Patent Publication Number: US-6906609-B1

Title: Planar transformer

Description:
FIELD OF THE INVENTION 
   Embodiments of the invention relate to transformers and transformer assemblies. 
   BACKGROUND OF THE INVENTION 
   Small switch mode AC/DC power supplies or adapters are now starting to replace 50/60 Hz transformer “linear” adapters. They are lighter, smaller, and are cost competitive with the “linear” adapters. One of the main areas of use for these adapters is as battery chargers for GSM and other types of cellular telephones. With the standby power consumption of these telephones getting continuously lower, the battery sizes for these telephones are also getting smaller. A two-watt adapter charger is adequate for charging such a battery in only a few hours. 
   Because of the very low cost of the linear chargers, only the lowest cost “switching” topology is capable of competing in terms of cost. This topology is usually a self-oscillating fly back converter using a high voltage bipolar transistor as a main switch.  FIG. 18  shows a functional diagram of a typical self-oscillating converter. 
   The transformer is both a costly and physically large part of a power supply. The large size is due in part to the safety creepage and clearances required between the primary and secondary windings of the transformer. Creepage and clearance distances are a significant factor in determining the physical size of the transformer. While triple insulation on the secondary wire can be used to keep the size of the transformer small, the use of triple insulation is expensive. The concentric winding arrangement of the transformer&#39;s windings also results in high common mode EMI, which usually requires an electrostatic shield winding and a common mode filter capacitor. 
   Embodiments of the invention address these and other problems. 
   SUMMARY OF THE INVENTION 
   Embodiments of the invention are directed to transformers and transformer assemblies, especially planar transformers for small switch-mode isolated adapters. 
   One embodiment of the invention is directed to a transformer having at least one primary winding and one secondary winding wound about a common axis comprising: a first bobbin member including a first body portion defining a first hollow region, and axially spaced walls extending radially away from the first body portion; and a second bobbin member including a second body portion defining a second hollow region, axially spaced walls extending radially away from the second body portion, and a flange on one of said axially spaced walls and extending away from the other axial spaced wall of the second bobbin member; and wherein the first bobbin member is disposed adjacent to the second bobbin member and is partially enclosed by the flange, said primary and secondary windings respectively wound about said first and second body portions. 
   An alternative embodiment of the invention is directed to a transformer having at least one primary winding and one secondary winding wound about a common axis comprising: a first bobbin member including a first body portion defining a first hollow region, axially spaced walls extending radially away from the first body portion, and a structure adapted to receive a printed circuit board (PCB) so that the printed circuit board is disposed parallel to the walls of the first bobbin member; and a second bobbin member including a second body portion defining a second hollow region which is aligned with the first hollow region, and axially spaced walls extending radially away from the second body portion, wherein the first bobbin member is disposed adjacent to the second bobbin member, the primary and secondary windings respectively wound about said first and second body portions. 
   These and other embodiments are described with reference to the foregoing Figures and Detailed Description 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1  to  3  show different isometric views of a transformer according to an embodiment of the invention. 
       FIG. 4  is an exploded isometric view of a transformer according to an embodiment of the invention. 
       FIG. 5  is an exposed side view of a transformer according to an embodiment of the invention. 
     FIGS.  6 ( a ) to  6 ( h ),  7 , and  8 ( a ) to  8 ( h ) show various views of exemplary bobbin members according to an embodiment of the invention. 
       FIG. 9  shows a top view of a transformer according to an embodiment of the invention. 
       FIG. 10  is a cross-sectional view along the line A—A of the transformer shown in FIG.  9 .  FIG. 10  shows one example of cooperatively arranged structures in a transformer to, e.g., increase creepage distance. 
       FIG. 11  is a cross-sectional perspective view of a transformer according to an embodiment of the invention. 
       FIGS. 12 and 13  are isometric views of bobbin members according to an embodiment of the invention. 
       FIG. 14  is a side view of a transformer on top of a circuit board. 
       FIG. 15  is a side view of a circuit board coupled to a side of a transformer. 
       FIG. 16  is a side view of a transformer disposed between two circuit boards. 
       FIG. 17  is a top view of a transformer disposed between two circuit boards. 
       FIG. 18  is a circuit diagram including a flyback transformer. 
   

   For clarity of illustration, some drawings may not be to scale. Also, in the Figures, like numerals are intended to designate like elements. 
   DETAILED DESCRIPTION 
   The transformers according to embodiments of the invention are smaller and have a lower profile than many conventional transformers, and meet or exceed the safety and creepage requirements of many countries. The height of a transformer, in particular, is an important factor to consider when designing a device such as cellular phone charger. 
     FIG. 1  shows a transformer  100  including a first bobbin member  40  and a second bobbin member  20  including at least a primary and secondary winding. The first and second bobbin members  40 ,  20  are adjacent to each other, and are coupled together. In some embodiments, the second bobbin member  20  may occupy a larger area than the first bobbin member  40 . As shown in  FIG. 1 , the first bobbin member  40  may be disposed on and may be partially enclosed by the second bobbin member  20 . Both the first and second bobbin members  40 ,  20  may include portions formed from molded plastic. 
   Any suitable wiring, such as enameled copper wiring, may be used for the windings. Moreover, any suitable number of windings may be present on the first or the second body portions of the first and second bobbin members. For example, an auxiliary winding may be provided over or under the primary winding such that it is closest to a transistor collector end of the winding. This winding further shields the noisiest end of the primary winding. 
   The first bobbin member  40  comprises a first body portion (not shown) having a hollow region. Walls  46 ,  47  on respective ends of the first body portion partially define a winding region for at least one winding. For example, a winding  51  (e.g., a primary winding) is disposed between the walls  46 ,  47  and around the first body portion. Both the winding  51  and the first body portion upon which it rests are disposed around an axis  105 . The walls  46 ,  47  are axially spaced from each other and extend in a radial direction away from the first body portion. A number of pins  91  may be present in a number of pin supports  95 , which may be integral with a wall  46  of the first bobbin member  40 . Each of the pins  91  may be in communication with one or more windings  51  disposed around the first body portion. Wires may pass through slots between the pin supports  95 . The pins  91  may be used to couple the transformer  100  to an external electrical device such as a printed circuit board. 
   The second bobbin member  20  comprises a second body portion (not shown) having a hollow region. The second body portion is disposed between walls  26 ,  27  which, along with the exterior surface of the second body portion, define a winding region for at least one winding. The winding  52  on the second body portion (e.g., a secondary winding) is disposed between the walls  26 ,  27  of the second bobbin member  20  and around the second body portion. The winding  52  and the hollow body portion are both disposed around the axis  105 . The walls  26 ,  27  of the second bobbin member  20  are axially spaced from each other and extend in a radial direction away from the hollow body portion. As shown in  FIG. 1 , the walls  26 ,  27  of the second bobbin member  20  may have a larger major surface area than the walls  46 ,  47  of the first bobbin member  40 . 
   A flange  21  is disposed on one wall  27  of the second bobbin member  20  and may extend in a direction away from the other wall  26  of the second bobbin member  20 . The flange  21  may be located at any suitable region on the wall  27  of the second bobbin member  20 . For example, the flange  21  can be at the side of the transformer  100  opposite the outer leg of the core  70 . Preferably, the flange  21  is located at the edges of the wall  27  (e.g., an inner wall) upon which it is disposed. In the transformer  100  shown in  FIG. 1 , the flange  21  includes two flange portions  21 ( a ),  21 ( b ). The flange portions  21 ( a ),  21 ( b ) are substantially perpendicular to each other and each is perpendicular to the walls  26 ,  27 . 
   The flange  21  advantageously increases the creepage distance between the two windings  51 ,  52  at regions of the transformer  100 . Lengthening the creepage path (i.e., the path across the surface of a dielectric between two conductors) reduces the possibility of damage due to, e.g., arcing between the windings on the first and second bobbin members  40 ,  20 . In the transformer  100  shown in  FIG. 1 , for example, the creepage path begins at the winding  52  on the second bobbin member  20 , passes outwardly across the lower surface of the wall  27 , up the face of the flange portion  21 ( a ), down the opposite face of the flange portion  21 ( a ), across the upper surface of the wall  27 , and to the coil  51  on the first bobbin member  40 . In embodiments of the invention, the creepage distance can be increased without increasing the length or width of the walls of the first and second bobbin members  40 ,  20 . 
   Optionally, the transformer embodiments may include one or more structures for receiving a circuit board (not shown). The circuit boards can be mounted using the structures so that the mounted boards are disposed generally parallel to the walls of the bobbin members  20 ,  40 . In  FIG. 1 , for example, a structure  80  for receiving a circuit board is present on the flange  21 . This structure  80  includes two protrusions extending away from an outer surface of one of the flange portions  21 ( a ). When a circuit board is mounted to the flange portion  21 ( a ), the circuit board is sandwiched between the protrusions and is parallel to the walls  26 ,  27 ,  46 ,  47  of the bobbin members  20 ,  40 . 
   A core  70  such as a ferrite core passes through the first and second hollow portions of the first and second bobbin members  40 ,  20 . The core  70  may be formed from portions having any suitable shape. For example, the core  70  may be formed by using two U-shaped core portions  4  coupled together to form a ring. Alternatively, the core  70  may be formed by coupling a U-shaped core portion and an I-shaped bar to form a ring. The core may also be formed from E-shaped core portions. For example, the core  70  may include two E type core portions coupled together or an E and an I type core coupled together. 
   The core  70  may have a potential which is between (e.g., halfway between) the potentials of the windings  51 ,  52  on the first and second bobbin members  40 ,  20 . In preferred embodiments, the first and second bobbin members  40 ,  20  may also include additional flanges to increase the creepage distance between the core  70  and the windings  51 ,  52  on respective bobbin members  40 ,  20 . For example, the first bobbin member  40  may include a flange  43  which increases the creepage distance between the winding  51  on the first bobbin member  40 , and the core  70 . In this example, the flange  43  may have a number of flange portions and these flange portions may be closely adjacent the core  70  to shield portions of the core  70  from the winding  51 . Preferably, the flange  43  conforms to the outer surface of the core  70 . In the example shown in  FIG. 1 , the flange portions are on a wall  46  of the first bobbin member  40  and are perpendicular to the wall  46 . 
   Preferably, as shown in  FIG. 1 , a flange portion  21 ( b ) of the second bobbin member can extend beyond the back of the winding (e.g., a primary winding)  51  on the first bobbin member  40 . Alternatively or additionally, a flange portion  21 ( a ) of the second bobbin member can extend beyond the side of the winding  51 . Extra creepage distance is provided by these flange portions and the transformer height can be reduced. If desired, the creepage distance between elements in the transformer may be increased in other ways. For example, the walls of the first and second bobbin members can be made wider to increase the creepage distance between respective windings on the first and second bobbin members  40 ,  20 . In another example, additional flanges may be present on the walls of the bobbin members. For example, flanges may be on the outer walls  26 ,  46  of the first and second bobbin members  40 ,  20  at the core side of the transformer on either or both sides of the core  70 . This could result in a slight increase in the height of the core, but can make the transformer narrower. This may be particularly useful for EE or EI type cores. 
     FIG. 2  shows another view of the transformer  100 . In  FIG. 2 , the outer surface of the second bobbin member  20  is shown more clearly. The second bobbin member  20  includes pins  92  which are electrically coupled to the winding on the second bobbin member  20 . A flange  23  may be present on the outer wall  26  of the second bobbin member  20 . The flange  23  may be disposed adjacent to the core  70  to increase the creepage distance between the winding on the second bobbin member  20  and the core  70 . Ribs  24  may be present to provide structural support for the flange  23  disposed around the core  70 . The ribs  24  also increase the creepage distance between the windings on the first and second bobbin members  40 ,  20 , especially the portions of the windings exposed by the slots between the pin supports  95 . In this example, the first bobbin member  40  may include a recess  81  (e.g., a slot) for receiving a printed circuit board. 
     FIG. 3  shows yet another view of the transformer  100 . In  FIG. 3 , the winding  52  on the second bobbin member  20  is shown more clearly. In this embodiment, a slot  26  is provided between two pin supports  95 . The slot  26  allows the wire used for the winding  52  to start near the body portion of the bobbin member. Similar slots may be present on the first bobbin member. Also, as shown in  FIG. 23 , the second bobbin member  20  includes a recessed structure  83  for receiving a side-mounted printed circuit board (not shown). The recessed structure  83  is at a corner region of the second bobbin member  20 . 
     FIGS. 4 and 5  show exploded views of a preferred transformer embodiment. As shown in these Figures, a conductive layer  90  may optionally be provided between the first and second bobbin members  40 ,  20  of the taansformer  100  before they are fitted together. The conductive layer  90  can be in the form of a ring and may be a Faraday shield. Typically, the conductive layer  90  comprises a flat copper shield. The conductive layer  90  may include a tab  99 , which may be bent over and may be electrically coupled to one of the pins (e.g., a ground pin) on the first bobbin member  40 . Conductive charge can be removed from the region between the windings of the first and second bobbin members by using the conductive layer  90 . Charge can pass to the conductive layer  90 , through the tab  99  and to a pin coupled to the tab  99 . Advantageously, the thickness of the walls of the first and second bobbin members  40 ,  20  can be reduced by using the conductive layer  90  between the bobbin members  40 ,  20 . Minimizing the wall thickness reduces any undesirable leakage inductance between the windings on the first and second bobbin members. Also, by minimizing the wall thickness, the height of the resulting transformer  100  can be reduced. The design also allows for the removal of a Y-capacitor (see e.g.,  FIG. 17 ) which might otherwise be needed. This is because the common mode EMI is significantly reduced by the presence of the EMI shield. 
   With reference to  FIGS. 4 and 5 , the core  70  may include two core portions  70 ( a ),  70 ( b ). In this example, both core portions  70 ( a ),  70 ( b ) are U-shaped. When the ends of the U-shaped core portions are joined together, they form a ring. One end of the ring passes through hollow portions in the first and second bobbin members  40 ,  20 , while the other end of the ring is outside of the first and second bobbin members  40 ,  20 . 
   FIGS.  6 ( a ) to  6 ( h ) show various views of an exemplary second bobbin member  20 . Many of the elements shown in FIGS.  6 ( a ) to  6 ( h ) are already described above. However, FIG.  6 ( b ) more clearly shows the second body portion  29  including a second hollow portion. The second body portion  29  is disposed between two walls  26 ,  27 . A flange  21  including perpendicular flange portions  21 ( a ),  21 ( b ) is at an outer edge of one of the walls  27  and extends away from the other wall  26 . In this example, the flange  21  is substantially perpendicular to the orientation of the walls  26 ,  27 . 
   The second body portion  29  is preferably adapted to receive, and is preferably cooperatively arranged with, a tubular portion  49  on the first bobbin member  40  (see FIG.  7 ). For example, the second body portion  29  may be, for example, in the form of a cylinder which has a wider diameter than a cylindrical tubular portion  49 . The tubular portion  49  of the first bobbin member  40  can be inserted within the hollow region of the second body portion  29  so that the first and the second bobbin members  40 ,  20  are coupled together. Advantageously, the first and the second bobbin members  40 ,  20  may be coupled together without the need to use a shroud to hold the first and second bobbin members together. Since a shroud can be excluded in preferred embodiments of the invention, the size of the transformer can be reduced by the space which might otherwise be taken up by the shroud. Moreover, the tubular portion  49  can increase the creepage distance between a conductive layer (e.g., a Faraday shield) between the first and second bobbin members, and the core passing through the bobbin member. 
   The tubular portion  49  shown in  FIG. 7  has a rectangular cross-section. However, it is understood that the tubular portion  49  can have any suitable cross-sectional shape including a circular or square cross-section. The tubular portion  49  includes a hollow region through which a core (not shown) passes. 
   Exemplary pin supports  94  are more clearly shown in FIG.  7 . The pin supports  94  may be provided to support to a plurality of pins (not shown). The pins may be inserted through holes in the individual pin supports. For instance, as shown in  FIG. 9 , four pins are disposed to one side of the transformer  100 , while two pins may be disposed on the other side of the transformer  100 . Of course, the number of pins shown is for illustration purposes and the transformer  100  may include any suitable number of pins. Various views of a first bobbin member  40  are shown in FIGS.  8 ( a ) to  8 ( h ). Many of the elements shown in these Figures are described in detail above. 
   As noted above, portions of the first and second bobbin members  40 ,  20  may be cooperatively structured so that the first and second bobbin members  40 ,  20  can be joined together. Exemplary cooperative structures are shown in  FIGS. 10 and 11 .  FIGS. 10 and 11  show a first bobbin member  40  and a second bobbin member  20 . The second bobbin member  20  includes a second body portion  29  including two sections  29 ( a ),  29 ( b ) which form a recess. The walls  26 ,  27  of the second bobbin member  20  are axially spaced from each other (e.g., with respect to the axis  105 ) and extend away from the second body portion  29  in a radial direction. The first bobbin member  40  includes a first body portion  45  coupled to a pair of walls  46 ,  47 . The walls  46 ,  47  extend away from the first body portion  45  in a radial direction and are axially spaced from one another. A portion of a ring-shaped core  70  is disposed within hollow regions of the first and second body portions  29 ,  45 , while an opposing portion of the core  70  extends past the outer edges of the walls  26 ,  27 ,  46 ,  47 . A flange portion  21 ( b ) is on a wall  27  of the second bobbin member  20  and extends away from the other wall  26  of the second bobbin member  20 . The flange portion  21 ( b ) partially encloses the first bobbin member  40  and the winding (not shown) thereon. 
   Specific features of the cooperatively structured portions of the first and second bobbin members are more clearly shown in  FIGS. 12 and 13 . The first bobbin member  40  includes a tubular portion  49  including a ledge  49 ( a ). The second bobbin member  20  includes a second body portion  29  with sections  29 ( a ),  29 ( b ) forming a recess. The recess receives the tubular portion  49  of the first bobbin member  40 . When the first and second bobbin members  40 ,  20  are coupled together, the ledge  49 ( a ) can abut an inner section  29 ( b ) of the second body portion  29  of the second bobbin member  20 . 
   The cooperatively arranged structures shown in  FIGS. 10  to  13  are especially suitable for increasing the creepage distance from, e.g., the core and a conductive layer (e.g., a Faraday shield) disposed between the first and second bobbin members  40 ,  20 . For instance, with reference to  FIG. 10 , the creepage path between the core portion along the axis  105  and the conductive layer  90  passes from the core  70 , down the inner face of the tubular portion  49 , up the outer face of the tubular portion  49 , and to the conductive layer  90 . If, for example, the section  29 ( b ) of the second body portion  29  is not present (as in some embodiments), the creepage path would extend from the core  70 , up the outer face of the section  29 ( a ) of the second body portion  29 , and to the conductive layer  90 . Accordingly, the embodiments shown and described with reference to  FIGS. 10  to  13  are especially desirable to increase the creepage distance between the core and a conductive layer disposed between the bobbin members  20 ,  40 . By doing so, very low profile transformers can be made. 
   As noted above, embodiments of the invention may also provide for a structure adapted to receive a printed circuit board. The structure is on the side of the transformer and receives the circuit board so that the circuit board is substantially parallel to the walls of the bobbin members of the transformer. In this regard, circuit board receiving structures may be present at any or all of the side surfaces of the first or the second bobbin members. By providing a side mounting structure on the transformer, the overall thickness of a device (e.g., a cellular phone, power supply) using the transformer can be reduced. For example, with reference to  FIG. 14 , in a typical power supply, a transformer  100  is disposed on a circuit board  101  having a thickness “t”. The combined thickness of the circuit board and the transformer  100  is equal to “T”. When the circuit board  101  is disposed at the side of the transformer  100  as shown in  FIG. 15 , the thickness taken up by the circuit board  101  and the transformer  100  is limited to the thickness of the transformer, or “T-t”. In embodiments of the invention, the thickness of the power supply can be reduced by the thickness of the circuit board in comparison to many conventional power supplies. More particularly, in embodiments of the invention, the height of the power supply (i.e., printed circuit board, transformer, and other components) can be limited to the height of the transformer. 
   Providing side mounting structures on a transformer can provide other advantages. For example, with reference to  FIGS. 16 and 17 , two circuit boards  101 ,  102  are disposed on opposite sides of a transformer  100 . The circuit boards  101 ,  102  may be within their own respective planes and parallel to each other, or they may be within the same plane. Circuits are not disposed above or below the transformer. Greater isolation is provided between the windings in the transformer  100 , thus reducing noise. 
   The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed. Moreover, any one or more features of any embodiment of the invention may be combined with any one or more other features of any other embodiment of the invention, without departing from the scope of the invention.