Abstract:
A transformer comprises a core formed by laminations, each having a rectangular plate with two spaced windows for receiving a set of coils passing through the windows in turn; a pair of terminal supports made of dielectric material, the supports extending through the core laminations down to the bottom thereof and being terminated by L-shaped brackets for mounting on the surface of the circuit board, the brackets spanning the width of the transformer and carrying two arrays of pins extending from the output transformer for providing connections of the coil ends with other components of the electronic device; a hexahedron mold block of thermoplastic resin with glass fiber for encapsulating the transformer leaving the pins for electrical connections.

Description:
BACKGROUND OF THE INVENTION 
   A. Field of the Invention 
   The present invention relates to transformers. More particularly, the present invention relates to a transformer encapsulated in a protective mold of fiberglass embedded resin. 
   B. Description of the Prior Art 
   A transformer comprises two or more coupled windings of a wire and a laminated bobbin or core for holding the windings to concentrate magnetic flux. An alternating voltage (AC) applied to one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Varying the relative number of turns between primary and secondary windings determines the ratio of the input and output voltages, thus transforming the voltage by stepping it up or down between circuits. 
   The transformer transforms electrical current received from a primary coil side of circuit into a magnetic flux, which is transferred in a different current through the core to a secondary coil side of circuit ideally without any movement between the transformer parts. Its purpose is to change the electricity into a desired value wherein between the change of voltage and the corresponding change of the current ampere the voltage change is mainly used. 
   When the transformer is used to power in a power circuit, the power transformer is primarily desired to function better for the simple and sophisticated operation of higher end device circuitries. For example, audio devices need signal flowing through a pure and simple path as well as a stable clean DC as in illuminating devices. To this end, transformers use the laminated metal core. Metal cores easily create magnetic flux and increases effectiveness of the transformer. Most metal cores are made from silicon steel sheets for their superior electrical properties in creating the magnetic flux with ease. Also as a measure to reduce Foucault current or Eddie current layers of 0.3 mm thin metal plates form the core. The superimposed surfaces of the metal layers are insulated from each other and carefully bonded together avoiding any gaps to deteriorate efficiency. 
   Generally, when an electric current flows through a conducting line, certain magnetic field is formed around the conductor. And moving the current on conductor with respect to the magnetic field will induce an electric voltage. Alternating current with the frequency of 60 Hz applied to the transformer primary winding will result in a magnetic field expanding and contracting around the coil winding. Such magnetic field movement induces a voltage across the ends of the secondary coil winding. Induced secondary coil voltage is determined by the winding ratio between the primary and secondary coils. By having various winding ratios desired output voltages might be obtained simultaneously. 
   Transformers are adapted to have multiple secondary coils for providing various voltages needed by various circuit components. However, transformers themselves inherently have the problem of producing 60 Hz hum to be introduced into circuits around the transformers. 
   Besides its low cost to manufacture, because transformers in the open type of system have quality issues including emission of noise and unprotected impacts from external forces encapsulation of transformer has been performed using materials and structures with unlimited varieties. 
   With special consideration on reliable voltage insulation among others, mold-in type transformers currently available generally comprise a dielectric sheet, a first dielectric tape for fastening a core, a second dielectric tape for insulating windings around a bobbin or core and a volatile dipping solution. However, this method of making transformers needs the initial step of aligning the dielectric sheet to make a good fit with the winding bobbin and subsequent steps of securing the core with wraps of the dielectric tape and then another tape for insulating the windings which renders the whole process relatively complex. The final step has been dipping the transformer assembly into a bath of dielectric liquid followed by a curing step wherein the dielectric liquid is a volatile material like a liquid varnish that leaves little ingredients on the transformer product resulting in poor insulation voltage and insulation resistance and thus leaving the coils unprotected from contacting objects that may break the windings. 
   Different from the tape and varnish approach, potting consists of placing components in a potting cup then pouring a potting compound into the vessel. This compound may be either air- or oven-cured depending upon the type of material used. This manufacturing process traditionally has offered superior levels of thermal conductivity and corona resistance. Still, potting tends to be very labor and time intensive. 
   However, manufacturing transformers can be more simplified taking advantage of newly introduced thermoplastics. The three dimensional moldability of thermoplastics offers ways to add additional functionality into the part without adding extra components or manufacturing steps. No harmful volatile organic compounds (VOCs) are released as they are with many potting compounds. In addition, the process has a faster cycle time because thermoplastics eliminate several steps. A potting cup is no longer needed as with the insertion of the delicate components into the cup, and the labor-intensive potting operation or the oven-curing step. While potting cure times can last from one hour to days, thermoplastic encapsulation cycle times are generally from ten to sixty seconds that is a dramatic reduction. Thermoplastics generally perform better in thermal cycle, have a smaller size and lighter weight, and are more durable. 
   Encapsulation with standard thermoplastics on transformers typically leads to hot spot temperature reductions relative to open structures, due to the inherent thermal transfer advantages of conduction relative to convection. Use of a thermally conductive plastic can provide even greater thermal benefits. Key to this behavior is the intimate contact with the windings enabled by encapsulation. For reference, air has a low thermal conductivity of approximately 0.01 W/mK, and standard thermoplastics are 0.1 to 0.35 W/mK. Thermally conductive polymers can be as high as from 0.5 W/mK to 50 W/mK. 
   Subsequent benefits of providing a good thermal conductivity include reduced component sizes, faster electrical response times and longer component life. Thermoplastic encapsulation of transformers enables the design of a system with improved acoustic and vibratory characteristics. Thermoplastic encapsulation tends to dampen transformer vibration. Thermoplastic components are more rugged and pass higher voltage withstand tests so they are safer in operation. With encapsulation, a newly unified structure is produced where its resonant frequencies can be controlled and shifted using a proprietary process control technology and by the tailoring of the encapsulant stiffness and loss factor properties. Depending on the specific system requirements, encapsulant properties are modified to develop polymers ranging from ultra stiff composites to flexible thermoplastic elastomers. An encapsulating composition comprises a glass reinforced resin or a liquid crystal polymer. Injection molding to encapsulate transformers provides significant advantages in attenuation of noise and vibration and an ability to shift the frequencies of peak amplitude. 
   Also, in an electric device like ballast, the transformer is normally enclosed in a metal housing with a too tight space to dissipate heat, which is not only a waste of the electric energy but also a cause to shorten the life of the transformer resulting in early failure of the surrounding circuits in the same housing. Such conventional configurations do not allow ready dissipation of heat due to lack of space in the compact housing and confinement of the transformer by the surrounding components. 
   To solve this problem, the present invention provides a universal transformer geometry that may be applied to wide varieties of available materials in order to maximize a transformer performance against harmful heat and noise. 
   SUMMARY OF THE INVENTION 
   The general object of the present invention is to provide a shielded enclosure for a transformer through a molding process to improve moisture-resistance, insulation with respect to the surrounding components and between the primary and secondary windings and the strength of the shielding walls formed. 
   The secondary object of the present invention is to provide an effective transformer enclosure to block undesirable electromagnetic radiation or conduction from entering or exiting the enclosure. 
   The enclosed transformer of the present invention is excellent for mounting on a compact circuit board where a plurality of electrical components, such as connectors, transistors, diodes, and the like are closely positioned with minimal clearance among them. 
   The present invention is an improvement to conventional enclosed transformer for electronic devices. The transformer comprises a core formed by laminations, each having a rectangular plate with two spaced windows for receiving a set of coils passing through the windows in turn; a pair of terminal supports made of dielectric material, the supports extending through the core laminations down to the bottom thereof and being terminated by L-shaped brackets for surface mounting on the surface of the circuit board, the brackets spanning the width of the transformer and carrying two arrays of pins extending from the output transformer for providing connections of the coil ends with other components of the electronic device; a hexahedron mold block of thermoplastic resin with glass fiber for encapsulating the transformer leaving the pins for electrical connections and at least four legs protruding downwardly of the transformer at corresponding thru-holes of the circuit board and fixed thereto by soldering as with other components. 
   In another embodiment of the present invention, the transformer has one sided terminal support for mounting on the printed circuit board at its edge away from the rest of the electronic components and the terminal support comes to hug the circuit board at its top surface and a side wall area and fixed thereto in a cantilevered mounting with an array of pins from the transformer make direct contact with the trace of the circuit board, whereby dissipation of heat from the operating transformer is less hindered by other components but expedited through the compact housing in closer proximity. 
   The objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing a dual in line package type transformer assembly for molding in accordance with a first embodiment of the present invention. 
       FIG. 2  is a partially exploded perspective view of the transformer of  FIG. 1  enclosed. 
       FIG. 3  is a perspective view of a second embodiment of the transformer of the present invention with a single line of terminals before an encapsulation. 
       FIG. 4  is a partially exploded perspective view of the transformer of  FIG. 3  after encapsulation. 
       FIG. 5  is a perspective view of the transformer of  FIG. 4  mounted at an edge of a ballast circuit board. 
       FIG. 6   a  is a perspective view of the transformer that has a vertical core orientation as opposed to the horizontal core orientation shown in  FIGS. 1 through 5 . 
       FIG. 6   b  is a perspective view of the transformer that has a vertical core orientation as opposed to the horizontal core orientation shown in  FIGS. 1 through 5 . Similar reference numbers denote corresponding features throughout the attached drawings. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a transformer  10  of the present invention is for use at power or audio frequencies and typically has a core  12  made of high permeability silicon steel. Although the exterior of the core  12  is shown as flatly rectangular at all four longitudinal sides, can either be made of multiple thin steel layers stacked alternately with dielectric layers in a longitudinal direction or can be made of one or more blocks or sections of ferrite core material. The ferrite core material can be made by ceramic production methods. When the transformer is used to power in a power circuit, the power transformer is primarily desired to function better for the simple and sophisticated operation of higher end device circuitries. For example, audio devices need signal flowing through a pure and simple path as well as a stable clean DC as in illuminating devices. To this end, transformers use the laminated metal core. Metal cores easily create magnetic flux and increases effectiveness of the transformer. Most metal cores are made from silicon steel sheets for their superior electrical properties in creating the magnetic flux with ease. Also as a measure to reduce Foucault current or Eddie current layers of 0.3 mm thin metal plates form the core  12 . The superimposed surfaces of the metal layers are insulated from each other and carefully bonded together avoiding any gaps to deteriorate efficiency. 
   For simplicity, the individual lines to depict the multiple sheets were omitted from the drawings. Latitudinally, the transformer  10  may be divided at a straight middle line by two E shaped cutouts joined facing opposite to each other leaving two rectangular through holes in the transformer  10 . A bobbin  14  shaped as H is positioned in the two rectangular holes not shown. 
   Like most small power transformers, the transformer  10  is shell-type, in which the iron core  12  surrounds copper wires  16 , which have been wound about the bobbin  14 . The ideal would be for the core iron to completely surround the windings, although this is impractical. The compromise is to divide the magnetic circuit into two return paths on opposite sides of the core, as may be done with E and E laminations. As to the lamination pattern, the common E and I shapes may be also employed. So, the core  12  may have a parting line (not shown) through the thickness at its middle or one of the lateral sides near the bracket  20 . 
   As the E and E laminations are bonded together, the core  12  is tightly held by the plastic bobbin  14  holding the windings  16 . Alternatively, the core  12  can also be made of one or more sections of ferrite core material similarly bonded together and tightly held by the plastic bobbin  14  holding the windings  16 . The laminations are glued together by the insulating substance or the ferrite core material is glued or taped or otherwise adhered together. The windings on this transformer are wound one over the other, the primary winding may be first wound on the bobbin followed by the secondary winding on top of the primary winding maintaining electrical isolation at each and every turns of the copper wires. The ends of the wires  16  are soldered to multiple terminal pins  24 . The turns of the windings  16  must be insulated from each other to ensure that the current travels through the entire winding. The potential difference between adjacent turns is usually small, so that enamel insulation may suffice for small power transformers. Supplemental sheet or tape insulation usually employed between winding layers may be omitted thanks to the subsequent encapsulation over the optimal contour of the transformer  10  for encapsulation. 
   The present invention is an improvement to conventional enclosed transformer for mounting on electronic circuit boards. The bobbin  14  of the transformer  10  also comprises a pair of terminal supports  18  made of a dielectric material, the supports  18  extending throughout the core laminations and being terminated by L-shaped brackets  20  for surface mounting on the surface of the circuit board. The brackets  20  span the width of the transformer and carry at least one array of terminal pins  22  extending from the output transformer  10  for providing connections of the coil ends with other components of the electronic device. 
   The transformer is completely sealed against moisture ingress. Thus the encapsulant serves as a cooling medium to remove heat from the core and coil. By impregnating the transformer with encapsulant resin, air spaces within the windings are replaced with the resin, thereby sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers suitable for damp or dusty environments. 
   In  FIG. 2 , a hexahedral mold block  26  of thermoplastic resin with glass fiber encapsulates the transformer  10  by injection molding leaving the pins  24  for electrical connections and at least four pins  24  protruding downwardly of the transformer  10  at corresponding thru-holes of the circuit board and fixed thereto by soldering as with other components. 
   The encapsulated transformer  10  comprises two or more coupled windings  16  of a wire and laminated bobbin  14  in the core  12  for holding the windings  16  to concentrate magnetic flux. An alternating voltage (AC) applied to one winding creates a time-varying magnetic flux in the core  12 , which induces a voltage in the other windings. Varying the relative number of turns between primary and secondary windings determines the ratio of the input and output voltages, thus transforming the voltage by stepping it up or down between circuits. 
   The transformer transforms electrical current received from a primary coil side of circuit into a magnetic flux, which is transferred in a different current through the core to a secondary coil side of circuit ideally without any movement between the transformer parts. Its purpose is to change the electricity into a desired value wherein between the change of voltage and the corresponding change of the current ampere the voltage change is mainly used. 
   Transformers are adapted to have multiple secondary coils for providing various voltages needed by various circuit components. Both the primary and secondary windings on the transformer  10  may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio. The taps may be connected to an automatic, on-load tap changer for voltage regulation of distribution circuits. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Besides its low cost to manufacture, because transformers in the open type of system have quality issues including emission of noise and unprotected impacts from external forces encapsulation of transformer has been performed using materials and structures with unlimited varieties. 
   With special consideration on reliable voltage insulation among others, mold-in type transformers currently available generally comprise a dielectric sheet, a first dielectric tape for fastening a core, a second dielectric tape for insulating windings around a bobbin or core and a volatile dipping solution. However, this method of making transformers needs the initial step of aligning the dielectric sheet to make a good fit with the winding bobbin and subsequent steps of securing the core with wraps of the dielectric tape and then another tape for insulating the windings which renders the whole process relatively complex. The final step has been dipping the transformer assembly into a bath of dielectric liquid followed by a curing step wherein the dielectric liquid is a volatile material like a liquid varnish that leaves little ingredients on the transformer product resulting in poor insulation voltage and insulation resistance and thus leaving the coils unprotected from contacting objects that may break the windings. 
   Different from the tape and varnish approach, potting consists of placing components in a potting cup then pouring a potting compound into the vessel. This compound may be either air- or oven-cured depending upon the type of material used. This manufacturing process traditionally has offered superior levels of thermal conductivity and corona resistance. Still, potting tends to be very labor and time intensive. 
   However, manufacturing transformers can be more simplified taking advantage of newly introduced thermoplastics as applied to the simplified profile of the present transformer. The three dimensional moldability of thermoplastics offers ways to add additional functionality into the part without adding extra components or manufacturing steps. No harmful volatile organic compounds (VOCs) are released as they are with many potting compounds. In addition, the process has a faster cycle time because thermoplastics eliminate several steps. A potting cup is no longer needed as with the insertion of the delicate components into the cup, and the labor-intensive potting operation or the oven-curing step. While potting cure times can last from an hour to days, thermoplastic encapsulation cycle times can be generally from ten to sixty seconds that is a dramatic reduction. Thermoplastics generally perform better in thermal cycle, have a smaller size and lighter weight, and are more durable. 
   Encapsulation with standard thermoplastics on transformers typically leads to hot spot temperature reductions relative to open structures, due to the inherent thermal transfer advantages of conduction relative to convection. Use of a thermally conductive plastic can provide even greater thermal benefits. Key to this behavior is the intimate contact with the windings enabled by encapsulation. For reference, air has a low thermal conductivity of approximately 0.01 W/mK, and standard thermoplastics are 0.1 to 0.35 W/mK. Thermally conductive polymers can be as high as from 0.5 W/mK to 50 W/mK. 
   Subsequent benefits of providing a good thermal conductivity include reduced component sizes, faster electrical response times and longer component life. Thermoplastic encapsulation of transformers enables the design of a system with improved acoustic and vibratory characteristics. Thermoplastic encapsulation tends to dampen transformer vibration. Thermoplastic components are more rugged and pass higher voltage withstand tests so they are safer in operation. With encapsulation, a newly unified structure is produced where its resonant frequencies can be controlled and shifted using a proprietary process control technology and by the tailoring of the encapsulant stiffness and loss factor properties. Depending on the specific system requirements, encapsulant properties are modified to develop polymers ranging from ultra stiff composites to flexible thermoplastic elastomers. 
   In comparison,  FIGS. 3 and 4  show new transformer  100  having all the equivalent components of transformer  10  but adapted to a different circuit board, which can be relatively shorter. The transformer  100  comprises core  112 , coil  116  and a single terminal support  120  at one side of the transformer  100 . The terminal support  120  becomes a bracket through which an array of pins  122  protrudes from the transformer  100 . The other terminal support  130  extends in the same direction as that of the support  120  but terminates short at a straight end. 
   Referring to  FIG. 5 , the transformer  100  may be mounted at an edge of ballast or other circuit board  122  whereby the overall size of the circuit board is reduced. The printed trace of circuit may be designed at the bottom area of the board  122  near the edge so that it maintains the functional connections with the components  124 . This edgewise mounting of the transformer  100  allows more intimate connections between the transformer  100  and the circuit board  122 . The L-shaped bracket  120  hugs an output side edge of the circuit board instead of being surface mounted as with the transformer  10  of  FIG. 1 . The thus cantilevered transformer  100  is located near an end wall of the circuit board and physically distanced from the rest of the board  122  so that heat from the output transformer  100  can be dissipated faster through its encapsulation of  FIG. 4  in conjunction with the atmosphere and thereby prolong the life of the circuit board  122  according to the present invention. As described above, transformers may now be encapsulated without the laborous conventional methods and time to manufacture. 
   The a pair of “E-shaped” ferrite core material is typically oriented so that they form a plane of orientation parallel to the board. The horizontal configuration contrasts with a vertical orientation not shown in  FIG. 1 . The vertical orientation is where the ferrite core material is oriented so that there is a plane of orientation perpendicular to the board. 
   The transformer can also have a vertically oriented core as shown in  FIG. 6   a  which is encapsulated as shown in  FIG. 6   b . The vertically oriented core is contrasted with the horizontally oriented core of  FIG. 3 ,  FIG. 4 . 
   Therefore, while the presently preferred form of transformer has been shown and described, it is to be understood that the present invention is not limited to the sole embodiment describe above, but encompasses any and all embodiments within the scope of the following claims.