Abstract:
A planar magnetic winding structure such as a transformer or inductor having an air gap in the core has a “keep away” region of 2 to 3 times the gap height in which there are no windings, reducing high frequency winding losses by 35 percent or more, without appreciable increases in low frequency winding losses. Such structures are useful, for example, in electronic ballasts for the lighting industry.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to low profile magnetic components, and more particularly relates to such components including planar magnetic winding structures, such as inductors and transformers, in which the windings are composed of stacks of interconnected layers of conductor patterns. 
     The trend toward miniaturization in power electronics requires high power density and low profile magnetic components, which in most cases are the bulkiest circuit components. Reducing the size of such components requires higher drive frequencies, in order to obtain the same performance as a larger magnetic component. All of these factors of smaller size, higher power density and higher operating frequency lead to considerably increased heat generation. As the size of such components decreases, the available surface area through which the heat can be guided towards the environment decreases rapidly, while the total losses remain about the same. 
     Planar winding structures consist of a stack of layers each containing part of the total winding structure, an insulating layer used to prevent electrical contact between the turns in adjacent layers, usually consisting of a flexible, non-conducting, low permittivity, high temperature resistant polymer, and a contacting structure that permits electrical contact between turns in adjacent layers where needed. The winding structures are optimized with respect to winding losses, and are usually made by etching or stamping or sometimes by folding. Contacts are usually made by soldering or via plating. 
     The use of such magnetic components is always accompanied by dissapative losses in the core and windings. Such losses decrease the efficiency of the magnetic component and the electronic circuit of which it is a part, and increase the temperature of the component and the surrounding area, changing the electrical characteristics and reducing the lifetime of the component and other components in thermal contact with it. 
     In general, winding losses are due to the interaction of the winding current with a local magnetic field, due largely to leakage flux from the windings and to stray fields near the gaps in the core. The leakage flux is largely determined by the dimensions of the winding structure and is more or less evenly distributed over the winding window. However, the stray fields near the gap are local in nature and can give rise to large local increases in winding losses. In addition, such local losses will give rise to local thermal runaway phenomena, so called “hot spots”. 
     In planar winding structures, air gaps which induce fringing fields can form a significant portion of the core window height. High frequency magnetic field components in the winding current then induce large eddy currents in the conductors near the air gaps, and as a result, current crowds toward the edges of these conductors. The eddy currents are most severe for the conductors adjacent to the gaps because the fringing fields are strongest in these regions, and therefore it is preferred to locate the windings as far away from the gaps as possible. 
     The one-dimensional analytical loss equations popularly used for calculating high frequency losses in ungapped transformer designs do not adequately predict the winding losses of a gapped structure due to the presence of these air gaps. 
     It is difficult to minimize these gap-related losses using traditional wire wound construction techniques, since it is difficult to achieve the desired separation between air gaps and windings in a reproducible way. 
     Increasing the spacing between the windings and the air gaps inevitably decreases the cross-sectional area available for the winding in a fixed core geometry. This reduction in winding area increases the DC resistance of the coil, and depending on the balance of low-frequency and high-frequency currents in the core, the benefits of reducing high-frequency losses by avoiding the keep-away regions may be offset by increased DC losses. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide a low profile magnetic component with a planar magnetic winding structure, having reduced winding losses. 
     It is another object of the invention to provide such a component containing air gaps in the core, having reduced winding losses. 
     According to the invention, there is provided a planar magnetic winding structure comprising a core of two or more core components having mutually facing planar surfaces separated by at least one air gap having a height g, and a stack of winding layers, each layer including one or more turns, in which structure the edges of the windings are separated from the air gap by a distance of at least 2g, and preferably by a distance of at least 3g. 
     According to one embodiment of the invention, the core of the planar magnetic winding structure comprises a first lower core component having a planar portion and two or more spaced-apart upstanding portions having planar upper surfaces, the upstanding portions defining a space to accommodate the stack of winding layers, the core also comprising. a second upper core component having a planar lower surface, the planar upper surfaces of the upstanding portions of the first core component and the planar lower surface of the second core component defining the air gap g. 
     Preferably, the stack of winding layers in the space between the upstanding portions of the lower core component comprises two or more sub-stacks, each sub-stack having the same number of turns in each layer, the sub-stacks proximal to the air gap g having a smaller number of windings per layer than the sub-stacks distal from the air gap g. 
     Most preferably, the number of turns per layer in each successive sub-stack from the most distal sub-stack to the most proximal sub-stack is smaller than the number of turns per layer in the preceding sub-stack. 
     A planar magnetic winding structure in accordance with the invention, such as a transformer or inductor, having an air gap in the core with a “keep away” region of 2 to 3 times the gap height in which there are no windings, reduces high frequency winding losses by 35 percent or more, without appreciable increases in low frequency winding losses. Such structures are useful, for example, in electronic ballasts for the lighting industry. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1 a ,  1   b  and  1   c  are an expanded perspective view, a top view and a side view, respectively, of a schematic representation of a planar transformer construction of the prior art; 
     FIGS. 2 a ,  2   b  and  2   c  are side section views of a schematic representation of a planar transformer construction of the type shown in FIG. 1, having a winding configuration of the prior art, a stepped winding configuration of the invention, and a tapered winding configuration of the invention, respectively; 
     FIG. 3 is a graph of planar inductor winding resistivity in ohms versus frequency in Hz for the planar transformer constructions of FIGS. 2 a  through  2   c ; and 
     FIGS. 4 is a bar graph of relative inductor winding losses in percent versus frequency in kHz for the planar transformer constructions of. FIGS. 2 a  and  2   c.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be further elucidated by a detailed description of certain preferred embodiments of the invention, in conjunction with the drawings, in which the same reference numerals are used to indicate similar features or elements in different figures. 
     Referring now to FIGS. 1 a  through  1   c , there is illustrated a planar transformer construction  10  of the prior art, including a composite ferrite core made up of a bottom “E” core  11 , so named for the E-shape resulting from the upstanding portions  13 ,  14  and  15  on the base portion  12 , and a top “I” core  16 , having a planar configuration. Arranged in the spaces between the upstanding portions  13 ,  14  and  15  is a stack of winding layers  17 ,  18  and  19 , separated by insulating layers  20  through  25 , and provided with external connections through lead frames  26  and  27 . While only three exemplary winding layers are shown, it will be appreciated that in practice there may be eleven or more such layers, with a corresponding increase in the number of insulating layers. When bonded together in a known manner, these layers form a unitary winding structure  28 . 
     FIGS. 2 a  through  2   c  are cross sectional views of the right half of such a structure, including an upstanding portion  15  and one half of central upstanding portion  14  of E core portion  11 , with three different arrangements of the turns in the stacks of winding layers. The construction is assembled so as to result in an air gap between the lower E core portion  11  and upper I core portion  16 , the air gap having a height g. 
     Shown in FIG. 2 a  is a typical winding arrangement of the prior art. A single stack  30  of eleven winding layers L 1  through L 11  is arranged in the space between the upstanding portions  14  and  15  of core portion  11 , each layer having four turns vertically aligned with the turns in the other layers to result in four columns of turns C 1  through C 4 , for a total of  44  turns. Such an arrangement has been found to result in significant losses due to fringing fields in the area of the turns adjacent to the air gap. 
     In accordance with the invention, it has been found that such losses may be significantly reduced by arranging the winding pattern so that the edges of the windings are at least a distance 2g, and preferably a distance 3g, from point A in the lower surface of the I core  16 . directly above the inner surface  31  of upstanding wall  14 . This distance defines a “keep away” region which, when kept clear of windings in accordance with the teachings of the invention, has been found to reduce high frequency losses by up to 30 percent, without significantly increasing low frequency losses. 
     One such arrangement for reducing such high frequency losses is shown in FIG. 2 b . Stack  30  is composed of two sub-stacks, a lower sub-stack I and an upper sub-stack II. As in FIG. 2 a , sub-stack I is composed of four columns C 1  through C 4  of turns. However, the columns are formed of eight layers L 1  through L 8 . The upper sub-stack has three columns C 5  through C 7  of four layers L 9  through L 12 . Thus, this arrangement also has a total of  44  turns. However, the upper sub-stack is arranged centered over the lower sub-stack, resulting in a stepped arrangement, in which the edges of the upper turns do not extend into the “keep away” region. 
     Another such arrangement for reducing such high frequency losses is shown in FIG. 2 c . Stack  30  is composed of three sub-stacks, a lower sub-stack I, a middle sub-stack II and an upper sub-stack III. As in FIGS. 2 a  and  2   b , sub-stack I is composed of four columns C 1  through C 4  of turns. As in FIG. 2 b , the columns are formed of eight layers L 1  through L 8 . However, the widths of the outermost turns in layer L 8  have been slightly reduced so that their outer edges approximately coincide with a low frequency curved flux line S associated with the gap-forming surfaces  32  and  33 , which flux line S passes approximately through the outer edges of the outermost turns of layer L 7 . The middle sub-stack II has three columns C 5  through C 7  of two layers L 9  and L 10 . The widths of the outermost turns are also adjusted so that their outer edges fall approximately on line S. The upper sub-stack III has two columns C 8  and C 9  of three layers L 11  through L 13 . Again, the outer edges of the turns fall approximately on line S, resulting in an inwardly “tapered” arrangement of a total of  44  turns, in which the edges of the upper turns do not extend into the “keep away” region as defined by line S. 
     All three of the above winding configurations, referred to hereinafter as Cases  1  through  3 , were fit into a Philips E18/4/10 planar EI core, and were compared using a 2D finite element analysis. The air gap, as well as the spacing d between the inner surfaces  31  of the upstanding core portions and the outermost turns of the stack in case  1  and the lower sub-stack I in cases  2  and  3 , were kept constant, at 0.5 mm and 0.5 mm, respectively. The smallest distance between the turn edges and point A was 0.5 mm for case  1 , 1.0 mm or 2g for case  2  and 1.5 mm or 3g for case  3 . The loss densities per meter were computed along the Z axis at 120 Hz (DC loss) and 200 kHz (AC loss). The results are given in the table below. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                                    winding loss 
                 winding loss 
                 winding loss 
               
               
                   
                 (W/m) 
                 (W/m) 
                 (W/m) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 frequency (Hz) 
                 case 1 
                 case 2 
                 case 3 
               
               
                   
                 120 
                 5.5 
                 5.5 
                 5.4 
               
               
                   
                 200 k 
                 94.2 
                 73.2 
                 68 
               
               
                   
                 AC/DC ratio 
                 17.1 
                 13.3 
                 12.6 
               
               
                   
                   
               
             
          
         
       
     
     As may be seen from the table, AC loss in cases  2  and  3  are only 78 percent and 72 percent, respectively, of case  1 , while DC loss remains approximately the same. 
     FIG. 3 illustrates graphically the winding losses in terms of winding resistivity in ohms as a function of the applied excitation frequency in Hz, in accordance with the above 2D FEA modeling experiment. As may be seen, the losses increase gradually with increasing frequency, but at different rates, case  2  exhibiting a lower growth rate than case  1  and case  3  exhibiting the lowest growth rate. 
     FIG. 4 presents the same data in bar graph form, but only for Cases  1  and  3 , and only for the frequencies appropriate for switched mode power supplies or miniaturized electronic ballasts, the applications of greatest interest at the present time. From the graph, it can be seen that in this frequency range, the differences in loss are significant. For example, at 250 kHz, Case  3  has a 35 percent lower winding loss than Case  1 . At higher frequencies, this difference is even greater. 
     Thus, it can be seen that maintaining a “keep away” region of at least 2g can significantly reduce high frequency losses without appreciably increasing low frequency losses, while optimal arrangement of the windings can result in even further reductions in high frequency losses. 
     The invention has been necessarily described in terms of a limited number of embodiments and variations of embodiments. Other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be encompassed within the scope of the appended claims. For example, other core configurations are possible, such as two U-shaped cores in which the gaps are located adjacent the middle of the winding. More advantageous would be configurations having multiple gaps, such as a core having upper and lower planar core portions and two or more upstanding portions, each upstanding portion forming gaps with the uppper and lower planar core portions. With gaps at both the upper and lower ends of the winding, the height g of each gap can be half that in the E core configuration, reducing the reach of the low frequency flux lines of the fringing fields by the square root of g. Further numbers of gaps would enable further reductions. 
     As used herein, terms such as “upper”, “lower”, “upstanding”, “vertical” and “horizontal”, are used to indicate the relative locations of the various elements in the conventional orientation shown in the drawings, but do not in any way limit the use of the various constructions of the invention to such illustrated orientation.