Patent Publication Number: US-2018040410-A1

Title: Isolated switching power supply

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to an isolated switching power supply. 
     Background Art 
     An isolated switching power supply is well-known, such as a flyback, forward, push-pull, or full-bridge isolated DC-DC switching power supply, an isolated AC-DC switching power supply, and an isolated DC-AC inverter, wherein an isolation transformer electrically isolates input and output. Along with the reduction in the size of electronic devices in recent years, there is an increasing demand for reducing the size of the isolated switching power supply in an electronic device including the isolated switching power supply as a power supply. 
     An example of prior art corresponding to the demand includes a well-known isolated switching power supply using a multiplayer board transformer as an isolation transformer. In the multilayer board transformer, coil patterns are formed on surfaces of a plurality of base materials, and the base materials are laminated and integrated through insulating layers, for example. A primary winding and a secondary winding are formed by the coil patterns (for example, see Patent Document 1). The multilayer board transformer can be used to reduce the size and the thickness of the isolation transformer, and this can reduce the size of the isolated switching power supply. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Laid-Open No. 2010-093174 
     SUMMARY 
     Problems to be Solved 
     In the isolated switching power supply, a resonant inverter is adopted as an inverter circuit in some cases, and LC resonance is used to perform soft switching. The soft switching is a technique of using the LC resonance to reduce the switching loss by performing switching at a zero crossing point where the current or the voltage is 0. In the soft switching using the LC resonance, constants of a coil and a capacitor constituting an LC resonant circuit can be set to appropriate constants to realize switching at the zero crossing point where the current or the voltage is 0. 
     However, in the soft switching using the LC resonance, the inverter circuit requires the capacitor and the coil constituting the LC resonant circuit. Particularly, a choke coil used as a coil of the LC resonant circuit is often large-sized and expensive in general, and this may lead to a problem of an increase in the cost or the size of the isolated switching power supply. 
     An object of the present disclosure is to provide a small-sized isolated switching power supply with less switching loss at a low cost. 
     Means for Solving the Problems 
     &lt;First Aspect&gt; 
     A first aspect of the present disclosure provides an isolated switching power supply including: a first circuit board provided with a pattern of a primary winding constituting an isolation transformer; a second circuit board provided with a pattern of a secondary winding constituting the isolation transformer, wherein a part provided with the secondary winding is arranged to face a part provided with the primary winding of the first circuit board at a predetermined interval; and a core inserted into the primary winding of the first circuit board and the secondary winding of the second circuit board and made of a magnetic body constituting the isolation transformer. 
     The pattern of the primary winding constituting the isolation transformer is formed on the first circuit board, and the pattern of the secondary winding constituting the isolation transformer is formed on the second circuit board. The part provided with the primary winding of the first circuit board and the part provided with the secondary winding of the second circuit board are arranged to face each other at the predetermined interval. Therefore, the interval between the first circuit board and the second circuit board can be adjusted to adjust a degree of coupling (coupling coefficient) of the primary winding and the secondary winding of the isolation transformer, and this can adjust a leakage inductance of the isolation transformer to a desired value. 
     As described, in the isolated switching power supply, the soft switching using the LC resonance can realize the reduction in the switching loss through the switching at the zero crossing point where the current or the voltage is 0. The present disclosure can use the leakage inductance of the isolation transformer in the soft switching using the LC resonance. More specifically, the leakage inductance of the isolation transformer can be used as an inductance of the LC resonance or as part of the inductance of the LC resonance. The interval between the first circuit board and the second circuit board can be adjusted to adjust the leakage inductance of the isolation transformer to a desired value, and a constant of the LC resonant circuit of the soft switching can be highly accurately set to an appropriate constant. 
     In this way, the isolated switching power supply according to the present disclosure allows using the leakage inductance of the isolation transformer in the LC resonance of the soft switching. More specifically, the isolated switching power supply according to the present disclosure can eliminate the need to mount a choke coil that is generally large and expensive or can reduce the size of a conventional choke coil, thereby allowing to perform the soft switching using the LC resonance. This can reduce the possibility of an increase in the cost or an increase in the size while realizing the reduction in the switching loss through the switching at the zero crossing point in the isolated switching power supply. 
     As a result, the first aspect of the present disclosure can obtain an effect that a small-sized isolated switching power supply with less switching loss can be provided at a low cost. 
     &lt;Second Aspect&gt; 
     A second aspect of the present disclosure provides the isolated switching power supply according to the first aspect of the present disclosure, wherein the first circuit board includes an inverter circuit, and the second circuit board includes a converter circuit. 
     According to the second aspect of the present disclosure, the inverter circuit constituting a circuit on a primary side (input side) is mounted on the first circuit board, and the converter circuit constituting a circuit on a secondary side (output side) is mounted on the second circuit board. This can more efficiently reduce the space. Therefore, the size and the cost of the isolated switching power supply can be further reduced. 
     &lt;Third Aspect&gt; 
     A third aspect of the present disclosure provides the isolated switching power supply according to the first or second aspect of the present disclosure, further including an interval defining member provided between the first circuit board and the second circuit board and configured to define the predetermined interval between the part provided with the primary winding of the first circuit board and the part provided with the secondary winding of the second circuit board. 
     According to the third aspect of the present disclosure, the interval defining member is interposed between the part provided with the primary winding of the first circuit board and the part provided with the secondary winding of the second circuit board to directly define the interval. This can further highly accurately set the leakage inductance of the isolation transformer to the desired value. 
     Advantageous Effects 
     The embodiments of the present disclosure can obtain an effect that a small-sized isolated switching power supply with less switching loss can be provided at a low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating main parts of a circuit of an isolated switching power supply according to the present disclosure. 
         FIG. 2  is a perspective view illustrating the main parts of the isolated switching power supply according to the present disclosure. 
         FIG. 3  is an exploded perspective view illustrating the main parts of the isolated switching power supply according to the present disclosure. 
         FIG. 4  is a front view illustrating the main parts of the isolated switching power supply according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
     Note that the present invention is not particularly limited to the embodiment described below, and it is obvious that various modifications can be made within the scope of the invention written in the claims. 
       FIG. 1  is a circuit diagram illustrating main parts of a circuit of an isolated switching power supply according to the present disclosure. 
     An example of the isolated switching power supply according to the present disclosure is a full-bridge isolated DC-DC converter. However, the isolated switching power supply is not particularly limited to the full-bridge isolated DC-DC converter. For example, the present disclosure can be implemented in isolated DC-DC converters in various formats, such as flyback, forward, and push-pull converters, and the present disclosure can also be implemented in an isolated AC-DC converter, an isolated DC-AC inverter, and the like. 
     The isolated switching power supply according to the present disclosure includes an inverter circuit  11 , a converter circuit  21 , and an isolation transformer T 1 . 
     The inverter circuit  11  includes field effect transistors (FETs) Q 1  to Q 4  and capacitors C 1  to C 5 . The field effect transistors Q 1  to Q 4  as switching elements constitute a well-known full-bridge inverter circuit. The field effect transistors Q 1  to Q 4  are turned on or off at the same time by gate signals output by a control circuit not shown and are turned on or off such that the field effect transistors Q 2  and Q 3  are in opposite phase to the field effect transistors Q 1  and Q 4 . The capacitor C 1  is connected between a + input terminal VIN+ and a − input terminal VIN−. The capacitors C 2  to C 4  are connected in parallel between source and drain terminals of the field effect transistors Q 1  to Q 4  and constitute an LC resonant circuit in soft switching. The alternating current generated by the full-bridge inverter circuit flows to the converter circuit  21  through the isolation transformer T 1 . 
     Note that the capacitors C 2  to C 4  may be constituted by parasitic capacitances between drains and sources of the field effect transistors Q 1  to Q 4 . 
     The inverter circuit  11  further includes a leakage inductance L 1 . The leakage inductance L 1  does not exist as an electronic component provided with a choke coil or the like, but is an inductance component generated on a primary side of the isolation transformer T 1  as a component equivalent to a coil. The leakage inductance L 1  constitutes the LC resonant circuit in the soft switching along with the capacitors C 2  to C 4 . 
     The converter circuit  21  includes field effect transistors Q 5  and Q 6 , a coil L 2 , and a capacitor C 6 . The field effect transistors Q 5  and Q 6  are turned on or off by gate signals output by a control circuit not shown and constitute a well-known synchronous rectifier circuit along with the coil L 2  and the capacitor C 6 . The power converted to a predetermined DC voltage by the synchronous rectifier circuit is output from a + output terminal VOUT+ and a − output terminal VOUT−. 
     The isolation transformer T 1  includes a primary winding L 11 , secondary windings L 21  and L 22 , and a core  30 . A winding finish end of the primary winding L 11  is connected to a connection point of the source terminal of the field effect transistor Q 1  and the drain terminal of the field effect transistor Q 2 , and a winding start end of the primary winding L 11  is connected to a connection point of the source terminal of the field effect transistor Q 3  and the drain terminal of the field effect transistor Q 4 . A winding finish end of the secondary winding L 21  and a winding start end of the secondary winding L 22  are connected, and the connection point (center tap) of the secondary windings L 21  and L 22  are connected to one end side of the coil L 2 . A winding start end of the secondary winding L 21  is connected to the drain terminal of the field effect transistor Q 6 . A winding finish end of the secondary winding L 22  is connected to the drain terminal of the field effect transistor Q 5 . 
       FIG. 2  is a perspective view illustrating main parts of the isolated switching power supply according to the present disclosure.  FIG. 3  is an exploded perspective view of the main parts.  FIG. 4  is a front view of the main parts. 
     The isolated switching power supply according to the present disclosure includes a first circuit board  10 , a second circuit board  20 , the core  30 , and a plurality of spacers  41 . 
     The first circuit board  10  is provided with the inverter circuit  11 . More specifically, the first circuit board  10  is provided with the field effect transistors Q 1  to Q 4  constituting the inverter circuit  11 . The first circuit board  10  is also provided with the capacitors C 1  to C 5  constituting the inverter circuit  11  (not shown in  FIGS. 2 to 4 ). Furthermore, the first circuit board  10  is a multilayer board, and a pattern of the primary winding L 11  constituting the isolation transformer T 1  is formed. 
     The second circuit board  20  is provided with the converter circuit  21 . More specifically, the second circuit board  20  is provided with the field effect transistors Q 5  and Q 6  constituting the converter circuit  21 . The second circuit board  20  is also provided with the coil L 2  and the capacitor C 6  constituting the converter circuit  21  (not shown in  FIGS. 2 to 4 ). Furthermore, the second circuit board  20  is a multilayer board, and patterns of the secondary windings L 21  and L 22  constituting the isolation transformer T 1  are formed. 
     The first circuit board  10  and the second circuit board  20  are arranged such that a portion  12  provided with the pattern of the primary winding L 11  of the first circuit board  10  (hereinafter, referred to as “primary winding pattern portion  12 ”) and a portion  22  provided with the patterns of the secondary windings L 21  and L 22  of the second circuit board  20  (hereinafter, referred to as “secondary winding pattern portion  22 ”) face each other at a predetermined interval G. The primary winding pattern portion  12  of the first circuit board  10  is provided with a rectangular insertion hole  13  at a part corresponding to the inside of the primary winding L 11 . The secondary winding pattern portion  22  of the second circuit board  20  is provided with an insertion hole  23  in the same shape and the same size as the insertion hole  13  of the first circuit board  10 , at a part corresponding to the inside of the secondary windings L 21  and L 22 . 
     The core  30  is a member formed by a magnetic body constituting the isolation transformer T 1  and includes a first core portion  31  and a second core portion  32 . The first core portion  31  is provided with a rectangular columnar protrusion  311  at the center. The core  30  is arranged on the insertion hole  13  of the first circuit board  10  and the insertion hole  23  of the second circuit board  20 , with the protrusion  311  of the first core portion  31  being inserted. 
     The plurality of spacers  41  as “interval defining members” are provided between the first circuit board  10  and the second circuit board  20 . The plurality of spacers  41  are members that define the predetermined interval G between the primary winding pattern portion  12  provided with the primary winding L 11  of the first circuit board  10  and the secondary winding pattern portion  22  provided with the secondary windings L 21  and L 22  of the second circuit board  20 . The first circuit board  10  and the second circuit board  20  are screwed and connected to the plurality of spacers  41 . 
     It is obvious that the “interval defining members” are not particularly limited to the spacers  41 , and the “interval defining members” may be any members as long as the members can define the predetermined interval G between the first circuit board  10  and the second circuit board  20 . For example, an insulation sheet or the like may be interposed between the first circuit board  10  and the second circuit board  20 . 
     In the isolated switching power supply according to the present disclosure described above, the primary winding L 11  constituting the isolation transformer T 1  is formed by a pattern on the first circuit board  10 , and the secondary windings L 21  and L 22  constituting the isolation transformer T 1  are formed by patterns on the second circuit board  20 . The primary winding pattern portion  12  provided with the primary winding L 11  of the first circuit board  10  and the secondary winding pattern portion  22  provided with the secondary windings L 21  and L 22  of the second circuit board  20  are arranged to face each other at the predetermined interval G. 
     Therefore, the interval G between the first circuit board  10  and the second circuit board  20  can be adjusted to adjust a degree of coupling (coupling coefficient) of the primary winding L 11  and the secondary windings L 21  and L 22  of the isolation transformer T 1 , and this can adjust the leakage inductance L 1  of the isolation transformer T 1  to a desired value. More specifically, the length of the plurality of spacers  41  can be adjusted to adjust the interval G, for example. The plurality of spacers  41  are interposed between the primary winding pattern portion  12  of the first circuit board  10  and the secondary winding pattern portion  22  of the second circuit board  20  to directly define the interval G. This can further highly accurately set the leakage inductance L 1  of the isolation transformer T 1  to the desired value. 
     The isolated switching power supply according to the present disclosure can use the leakage inductance L 1  on the primary side of the isolation transformer T 1  in the soft switching using the LC resonance. Therefore, the leakage inductance L 1  on the primary side of the isolation transformer T 1  can be used as an inductance of the LC resonance or as part of the inductance of the LC resonance. The interval G between the first circuit board  10  and the second circuit board  20  can be adjusted to adjust the leakage inductance L 1  on the primary side of the isolation transformer T 1  to the desired value, and a constant of the LC resonance circuit of the soft switching can be highly accurately set to an appropriate constant. 
     In this way, the isolated switching power supply according to the present disclosure allows to use the leakage inductance L 1  on the primary side of the isolation transformer T 1  in the LC resonance of the soft switching. More specifically, the isolated switching power supply according to the present disclosure can eliminate the need to mount a choke coil that is generally large and expensive or can reduce the size of a conventional choke coil, thereby allowing to perform the soft switching using the LC resonance. This can reduce the possibility of an increase in the cost or an increase in the size while realizing a reduction in the switching loss through the switching at a zero crossing point in the isolated switching power supply. Therefore, according to the present disclosure, a small-sized isolated switching power supply with less switching loss can be provided at a low cost. 
     In the present disclosure, it is preferable to mount the inverter circuit  11  constituting the circuit on the primary side (input side) on the first circuit board  10  and to mount the converter circuit  21  constituting the circuit on the secondary side (output side) on the second circuit board  20 . This can more efficiently reduce the space, and the size and the cost of the isolated switching power supply can be further reduced. 
     According to the present disclosure, the primary winding L 11  and the secondary windings L 21  and L 22  constituting the isolation transformer T 1  are separately provided on the first circuit board  10  and the second circuit board  20 , respectively, and the heat generated by the isolation transformer T 1  can be efficiently dissipated. This can reduce the size of a heat dissipating component of the isolation transformer T 1 , and the cost of the isolated switching power supply can be further reduced. Adopting the configuration can individually set the thicknesses, the shapes, and the like of the primary winding L 11  and the secondary windings L 21  and L 22 , and the degree of freedom of design can be improved. Adopting the configuration also allows using a multilayer board with relatively few layers when the isolation transformer T 1  is constituted by the multilayer board, and the manufacturing cost of the isolation transformer T 1  based on the multilayer board can be significantly reduced. 
     EXPLANATION OF REFERENCE SIGNS 
       10  circuit board 
       11  inverter circuit 
       20  circuit board 
       21  converter circuit 
       30  core 
       41  spacer 
     C 1  to C 6  capacitors 
     L 1  leakage inductance 
     L 2  coil 
     L 11  primary winding 
     L 21 , L 22  secondary windings 
     Q 1  to Q 6  field effect transistors 
     T 1  isolation transformer