Patent Publication Number: US-7225018-B2

Title: Flyback transformer wire attach method to printed circuit board

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a division of U.S. patent application Ser. No. 11/189,599, filed on Jul. 26, 2005 now U.S. Pat. No. 7,120,492, which is a division of U.S. patent application Ser. No. 10/625,827, filed on Jul. 23, 2003, now issued as U.S. Pat. No. 6,927,663. These applications are incorporated herein by reference. 

   TECHNICAL FIELD 
   This document relates to flyback transformers, and in particular, to a surface mountable flyback toroidal transformer for use in an implantable medical device. 
   BACKGROUND 
   Flyback transformers are used with flyback power converters to provide electrical isolation between the power source and load. The transformers are comprised of primary windings and secondary windings around a common core of magnetic material. The voltage produced in the secondary winding is related to the voltage in the primary winding by the ratio of turns between the primary and secondary windings. In a toroidal transformer, the windings are formed around a toroid-shaped core. 
   It is important to minimize the size of components, including transformers, used in implantable medical devices to reduce the overall size of the implantable device for patient comfort. Also, because implantable devices are generally battery powered it is important to use battery power efficiently by reducing losses due to resistance and thereby extend the battery life. 
   Miniaturized transformers are often formed by forming the windings around a bobbin to hold the wires and a core is then slipped into the bobbin. Often, the windings are formed around the toroid-shaped core. The bobbin and/or core are placed in a package with the winding wires wrapped around package leads. The packaged transformer is then mounted onto a main circuit board. The package leads add height to the final transformer assembly. Also, the leads add resistance to the transformer assembly and the efficiency of the transformer is reduced by the energy loss resulting from the lead resistance. What is needed is a transformer assembly that reduces or eliminates the disadvantages of using a transformer assembly that includes leads. 
   SUMMARY 
   This document discusses a surface mountable toroidal transformer and a method for making the transformer. The transformer comprises a toroid-shaped core comprising magnetic material, at least one primary coil of insulated wire wound around the core where the primary coil is electrically isolated from a transformer load, at least one secondary coil of insulated wire wound around the core, and a printed circuit board (PCB) where the PCB includes a plurality of vias in communication with bonding pads. Wire ends of the at least one primary and at least one secondary coils are attached to the vias and the toroid-shaped core and coils are affixed to the PCB such that a center axis of the toroid is substantially perpendicular to the PCB. 
   The method of forming a transformer comprises winding insulated wire around a magnetic core to form at least one secondary transformer coil, winding insulated wire around the magnetic core to form at least one primary transformer coil, attaching wire ends of the at least one primary coil to at least first and second vias of a PCB, attaching wire ends of the at least one secondary coil to at least third and fourth vias of the PCB, affixing the coils and the magnetic core to the first side of the PCB, and attaching the coil wire ends flush to a surface of the vias on the second side of the PCB. 
   This summary is intended to provide an overview of the subject matter of the present application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the subject matter of the preset patent application. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings like numerals refer to like components throughout the several views. 
       FIG. 1  is a drawing of a toroidal transformer mounted on a printed circuit board (PCB). 
       FIG. 2  is a drawing of a PCB used with a toroidal transformer showing vias and bonding pads. 
       FIG. 3  is an electrical schematic of a transformer using multiple primary and secondary coil windings. 
       FIG. 4  is an exploded view of a PCB, a toroid-shaped core and a cover. 
       FIG. 5  is a flow chart of a method of forming a transformer. 
       FIG. 6  is a generalized schematic diagram of one embodiment of a portion of a cardiac rhythm management system  600 . 
       FIG. 7  shows a generalized schematic of one embodiment of a voltage generator used in an implantable pulse generator. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and specific embodiments in which the invention may be practiced are shown by way of illustration. It is to be understood that other embodiments may be used and structural changes may be made without departing from the scope of the present invention. 
   As stated previously, this document discusses a surface mountable toroidal transformer and a method for making the transformer.  FIG. 1  is a drawing of an embodiment of a toroidal transformer  100 . A toroid-shaped core of magnetic material  110  is mounted on a printed circuit board (PCB)  120 . In one embodiment, the PCB is FR 4 . In another embodiment, the PCB comprises ceramic material. In a further embodiment, the PCB comprises flexible circuit tape. The center axis of the core  110  is substantially perpendicular to the plane of the PCB  120 . In one embodiment, the core is a molypermalloy core. In another or the same embodiment, the core is mounted on the PCB using epoxy. The PCB includes vias  130 .  FIG. 1  also shows simplified drawings of a primary coil  150  and a secondary coil  160  comprised of insulated wire wound around the core  110 . The coils  150 ,  160  are attached to vias  130  by soldering. 
     FIG. 2  shows a second side of the PCB  120 . The vias  130  are in communication with bonding pads  240 . The bonding pads  240  are then bonded to a main circuit board. Thus the transformer assembly eliminates package leads and reduces the height of the assembly. 
   The simplified drawing of  FIG. 1  shows only 4 vias  130  of the PCB  120  being used. Additional vias  130  are used to accommodate additional windings.  FIG. 3  shows an electrical schematic  300  of a transformer using multiple primary and secondary coil windings  150 ,  160 . In the embodiment shown in the schematic, the primary coil  150  is comprised of four windings of N 1  turns (T) electrically connected in parallel to vias in communication with bonding pads five and six of the PCB. Connecting the windings in parallel reduces the net resistance of the primary coil  150 . In another embodiment, the resistance is further reduced by using a heavier gauge wire to form the primary windings. In an example of this embodiment, the wire used in the primary coil is 30AWG and the wire in the secondary coil is 42AWG. In another embodiment, the net resistance is 15 milliohms. Thus the assembly increases the efficiency of the transformer by reducing the resistance of primary coil  150  and by eliminating the use of package leads in the assembly. 
   The finished transformer assembly is then connected to a higher level assembly of an electronic device. In one embodiment, the bonding pads of the transformer are solder-bumped. The transformer is then surface mounted to the higher level assembly by placing the bonding pads in position and reflowing the solder to make an electrical connection. The solder used for solder bumping is different than the solder used for attaching the coil ends. The solder used for bumping has a lower melting temperature so that the solder used to attach the vias does not melt and reflow while the transformer PCB is being attached to the higher level assembly. In another embodiment, the bonding pads are on the same side of the PCB as the toroid and the electrical connection to the higher level assembly is made by wire bonding the transformer to pads of the higher level assembly. 
   The schematic in  FIG. 3  also shows multiple secondary coils  160 ,  161 ,  162  and  163 . The ends of each of the coils are attached to vias  130  of the PCB  120 . Thus four voltages related to the ratio of turns between the primary coil  150  and secondary coils  160 ,  161 ,  162 ,  163  are available on the PCB. In the embodiment shown, coils  160 ,  161  and  162  are comprised of N 2  turns and coil  163  is comprised of N 3  turns. Thus it is not necessary for all of the secondary coils to be identical and different voltage magnitudes are available on the secondary coils  160 ,  161 ,  162 , and  163 . Also, the dot references of the secondary coils  160 ,  161 ,  162 ,  163  in the schematic indicate that the winding orientation of the secondary coils is different from the primary windings  150 . This is accomplished, for example, by orienting the primary windings clockwise and the secondary windings counter-clockwise. One of ordinary skill in the art would understand, upon reading and comprehending this disclosure, that various embodiments of the primary and secondary coils  150 ,  160  include various combinations of windings, winding turns, winding orientations and electrical connections. 
     FIG. 4  is an exploded view of an embodiment of a transformer assembly  400  that includes a cover  410 . In one embodiment, the cover  410  is plastic. In another embodiment, the cover  410  includes an electrically conductive material. In another embodiment, the cover  410  of electrically conductive material is connected to a DC voltage level, such as electrical ground for example, to form an electromagnetic shield. Shielding the transformer reduces electromagnetic interference within the medical device if the transformer is used in a flyback power converter circuit. 
     FIG. 5  is a flow chart of a method of forming a transformer. At  510 , insulated wire is wound around a magnetic core to form at least one secondary transformer coil. At  520 , insulated wire is wound around the magnetic core to form at least one primary transformer coil, the primary coil electrically isolated from a transformer load. At  530 , wire ends of the at least one primary coil are attached to at least first and second vias of a printed circuit board (PCB), the PCB having a first and second side, the vias being in communication with bonding pads on the PCB. At  540 , wire ends of the at least one secondary coil are attached to at least third and fourth vias of the PCB. At  550 , the coils and the magnetic core are affixed to the first side of the PCB and the coil wire ends are attached flush to a surface of the vias on the second side of the PCB. 
   In one embodiment, attaching wire ends flush to a surface of the vias of the PCB includes, inserting the primary and secondary coil wire ends into vias, cutting the wire ends to length, and soldering the wire ends to the vias. In another embodiment, the wire ends are cut to extend about one-eighth of an inch beyond the board after soldering. The coil ends are then ground flush to the surface. In another embodiment, soldering the wire ends to the vias includes melting the insulation off of the wire ends as the ends are soldered. 
     FIG. 6  is a generalized schematic diagram of one embodiment of a portion of a cardiac rhythm management system  600 . Various embodiments of system  600  include external or implantable pulse generators, pacer/defibrillators, cardioverters, defibrillators, any combination of the foregoing, or any other system using or maintaining cardiac rhythms. 
   In the embodiment of  FIG. 6 , cardiac rhythm management system  600  includes an implantable pulse generator  605  coupled to heart  610  via one or more endocardial or epicardial leads, such as a pacing lead or a defibrillation lead  615 . Defibrillation lead  615  includes one or more defibrillation electrodes, such as for delivering defibrillation counter-shock (“shock”) therapy via first defibrillation electrode  620 A and/or second defibrillation electrode  620 B. Defibrillation lead  615  may also include additional electrodes, such as for delivering pacing therapy via first pacing electrode  625 A (e.g., a “tip” electrode) and/or second pacing electrode  625 B (e.g., a “ring” electrode). Defibrillation electrodes  620 A–B and pacing electrodes  625 A–B are typically disposed in or near one or more chambers of heart  610 . 
     FIG. 7  shows a schematic of an embodiment of a voltage generator  700  used in an implantable pulse generator  605  that includes an implantable cardioverter defibrillator (ICD). The voltage generator  700  uses a transformer  100  mounted on a circuit board. The voltage generator  700  includes a power source  710  having output terminals. Across the terminals of the power source  710  is a circuit board mounted non-lead transformer  100  connected to a main circuit board of the ICD and a switch  720  connected in series with the primary coil of the transformer  100 . Diode  730  is connected in series with the secondary coil of transformer  100  across the terminals of a load. The load includes a capacitive storage element  740  connected by switch  750  to leads adaptable for connection to a heart  760 . Switch  750  is closed to deliver the energy stored in the capacitive element to the heart  760  to perform cardioversion. Sense circuit  770  monitors the voltage of the capacitive element. In one embodiment, the sense circuit  770  is a comparator and a known voltage reference. 
   The topology of the voltage generator  700  shown is a flyback voltage converter. When switch  720  is closed, diode  730  is reverse biased and energy from the power source  710  is delivered to the transformer through the primary winding, storing energy in the core of the transformer  100 . When switch  720  is opened, diode  730  becomes forward biased and energy stored in the core is delivered to the capacitive load  740 . The voltage transfer from the power source  710  to the load  740  is dependent on the duty ratio of the switch  720 . The output signal of the sense circuit  770  is connected to controller  780 . When the voltage of the capacitive element  740  is below a selected threshold voltage, the sense circuit  770  signals the controller to deliver energy from the power source by repeatedly cycling switch  720 . When the output voltage of the power supply is above or equal to the selected threshold voltage, the sense circuit  770  signals the controller  780  to open the switch  720 . The switching methodology used to charge the transformer shows the advantage of minimizing the resistance of the primary coil to increase the efficiency of the transformer. 
   Although specific examples have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement calculated to achieve the same purpose could be substituted for the specific example shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and their legal equivalents.