Abstract:
A switching power supply causes fewer losses, being capable of suppressing surges generated at a switching element, and providing improved conduction noise characteristics. The present invention is a switching power supply, wherein voltages and currents are varied by turning said switching element on and off to convert input voltages into output voltages, comprising a distributed parameter line with a line length of about 
       n   4       
 
(n is an integer of one or more) of a wavelength peculiar to a potential point at said fluctuating potential point in said switching power supply.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a switching power supply, wherein electric power is controlled by turning a switching element on and off, and to a distributed parameter structure for use in said switching power supply. 
   2. Description of the Prior Art 
     FIG. 1  shows a schematic block diagram of a fly-back converter, which is an example of prior art switching power supplies. 
   In  FIG. 1 , an input power supply Vin is connected to an X capacitor C 6 , to a line filter T 2  which is a filter inductor, to a diode bridge D 3 , and to a bulk capacitor C 5 . 
   A voltage Vb of the bulk capacitor C 5  is connected to one end of a primary winding N 1  of a transformer T 1 , the other end of the primary winding N 1  of the transformer T 1  being connected to a voltage Vds of the drain of a switching element Q 1 , and the source of the switching element Q 1  is connected to a stable potential COM. 
   A capacitor C 1 , a resistance R 1 , and a diode D 1  configure a CRD snubber circuit  10  which is a primary snubber circuit. The anode of the diode D 1  is connected to the drain Vds of the switching element Q 1 , one end of the resistance R 1  and one end of the capacitor C 1  being connected to the cathode of the capacitor D 1 , while the other end of the resistance R 1  and the other end of the capacitor C 1  being connected to the voltage Vb. 
   The CRD snubber circuit  10  branches off at the voltage Vds in the main line, which is a route from the primary winding N 1  of the transformer T 1  to the drain of the switching element Q 1 . 
   A secondary winding N 2  of the transformer T 1  is connected to an output Vout via a rectifier circuit of a diode D 2  and a smoothing circuit of a capacitor  4 . 
   A capacitor C 2  and a resistance R 2  are connected serially to configure a CR snubber circuit which is a secondary snubber circuit, and are connected in parallel to the diode D 2 . 
   Behaviors of the above-mentioned prior art embodiment in  FIG. 1  will be explained hereunder. The input power supply Vin is rectified at the diode bridge D 3 , smoothed at the bulk capacitor C 5 , and becomes the voltage Vb. The switching element Q 1  turns on and off the voltage Vb that is applied to the primary winding N 1  of the transformer T 1 . A voltage induced at the secondary winding N 2  of the transformer T 1  is rectified at the diode D 2 , smoothed at the capacitor C 4 , and becomes the output voltage Vout. 
   The voltage Vds changes in square waves when the switching element Q 1  is turned on and off. Also, a surge is generated when the switching element Q 1  is turned on and off. The surge is influenced by the parasitic inductance and parasitic capacity of the transformer T 1  and by the output capacity and switching characteristics of the switching element Q 1 . The CRD snubber circuit  10  suppresses a voltage surge, which is generated when the switching element Q 1  is turned off. 
   More specifically, when the switching element Q 1  is turned on, the voltage Vds is low and the diode D 1  is turned off. Then, when the switching element Q 1  is turned off, the voltage Vds increases while a voltage surge is generated. When the voltage Vds increases, the diode D 1  is turned on while the capacitor C 1  provides an electric charge. Increase of the voltage Vds is suppressed when the capacitor C 1  provides an electric charge. Electric charges of the capacitor C 1  are discharged at the resistance R 1 . 
   A part of the noise that is generated when the switching element Q 1  is turned on or off is passed on to the input power supply Vin via the diode bridge D 3 , the line filter T 2 , and the X capacitor C 6 . The main inductance of the line filter T 2  attenuates common mode elements of noise. The leaked inductance of the line filter T 2  and the X capacitor C 6  attenuate normal mode elements of noise. 
     FIG. 2  illustrates the waveforms of the voltage Vds that was generated when the switching element Q 1  was turned off in the prior art embodiment of  FIG. 1. A  voltage surge was generated when the frequency was approximately 7 MHz. When the diode D 1  was turned on at the high voltage point P, the capacitor C 1  clamped the oscillations of the voltage surge of the voltage Vds. The amplitude of the voltage surge of the voltage Vds was attenuated gradually as the energy became heat, noise, and others. 
     FIG. 3  illustrates conduction noise characteristics of the prior art embodiment in FIG.  1 . In  FIG. 3 , the portion A shows the noise which peaked at the frequency of 8 MHz. The portion A was generated when a voltage surge of the voltage Vds in  FIG. 2  became conduction noise. The reason why frequencies did not match in  FIGS. 2 and 3  was that they were mainly influenced by parasitic capacities of probes when waveforms were measured. 
   Also, the CR snubber circuit comprising the capacitor C 2  and the resistance R 2  suppresses a voltage surge generated at the diode D 2 . 
   Moreover, some of the prior art switching power supplies have wirings equipped at their transformers in order to eliminate common mode signals (for example, see the Japanese Utility Model Gazette 1988-30230 according to the concept proposed by the present applicant). 
   An object of such prior art embodiments is to realize an insulated DC power supply circuit that is less influenced by common mode signals by means of windings of a transformer. The object, however, cannot be a cause or a motivation of suppression of surges generated at a switching element. Furthermore, the object does not include any intention to add windings to filter inductors. 
   On the other hand, FIG.  4 ( a ) and FIG.  4 ( b ) are schematic diagrams of a prior art micro strip line and show a distributed parameter structure. 
   FIG.  4 ( a ) shows a perspective diagram. A distributed parameter line Z 1  branches off at the point S in the main line which runs from the input port to the output port. The distributed parameter line Z 1  is open-ended and becomes an open stub. 
   When the line length L of the distributed parameter line Z 1  is 
         1   4     ·   λ       
 
(λ is a wavelength), the distributed parameter line Z 1  acts as a filter for the wavelength λ and separates specific frequency elements of signals propagated in the main line.
 
   FIG.  4 ( b ) shows a cross section. The distributed parameter line Z 1  is formed by a conductor of the width W and the thickness t on a flat plate. A stable potential surface Z 2  is connected to a stable potential GND, is formed by a conductor on a flat plate which is sufficiently wider than the distributed parameter line Z 1 , and is arranged in parallel with the distributed parameter line Z 1 . A dielectric Z 3  having the thickness h and the relative dielectric constant ∈r is formed so that it is placed between the distributed parameter line Z 1  and the stable potential surface Z 2 . 
   Accordingly, in the prior art distributed parameter structure, the distributed parameter line Z 1  is formed as a linear and flat conductor on a flat surface. The stable potential surface Z 2  is formed as a flat conductor. 
   Next, the distributed parameter line Z 1  is explained in detail. A frequency f and a wavelength λ have approximately the following relationship:
 
λ= C/f/Sqrt  (∈ r )
 
   Here, C is the speed of light (3*10 8  m/s) and ∈r is the relative dielectric constant of the dielectric Z 3  (4.21 in the case of polyurethane). A wavelength in the dielectric Z 3  is proportional to the inverse number of the square root of the relative dielectric constant ∈r. That is, the wavelength is reduced to 1/Sqrt (∈r) in comparison with the wavelength in vacuum. 
   For example, when f=7 MHz and ∈r=4.21 are given, λ=20.9 m is produced and consequently 
           1   4     ·   λ     =     5.22   ⁢           ⁢   m         
 
is obtained. The characteristics of the distributed parameter line Z 1  are almost determined by its line length L. Influences of its width W and thickness t, the thickness h of the dielectric Z 3 , and others are small.
 
   However, these switching power supplies have problems such as increased losses due to a resistance R 1  and deterioration of conduction noise characteristics. 
   In addition, if a prior art distributed parameter structure Z 1  is applied in a wavelength corresponding to a frequency band (MHz band) that is liable to cause a problem for switching power supplies, there is another problem of larger size. 
   More specifically, if a distributed parameter line Z 1  of 5.22 m is formed linearly, a switching power supply becomes larger in size and consequently impractical. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to solve the above-mentioned problems by providing a switching power supply, which causes fewer losses, is capable of suppressing surges generated at a switching element, and provides improved conduction noise characteristics. 
   Another object of the present invention is to provide a distributed parameter structure, whereby the size of the switching power supply can be reduced for wavelengths corresponding to bands of low frequencies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of a prior art switching power supply. 
       FIG. 2  illustrates waveforms of the voltage Vds of the prior art embodiment in FIG.  1 . 
       FIG. 3  illustrates conduction noise characteristics of the prior art embodiment in FIG.  1 . 
     FIGS.  4 ( a ) and  4 ( b ) are schematic diagrams of a prior art micro strip line, with  4 ( a ) being a perspective diagram and ( b ) being a cross section. 
       FIG. 5  is a schematic block diagram showing an embodiment of the present invention. 
       FIG. 6  is a schematic diagram showing an embodiment of a distributed parameter snubber circuit  20 . 
       FIG. 7  is a schematic diagram showing another embodiment of the distributed parameter snubber circuit  20 . 
       FIG. 8  illustrates voltage and amplitude characteristics of an open stub. 
       FIG. 9  illustrates impedance characteristics of the distributed parameter snubber circuit  20 . 
       FIG. 10  illustrates waveforms of the voltage Vds of the embodiment in FIG.  5 . 
       FIG. 11  illustrates conduction noise characteristics of the embodiment in FIG.  5 . 
       FIG. 12  is a schematic block showing a second embodiment of the present invention. 
       FIG. 13  is a cross section of a wiring structure of a compound magnetic element  30 . 
       FIG. 14  is a cross section of another wiring structure of the compound magnetic element  30 . 
       FIG. 15  is a schematic block diagram showing a third embodiment of the present invention. 
       FIG. 16  is a cross section of a wiring structure of a compound magnetic element  40 . 
       FIG. 17  is an external perspective diagram of the compound magnetic element  40 . 
       FIG. 18  is a schematic block diagram showing a fourth embodiment of the present invention. 
       FIG. 19  illustrates attenuation characteristics of a line filter T 2  and a compound magnetic element  50 . 
       FIG. 20  illustrates conduction noise characteristics of the embodiment in FIG.  18 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described in detail with reference to FIG.  5 .  FIG. 5  is a schematic block diagram showing an embodiment of a switching power supply according to the present invention. The same elements of said embodiment as those of a prior art embodiment in  FIG. 1  will be given the same symbols and their explanations will be omitted. 
   The embodiment in  FIG. 5  is characterized in that a distributed parameter snubber circuit  20  is provided as a primary snubber circuit, comprising a distributed parameter line Z 1 , a stable potential surface Z 2 , and a dielectric Z 3 . 
   In the distributed parameter snubber circuit  20 , the distributed parameter line Z 1 , the stable potential surface Z 2 , and the dielectric Z 3  configure a distributed parameter filtering circuit. 
   In the main line which is a route from a primary winding N 1  of a transformer T 1  to the drain of a switching element Q 1 , the distributed parameter snubber circuit  20  configures a filter on the distributed parameter line Z 1  branching off at the voltage Vds, and is formed in an open stub. 
   One end of the distributed parameter line Z 1  is connected to the voltage Vds which is a fluctuating potential point in a switching power supply, and the other end of the distributed parameter line Z 1  is open. The distributed parameter line Z 1  has a line length which is one fourth of a wavelength corresponding to the 7 MHz frequency of a voltage surge peculiar to the voltage Vds. 
   The stable potential surface Z 2  is arranged adjacent to the distributed parameter line Z 1  and is connected to a voltage Vb which is a stable potential. The dielectric Z 3  is provided between the distributed parameter line Z 1  and the stable potential surface Z 2 . 
     FIG. 6  is a schematic diagram showing an embodiment of the distributed parameter snubber circuit  20  and shows a distributed parameter structure. The distributed parameter line Z 1  is formed windingly in a folded structure on a film  21 , is arranged adjacent to a dielectric Z 3  which is formed in foils, is further arranged adjacent to a stable potential surface Z 2  with equal potential surfaces which is formed in foils, is still further arranged adjacent to an insulating film  22 , and at the same time forms all of the foregoing in a winding and folded manner like a film capacitor. 
   One end of the distributed parameter line Z 1  is connected to a drawing line  23  and the drawing line  23  is connected to the voltage Vds, which is a fluctuating potential point in the switching power supply. The other end of the distributed parameter line Z 1  is open. 
   The drawing line  23  is a part of the distributed parameter line Z 1  and the line length of the drawing line  23  becomes a part of the line length of the distributed parameter line Z 1 . 
   Similarly, the stable potential surface Z 2  is connected to a drawing line  24  and the drawing line  24  is connected to the voltage Vb, which is a stable potential in the switching power supply. 
     FIG. 7  is a schematic diagram showing another embodiment of the distributed parameter snubber circuit  20  and shows a distributed parameter structure. The schematic diagram of  FIG. 7  is characterized in that the distributed parameter snubber circuit  20  comprises multilayer print coils. 
   The distributed parameter line Z 1  is formed in a whirling manner of a winding structure on a wiring layer  26 , is arranged adjacent to a dielectric Z 3 , is further arranged adjacent to a wiring layer  27  on which a stable potential surface Z 2  is arranged, and forms a structure in which all of the foregoing are stacked like multilayer print coils. 
   One end of the distributed parameter line Z 1  is connected to a connection hole  28 , the connection hole  28  is connected to a means  29  for linking connection holes, and the means  29  for linking connection holes is connected to a connection hole P 11  of a printed circuit board  25  of the switching power supply and is connected to the voltage Vds which is a fluctuating potential point in the switching power supply. The other end of the distributed parameter line Z 1  is open. 
   The connection hole  28 , the means  29  for linking connection holes, and the connection hole P 11  constitute a part of the distributed parameter line Z 1 , and their line lengths constitute a part of the line length of the distributed parameter line Z 1 . 
   Similarly, the stable potential surface Z 2  is connected to a connection hole P 23  of the printed circuit board  25  of the switching power supply via connection holes and a means for linking connection holes, and is connected to the voltage Vb which is a stable potential in the switching power supply. 
   Next, the line length of the distributed parameter line Z 1  is explained. As in a prior art embodiment of  FIG. 4 , when f=7 MHz and ∈r=4.21 are given, λ=20.9 m is produced and consequently 
           1   4     ·   λ     =     5.22   ⁢           ⁢   m         
 
is obtained.
 
     FIG. 8  illustrates voltage and amplitude characteristics of an open stub in the distributed parameter snubber circuit  20 . The open stub branches off at the voltage Vds in the main line which is a route from the primary winding N 1  of the transformer T 1  to the drain of the switching element Q 1 . The voltage Vds at the branch point of the open stub is a connection point among the primary winding N 1  of the transformer T 1 , the drain end of the switching element Q 1 , and the distributed parameter line Z 1 . Although the schematic diagram of  FIG. 6  shows that the distributed parameter line Z 1  is linearly arranged on a flat surface, it is actually formed as a distributed parameter structure, which is a winding or folded structure. 
   The amplitude of a frequency, for which the line length L of the distributed parameter line Z 1  is 
           1   4     ·   λ     ,       
 
becomes largest at an open end and zero at a branch end, while the branch end acts as a filter. Similarly, the branch end also acts as a filter for frequencies for which the line length of the distributed parameter line Z 1  is 
             3   4     ·   λ     ⁢           ⁢   or   ⁢           ⁢       5   4     ·   λ       ,       
 
and separates specific frequency elements of signals that propagate on the route from the primary winding N 1  of the transformer T 1  to the drain of the switching element Q 1 .
 
   Similarly, in the voltage and amplitude characteristics (not shown in a figure) of a short stub comprising short-circuit ends, the amplitude of the frequency for which the line length of the distributed parameter line Z 1  is 
         1   2     ·   λ       
 
becomes largest at the center of the distributed parameter line Z 1  and zero at a branch end and a short circuit end, while the branch end acts as a filter.
 
     FIG. 9  illustrates impedance characteristics of the distributed parameter snubber circuit  20 . The amplitude (gain) becomes the lowest and the phase is reversed at the 7 MHz frequency almost at the point P. 
   As is evident from the above, in a frequency band (MHz band), which becomes a problem in a switching power supply, and in a low frequency band, characteristics of the distributed parameter line Z 1  are almost determined by its line length L, while influences of its width W, its thickness, and others are small. In addition, influences of whether it is arranged linearly or in coils or whether it is wound before folded or not are small. 
   Accordingly, with a distributed parameter structure where the distributed parameter line Z 1  is formed in a winding or folded structure, the size of the switching power supply can be reduced for wavelengths corresponding to bands of low frequencies. 
   Now, behaviors of such an embodiment in  FIG. 5  will be explained hereunder. Explanations of behaviors similar to those in a prior art embodiment in  FIG. 1  will be omitted. The distributed parameter snubber circuit  20  suppresses a voltage surge that is generated when the switching element Q 1  is turned off. 
   More specifically, when the switching element Q 1  is turned on, the voltage Vds becomes low. Next, when the switching element Q 1  is turned off, the voltage Vds rises and, at the same time, the voltage surge is generated. Elements of the 7 MHz frequency of the voltage surge become low in impedance and are suppressed at the voltage Vds of the branch end of the distributed parameter line Z 1 . On the other hand, elements of the 7 MHz frequency of the voltage surge become electromagnetic waves and are emitted at an open end of the distributed parameter line Z 1 , but are shielded at the stable potential surface Z 2 . 
     FIG. 10  illustrates waveforms of a voltage Vds when the switching element Q 1  is turned off in an embodiment of FIG.  5 . Elements of the 7 MHz frequency of the voltage surge disappear, while elements of almost 3 MHz are generated. Accordingly, peaks of the voltage surge of the voltage Vds are suppressed. Losses are rarely caused at the distributed parameter snubber circuit  20 . 
     FIG. 11  illustrates conduction noise characteristics of an embodiment of FIG.  5 . In comparison with a prior art embodiment of  FIG. 3 , the noise with its peak at the 8 MHz frequency of the portion A is lower. This is because 7 MHz elements of the voltage surge of the voltage Vds are suppressed by the distributed parameter snubber circuit  20 . 
   Accordingly, the distributed parameter snubber circuit  20  suppresses, with fewer losses, a voltage surge that is generated when the switching element Q 1  is turned off. 
   In addition, the distributed parameter snubber circuit  20  can be formed in a practical size. 
     FIG. 12  shows a schematic block diagram of a second embodiment of a switching power supply according to the present invention. The same symbols will be given to the same elements as those of an embodiment of FIG.  5  and explanations will be omitted. 
   The embodiment of  FIG. 12  is characterized in that a transformer and a primary snubber circuit are configured by a compound magnetic element  30  which comprises a distributed parameter line Z 1 , a stable potential surface Z 2 , a dielectric Z 3 , a primary winding N 1 , and a secondary winding N 2 . 
   More specifically, the primary winding N 1 , the secondary winding N 2 , the distributed parameter line Z 1 , and the stable potential surface Z 2  are wound around the same core in the compound magnetic element  30 . 
     FIG. 13  shows a cross section of a winding structure of the compound magnetic element  30  in the embodiment of FIG.  12 . In said cross section, the bottom shows the inside of the compound magnetic element  30 , while the top shows the outside of the compound magnetic element  30 . The respective windings and stable potential surfaces configure the layers. Starting from the inside, a secondary winding N 2 , a primary winding N 1 , a stable potential Z 2   a , a distributed parameter line Z 1 , and a stable potential surface Z 2   b  are arranged. The respective layers are arranged in a bobbin B and a core C is arranged outside the bobbin B. 
   One end of the distributed parameter line Z 1  is connected to the voltage Vds which is a fluctuating potential point in a switching power supply, while the other end is open. The distributed parameter line Z 1  has a line length which is one fourth of a wavelength corresponding to the 7 MHz frequency of a voltage surge peculiar to the voltage Vds. 
   The stable potential surface Z 2  comprises the stable potential Z 2   a  and the stable potential surface Z 2   b , is arranged adjacent to the distributed parameter line Z 1 , and is connected to the voltage Vb which is a stable potential. The stable potential Z 2   a  and the stable potential surface Z 2   b  are formed in foils and are formed as Faraday shields in one turn so that both their ends will not be short-circuited. 
   The distributed parameter line Z 1  is coated by polyurethane of the dielectric Z 3  and is arranged closely with the stable potential surface Z 2 . The stable potential Z 2   a  and the stable potential surface Z 2   b  are arranged across the distributed parameter line Z 1  and have a distributed parameter structure which is formed in coils of a winding structure. This structure enhances shield effects. 
     FIG. 14  shows a cross section of another winding structure in a compound magnetic element  30  of an embodiment of FIG.  12 . The cross section of  FIG. 14  is characterized in that, compared with the cross section of  FIG. 13 , a stable potential line Z 2   c  instead of the stable potential surface Z 2  is arranged adjacent to the distributed parameter line Z 1  and that the distributed parameter line Z 1  and the stable potential line Z 2   c  are formed in coils of a winding structure. Explanations of the same portions as those in the cross section of  FIG. 13  will be omitted. 
   More specifically, respective windings and layers of stable potential lines are, starting from the inside, the secondary winding N 2 , the primary winding N 1 , the distributed parameter line Z 1 , and the stable potential line Z 2   c.    
   The distributed parameter line Z 1  and the stable potential line Z 2   c  are bifilarly wound, are arranged in parallel and adjacent to the same line lengths, and at the same time have a stable parameter structure which forms the distributed parameter line Z 1  and the stable potential line Z 2   c  in a winding structure. 
   A connection end of the voltage Vds of the distributed parameter line Z 1  and an open end of the stable potential line Z 2   c  are matched, while an open end of the distributed parameter line Z 1  and a connection end of the voltage Vb, which is a stable potential of the stable potential line Z 2   c , are matched. 
   When the distributed parameter line Z 1  and the stable potential line Z 2   c  are bifilarly wound, the distributed parameter line Z 1  and the stable potential line Z 2   c  are connected closely and a voltage induced by the magnetic flux of the core C is cancelled so that preferred characteristics will be provided for suppressing surges. 
   Explanations of the foregoing behaviors of the embodiment of  FIG. 12  will be omitted, because they are the same as in the case of the embodiment of FIG.  5 . In comparison with the embodiment of  FIG. 5 , the embodiment of  FIG. 12  contains fewer elements, is smaller in size, and costs less. 
   Although the distributed parameter line Z 1  and the stable potential line Z 2   c  are bifilarly wound in the above example, the distributed parameter line Z 1  and the stable potential line Z 2   c  may be formed differently with a coaxial cable and have the distributed parameter structure in which they are formed in a winding structure. More specifically, either one of the distributed parameter line Z 1  and the stable potential line Z 2   c  is used as an internal conductor of the coaxial cable, while the other is used as an external conductor of the coaxial cable. 
     FIG. 15  is a schematic block diagram showing a third embodiment of a switching power supply according to the present invention. The same symbols will be given to the same elements as those of the embodiment of FIG.  12  and their explanations will be omitted. 
   An embodiment of  FIG. 15  is characterized in that a transformer, a primary snubber circuit, and a secondary snubber circuit are configured by a compound magnetic element  40  which comprises a distributed parameter line Z 1 , a stable potential surface Z 2 , a dielectric Z 3 , a primary winding N 1 , a secondary winding N 2 , a distributed parameter line Z 4 , a stable potential surface Z 5 , and a dielectric Z 6 . 
   The distributed parameter line Z 4 , the stable potential surface Z 5 , and the dielectric Z 6  configure a distributed parameter snubber circuit as in the case of the distributed parameter line Z 1 , the stable potential surface Z 2 , and the dielectric Z 3 , and suppress voltage surges generated at a diode D 2 . 
   One end of the distributed parameter line Z 4  is connected to the anode of the diode  2  which is a fluctuating potential point in a switching power supply, and the other end is open. The distributed parameter line Z 4  has a line length which is one fourth of a wavelength for a voltage surge peculiar to the anode of the diode D 2 . 
   The stable potential surface Z 5  is arranged adjacent to the distributed parameter line Z 4  and is connected to the stable potential GND. The dielectric Z 3  is provided between the distributed parameter line Z 1  and the stable potential surface Z 2 . 
   Since behaviors of the distributed parameter line Z 4 , the stable potential surface Z 5 , and the dielectric Z 6  are the same as those of the distributed parameter snubber circuit  20  of an embodiment in  FIG. 5 , their explanations will be omitted. 
     FIG. 16  shows a cross section of the winding structure in the compound magnetic element  40  of the embodiment of FIG.  15 . The cross section of  FIG. 16  is characterized in that windings and stable potential surfaces are arranged outside the core C and that they are formed windingly. 
   All windings and stable potential surfaces of the embodiment in  FIG. 15  may also be arranged inside the core C as shown in the cross sections of  FIGS. 13 and 14 . Similarly, windings and stable potential surfaces of the embodiment in  FIG. 12  may also be arranged outside the core C as shown in the cross section of  FIG. 16 , and similar effects can be obtained. 
   In the cross section of  FIG. 16 , the bottom shows the inside of the compound magnetic element  40 , while the top shows the outside of the compound magnetic element  40 . The respective windings and stable potential surfaces configure layers. A secondary winding N 2  and a primary winding N 1  are arranged in a bobbin Ba inside the core C, while a distributed parameter line Z 1 , a stable potential surface Z 2 , a distributed parameter line Z 4 , and a stable potential surface Z 6  are arranged in a bobbin Bb outside the core C, and are formed in a winding structure to provide a distributed parameter structure. 
   The stable potential surface Z 2  and the stable potential surface Z 5  are formed in foils and, because both their ends are short-circuited as short strings, their potential surfaces are equivalent and their potentials are stable. In addition, since magnetic fluxes generated by the primary winding N 1  and the secondary winding N 2  do not cross-link the distributed parameter line Z 1 , the stable potential surface Z 2 , the distributed parameter line Z 4 , and the stable potential surface Z 5 , electric potentials become stable and provide preferred characteristics to suppress surges. 
     FIG. 17  shows an external perspective diagram of a compound magnetic element  40  in  FIG. 16. A  bobbin Bb is arranged outside a core C. 
   Since behaviors of the embodiment in  FIG. 15  as shown in the above are the same as those of the embodiment in  FIG. 12 , their explanations will be omitted. In comparison with the embodiment of  FIG. 12 , the embodiment of  FIG. 15  requires fewer elements, becomes smaller in size costs less, and causes fewer losses. 
   Although the distributed parameter line Z 1 , the stable potential surface Z 2 , and the dielectric Z 3 , as well as the distributed parameter line Z 4 , the stable potential surface Z 5 , and the dielectric Z 6  configure open stubs in the above example, they may differently configure short stubs. 
   More specifically, one end of the distributed parameter line Z 1  is connected to the voltage Vds which is a fluctuating potential point in a switching power supply, while the other end is connected to the voltage Vb, which is a stable potential, and has a line length which is approximately a half of a wavelength in a voltage surge peculiar to the voltage Vds. One end of the distributed parameter line Z 4  is connected to the anode of a diode D 2  which is a fluctuating potential point in a switching power supply, while the other end is connected to a stable potential GND and has a line length which is approximately a half of a wavelength in a voltage surge peculiar to the anode of the diode D 2 . 
   Although explanations for the behaviors of such cases are omitted because they are the same as those of the aforementioned embodiment, a short stub can enhance shield effects because ends of the distributed parameter lines Z 1  and Z 4  become stable potentials and their amplitudes become largest at the centers of stable potential surfaces Z 2  and Z 5 . 
   While the above example uses a fly-back converter, a forward converter, a non-isolated converter, or other converter methods may be-used. Similar effects can also be obtained in these cases. 
   Also, while a snubber circuit and a transformer are combined as a compound magnetic element in the above example, the snubber circuit and a smoothing choke may be differently combined as a compound magnetic element. The size of an embodiment can also be reduced in this case. 
     FIG. 18  shows a schematic block diagram of a fourth embodiment of a switching power supply according to the present invention. The same symbols will be given to the same elements as those of the prior art embodiment of FIG.  1  and explanations will be omitted. 
   An embodiment of  FIG. 18  is characterized by the configuration of a compound magnetic element  50  which is formed by winding distributed parameter lines Z 7  to Z 11 , a stable potential surface Z 12 , a dielectric Z 13 , distributed parameter lines Z 14  to Z 18 , a stable potential surface Z 19 , and a dielectric Z 20  around the line filter T 2  which is a filter inductor in FIG.  1 . 
   The compound magnetic element  50  acts as a filter suppressing noise on a line. Distributed parameter lines can not only suppress surges as shown by embodiments of  FIGS. 1 ,  12 , and  15 , but also can suppress noise as shown in the embodiment of FIG.  18 . 
   More specifically, distributed parameter lines Z 7  to Z 11 , a stable potential surface Z 12 , and a dielectric Z 13 , as well as distributed parameter lines Z 14  to Z 18 , a stable potential surface Z 19 , and a dielectric Z 20  are arranged symmetrically. 
   One end of each of distributed parameter lines Z 7  to Z 11  is connected to the point Vc connecting with a line filter and a diode bridge D 3 , which is a fluctuating potential point in a switching power supply, while other ends are open. Similarly, one end of each of distributed parameter lines Z 14  to Z 18  is connected to the point Vd connecting with a line filter and a diode bridge D 3 , which is a fluctuating potential point in a switching power supply, while other ends are open. 
   In the route from the line filter to the diode bridge D 3  which is a main line, distributed parameter lines Z 7  to Z 11  and distributed parameter lines Z 14  to Z 18 , which branch off at connection points Vc and Vd, configure a filter and separate specific frequency elements of signals that propagate on the main line. 
   Distributed parameter lines Z 7  to Z 11  and distributed parameter lines Z 14  to Z 18  connect five distributed parameter lines in parallel respectively. The lengths of five distributed parameter lines are set to be 15 m, 1.5 m, 1 m, 0.75 m, and 0.5 m respectively. 
   Stable potential surfaces Z 12  and Z 19  are arranged adjacent to distributed parameter lines Z 7  to Z 11  and distributed parameter lines Z 14  to Z 18 , and are connected to the stable potential GND. Dielectrics Z 13  and Z 20  are to be provided between distributed parameter lines Z 7  to Z 11  and distributed parameter lines Z 14  to Z 18  on the one hand and stable potential surfaces Z 12  and Z 19  on the other. 
   More specifically, distributed parameter lines Z 7  to Z 11  and distributed parameter lines Z 14  to Z 18  are coated with polyurethane of dielectrics Z 13  and Z 20 , are closely arranged with stable potential surfaces Z 12  and Z 19 , and at the same time are in a distributed parameter structure where they are wound around the core of a line filter T 2 . 
   Distributed parameter lines are connected in parallel so that their filtering characteristics will be provided in a wide band. Lengths of distributed parameter lines are 15 m, 1.5 m, 1 m, 0.75 m, and 0.5 m, which correspond to frequencies 2.4 MHz, 24 MHz, 37 MHz, 49 MHz, and 73 MHz respectively according to ∈r=4.21. 
     FIG. 19  illustrates attenuation characteristics of a line filter T 2  and a compound magnetic element  50 . A characteristic Ca shows a characteristic of the line filter T 2  in the prior art embodiment of  FIG. 1. A  characteristic Cb shows a characteristic of the compound magnetic element  50  in the embodiment of FIG.  18 . 
   The characteristic Ca of the line filter T 2  has a resonant point at around 500 kHz and the largest attenuation value can be obtained near the resonant point. In a frequency higher than the resonant point, attenuation values increase and filtering characteristics deteriorate. 
   Although the characteristic Cb of the compound magnetic element  50  is equivalent to the characteristic Ca at a frequency lower than the resonant point of 500 kHz, its filtering characteristic is enhanced because the increase of attenuation values is suppressed by the effects of distributed parameter lines Z 7  to Z 11  and distributed parameter lines Z 14  to Z 18  at a frequency higher than the resonant point. 
   In the characteristic Cb, peak characteristics Pa and Pb can be observed at the frequencies 2.4 MHz and 24 MHz which are peculiar to distributed parameter lines. 
     FIG. 20  illustrates conduction noise characteristics of an embodiment of FIG.  18 . In comparison with a prior art embodiment of  FIG. 3 , the noise level decreases at a frequency higher than the resonant point of 500 kHz. Noise is suppressed by the compound magnetic element  50 . 
   Although the above example shows an arrangement of a distributed parameter line for each of the two lines of the line filter, which is a filtering inductor, to suppress common mode noise, similar effects can be obtained by arranging distributed parameter lines differently to normal mode chokes (not shown in a diagram), which are filtering inductors, in order to suppress normal mode noise. 
   As is evident from the foregoing explanations, the present invention provides the following effects: 
   According to one aspect of the present invention described in claim  1 , fluctuation of electric potentials in a switching power supply can be suppressed. Voltage surges and noise can be suppressed. 
   According to another aspect of the present invention described in claim  2 , emissions of electromagnetic waves from a distributed parameter line can be shielded. Accordingly, noise characteristics of a switching power supply can be improved. 
   According to yet another aspect of the present invention described in claim  3 , a length of a distributed parameter line can be minimized. Accordingly, a switching power supply can be made smaller and less costly. 
   According to yet another aspect of the present invention described in claim  4 , the other end of a distributed parameter line can be stabilized to enhance shield effects. Accordingly, noise characteristics of a switching power supply can be improved. 
   According to yet another aspect of the present invention described in claims  5  to  7 , the number of parts configuring a switching power supply can be reduced to make it smaller and less costly, and so it can be formed in a practical size. 
   According to yet another aspect of the present invention described in claim  8  or  9 , a distributed parameter structure is provided to enable the formation of smaller elements. Areas of implementation can thus be made smaller, and elements can be formed in a practical size in a switching power supply. 
   According to yet another aspect of the present invention described in claim  10 , electric potential fluctuations and noise in a wide band can be suppressed. Noise can also be suppressed in a switching power supply. 
   According to yet another aspect of the present invention described in claim  11 , a single element can suppress a plurality of voltage surges in a switching power supply. Common mode noise can also be suppressed in the switching power supply.