Patent Publication Number: US-8125306-B2

Title: Transformer set

Description:
FIELD OF THE INVENTION 
     The present invention relates to a transformer set, and more particularly to a transformer set for avoiding high-voltage spark. 
     BACKGROUND OF THE INVENTION 
     A transformer has become an essential electronic component for voltage regulation into required voltages for various kinds of electric appliances. 
       FIG. 1  is a schematic exploded view illustrating a conventional transformer. As shown in  FIG. 1 , the transformer  1  comprises a magnetic core assembly  11 , a first bobbin piece  12 , a second bobbin piece  13 , a primary winding coil  14  and a secondary winding coil  15 . The first bobbin piece  11  has a first side plate  16 . The second bobbin piece  13  has a second side plate  17  and plural partition plates  13   a . Several winding sections  13   b  are defined by every two adjacent partition plates  13   a . According to a voltage-dividing principle, the number of winding sections  13   b  may be varied depending on the voltage magnitude. In addition, a first base  16   a  and a second base  17   a  are extended from the first side plate  16  and the second side plate  17 , respectively. Several pins  18  and  19  are respectively arranged on the bottom surfaces of the first base  16   a  and the second base  17   a.    
     For winding the primary winding coil  14  on the first bobbin piece  11 , a first terminal of the primary winding coil  14  is firstly wound around and fixed on a pin  18   a  under the first base  16   a . The primary winding coil  14  is then successively wound on the first bobbin piece  11  in the direction distant from the first side plate  16 . Then, the primary winding coil  14  is returned back, and a second terminal of the primary winding coil  14  is wound around and fixed on another pin  18   b  under the first base  16   a . For winding the secondary winding coil  15  on the second bobbin piece  13 , a first terminal of the secondary winding coil  15  is firstly wound around and fixed on a pin  19   a  under the second base  17   a . The secondary winding coil  15  is then successively wound on the winding sections  13   b  of the second bobbin piece  13  in the direction distant from the second side plate  17 . Then, the secondary winding coil  15  is returned back, and a second terminal of the secondary winding coil  15  is returned to be wound around and fixed on another pin  19   b  under the second base  17   a . Moreover, due to the partition plate  13   a  of the second bobbin piece  13 , the primary winding coil  14  is separated from the secondary winding coil  15 , thereby maintaining an electrical safety distance and increasing leakage inductance of the transformer  1 . 
     The winding structure of the transformer  1 , however, still has some drawbacks. For example, since the primary winding coil  14  and the secondary winding coil  15  are returned back to be respectively fixed on the pins  18   b  and  19   b  under the first base  16   a  and the second base  17   a , portions of the primary winding coil  14  and the secondary winding coil  15  are exposed under the first bobbin piece  11  and the second bobbin piece  13 . Even if the exposed portions are covered by insulating material, the safety distance is also insufficient. Under this circumstance, the transformer  1  is readily suffered from high-voltage spark or short circuit and eventually has a breakdown. 
     For increasing the working voltage, two or more transformers are connected in parallel to collectively define a transformer set.  FIG. 2  is an exploded view illustrating a transformer set according to the prior art. As shown in  FIG. 2 , the transformer set comprises a first transformer  2  and a second transformer  3 , which are arranged in parallel with each other. The first transformer  2  comprises a first bobbin piece  22  and a second bobbin piece  23 . The second transformer  3  also comprises a first bobbin piece  32  and a second bobbin piece  33 . The first bobbin pieces  22  and  32  have the same structures. The second bobbin pieces  23  and  33  have the same structures. Since the first transformer  2  and the second transformer  3  have the same structures, only the first transformer  2  is illustrated for clarification herein. 
     The first bobbin piece  22  comprises a base  221 , a sheathing part  222  and a first channel  223 . A winding section  224  is arranged between the base  221  and the sheathing part  222  for winding a primary winding coil  24 . The sheathing part  222  is substantially a rectangular hollow structure with a receptacle  222   a . The first channel  223  runs through the base  221  and the winding section  224 . In addition, the first channel  223  is in communication with the receptacle  222   a  of the sheathing part  222 . The second bobbin piece  23  and the secondary winding coil  25  that is wound around the second bobbin piece  23  are accommodated within the receptacle  222   a  of the sheathing part  222 . As such, by the sheathing part  222 , the primary winding coil  24  and the secondary winding coil  25  are separated from each other, and the first channel  223  of the first bobbin piece  22  and the second channel  235  of the second bobbin piece  23  are in communication with each other. The second bobbin piece  23  comprises a first side plate  230 , a second side plate  238 , plural partition plates  232 , a slab  233  and a base  231 . The first side plate  230 , the second side plate  238 , the partition plates  232  and the base  231  have rectangular profiles. The first side plate  230  and the second side plate  238  are arranged on two opposite sides of the second bobbin piece  23 . In addition, the first side plate  230  and the second side plate  238  are perforated plates. The partition plates  232  are parallel with the first side plate  230  and the second side plate  238 . As such, the partition plates  232 , the first side plate  230 , the second side plate  238  and the slab  233  collectively define plural winding sections  234  for winding the secondary winding coil  25 . 
     The base  231  is extended from the first side plate  230  and connected with the first side plate  230 . In addition, a first pin  236  and a second pin  237  are disposed on the base  231  to be inserted into corresponding holes of a circuit board (not shown). The first pin  236  and the second pin  237  are L-shaped. The first pin  236  comprises a first coupling part  236   a  and a second coupling part  236   b . The second pin  237  comprises a first coupling part  237   a  and a second coupling part  237   b . For winding the secondary winding coil  25  on the second bobbin piece  23 , a first terminal of the secondary winding coil  25  is firstly wound around and fixed on the first coupling part  236   a  of the first pin  236 . The secondary winding coil  25  is then successively wound on the winding sections  234 , and then wound on an auxiliary part  239 . Then, the secondary winding coil  25  is returned back, and a second terminal of the secondary winding coil  25  is fixed onto the first coupling part  237   a  of the second pin  237 . 
     Please refer to  FIG. 2  again. The transformer set further comprises a magnetic core assembly  31 . The magnetic core assembly  31  comprises a first magnetic part  311  and a second magnetic part  312 . The first magnetic part  311  includes a first lateral leg  311   a  and a second lateral leg  311   b . The second magnetic part  312  includes a first lateral leg  312   a  and a second lateral leg  312   b . For connecting the first transformer  2  and the second transformer  3  in parallel, the first lateral leg  311   a  and the second lateral leg  311   b  of the first magnetic part  311  are respectively embedded into the first channel  223  of the transformer  2  and the first channel  323  of the transformer  3 , and the first lateral leg  312   a  and the second lateral leg  312   b  of the second magnetic part  312  are respectively embedded into the second channel  235  of the transformer  2  and the second channel  335  of the transformer  3 . As such, the transformers  2  and  3  are combined together to produce the transformer set. The transformer set, however, still has some drawbacks. For example, after the both terminals of the secondary winding coil  25  are respectively fixed on the first coupling parts  236   a  and  237   a  of the pins  236  and  237 , the L-shaped pins  236  and  237  are readily shifted. In this situation, the terminals of the secondary winding coil  25  are possibly broken. As known, the increase of wire diameter of the secondary winding coil  25  may reduce the possibility of breaking the terminals of the secondary winding coil  25 . However, since the wire diameter is increased, the bobbin pieces need to be re-designed and re-produced. The process of re-designing and re-producing the new bobbin pieces is time-consuming and costly. Moreover, since the bobbin pieces  22 ,  23 ,  32  and  33  are made of plastic material, the bases  221 ,  231 ,  321  and  331  are readily subject to deformation during the fabricating process because the structural strength is usually insufficient. 
     Moreover, since the auxiliary part  239  fails to facilitate fixing the returning portion of the secondary winding coil  25  at a specified position, the secondary winding coil  25  is readily abraded when the second bobbin piece  23  is accommodated within the sheathing part  222  of the first bobbin piece  22 . In a case that the distance between the returning portion of the secondary winding coil  25  and the secondary winding coil  25  wound on the winding section  234  is very short, the transformer  2  is readily suffered from high-voltage spark. Even if the distance between the returning portion of the secondary winding coil  25  and the secondary winding coil  25  wound on the winding section  234  is increased, the returning portion of the secondary winding coil  25  becomes close to the secondary winding coil  35 , and thus the possibility of causing high-voltage spark is increased. In this circumstance, the transformers  2  and  3  are readily damaged. 
     Therefore, there is a need of providing an improved transformer set so as to obviate the drawbacks encountered from the prior art. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a transformer set for minimizing the possibility of breaking the secondary winding coil. 
     Another object of the present invention provides a transformer set having enhanced structural strength. 
     A further object of the present invention provides a transformer set for avoiding high-voltage spark or short circuit so as to prevent damage of respective transformers of the transformer set. 
     In accordance with an aspect of the present invention, there is provided a transformer set. The transformer set includes a first bobbin piece, a second bobbin piece, a third bobbin piece, a fourth bobbin piece and a magnetic core assembly. The first bobbin piece is used for winding a first primary winding coil. The first bobbin piece has a first sheathing part. The second bobbin piece is partially accommodated within the first sheathing part of the first bobbin piece. The second bobbin piece includes a first base and plural first winding sections. The first base has a first pin and a second pin. A first terminal of the first secondary winding coil is fixed on the first pin, then the first secondary winding coil is successively wound on the first winding sections and returned back, and then a second terminal of the first secondary winding coil is fixed on the second pin. The third bobbin piece is used for winding a second primary winding coil. The third bobbin piece has a second sheathing part. The third bobbin piece and the first bobbin piece are arranged in bilateral symmetry. The fourth bobbin piece is partially accommodated within the second sheathing part of the third bobbin piece. The fourth bobbin piece and the second bobbin piece are arranged in bilateral symmetry. The fourth bobbin piece includes a second base and plural second winding sections. The second base has a third pin and a fourth pin. A first terminal of the second secondary winding coil is fixed on the third pin, then the second secondary winding coil is successively wound on the second winding sections and returned back, and a second terminal of the second secondary winding coil is fixed on the fourth pin, which is opposed to the second pin. The magnetic core assembly is partially embedded into the first bobbin piece, the second bobbin piece, the third bobbin piece and the fourth bobbin piece. The first primary winding coil, the first secondary winding coil, the second primary winding coil and the second secondary winding coil interact with the magnetic core assembly to achieve the purpose of voltage regulation. 
     In accordance with another aspect of the present invention, there is provided a transformer set. The transformer set includes a first bobbin piece, a second bobbin piece, a third bobbin piece, a fourth bobbin piece and a magnetic core assembly. The first bobbin piece is used for winding a first primary winding coil. The first bobbin piece has a first sheathing part. The second bobbin piece is partially accommodated within the first sheathing part of the first bobbin piece. The second bobbin piece includes a first base, plural first auxiliary ribs and plural first winding sections. The first base has a first pin and a second pin. A first terminal of the first secondary winding coil is fixed on the first pin, then the first secondary winding coil is successively wound on the first winding sections and returned back in the assistance of the first auxiliary ribs, and then a second terminal of the first secondary winding coil is fixed on the second pin. The third bobbin piece is used for winding a second primary winding coil. The third bobbin piece has a second sheathing part. The third bobbin piece and the first bobbin piece are arranged in bilateral symmetry. The fourth bobbin piece is partially accommodated within the second sheathing part of the third bobbin piece. The fourth bobbin piece and the second bobbin piece are arranged in bilateral symmetry. The fourth bobbin piece includes a second base, plural second auxiliary ribs and plural second winding sections. The second base has a third pin and a fourth pin. A first terminal of the second secondary winding coil is fixed on the third pin, then the second secondary winding coil is successively wound on the second winding sections and returned back in the assistance of the second auxiliary ribs, and a second terminal of the second secondary winding coil is fixed on the fourth pin. The magnetic core assembly is partially embedded into the first bobbin piece, the second bobbin piece, the third bobbin piece and the fourth bobbin piece. The first primary winding coil, the first secondary winding coil, the second primary winding coil and the second secondary winding coil interact with the magnetic core assembly to achieve the purpose of voltage regulation. 
     The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic exploded view illustrating a conventional transformer; 
         FIG. 2  is an exploded view illustrating a transformer set according to the prior art; 
         FIG. 3  is an exploded view illustrating a transformer set according to an embodiment of the present invention; 
         FIG. 4  is an enlarged fragmentary view illustrating the second bobbing piece of the transformer set as shown in  FIG. 3 ; and 
         FIG. 5  is a schematic assembled view illustrating the transformer set of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
       FIG. 3  is an exploded view illustrating a transformer set according to an embodiment of the present invention. As shown in  FIG. 3 , the transformer set  4  comprises a magnetic core assembly  41 , a first bobbin piece  42 , a second bobbin piece  43 , a third bobbin piece  44 , a fourth bobbin piece  45 , a first primary winding coil  46 , a first secondary winding coil  47 , a second primary winding coil  48  and a second secondary winding coil  49 . 
     The magnetic core assembly  41  comprises a first magnetic part  411  and a second magnetic part  412 . The first magnetic part  411  includes a first lateral leg  411   a  and a second lateral leg  411   b . The second magnetic part  412  includes a first lateral leg  412   a  and a second lateral leg  412   b . The first lateral leg  411   a  of the first magnetic part  411  is embedded within the first bobbin piece  42 . The first lateral leg  412   a  of the second magnetic part  412  is embedded within the second bobbin piece  43 . The second lateral leg  411   b  of the first magnetic part  411  is embedded within the third bobbin piece  44 . The second lateral leg  412   b  of the second magnetic part  412  is embedded within the fourth bobbin piece  45 . As such, the first primary winding coil  46 , the first secondary winding coil  47 , the second primary winding coil  48  and the second secondary winding coil  49  interact with the magnetic core assembly  41  to achieve the purpose of voltage regulation. 
     In this embodiment, the first bobbin piece  42  comprises a base  421 , a first sheathing part  422  and a first channel  423 . A winding section  424  is arranged between the base  421  and the sheathing part  422  for winding the first primary winding coil  46 . It is preferred that the first sheathing part  422 , the winding section  424  and the base  421  of the first bobbin piece  42  are integrally formed. The first sheathing part  422  is substantially a rectangular hollow structure with a receptacle  422   a . In addition, a third partition plate  422   b  is extended from an inner surface of the first sheathing part  422  and a slot is formed between another inner surface of the first sheathing part  422  and the top edge of the third partition plate  422   b . The first channel  423  runs through the base  421  and the winding section  424 . In addition, the first channel  423  is in communication with the receptacle  422   a  of the first sheathing part  422 . The second bobbin piece  43  and the first secondary winding coil  47  that is wound around the second bobbin piece  43  are accommodated within the receptacle  422   a  of the first sheathing part  422 . As such, by the first sheathing part  422 , the first primary winding coil  46  and the first secondary winding coil  47  are separated from each other, and the first channel  423  of the first bobbin piece  42  and the second channel  435  of the second bobbin piece  43  are in communication with each other. 
     In some embodiments, the first channel  423  of the first bobbin piece  42  and the second channel  435  of the second bobbin piece  43  are separated from each other by a separation wall (not shown), so that the first channel  423  and the second channel  435  are not in communication with each other. In this circumstance, the first sheathing part  422  has five sidewalls to define a rectangular hollow structure, wherein the first sheathing part  422  is opened to the side opposed to the winding section  424 . As such, the second bobbin piece  43  is partially accommodated within the receptacle  422   a  of the first sheathing part  422 . The sidewall  422   c  of the first sheathing part  422  that is adjacent to the winding section  424  is served as the separation wall for insulating the second magnetic part  412  from the first primary winding coil  46  and insulating the first magnetic part  411  from the first secondary winding coil  47 . Moreover, by changing the thickness of the sidewall  422   c  (i.e. the thickness of the separation wall for separating the first primary side from the first secondary side), the leakage inductance of the transformer set is adjustable. Since the sheathing part and the separation wall can separate the first primary side from the first secondary side, the creepage distance is increased, the distance between the first primary side and the first secondary side is shortened, and the overall length of the transformer set is reduced. 
     Please refer to  FIG. 3  again. The second bobbin piece  43  comprises a first side plate  430 , a second side plate  438 , a plurality of first partition plates  432 , a first slab  433  and a first base  431 . The first side plate  430 , the second side plate  438 , the first partition plates  432 , the first slab  433  and the base  431  have rectangular profiles. The first side plate  430  and the second side plate  438  are arranged on two opposite sides of the second bobbin piece  43 . In addition, the first side plate  430  and the second side plate  438  are perforated plates. It is preferred that the first side plate  430 , the second side plate  438 , the first partition plates  432 , the first slab  433  and the base  431  are integrally formed. 
     In some embodiments, the first partition plates  432  are parallel with the first side plate  430  and the second side plate  438 . The first slab  433  is arranged between the first side plate  430  and the second side plate  438 . In addition, the first slab  433  is connected with the first side plate  430 , the second side plate  438  and the first partition plates  432 , thereby defining a second channel  435  for partially accommodating the second magnetic part  412 . In addition, the first side plate  430 , the second side plate  438 , the first partition plates  432  and the first slab  433  collectively define plural first winding sections  434  for winding the first secondary winding coil  47 . 
     The first base  431  is extended from the first side plate  430  and connected with the first side plate  430 . The first base  431  has an opening  431   c  in communication with the second channel  435 . In addition, the first base  431  further comprises a third slab  431   a  and a fourth slab  431   b . The third slab  431   a  and the fourth slab  431   b  are opposed to each other, and connected with the first side plate  430 . In addition, since the third slab  431   a  and the fourth slab  431   b  are perpendicular to the first side plate  430 , the structural strength of the bilateral sides of the first base  431  is enhanced, and the possibility of deforming the second bobbin piece  43  during the fabricating process is reduced. Similarly, the base  421  of the first bobbin piece  42  further comprises a third slab (not shown) and a fourth slab (not shown) arranged on two opposite sides of the base  421 . 
     Please refer to  FIG. 3  again. A first pin  436  and a second pin  437  are disposed on the first base  431  of the second bobbin piece  43  to be inserted into corresponding holes of a circuit board (not shown). In some embodiments, the first pin  436  and the second pin  437  are L-shaped. The first pin  436  and the second pin  437  are inserted into corresponding holes of the first base  431 . The first pin  436  comprises a first coupling part  436   a  and a second coupling part  436   b . The second pin  437  comprises a first coupling part  437   a  and a second coupling part  437   b . The second coupling part  436   b  of the first pin  436  and the second coupling part  437   b  of the second pin  437  are protruded downwardly and inserted into corresponding holes of the circuit board. The first coupling parts  436   a ,  437   a  and the second coupling part  436   b ,  437   b  are made of conductive material such as copper or aluminum. The first coupling parts  436   a  and the second coupling part  436   b  of the first pin  436  are integrally formed such that the first pin  436  is L-shaped. Similarly, the first coupling part  437   a  and the second coupling part  437   b  of the second pin  437  are integrally formed such that the second pin  437  is L-shaped. The L-shaped pins  436  and  437  could be easily assembled on the first base  431 . Moreover, a first auxiliary pin  436   c  and a second auxiliary pin  437   c  are arranged beside the first pin  436  and the second pin  437 , respectively. The first auxiliary pin  436   c  and the second auxiliary pin  437   c  may facilitate winding the terminals of the first secondary winding coil  47  on the first pin  436  and the second pin  437 . 
     Hereinafter, a process of winding the first secondary winding coil  47  on the second bobbin piece  43  will be illustrated with reference to  FIG. 3 . First of all, a first terminal of the first secondary winding coil  47  is wound around and fixed on the first coupling part  436   a  of the first pin  436 . Then, the first secondary winding coil  47  is wound around the first auxiliary pin  436   c , and successively wound on the first winding sections  434  through the notches  439  of respective first partition plates  432 . After the first secondary winding coil  47  is wound on the first winding section  434  that is adjacent to the second side plate  438 , the first secondary winding coil  47  is returned back in the assistance of the first auxiliary rib  438   a  of the second side plate  438  and the first auxiliary ribs  432   a  of the first partition plates  432 . Then, a second terminal of the first secondary winding coil  47  is wound around the second auxiliary pin  437   c  and fixed on the first coupling part  437   a  of the second pin  437 . 
       FIG. 4  is an enlarged fragmentary view illustrating the second bobbin piece of the transformer set as shown in  FIG. 3 . In this embodiment, the first auxiliary ribs  438   a  and  432   a  are protruding blocks. In addition, respectively notches  438   b  and  432   b  are formed in the upper peripheries of the protruding blocks. When the first secondary winding coil  47  is returned back, the returning portion of the first secondary winding coil  47  is received within the notches  438   b  and  432   b  so that the returning portion of the first secondary winding coil  47  is initially fixed. In addition, for combining the second bobbin piece  43  with the first bobbin piece  42 , the second bobbin piece  43  is moved toward and partially accommodated within the first sheathing part  422  of the first bobbin piece  42  so that the lower peripheries  438   c  and  432   c  of the first auxiliary ribs  438   a  and  432   a  are sustained against the third partition plate  422   b  of the first sheathing part  422 . As such, after the second bobbin piece  43  is accommodated within the first sheathing part  422  of the first bobbin piece  42 , the returning portion of the first secondary winding coil  47  is received within the notches  438   b  and  432   b . Since the returning portion of the first secondary winding coil  47  is not in direct contact with the third partition plate  422   b  of the first sheathing part  422 , the safety distance between the returning portion of the first secondary winding coil  47  and the first secondary winding coil  47  wound on the first winding section  434  will be sufficient. In this circumstance, the problem of causing the high-voltage spark is avoided. 
     Please refer to  FIG. 3  again. The third bobbin piece  44  and the first bobbin piece  42  are arranged in bilateral symmetry. The configurations of the base  441 , the second sheathing part  442 , the receptacle  442   a , the fourth partition plate  442   b  and the third channel  443  of the third bobbin piece  44  are similar to those of the first bobbin piece  42 , and are not redundantly described herein. Whereas, the fourth partition plate  442   b  is opposed to the third partition plate  422   b . In addition, a first engaging element  441   a  is formed on the base  441 , and a second engaging element  421   b  is formed on the base  421  of the first bobbin piece  42 . Due to the engagement between the first engaging element  441   a  and the second engaging element  421   b , the third bobbin piece  44  and the first bobbin piece  42  are combined together. In this embodiment, the first engaging element  441   a  is a concave structure, and the second engaging element  421   b  is a convex structure corresponding to the concave structure of the first engaging element  441   a.    
     Similarly, the fourth bobbin piece  45  and the second bobbin piece  43  are arranged in bilateral symmetry. Whereas, the second auxiliary ribs  458   a  and  452   a  of the fourth bobbin piece  45  are opposed to the first auxiliary ribs  438   a  and  432   b  of the second bobbin piece  43 . The fourth pin  457  of the fourth bobbin piece  45  is opposed to the second pin  437  of the second bobbin piece  43 . For winding the second secondary winding coil  49  on the fourth bobbin piece  45 , a first terminal of the second secondary winding coil  49  is firstly wound around and fixed on the third pin  456 . Then, the second secondary winding coil  49  is successively wound on the second winding sections  454 . Then, the second secondary winding coil  49  is returned back in the assistance of the second auxiliary ribs  458   a  and  452   a . Then, a second terminal of the second secondary winding coil  49  is fixed on the fourth pin  457 . By means of the first auxiliary ribs  438   a ,  432   b  and the second auxiliary ribs  458   a ,  452   a , the safety distance between the returning portion of the first secondary winding coil  47  and the first secondary winding coil  47  wound on the first winding section  434  will be increased and the safety distance between the returning portion of the second secondary winding coil  49  and the second secondary winding coil  49  wound on the second winding section  454  will be increased. In this circumstance, the problems of causing high-voltage spark from the returning portions of the first secondary winding coil  47  and the second secondary winding coil  49  will be effectively avoided. 
       FIG. 5  is a schematic assembled view illustrating the transformer set of the present invention. Hereinafter, a process of assembling the transformer set  4  will be illustrated with reference to  FIGS. 3 ,  4  and  5 . Firstly, the second side plate  438  and the first secondary winding coil  47  wound on the second bobbin piece  43  are inserted into the receptacle  422   a  of the first sheathing part  422  of the first bobbin piece  42 , and the second side plate  458  and the second secondary winding coil  49  wound on the fourth bobbin piece  45  are inserted into the receptacle  442   a  of the second sheathing part  442  of the third bobbin piece  44 . As such, the first primary winding coil  46  and the first secondary winding coil  47  are isolated from each other by the first sheathing part  422 , and the second primary winding coil  48  and the second secondary winding coil  49  are isolated from each other by the second sheathing part  442 . Then, due to the engagement between the first engaging element  441   a  and the second engaging element  421   b , the third bobbin piece  44  and the first bobbin piece  42  are combined together. In some embodiments, the second bobbin piece  43  and the fourth bobbin piece  45  have plural protrusions  430   a  and  450   a , respectively; and the first bobbin piece  42  and the third bobbin piece  44  have indentions corresponding to the protrusions  430   a  and  450   a . The protrusions  430   a  and  450   a  are engaged with the indentions so that the evenness of the transformer set is enhanced. Then, the first lateral leg  411   a  of the first magnetic part  411  is embedded within the first bobbin piece  42 , the first lateral leg  412   a  of the second magnetic part  412  is embedded within the second bobbin piece  43 , the second lateral leg  411   b  of the first magnetic part  411  is embedded within the third bobbin piece  44 , and the second lateral leg  412   b  of the second magnetic part  412  is embedded within the fourth bobbin piece  45 . The resulting structure of the transformer set  4  is shown in  FIG. 5 . 
     From the above description, the transformer set of the present invention has increased safety distance. By means of the first auxiliary ribs and the second auxiliary ribs, the safety distance between the returning portion of the first secondary winding coil and the first secondary winding coil wound on the second bobbin piece is increased, and the safety distance between the returning portion of the second secondary winding coil and the second secondary winding coil wound on the fourth bobbin piece is increased. As such, the problems of causing high-voltage spark from the returning portions of the first secondary winding coil and the second secondary winding coil will be effectively avoided. Moreover, after the first secondary winding coil and the second secondary winding coil are respectively accommodated within the sheathing parts of the first bobbin piece and the third bobbin piece, the possibility of abrading the winding coils is minimized. As a consequence, the yield of the transformer set is enhanced. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.