Abstract:
A low-power planar transformer for cell phone chargers or the like includes a primary bobbin structure carrying connection pins aligned for mounting the transformer in a through-hole configuration or directly on a circuit board, and a secondary bobbin structure that fits wholly within the primary bobbin structure.

Description:
FIELD OF THE INVENTION 
     This invention relates to low-power transformers for use in cell phone chargers or the like, and more particularly to a planar transformer suitable for installation on a printed circuit board in miniaturized electronic equipment. 
     BACKGROUND OF THE INVENTION 
     Power supplies for miniaturized electronic equipment such as cell phone chargers typically involve converter circuits, such as, for example, flyback converters, that require a very low-power (2-3 watts) transformer for their operation. Because of the space constraints in such equipment, the transformer is typically a limiting factor in the power supply design. Inasmuch as the power supply usually takes the form of a low-profile printed circuit board, it is desirable to make the transformer as small and flat, or planar, as possible, and to so arrange its pins that it can either be mounted in a through-hole configuration or soldered directly to pads on the board. 
     The present invention is an improvement over my copending application Ser. No. 09/545,367 filed Apr. 7, 2000 and assigned to the assignee of this application. In that application, I disclosed a transformer construction in which the primary and secondary winding bobbin structures are positioned one above the other in the axial direction (i.e., a direction perpendicular to the plane of the transformer on each side of an electrostatic shield). The construction of the &#39;367 application is relatively flat when mounted to an edge of a circuit board, but loses that advantage when mounted in a through-hole configuration by pins. It is therefore desirable to provide a transformer that can be mounted on a circuit board by pins yet lies as flatly against the board as possible. 
     SUMMARY OF THE INVENTION 
     The present invention fills the above-stated need by providing a transformer construction in which the secondary bobbin structure fits wholly inside the primary bobbin structure, yet is electrostatically shielded therefrom and so configured as to minimize creepage and leakage inductance between the primary and secondary windings. The primary bobbin structure and the secondary bobbin structure are also so configured as to easily snap together during assembly of the transformer and are light and sturdy enough to be impact-resistant if the power supply or the equipment using it is dropped. 
     Broadly stated, the present invention comprises a planar transformer for printed circuit boards, comprising: a primary bobbin structure; a secondary bobbin structure; and a core extendable through a central opening in each of said bobbin structures; wherein the primary bobbin structure includes a body surrounding said opening, a recess formed in said body around said opening, a primary bobbin supported by said body, and primary and secondary connection pins for mounting said primary bobbin structure on a circuit board; and wherein the secondary bobbin structure forms a secondary bobbin that is lockingly insertable into said recess so as to be contained therein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of the transformer according to the present invention; 
     FIG. 2 is a vertical section of the assembled primary and secondary bobbin structures along line  2 — 2  of FIG. 1; and 
     FIG. 3 is a vertical section along line  3 — 3  of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As best seen in FIG. 1, the transformer  10  of this invention is constructed of a primary bobbin structure  12 , a secondary bobbin structure  14 , and an electrostatic shield  16  interposed between them. A two-piece core  18   a ,  18   b  extends around the bobbins  12 ,  14  and has a central leg  20   a ,  20   b  that passes through a central opening  22   a ,  22   b  and  22   c  (see also FIGS. 2 and 3) formed, respectively, in bobbins  12 ,  14  and in the electrostatic shield  16 . 
     The primary bobbin structure  12  includes a hollow body  24  which supports on its upper side a primary bobbin  28 . The outer ends  30 ,  32  of the bobbin structure  12  are thickened to receive the contact pins  34   a ,  34   b  and  34   c  by which the transformer  10  is mounted to a circuit board  36  (FIGS.  2  and  3 ). 
     The primary winding  38  of the transformer  10  (as seen in FIGS. 2 and 3) is wound about the primary bobbin  28 . Its ends are brought around the edge  40  of the primary bobbin structure  12  and are attached to the pins  34   a . The windings of the transformer  10  are preferably formed from triple insulated wire to provide adequate high voltage isolation. An auxiliary winding  42  such as the control winding for the flyback circuit, or “dummy” windings (windings terminated, only at one end, to a high frequency ground point on the primary winding to improve faraday shielding) may be wound over and/or under the primary winding  38  and may be connected to the pins  34   b . One of the pins  34   b  may be the ground connection to which the prongs  44  of the flange  46  on the electrostatic shield  16  are connected. 
     The secondary winding  48  is wound on a secondary bobbin  50  which forms the central part of the secondary bobbin structure  14 , and which surrounds the central opening  22   b . The horizontal hollow flange  52  of the secondary bobbin  50  is equipped with lateral tabs  54   a ,  54   b  which clip into the clips  56   a ,  56   b  (FIG. 1) formed on the primary bobbin structure  12 . The ends of the secondary winding  48  are attached to the pins  34   c  after the secondary bobbin structure  14  is assembled with the primary bobbin structure  12 . It will be understood that other windings may also be wound on the secondary bobbin  50  as described above, or that the primary and secondary windings may be interchanged insofar as the circuit is concerned. 
     In the manufacture of the transformer  10 , the primary winding  38  and any additional windings such as the auxiliary winding  42  are first wound onto the primary bobbin  28 , and their ends are attached to the pins  34   a  and  34   b . Next, the electrostatic shield  16  is inserted into the well or recess  58  of the primary bobbin structure, and its prongs  44  are engaged with one of the pins  34   b.    
     The secondary winding or windings  48  are now wound onto the secondary bobbin  50 , and their ends passed through the slot  60  (FIG.  1 ). The secondary winding structure  14  is now pushed up into the recess  58  until the tabs  54   a ,  54   b  clip into the clips  56   a ,  56   b  and the flange  62  of the structure  14  passes through the slot  64  of the primary bobbin structure  12  and locks onto it as shown in FIG.  2 . The secondary bobbin structure  14  is now wholly contained within the recess  58  of the primary bobbin structure  12 , and the winding ends protruding from slot  60  can be attached to the secondary pins  34   c.    
     The winding ends and electrostatic shield prongs can now be dip-soldered onto their respective pins, and the two core halves  18   a  and  18   b  can be passed around and through the assembled primary and secondary structures, and joined together. A complete 2-watt transformer may be 25 mm×15 mm×9 mm in size. 
     Although the electrostatic shield  16  has been shown herein with the flange  46  on only one of the short sides of the shield  16 , it will be understood that the construction of this invention allows the electrostatic shield  16  to have flanges  46  also on its two long sides, or even the sides defining the opening  22   c , if this is desired. 
     The relatively large overhang of the bobbin flanges  66  is effective in reducing creepage. The relatively large winding area allows the use of fewer turns for winding, which makes the use of triple-insulated wire practical. 
     It will be appreciated that the inventive transformer construction has a number of advantages over the prior art. These include, among others, a low profile; a low primary-to-secondary capacitance to minimize “earth leakage” current; light weight to prevent circuit board breakage when dropped; small size for improved equipment miniaturization; and pin alignment for through-hole and surface mount of the transformer on the circuit board. Although only a preferred embodiment of the invention has been shown and described herein, it will be understood by those skilled in the art that the invention can be carried out in a variety of ways within the scope of the following claims.