Abstract:
An object of the invention is to provide a PCB transformer having plural output channels, which can suppress a voltage fluctuation in each output channel to supply a stable output without the need for a larger body, even though an input load fluctuates. The PCB transformer include, a core having a core axis, a first layer including a winding for each input line separately wound around said core as plural input coils spaced from each other along the core axis, and a second layer including a winding for an output line corresponding to each output channel separately wound on said first layer as plural output coils spaced from each other along the core axis. One of the input coils and one of the output coils are disposed in one of winding regions defined along the core axis. In each the winding region, one coil having a narrower width of the input coil or the output coil is disposed within a width of another coil along said core axis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a transformer for being mounted on a printed circuit board (hereinafter, it is referred as “PCB transformer”.), and particularly to a multi-channel insulated power PCB transformer having plural output channels. 
     2. Description of the Related Art 
     When an electronic device comprises a plurality of control circuits, each control circuit is individually supplied with its electric power from the power circuit. Such power circuit includes a multi-channel insulated power transformer which can produce a plurality of independent power from a single power source. The multi-channel insulated power transformer has, typically, coil structures comprising input (primary) coils for being supplied with the electric power from the outside of the device and output (secondary) coils, which is independent each other, for being connected to the control circuit. For the purpose of downsizing electric devices, there has been a demand to make a smaller insulated power transformer having plural output channels for being mounted on a circuit board, i.e. PCB transformer. In such a small power transformer, as compared with a larger power transformer, it is more important to supply highly accurate output power to each output channel to stabilize a drive efficiency. 
     For example, Japanese Utility Model Kokai No. 04-94713 discloses coil structures in a PCB transformer. An input coil comprises a first half coil and a second half coil, and a plurality of output coils corresponding to each channel are inserted between these input coils that face each other in a radial direction. With respect to the coil structures, the reference mentions that a magnetic flux formed by two input coils can be efficiently coupled to output coils so as to provide higher drive efficiency to the PCB transformer. 
     For example, Japanese Patent Kokai No. 2000-299233 also discloses coil structures in a PCB transformer. An input coil for one input line is divided into two input coils that face each other in a radial direction and plurality of output coils corresponding to each output channel are inserted between these two input coils. One of the windings in each output coil is mutually disposed on a core along its long axis. The output winding of each output coil is wound by non-inductive winding, such as bifilar-winding or trifilar-winding. According to this structures, a rectification smoothing circuit of each output channel can be suppressed in its peak value. Such power circuit can be smaller and more stable. Further, regulation characteristics are improved in an output voltage of each output channel. 
     In typicall power circuits, the input coil has a larger number of windings than the output coil corresponding to each output channel so that the output (secondary) side is a higher voltage than the input (primary) side. When the transformer has more output channels, the total number of output coil windings corresponding to output channels becomes larger. As mentioned in the above references, if the input coil is divided into two coils, output coils may not be accommodated within the width of the input coils. In this case, for example, a width of the output coil may be decreased by forming the output coils with double layers piled in a radial direction. However, the body of the transformer becomes larger, as the output coils become thicker. Such large body is not preferred in view of an accommodation space for mounting the transformer on a circuit board. 
     Further, since the relative position against the input coil is quite different in each output coil, a load variation in the input coil provides a different effect to output coils. In this case, the voltage variation should be individually compensated in each output channel. As a result, the power circuits become larger. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the problem as mentioned above. Objects of the invention are to provide a PCB transformer, which can suppress a voltage fluctuation in each output channel to supply a stable output without the need for a larger body, even though an input load fluctuates. 
     The PCB transformer having plural output channels comprises a core having a core axis, a first layer for input coils and a second layer for output coils. The first layer includes windings for input lines separately wound around the core as input coils spaced from each other along the core axis. The second layer includes windings for output lines corresponding to output channels separately wound on the first layer as plural output coils spaced from each other along the core axis. One of the input coils and one of the output coils are disposed in one of a plurality of winding regions defined along the core axis. In each of the plurality of winding regions, one coil having a narrower width of the input coil or the output coil is disposed within a width of another coil along the core axis. 
     According to the present invention, a magnetic coupling can be enhanced between the input coils and the output coils without the need for a larger body of the transformer, especially a larger height along a radial direction of the core. That is, a magnetic leakage flux can be reduced to obtain high driving efficiency as a power transformer. Further, even if a power load fluctuates in the input coil, low magnetic leakage flux can suppress the voltage fluctuation in each output channel to provide stable output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective and cross-sectional view illustrating a PCB transformer according to one embodiment of the present invention; 
         FIG. 2  is a fragmentary cross-sectional view illustrating the PCB transformer according to one embodiment of the present invention; 
         FIG. 3  is a fragmentary cross-sectional schematic view illustrating the PCB transformer according to one embodiment of the present invention; 
         FIG. 4  is a circuit diagram in the PCB transformer according to one embodiment of the present invention; 
         FIG. 5  is a schematic view illustrating coil structures in a PCB transformer according to prior art; and 
         FIG. 6  is a schematic view illustrating coil structures in a PCB transformer according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a PCB transformer according to the present invention, an input winding for an input (primary) line is separately wound around a core as plural input coils spaced from each other along a longitudinal direction of the core. That is, these input coils are spaced from each other along the core axis by “winding in a division space”. An output winding for an output (secondary) line corresponding to one output channel is wound as an output coil with correspondence to one input coil wound by “winding with a division space”. In this coil structure, it is easy to control a magnetic field produced by the input coil, and a magnetic coupling can be enhanced between the input and output coils. Further, a magnetic leakage flux can be reduced to stabilize outputs of the transformer. The transformer can become smaller since the input coil is not divided to several layers. 
     Embodiments of a PCB transformer according to the present invention will be described hereinafter in detail with reference to  FIGS. 1 to 4 . 
     The PCB transformer  1  has a rectangular body. Its longitudinal direction is defined as a Z-axis direction. The X-axis and Y-axis are defined along two mutually perpendicular sides in a cross section of the PCB transformer  1  being perpendicular in the Z-axis. 
     As illustrated in  FIG. 1 , a bobbin  10  is made from insulation materials such as plastics and covers four faces other than two opposed mutually sides (the two sides located in both ends of the X-axis) in six faces of a rectangular core  12  made from a core material, such as a ferrite. The rectangular core  12  is accommodated without a space into a center through hole  10   a , which has an approximately rectangular shape in a section, in a center portion of the bobbin  10 . A pair of rectangular plate flanges  10   b  faced each other and extends radially in a Y-Z plane from edges of both aperture ends of the center through hole  10   a . A protrusion  10   c  is formed along a Y-axis edge of the flange  10   b  toward the outside. The protrusion  10   c  supports plural metal reed terminals  14  for connecting each of the coils to a printed circuit board. 
     As illustrated in  FIGS. 1 to 4 , a non-controlling input (primary) winding Np 1  is separately wound in the most inner part of bobbin  10  within three winding regions A, B and C spaced from each other along the X-axis. Hereinafter, a coil formed by winding a winding U within a winding region L is referred to a “coil U-L”. For example, a coil of Np 1 -A is formed by winding a winding Np 1  within the winding region A. The coils of Np 1 -B and Np 1 -C are formed by winding the same winding Np 1  within winding regions B and C, respectively. A first coil layer  40  includes coils Np 1 -A, Np 1 -B and Np 1 -C around a core  12 . The winding regions A, B and C have the same width along the X-axis corresponding to the widest width in coils wound within these winding regions. All coils around the bobbin  10  are wound so as to be disposed within the width of either one of the winding regions A, B and C. 
     As described below, a magnetic field density increases in the winding region B by an influence of magnetic fields produced by the input coil within the two winding regions A and C located in both sides of the winding region B. It is, therefore, preferred that the number of winding of the input coil Np 1  in the winding region B is less than the number of winding coils in the winding regions A and C. 
     An insulation sheet  22   a  is disposed on the first coil layer  40  to cover the coils Np 1 -A, Np 1 -B and Np 1 -C to prevent a short circuit with coils thereon. 
     The output (secondary) windings Ns 1 , Ns 4  and Ns 3  corresponding to the first, fourth and third channels are wound within the winding regions A, B and C, respectively, on the insulation sheet  22   a  to compose a second coil layer  41 . Each output winding Ns 1 , Ns 4  and Ns 3  is wound with a high density winding within the winding regions A, B and C, respectively, as coils Ns 1 -A, Ns 4 -B and Ns 3 -C so that each of the output windings are arranged side by side with high density and no space therebetween along the X-axis direction. 
     When the coil Ns 1 -A has a wider width along the X-axis than the coil Np 1 -A, the coil Np 1 -A is accommodated in the inside of the width of the coil Ns 1 -A. On the other hand, the Ns 1 -A coil is accommodated in the inside along the width of the Np 1 -A coil, when the Ns 1 -A coil has a smaller width along the X-axis than the Np 1 -A coil. 
     Preferably, the Ns 1 -A and Np 1 -A coils are disposed within the same winding region A, so that these coils have the same center position along the width direction. More preferably, the Ns 1 -A coil and Np 1 -A coil are disposed so that the same center position of these coils corresponds to the center position of the winding region A. 
     Similarly, when the Ns 4 -B coil has a larger width than the Np 1 -B coil, the Np 1 -B coil is accommodated in the inside of the width of the Ns 4 -B coil. Also, the Ns 4 -B coil is accommodated in the inside of the width of the Np 1 -B coil, when the Ns 4 -B coil has a smaller width along the X-axis than the Np 1 -B coil. 
     Preferably, the Ns 4 -B and Np 1 -B coils are disposed within the same winding region C so that the center positions along the width direction of these coils are located at the same position. More preferably, the Ns 4 -B and Np 1 -B coils are disposed so that the center positions along the width direction of these coils correspond to a center position of the width direction of the winding region B. 
     Further, when the Ns 3 -C coil has a larger width than the Np 1 -C coil, the Np 1 -C coil is accommodated in the inside of the width of the Ns 3 -C coil. Also, the Ns 3 -C coil is accommodated in the inside of the width of the Np 1 -C coil, when the Ns 3 -C coil has a smaller width along the X-axis than the Np 1 -C coil. 
     Preferably, the Ns 3 -C and Np 1 -C coils are disposed within the same winding region C so that the center positions along the width direction of these coils are located at the same position. More preferably, the Ns 3 -C and Np 1 -C coils are disposed so that the center positions along the width direction of these coils correspond to a center position of the width direction of the winding region C. 
     As mentioned above, the first coil layer  40  including the input winding Np 1  makes a set with the second coil layer  41  including the output windings Ns 1 , Ns 4  and Ns 3 . One coil included in the first coil layer  40  and one coil included in the second coil layer  41  are accommodated in the same winding region selected from winding regions on the core  10 . Typically, Np 1 -A, Np 1 -B and Np 1 -C coils are wound with the Np 1  winding with two turns, one turn and two turns, respectively. Also, Ns 1 -A, Ns 4 -B and Ns 3 -C coils are wound with six turns with the Ns 1 , Ns 4  and Ns 3  windings, respectively. 
     Further, an insulation sheet  22   b  is disposed on the second coil layer  41  to cover surfaces of Ns 1 -A, Ns 4 -B and Ns 3 -C coils to prevent a short circuit with coils thereon. 
     As illustrated in  FIG. 4 , when the transformer  1  has a controlling input (feedback) winding Ns 0  which does not depend on the non-controlling input winding Np 1 , the controlling input winding Ns 0  is wound on the insulation sheet  22   b . The voltage in windings of Ns 1  to Ns 7  can be controlled by a voltage produced in the controlling input winding Ns 0 . For the feedback controll, the Ns 1  to Ns 7  windings should be coupled more firmly with Ns 1  through a magnetic flux. In detail, the controlling winding Ns 0  is separately wound in three winding regions A, B and C spaced from each other. The coils Ns 0 -A, Ns 0 -B and Ns 0 -C are wound within the winding regions A, B and C to compose a third coil layer  42 . 
     An insulation sheet  22   c  is disposed on the third coil layer  42  to cover surfaces of Ns 0 -A, Ns 0 -B and Ns 0 -C coils to prevent a short circuit with coils thereon. 
     The output windings Ns 2 , Ns 5  and Ns 6  for the second, fifth and sixth output channels are added to output windings in the second coil layer  41 . The output windings Ns 2 , Ns 5  and Ns 6  are wound within the winding regions A, B and C, respectively, on the insulation sheet  22   c  to compose the fourth coil layer  43  so that one turn of these windings is arranged side by side in high density along the longitude direction of the core within each winding region. 
     When the Ns 2 -A coil has a larger width than the Ns 0 -A coil, the Ns 0 -A coil is accommodated in the inside of the width of the Ns 2 -A coil. On the other hand, the Ns 2 -A coil is accommodated in the inside along the width of the Ns 0 -A coil, when the Ns 2 -A coil has a smaller width than the Ns 0 -A coil. 
     Preferably, the Ns 0 -A and Ns 2 -A coils are disposed within the same winding region A so that the center positions of these coils along the width direction are located at the same position. More preferably, the Ns 0 -A and Ns 2 -A coils are disposed so that the center positions of these coils along the width direction correspond to a center position of the winding region A along the width direction 
     Similarly, when the Ns 5 -B coil has a larger width than the Ns 0 -B coil, the Ns 0 -B coil coil is accommodated in the inside of the width of the Ns 5 -B coil. Also, the Ns 5 -B coil is accommodated in the inside along the width of the Ns 0 -B coil, when the Ns 5 -B coil has a smaller width than the Ns 0 -B coil. 
     Preferably, the Ns 5 -B and Ns 0 -B coils are disposed within the same winding region B so that the center positions along the width direction of these coils are located at the same position. More preferably, the Ns 5 -B and Ns 0 -B coils are disposed so that the center positions along the width direction of these coils correspond to a center position of the width direction of the winding region B. 
     Further, when the Ns 6 -C coil has a larger width than the Ns 0 -C coil, the Ns 0 -C coil is accommodated in the inside of the width of the Ns 6 -C coil. Also, the Ns 6 -C is accommodated in the inside along the width of the Ns 0 -C coil, when the Ns 6 -C coil has a smaller width than the Ns 0 -C coil. 
     Preferably, the Ns 6 -C and Ns 0 -C coils are disposed within the same winding region C so that the center positions along the width direction of these coils are located at the same position. More preferably, the Ns 6 -C and Ns 0 -C coils are disposed so that the center positions along the width direction of these coils correspond to a center position of the width direction of the winding region C. 
     The third coil layer  42  including the input coil winding Ns 0  makes a set with the fourth coil layer  43  including the output coil windings Ns 2 , Ns 5  and Ns 6 . One coil included in the third coil layer  42  and one coil included in the fourth coil layer  43  are disposed in one of winding regions A, B and C so that these coils face each other. 
     As described below, a magnetic flux density is relatively high in the-winding region B because of an influence of a magnetic flux produced by the input coils in the winding regions A and C located on both sides of the winding region B. Preferably, the number of windings in the input winding Ns 0  within the winding region B is less than the number of windings within the winding regions A and C. Typically, the Ns 0 -A, Ns 0 -B and Ns 0 -C coils include four turns, one turn and three turns of Ns 0  winding, respectively. The Ns 2 -A, Ns 5 -B and Ns 6 -C coils include six turns of Ns 2 , Ns 5  and Ns 6  windings, respectively. 
     An insulation sheet  22   d  is disposed on the fourth coil layer  43  to cover surfaces of the Ns 2 -A, Ns 5 -B and Ns 6 -C coils to prevent a short circuit with coils thereon. 
     The second non-controlling input winding Np 2  is an input (primary) winding independent of the non-controlling input winding Np 1  and is wound on the insulation sheet  22   d . Specifically, the non-controlling input winding Np 2  is separately wound within three winding regions A, B and C spaced from each other along the X-axis. That is, Np 2 -A, Np 2 -B and Np 2 -C coils are formed in the winding regions A, B and C to compose the fifth coil layer  44 . Preferably, the Np 2 -A, Np 2 -B and Np 2 -C coils are, respectively, disposed within the winding regions A, B and C so that the center positions along the width direction of these coils are located at the same position. The Ns 2 -A, Ns 5 -B and Ns 6 -C coils include four turns, one turn and three turns of Ns 2  winding, respectively. 
     An insulation sheet  22   e  is disposed on the fifth coil layer  44  to cover Np 2 -A, Np 2 -B and Np 2 -C coils. 
     Finally, the controlling (feedback) output winding Ns 7  corresponding to the controlling (feedback) input winding Ns 0  is wound on the insulation sheet  22   e  within the winding region B of the center to compose the Ns 7 -B coil. The sixth coil layer  45  includes only the Ns 7 -B coil. Preferably, the center position of the Ns 7 -B coil in the width direction is the center position of the winding region B. A poly-imide tape is wound on the Ns 7 -B coil. 
     Characteristics of a magnetic field produced by the above coil structure in the PCB transformer according to the present invention will be described. 
     As shown in  FIG. 5 , in the coil structures of the conventional transformer, input (primary) windings are disposed with an equal interval in input coil layers  51   a ,  51   b  and  51   c . On the other hand, output (secondary) windings  53   a ,  53   b ,  53   c ,  54   a ,  54   b  and  54   c  in output coil layers  52   a  and  52   b  are disposed with an equal intervals to insulate each other. A magnetic flux (magnetic energy) produced in the input coil layers  51   a ,  51   b  and  51   c  non-uniformly passes through output coil layers  52   a  and  52   b . Especially, the magnetic flux passing through coils  53   b  and  54   b  located in the center of the transformer is larger than the magnetic flux passing through coils  53   a ,  54   a ,  53   c  and  54   c  located in the ends of the transformer. That is, the “coupling degree” is high in coils  53   b  and  54   b . The dispersion of the coupling degree causes a fluctuation of output voltage in output channels. 
     As shown in  FIG. 6 , in the coil structures of the transformer according to the present invention, the input windings are separately wound within winding regions A, B and C spaced from each other and the output winding of each channel is also wound as a single coil within each winding region. That is, a magnetic flux produced by three input coils Np 1 -A, Ns 0 -A and Np 2 -A in the winding region A passes through output coils Ns 1 -A and Ns 2 -A in the winding region A. Further, one part of the magnetic flux passes through output coils Ns 4 -B and Ns 5 -B in the winding region B. Also, a magnetic flux produced by three input coils Np 1 -C, Ns 0 -C and Np 2 -C in the winding region C passes through output coils Ns 3 -C and Ns 6 -C in the winding region C and the one part passes through output coils Ns 4 -B and Ns 5 -B in the winding region B. The output coils Ns 4 -B and Ns 5 -B in the winding region B receives the influence of a magnetic flux from not only the input coils Np 1 -B, Ns 0 -B and Np 2 -B in the winding region B but also the winding regions A and B. When the three input coils Np 1 -B, Ns 0 -B and Np 2 -B in the winding region B have a fewer number of turns than the input coils Np 1 -A, Ns 0 -A and Np 2 -A in the winding region A, and less turns than the input coils Np 1 -C, Ns 0 -C and Np 2 -C in the winding region C, the magnetic flux produced in the three input coils in the winding region B is weaker than the magnetic flux produced in other winding regions. 
     Thus, the magnetic flux passing through output coils Ns 1 -A, Ns 2 -A, Ns 4 -B, Ns 5 -B, Ns 3 -C and Ns 6 -C can be kept approximately uniform. The fluctuation of output voltage in output channels can be suppressed by controlling a magnetic coupling degree in a coil of each output channel. 
     According to the present invention, the influence of magnetic flux from each winding region can be independently calculated since winding regions are wholly divided. It is, therefore, easy to design input coils, such as the number of turns, for making the magnetic flux passing through a coil of each output channel uniform. 
     This application is based on a Japanese patent application No. 2005-240847 which is incorporated herein by reference.