  (D=Ton/T) and T is a switching period of the switch Tr 1 . The output voltage Vo of the smoothing capacitor Co is expressed by Vo=Vi (1+A×D). Accordingly, managing the ON-duty D results in controlling the output voltage Vo. 
     Thereafter, the controller  100  issues a control signal Tr 2   g  to turn on the switch Tr 2 , and after the predetermined overlapping period, issues a control signal Tr 1 g to turn off the switch Tr 1 . This results in causing a current passing through a path extending along Vi+,  106   a,  Tr 2 , and Vi−, to linearly increase a current to the switch Tr 2 . At the same time, the secondary winding  106   c  of the transformer T 4  generates a voltage to increase and pass the current L 3   i  clockwise through a path extending along  106   c,    105   c,  L 3 , and  106   c.    
     The current L 3   i  causes according to the law of equal ampere-turns of the transformer, to accumulate energy in the reactor L 3  and the same current passes through the secondary winding  105   c  of the transformer T 3 . As a result, the primary winding  105   a  and coiled winding  105   b  of the transformer T 3  induce voltages depending on the numbers of turns thereof. 
     When the transformer T 3  has a turn ratio A as defined by A=(np+np1)/np, a current having a value of the current of the switch Tr 2  divided by A passes through a path extending along Vi+,  105   a,    105   b,  D 1 , Co, and Vi−. The current to the diode D 1  passes until the switch Tr 1  turns on. The output voltage Vo of the smoothing capacitor Co is the sum of a voltage (an input voltage) of the DC power source Vi, a voltage generated by the primary winding  105   a  of the transformer T 3 , and a voltage generated by the coiled winding  105   b  of the transformer T 3 . A voltage generated on the transformer T 3  is expressed by A×Vi×D, where D is an ON-duty of the switch Tr 2  (D=Ton/T), and T is a switching period of the switch Tr 2 . The output voltage Vo of the smoothing capacitor Co is expressed by Vo=Vi (1+A×D). Accordingly, managing the ON-duty D results in controlling the output voltage Vo. 
     The DC-DC converter of the related art illustrated in  FIG. 1  is known as a multiphase transformer-linked step-up chopper circuit whose example is disclosed in Japanese Unexamined Patent Application Publication No. 2010-004704 (Patent Literature 1) (refer to Patent Literature 1). The DC-DC converter connects two independent phases to each other through a transformer. This reduces the number of required cores from two to one to carry out a step-up operation. 
     The coupling transformer  20  has a core  21  that is a combination of two E-shaped core members faced in an extending planar direction. The core  21  has side legs  22  and  23 , a center leg  24 , and a gap  25 . Around the side leg  22 , a winding  31  is wound, and around the side leg  23 , a winding  32  is wound. A current i 1  passes through the winding  31  and a current i 2  the winding  32 . 
     SUMMARY OF INVENTION 
     Problems to be Solved by Invention 
     The coupling transformer  20 , however, leaks a magnetic flux component φ1k (φ is a Greek letter “phi”) outside the windings  31  and  32  as illustrated in  FIG. 3 . Also, the gap  25  of the core  21  leaks a magnetic flux component φfr due to a fringing effect. Namely, the coupling transformer  20  of the related art causes large leakage flux to enlarge differences from theoretical values. 
     The present invention is able to provide a DC-DC converter having a coupling transformer that substantially realizes a design based on theoretical values. 
     Means to Solve Problems 
     According to a technical aspect of the present invention, the DC-DC converter includes a coupling transformer having a first winding and a second winding, a first switch connected through the first winding to both ends of a DC power source, a second switch connected through the second winding to the both ends of the DC power source, a first series circuit connected to both ends of the first switch and including a first diode and a smoothing capacitor, a second series circuit connected to both ends of the second switch and including a second diode and the smoothing capacitor, and a controller that alternately turns on the first and second switches and simultaneously turns on the first and second switches for a predetermined overlapping period on every half cycle. The coupling transformer has an I-shaped core, two E-shaped cores holding the I-shaped core between them, a first gap formed between a center leg of one of the E-shaped cores and the I-shaped core, a second gap formed between a center leg of the other E-shaped core and the I-shaped core, and the first and second windings wound around the I-shaped core. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a DC-DC converter according to a related art. 
         FIG. 2  is an equivalent circuit diagram illustrating a coupling transformer in the DC-DC converter of the related art illustrated in  FIG. 1 . 
         FIG. 3  is a view explaining the cause of a gap length increase in the DC-DC converter of the related art illustrated in  FIG. 1 . 
         FIG. 4  is a circuit diagram illustrating a DC-DC converter according to Embodiment 1. 
         FIG. 5  is a schematic view illustrating a coupling transformer with an EEI core in the DC-DC converter according to Embodiment 1. 
         FIG. 6  is a comparative view illustrating a gap length of the related art and that of Embodiment 1. 
         FIG. 7  is a comparative view illustrating a winding method of the coupling transformer of the related art and that of Embodiment 2. 
     
    
    
     MODE OF IMPLEMENTING INVENTION 
     DC-DC converters according to embodiments of the present invention will be explained in detail with reference to the drawings. 
     The DC-DC converters of the present invention are characterized in that each employs two E-shaped cores and an I-shaped core to realize a coupling transformer that reduces leakage flux and substantially realizes a design based on theoretical values. 
     Embodiment 1 
       FIG. 4  is a circuit diagram illustrating a DC-DC converter according to Embodiment 1.  FIG. 5  is a schematic view illustrating a coupling transformer that employs an EEI core and is incorporated in the DC-DC converter of Embodiment 1. The embodiment is characterized in that it employs, instead of the coupling transformer  20  of the related art illustrated in  FIGS. 1 to 3 , the coupling transformer  1  illustrated in  FIG. 5 . 
     The remaining configuration of  FIG. 4  is the same as that of  FIG. 1 , and therefore, like parts are represented with like reference marks to omit the detailed explanations thereof. Only the coupling transformer  1  will be explained here. 
     The coupling transformer  1  illustrated in  FIG. 5  has an I-shaped core  4  and two E-shaped cores  2  and  3  that hold the I-shaped core  4  between them. The two E-shaped cores  2  and  3  are integrated into one so that center legs  2   a  and  3   a  thereof face each other in an extending planar direction with the I-shaped core  4  interposed between them. More precisely, a first gap  5  is formed between the center leg  2   a  of the E-shaped core  2  and the I-shaped core  4  and a second gap  5  is formed between the center leg  3   a  of the E-shaped core  3  and the I-shaped core  4 . Around the I-shaped core  4 , a winding  11  (a first winding) having the number of turns of n1 and a winding  12  (a second winding) having the number of turns of n2 are wound. A current i 1  passes through the winding  11  and a current i 2  passes through the winding  12 . As a result, as illustrated in  FIG. 5 , the inside of the integrated cores  3  and  4  forms four stable closed magnetic paths. 
       FIG. 6  is a comparative view illustrating a gap length of the related art and that of Embodiment 1, in which  FIG. 6(   a ) is of the related art and  FIG. 6(   b ) of Embodiment 1. A theoretical magnetic resistance value Rmg of a gap is expressed with the following expression: 
       Rmg=1 g/μo×S,
         where 1 g is a gap length, S is a sectional area, and μo is a magnetic permeability.       

     According to the coupling transformer  1  of the embodiment with such a configuration, the current it passes through the winding  11  and the current i 2  passes through the winding  12 . As illustrated in  FIG. 5 , the currents passing through the windings  11  and  12  generate magnetic flux along magnetic paths starting from the I-shaped core  4 , passing through the gaps  5  and E-shaped cores  2  and  3 , and returning to the I-shaped core  4 . Closed magnetic paths are formed to greatly reduce leakage magnetic flux and shorten a gap length. 
     In this way, the embodiment is able to provide a DC-DC converter having the coupling transformer that is capable of substantially realizing a design based on theoretical values. 
     On the other hand, the coupling transformer  20  of the related art illustrated in  FIG. 3  winds the windings  31  and  32  around the side legs  22  and  23 , and therefore, magnetic flux leaks outside the side legs  22  and  23 . This results in increasing leakage magnetic flux and expanding a difference between an actually measured value and a theoretical value. 
     Embodiment 2 
       FIG. 7  is a comparative view illustrating a winding method of the coupling transformer according to the related art and that according to Embodiment 2, in which  FIG. 7(   a ) is a schematic view of the coupling transformer  20  according to the related art and  FIG. 7(   b ) is of a coupling transformer according to Embodiment 2. 
     Except for the coupling transformer, the DC-DC converter of Embodiment 2 is the same as that illustrated in  FIG. 4 . 
     According to the coupling transformer  20  of the related art illustrated in  FIG. 7(   a ), the winding  31  having the number of turns of n1 is wound around the side leg  22  and the winding  32  having the number of turns of n2 is wound around the side leg  23 . 
     On the other hand, the coupling transformer of the embodiment illustrated in  FIG. 7(   b ) connects, between a positive electrode of a DC power source Vi and the collector of a switch Tr 1 , a series circuit in which a winding  31   a  (a first winding) is connected in series with a winding  31   b  (a second winding). A winding  32   a  (a third winding) is connected in series with a winding  32   b  (a fourth winding) and this series circuit is connected between the positive electrode of the DC power source Vi and the collector of a switch Tr 2 . 
     The coupling transformer has two E-shaped cores that are integrated into a θ-shape with respective center legs  24   a  being faced to each other in an extending planar direction. A gap  25   a  is formed between the center leg  24   a  of one of the E-shaped cores and the center leg  24   a  of the other E-shaped core. Around side legs  22  of the E-shaped cores, the windings  31   a  and  32   b  are wound, and around side legs  23  of the E-shaped cores, the windings  31   b  and  32   a  are wound. 
     The sum of the numbers of turns of the windings  31   a  and  31   b  is n1 and the sum of the numbers of turns of the windings  32   a  and  32   b  is n2. 
     Namely, windings  31  and  32  are each divided into two and the windings  31   a  and  32   b  are wound around the side legs  22  and the windings  31   b  and  32   a  around the side legs  23 . As results, magnetomotive force is distributed and a gap length is shortened, thereby the degree of coupling is improved. 
     In this way, the present invention is able to provide a DC-DC converter having the coupling transformer that is capable of reducing leakage magnetic flux and substantially realizing a design based on theoretical values. 
     (United States Designation) 
     In connection with United States designation, this international patent application claims the benefit of priority under 35 U.S.C. 119(a) to Japanese Patent Application No. 2012-060547 filed on Mar. 16, 2012 whose disclosed contents are cited herein.