Patent Publication Number: US-6664881-B1

Title: Efficient, low leakage inductance, multi-tap, RF transformer and method of making same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/168,073, filed Nov. 30, 1999, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to transformers, and more specifically, to an efficient, low leakage inductance, multi-tap, RF transformer. 
     2. Discussion of the Background 
     A transformer is a device that transfers electrical energy from one circuit to one or more other circuits, either increasing (stepping up) or decreasing (stepping down) a voltage. A transformer transfers energy through the process of electromagnetic induction. 
     A conventional transformer includes a first coil (the primary winding) and a second coil (the secondary winding). The primary winding and the secondary winding of a transformer are placed in close proximity to each other so that when a varying flux is produced in the primary winding the varying flux passes through the secondary winding. A varying flux can be produced in the primary winding by applying a varying voltage to the primary winding. As a result of the varying flux passing though the secondary winding, a voltage will be developed across the secondary winding through the process of electromagnetic induction. In this manner, voltage is transferred from the primary winding to the secondary winding. 
     FIG. 1 is an illustration of an ideal transformer  100 . As shown in FIG. 1, transformer  100  includes a first coil  101  and a second coil  102 . The first coil  101  is placed in close proximity to the second coil  102 . The first coil  101  will be referred to as the primary winding  101  and the second coil  102  will be referred to as the secondary winding  102 . Because transformer  100  is an ideal transformer (that is, it is 100% efficient), the relationship between the varying voltage developed across the secondary winding (V s ) and the varying voltage applied to the primary winding (V p ) is: V s =N s /N p  (V p ), where N s  is the number of turns in the secondary winding  102  and N p  is the number of turns in the primary winding  101 . 
     Unfortunately, unlike ideal transformers, realizable transformers are not 100% efficient. Realizable transformers have a characteristic called “leakage inductance,” which generally appears to be in series with the primary winding. The greater the leakage inductance of a transformer, the lower the transformer&#39;s efficiency. Consequently, in applications where high efficiency is demanded, the goal of the transformer designer is to reduce the leakage inductance as far as possible. However, the designs that have been developed to overcome the leakage inductance problem are difficult to manufacture, not versatile, and/or not able to transform energy efficiently over a wide range of frequencies. 
     Therefore, what is desired is an efficient, versatile, low leakage inductance transformer that is easy and inexpensive to manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low leakage inductance, versatile RF transformer with multiple input/output voltage ratios. 
     In one aspect, a transformer according to one embodiment includes a stack of conductors that has been shaped into the form of coil. A first group of the conductors form the primary winding of the transformer and the remaining conductors form the secondary winding. Preferably, to minimize leakage inductance, the group of conductors that forms the primary winding is interleaved with the group of conductors that forms the secondary winding. 
     A printed circuit board (PCB) is used to connect the conductors. More specifically, the PCB has a first set of plated slots and traces that are used to interconnect the conductors that form the primary winding, and the PCB has a second set of plated slots and traces that are used to interconnect the conductors that form the secondary winding. In one embodiment, the first set of traces connect in series the conductors that form the primary winding, and the second set of traces connect in series the conductors that form the secondary winding. 
     Advantageously, the PCB also has a number of input and output terminals (also referred to as thru-holes). The input terminals are connected to the primary winding and the output terminals are connected to the secondary winding. 
     In another aspect, the present invention provides an auto-transformer. The auto-transformer includes a plurality of conductors stacked on top of each other and formed into the shape of a coil. The auto-transformer also includes a PCB having a plurality of slots and a plurality of traces for electrically connecting the conductors. In one embodiment, the plurality of traces connect the conductors in series. There are also provided a number of input terminals and output terminals so that the user of the auto-transformer can select one from among many possible voltage ratios. 
    
    
     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
     FIG. 1 is an illustration of an ideal transformer. 
     FIG. 2 is a perspective view of a transformer according to one embodiment of the present invention. 
     FIG. 3 is a diagram of a printed circuit board (PCB) according to one embodiment. 
     FIG. 4 is an illustration of a conductor stack. 
     FIG. 5 shows the conductor stack being wound into the shape of a coil. 
     FIG. 6 is a side view of the transformer shown in FIG.  2 . 
     FIG. 7 is a circuit model of a transformer according to one embodiment. 
     FIG. 8 is a diagram of a PCB according to one embodiment. 
     FIG. 9 is a diagram of a conductor stack. 
     FIG. 10 is a circuit diagram of an auto-transformer according to one embodiment. 
     FIG. 11 is a diagram of a PCB for creating an auto-transformer. 
     FIG. 12 is a circuit diagram of an auto-transformer created using the PCB illustrated in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 illustrates a transformer  200  according to one embodiment of the present invention. Transformer  200  is a low leakage inductance, versatile RF transformer with multiple input/output voltage ratios. Transformer  200  has many applications. For example, and not by way of limitation, transformer  200  can be used with switching power supplies, RF induction power supplies, and RF plasma power supplies. The efficiency of transformer  200  is on the order of 99% to 99.7%. These efficiencies have been measured at power levels of 3 kilowatts (KW) to 10 KW. 
     As shown in FIG. 2, transformer  200  includes a stack of nine conductors  202 ( 1 )-( 9 ). However, the invention is not limited to any particular number of conductors  202 . The conductors  202  have been stacked on top of each other and wound around an axis to form a coil  250 . Each conductor  202  has two ends, and each end of each conductor  202  has been inserted into a plated slot  206  of a printed circuit board (PCB)  204 . Slots  206 ( 1 )-( 14 ) are shown in FIG.  2 . PCB  204  functions to form electrical connections between conductors  202 . 
     In a preferred embodiment, conductors  202  are made from thin strips of copper foil. Preferably, the width of the foil is about 1.5 inches, and the dimension of the slots  206  are {fraction (1/16)} inch by about 1.5 inch, however, other dimensions are contemplated. In an alternative embodiment, Litz wire can be used as the conductors. 
     FIG. 3 illustrates one embodiment of PCB  204  when nine conductors  202  are used to form transformer  200 . As shown in FIG. 3, PCB  204  includes eighteen slots  206 ( 1 )-( 18 ). Each slot  206  receives an end of one of the conductors  202 . For example, as shown in FIG. 2, slot  206 ( 9 ) receives one end of conductor  202 ( 9 ) and slot  206 ( 1 ) receives the other end of conductor  202 ( 9 ). 
     PCB  204  also includes conductive metal strips (also referred to as “traces”)  302 ( 1 )-( 8 ) and  306 (l)-( 7 ), and plated thru-holes  304 ( 1 )-( 8 ). Traces  302 ( 1 )-( 8 ) serve to connect a slot  206  to a thru-hole  304 . Traces  306 ( 1 )-( 7 ) serve to electrically connect a pair of slots  206 . For example, trace  306 ( 1 ) electrically connects slot  206 ( 3 ) with slot  206 ( 10 ), and trace  302 ( 1 ) connects slot  206 ( 2 ) with plated thru-hole  304 ( 1 ). Thus, if one end of conductor  202 ( 1 ) is inserted into slot  206 ( 3 ) and one end of conductor  202 ( 2 ) is inserted into slot  206 ( 10 ), then conductor  202 ( 1 ) and  202 ( 2 ) are electrically connected in series by trace  306 ( 1 ). Advantageously, PCB  204  is designed so that there are no trace crossovers. That is, there are no two traces that pass through the same point, which allows the utilization of all PCB layers to conduct current. 
     The process of constructing transformer  200  will now be described. Transformer  200  is constructed by first stacking conductors  202  on top of each other to form a conductor stack  400 , as shown in FIG.  4 . Preferably, each conductor  202  is coated with (or encased within) an electrically insulating material so as to electrically insulate the conductors from each other. Next, conductor stack  400  is formed into a coil  250  (or spiral) by winding conductor stack  400  around an axis, as shown in FIG.  5 . The number of times conductor stack  400  is wound around the axis depends on the application for which transformer  200  will be used. In one embodiment, conductor stack  400  is wound around the axis two times, as shown in FIG.  5 . 
     After forming conductor stack  400  into the shape of a coil, each end of each conductor  202  is inserted into one of the slots  206  of PCB  204 , as shown in FIG.  6 . In one configuration, transformer  200  is configured as follows. Slot  206 ( 1 ) receives one end of conductor  202 ( 1 ) and slot  206 ( 18 ) receives the other end. Slot  206 ( 2 ) receives one end of conductor  202 ( 2 ) and slot  206 ( 17 ) receives the other end. Slot  206 ( 3 ) receives one end of conductor  202 ( 3 ) and slot  206 ( 16 ) receives the other end. Slot  206 ( 4 ) receives one end of conductor  202 ( 4 ) and slot  206 ( 15 ) receives the other end. Slot  206 ( 5 ) receives one end of conductor  202 ( 5 ) and slot  206 ( 14 ) receives the other end. Slot  206 ( 6 ) receives one end of conductor  202 ( 6 ) and slot  206 ( 13 ) receives the other end. Slot  206 ( 7 ) receives one end of conductor  202 ( 7 ) and slot  206 ( 12 ) receives the other end. Slot  206 ( 8 ) receives one end of conductor  202 ( 8 ) and slot  206 ( 11 ) receives the other end. Slot  206 ( 9 ) receives one end of conductor  202 ( 9 ) and slot  206 ( 10 ) receives the other end. 
     The last step in the process of constructing transformer  200  is to secure each conductor  202  to PCB  204 . This can be accomplished by, among other ways, soldering each end of each conductor  202  to PCB  204  so that a good electrical connection is made and the end won&#39;t slip out of the slot  206  in which it was inserted. 
     FIG. 7 is an idealized circuit diagram of the transformer  200  that is formed using PCB  204  and the above described process and configuration. As shown in FIG. 7, transformer  200  includes a primary winding  702  and a secondary winding  704 . The primary winding consists of conductors  206 ( 2 ),  206 ( 4 ),  206 ( 6 ), and  206 ( 8 ) and traces  306 ( 2 ),  306 ( 4 ) and  306 ( 6 ), which connect conductors  206 ( 2 ),  206 ( 4 ),  206 ( 6 ), and  206 ( 8 ) in series. The secondary winding consists of conductors  206 ( 1 ),  206 ( 3 ),  206 ( 5 ),  206 ( 7 ), and  206 ( 9 ) and traces  306 ( 1 ), and  306 ( 7 ), which connect conductors  206 ( 1 ),  206 ( 3 ),  206 ( 5 ),  206 ( 7 ), and  206 ( 9 ) in series. 
     Traces  302  and thru-holes  304  (also referred to as input/output terminals) provide transformer  200  with versatility. For example, they enable transformer  200  to have a number of possible input to output voltage ratios. The possible input to output voltage ratios are: 1:1, 1:2, 1:3, 1:4, 1:5, 2:1, 2:3, 2:5, 3:1, 3:2, 3:4, 3:5, 4:1, 4:3, and 4:5. For example, to achieve a 1:1 input to output voltage ratio, the voltage input terminal pair would be input terminals  304 ( 1 ) and  304 ( 4 ), and the output terminal pair would be output terminals  304 ( 5 ) and  304 ( 7 ). Similarly, to achieve a voltage ratio of 1:3, the voltage input terminal pair would be input terminals  304 ( 2 ) and  304 ( 3 ), and the output terminal pair would be output terminals  304 ( 5 ) and  304 ( 6 ). The table below illustrates the relationship between the input terminal pairs, output terminal pairs, and the voltage ratio. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Input/Output Voltage Ratio 
                 Input Terminals 
                 Output Terminals 
               
               
                   
               
             
            
               
                 1:1 
                 304(2) &amp; 304(3) 
                 304(6) &amp; 304(7) 
               
               
                 1:2 
                 304(2) &amp; 304(3) 
                 304(6) &amp; 304(8) 
               
               
                 1:3 
                 304(2) &amp; 304(3) 
                 304(5) &amp; 304(6) 
               
               
                 1:4 
                 304(2) &amp; 304(3) 
                 304(5) &amp; 304(7) 
               
               
                 1:5 
                 304(2) &amp; 304(3) 
                 304(5) &amp; 304(8) 
               
               
                 2:1 
                 304(1) &amp; 304(2) 
                 304(6) &amp; 304(7) 
               
               
                 2:3 
                 304(1) &amp; 304(2) 
                 304(5) &amp; 304(6) 
               
               
                 2:5 
                 304(1) &amp; 304(2) 
                 304(5) &amp; 304(8) 
               
               
                 3:1 
                 304(1) &amp; 304(3) 
                 304(6) &amp; 304(7) 
               
               
                 3:2 
                 304(1) &amp; 304(3) 
                 304(6) &amp; 304(8) 
               
               
                 3:4 
                 304(1) &amp; 304(3) 
                 304(5) &amp; 304(7) 
               
               
                 3:5 
                 304(1) &amp; 304(3) 
                 304(5) &amp; 304(8) 
               
               
                 4:1 
                 304(1) &amp; 304(4) 
                 304(6) &amp; 304(7) 
               
               
                 4:3 
                 304(1) &amp; 304(4) 
                 304(5) &amp; 304(6) 
               
               
                 4:5 
                 304(1) &amp; 304(4) 
                 304(5) &amp; 304(8) 
               
               
                   
               
            
           
         
       
     
     FIG. 8 illustrates a printed circuit board (PCB)  800  according to another embodiment of the invention. PCB  800  is used to create an auto-transformer  1000  (see FIG.  10 ). An auto-transformer is constructed in the same way that transformer  200  is constructed. That is, a stack of conductors is wound into the shape of a coil, and each end of each conductor is electrically connected to PCB  800 . For example, given the stack of conductors  902 ( 1 )- 902 ( 4 ) shown in FIG. 9, an auto-transformer would be constructed as follows. First the stack of conductors  902  would be wound around an axis to form a shape of a coil. Next, ends  904 ,  906 ,  908  and  910  are inserted into plated slots  802 ( 1 ),  802 ( 2 ),  802 ( 3 ), and  802 ( 4 ), respectively. And ends  914 ,  916   918 , and  920  are inserted into plated slots  802 ( 8 ),  802 ( 7 ),  802 ( 6 ), and  802 ( 5 ), respectively. 
     FIG. 10 illustrates the resulting auto-transformer  1000 . As shown in FIG. 10, auto-transformer  1000  comprises a winding  1002  consisting of conductors  902 ( 1 )- 902 ( 4 ) and traces  804 ( 1 )- 804 ( 3 ) connected in series. The Auto-transformer also includes input terminals  808 ( 5 ) and  808 ( 6 ) connected to winding  1002  via traces  806 ( 5 ) and  806 ( 6 ), and a number of output terminals  808 ( 1 )- 808 ( 4 ) connected to winding  1002  via traces  806 ( 1 )- 806 ( 4 ). 
     FIG. 11 illustrates a PCB  1100 , which is identical to PCB  800  with the exception that PCB  1100  additionally includes traces  1104 ( 1 )-( 4 ) and  1106  and through holes  1102 ( 1 )-( 4 ). Like PCB  800 , PCB  1100  is used to create an auto-transformer  1200  (see FIG.  12 ). Auto-transformer  1200  is created in the same manner as auto-transformer  1000 . The difference between auto-transformer  1000  and  1200 , is that it is easier to set and change the input/output ratio of auto-transformer  1200 . That is, in auto-transformer  1200  the input/output voltage ratio is determined simply by using a jumper (not shown), or the like, to electrically connect one of the through hole pairs  808 ( 4 )&amp; 1102 ( 4 ),  808 ( 1 )&amp; 1102 ( 1 ), or  808 ( 3 )&amp; 1102 ( 3 ). 
     While various embodiments/variations of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.