Abstract:
In a high-voltage generating transformer having primary and secondary coil bobbins coaxially mounted on one another relative to a center core, an element wire of the primary coil is wound spirally around the primary coil bobbin in such a way that the both staring and terminating ends of the wire are disposed at the same end of the coil bobbin by forward and backward winding of the wire. This enables the transformer to have considerably reduced number of turns of the wires in the primary coil relative to that in the secondary coil, thus improving the voltage transformation ratio and obtaining a sufficiently high voltage at the secondary side.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a high-voltage generating transformer and particularly to a high-voltage generating transformer for use in a discharge lamp lighting circuit for a headlight of a vehicle. 
     A conventional high-voltage generating transformer of the type that a primary coil-wound bobbin and a secondary coil-wound bobbin are assembled coaxially relative to a center core has a large size because of providing a large transformer ratio (i.e., the ratio of the number of turns in the secondary winding to the number of the primary winding) to produce a high voltage at the secondary side of the transformer. 
     A typical attempt has been made to reduce in size and weight a conventional transformer by reducing the number of turns in its primary coil to 3-5 turns and correspondingly reducing the number of turns in the secondary coil. However, the attempt resulted in that the working efficiency of the transformer was decreased by an increased leakage of magnetic flux resulted from the decreased coverage ratio of the primary coil to the secondary coil. 
     As described above, the conventional high-voltage generating transformer having coaxially mounted primary and secondary coil-wound bobbins with a center core may be reduced in size and weight by considerably reducing the number of turns in the primary winding to 3-5 but encounters a problem of increasing leakage of magnetic flux and decreasing the working efficiency. This makes the transformer be unable to generate a sufficiently high secondary voltage. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a high-voltage generating transformer having coaxially mounted primary and secondary coil-wound bobbins with a center core, which is reduced in size and weight by minimizing the number of turns in the primary winding and, at the same time, can obtain a sufficiently high voltage at its secondary side, effectively preventing leakage of magnetic flux. This is realized by spirally winding an element wire around the primary coil bobbin in forward and backward direction to form thereon the primary coil whose axial length is long enough to meet with the secondary coil and whose starting and terminating ends are disposed at the same side of the bobbin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is illustrative of an arrangement of a discharge lamp socket and a high-voltage generating transformer used in a lamp-lighting circuit. 
     FIG. 2 is a block diagram of a discharge lamp lighting circuit. 
     FIG. 3 is a plan view of a high-voltage generating transformer embodying the present invention. 
     FIG. 4 is a side view of the high-voltage generating transformer of FIG.  3 . 
     FIG. 5 is a perspective illustration of the high-voltage generating transformer of FIG.  3 . 
     FIG. 6 is a front view of a coil unit of the high-voltage generating transformer of FIG.  3 . 
     FIG. 7 is a right-side view of a coil unit of the high-voltage generating transformer of FIG.  3 . 
     FIG. 8 is a left-side view of a coil unit of the high-voltage generating transformer of FIG.  3 . 
     FIG. 9 is a plan view of a primary coil bobbin of the high-voltage generating transformer of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is illustrative of an exemplified arrangement of a discharge lamp socket  1  and a high-voltage generating transformer  2  for a circuit for lighting a discharge lamp used as a headlight of a vehicle. 
     FIG. 2 is illustrative of a lamp-lighting circuit for lighting a discharge lamp  7 , which includes a control circuit CNT that receives a driving voltage E (DC 400V) when a lamp switch (not shown) is turned on and the controlled voltage is applied to the primary side of the high-voltage generating transformer  2  that in turn produces at its secondary side a high voltage (about 25 KV) for lighting the discharge lamp  7 . After firing the discharge lamp  7  by applying the high voltage for an initial firing period, the control circuit CNT operates to directly supply the discharge lamp with a working voltage (100V) through the secondary side coil of the high-voltage generating transformer T to maintain the lamp in a lightning mode. 
     In FIGS. 1 and 2, numeral  8  designates a high-voltage cable for the high-voltage side ( 2 E) of the high-voltage generating transformer  2  and numeral  9  designates a high-voltage cable for the low-voltage side ( 1 E) of the transformer. 
     The high-voltage generating transformer  2  according to the present invention is constructed as shown in FIGS. 3 to  8 . 
     As shown in FIGS. 3 to  6 , a coil bobbin  11  with a secondary coil  10  wound thereon and a rod-like core  14  inserted in the bobbin&#39;s hollow center is mounted in a coil case  15  made in the form of an open-top tub. After wiring of the coil ends as described later, all inside components are then integrally potted in the coil case  15  with insulating resin poured in a melted state and solidified therein. The coil case  15  is further provided at its side portion with a protecting cover  16  being channel-shaped in cross section, which fits on the case  12  by its spring force of both wings to shut off the possible effect of a high-voltage to other external circuit components. 
     In the transformer according to the present invention, as seen in FIG. 9, an element wire  121  is spirally wound around a primary coil bobbin  13  in forward and backward directions to form thereon a primary coil  12  of which the axial length is substantially equal to that of the secondary coil and of which the starting and terminating ends ( 1 S) and ( 1 E) are disposed at the same end of the bobbin  13 . The primary coil bobbin  13  has guiding grooves  17  spirally cut therearound for spiral winding the element wire  121  and a pin  18  formed at turning point thereof for supporting the returning part of the wire  121 . 
     The ratio of the number of turns in the primary coil  12  to the number of turns in the secondary coil is within the range of 1:90-100 to obtain a voltage of about 25 KV at the secondary side of the transformer. 
     The considerable reduction of the number of turns of the primary coil  12  enables the corresponding reduction of the number of turns of the secondary coil  10 , realizing the saving in size and weight of the whole transformer. In this case, the leakage of magnetic flux is effectively prevented since the primary coil has an axial length substantially equal to that of the secondary coil. The spiral winding of the element wire around the primary bobbin  13  along the forward and backward guiding grooves  17  eliminates the problem that a magnetic flux produced by the forward winding cancels a magnetic flux produced by the backward winding of the primary coil due to the opposite directions of both fluxes. Consequently, the transformer attains an improved efficiency of transforming a voltage between the primary and secondary windings and can produce a sufficiently high voltage at the secondary side. 
     As seen in FIG. 7, a secondary coil bobbin  11  is provided at one flanged end with a terminal pin  19  for connecting thereto the starting end ( 2 S) of the secondary coil ( 10 ) and a terminal pin  20  for connecting thereto a terminating end of the primary coil ( 1 E). The primary coil bobbin  13  is provided at one end with a seat  22  with a terminal pin  21  for securing the starting end ( 1 S) of the primary coil  12 . 
     The primary coil bobbin  13  is provided at one flanged end with grooves  23  and  24  formed thereon for temporally holding the starting end ( 1 S) and terminating end ( 1 E), respectively, of the primary coil wire. 
     The coil case  15  has through holes (not shown) in its bottom for fitting-in and projecting the terminal pins  19 - 21  of the coil unit when mounted in the coil case  15 . 
     The high-voltage generating transformer  2  can be directly mounted on a printed circuit board by connecting the terminal pins  19 - 21  projected from the bottom of the coil case  15 . 
     The secondary coil bobbin  11  is provided at its flanged end  111  with a terminal  25  connected with the terminal pin  20  securing thereto the terminating end ( 1 E) of the primary coil  12 . This terminal  25  is used for connecting thereto a high-voltage cable  9  of the low-voltage side of the transformer. 
     The secondary coil bobbin  11  is provided at its other flanged end  112  with a terminal  26  for securing thereto the terminating end ( 2 E) of the secondary coil  12 . This terminal  26  is also used for connecting thereto a high-voltage cable  8  of the high-voltage side of the transformer. 
     The coil case  15  is provided with a cable holder  27  formed as projecting therefrom for leading out the high-voltage cables  8  and  9  from the coil case  15 . The cable holder  27  has a base fitted in a supporting portion formed on the inside wall of the coil case  15  and integrally formed with insulating resin poured in a melted state and solidified in the coil case  15 . 
     In the high-voltage generating transformer, the coil assembly can be easily mounted in the coil case  15  with no need for laying therein additional wiring between the ends of the primary and secondary coils  10 ,  12 , their terminals and high-voltage cables  8 ,  9  that can easily be leading out of the coil case  15 . All mounting and wiring works can be effectively performed in good order. 
     As will be apparent from the foregoing, the high-voltage transformer according to the present invention has a primary coil-wound bobbin and a secondary coil bobbin, both of which are coaxially assembled about a center core and which primary coil is formed on the primary coil bobbin by spirally winding an element wire in the forward direction and the backward direction thereon to have both ends disposed at the same end of the bobbin and to have a very small number of turns in the coil but with the same axial length as that of the secondary coil. This construction can effectively prevent leakage of magnetic flux produced and can also eliminate a problem that the magnetic flux produced by the forward winding and the magnetic flux produced by the backward winding cancels each other due to the opposite directions of the fluxes. Therefore, the transformer can have an increased efficiency of transforming a voltage between the primary and secondary windings. Namely, the transformer can produce a sufficiently high voltage at its secondary side.