Abstract:
An electrical noise-filtering assembly includes first and second busbars. The first busbar is spaced from the second busbar at a fixed distance sufficient to induce the proximity effect. As arranged, high frequency noise having high order harmonics adhere to the surfaces spaced by the fixed distance. Noise can be removed efficiently by disposing low equivalent series resistance (ESR) noise filters between the surfaces where the noise harmonics concentrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electrical noise filtering, and more particularly, to filtering of high-frequency ripple noise from electrical circuits. 
     2. Description of the Related Art 
     When an alternating current (AC) passes through a conductor, depending on the alternating frequency, the current tends to crowd itself near the surface of the conductor. This phenomenon is called the “skin effect” and is known in the art. 
     FIG. 1 diagrammatically depicts the skin effect in which a cross-sectional area A of a conductor  2  carrying an AC current i in a direction  4  which is perpendicular to and out of the figure. Depending on the frequency of the AC current i, the current i tends to propagate near the surface S of the conductor  2 . The higher the frequency, the more the current i travels proximally to the surface S. Skin effect arises from the fact that the current i at different points in the cross-sectional area A does not encounter equal inductance. The current i confronts higher inductance, specifically self-inductance, near the center C of the cross-sectional area A. At the same time, there is much less self-inductance experienced by the current i near the skin surface S of the conductor  2 . As a consequence, an uneven current distribution results. The current density near the skin surface S is much higher than the corresponding current density near the center of the area A. With the majority of the current crowds near the conductor skin surface S which has a limited cross-sectional area in comparison to the total area A, the uneven current distribution in effect increases the overall resistance of the conductor  2 . 
     Two main factors contribute to the skin effect, they are, namely, the cross-sectional area of the conductor and the frequency of the current passing through the conductor. Referring back to FIG. 1, when the current i passes through a large cross-sectional area A, even at low frequency such as 60 Hz (Hertz), skin effect can be eminent. Alternatively, effective resistance also substantially increases when the AC current i alternates at high frequency, even with the cross-sectional area A relatively small in dimension. However, most detrimental of all is when the current i passes through the conductor  2  with a large cross-sectional area A and alternates at high frequency. 
     In reality, the combination of the aforementioned factors occurs in the design of power supplies and related circuits. For example, shown in FIG. 2 is a side elevational view, somewhat schematically, of a section of a pair of power busbars  6  and  8  driving a load  9 . The busbars  6  and  8  are in turn driven by a power supply  5 . As is known in the art, to supply a high current output with a manageable physical size, the power supply  5  needs to be built as a switch-mode power supply. Modern day power supplies are mostly designed to be miniaturized in size. To accomplish this end, switching frequencies of the power supplies are driven at very high ranges. The rapid switching power supply  5  is a source of high frequency noise which, if not properly controlled, is detrimental to the operation of the load  9 . The noise problem associated with the power supply  5  will be explained further below. 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  of FIG.  2 . The busbars  6  and  8  can be found in a high-current output power supply used to drive Internet routers, for example. The busbars  6  and  8  respectively carry currents  10  and  12  as shown in FIGS. 2 and 3. The directions of current flow for currents  10  and  12  are opposite to each other in this case. Depending on the load  9 , the currents  10  and  12  may assume high values. As such, the busbars  6  and  8  are normally designed with large cross-sectional areas. Each busbar  6  or  8  is intended to carry a direct current (DC). However, superimposed on the DC component of each current  10  or  12  is normally high-frequency noise. FIG. 4 graphically shows a typical current characteristic of the busbar  6  or  8 . As shown in FIG. 4, there is a DC component signified by the reference numeral  14 , and a noise component labeled  16 . The noise  16  comes from a variety of sources with a wide spectrum of frequency ranges. As mentioned above, modern day fast switching power supplies substantially aggravate the noise problem. For example, a major portion of the noise  16  may come from switching rectifiers and MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) which are key components of the power supply  5 . 
     The presence of the noise  16  is undesirable in several respects. Chief among all is the impairment of operational reliability of the load  9 . The noise  16  can cause false triggering of logical circuits, as well as erroneous electrical level processing of analog circuits in the load  9 . The noise  16 , if unmanaged, can render the load  9  totally malfunctional. 
     Not to be overlooked is the fact that the noise  16  is high-frequency in nature and the skin effect normally takes into effect. For reasons as stated above, the noise  16  tends to crowd itself near the skin surface of the busbars  6  and  8  when propagating through the busbars  6  and  8 , as diagrammatically shown in FIG.  3 . The noise currents passing through the limited cross-sectional areas near the busbar skins generate unwanted heat which further aggrevates the reliability of the components placed adjacent to the busbars  6  and  8 . Quite often, in high-frequency applications, the heat generated can be substantial. 
     For reasons as stated above, the noise  16  is normally filtered away before reaching any load. Heretofore, filtering of noise on the busbars  6  and  8  has mostly been accomplished by placing noise filtering capacitors  18  between the busbars  6  and  8 , irrespective of the distance between the busbars  6  and  8 . As a consequence, only a portion of the noise is filtered. That is, noise is filtered only in the areas at or near the capacitors  18 . The majority of noise  16  away from the capacitors, such as the noise at the three skin surfaces  6 A,  6 B and  6 C of the busbar  6  remain intact and may not be affected at all. Similarly, noise  16  clustering at the other three surface skins  8 A,  8 B and  8 C away from the capacitors  18  on the busbar  8  may also escape filtering and goes directly to the load  9 . 
     High frequency switching mode power supplies are commonly used to power electronic circuits. There has been a long-felt and increasing need to effectively and efficiently suppress the unwanted noise in practical applications. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the invention to provide a circuit scheme which efficiently filters away unwanted noise, thereby improving operational reliability and curtails wasteful heat generation. The objective of effective noise filtering without resorting to elaborate and expensive implementation is also sought. 
     The noise filtering assembly of the invention includes first and second busbars. The first busbar is proximally spaced from the second busbar at a fixed distance sufficient to induce proximity effect. As arranged, when current passes through the busbars, the high frequency noise having higher order harmonics adhere to the surfaces spaced by the fixed distance. Noise can be removed efficiently by disposing low equivalent resistance noise filters between the surfaces where the noise harmonics concentrate. 
     In other embodiments, more than two busbars are proximally disposed together so as to enhance the proximity effect. Noise filters are also disposed on the busbar surfaces where the noise concentrates. 
     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference numerals refer to like parts. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view illustrating the skin effect when a current passes through a conductor; 
     FIG. 2 is a schematic view showing a pair of busbars disposed between a power supply and a load; 
     FIG. 3 is a cross-sectional view taken along the line  3 — 3  of FIG. 2; 
     FIG. 4 is a graph illustrating a typical current characteristic of the busbars shown in FIGS. 1-4; 
     FIG. 5 is a cross-sectional view illustrating the proximity effect when two conductors carrying currents running in opposite directions are proximally positioned together; 
     FIG. 6 is a cross-sectional view illustrating the proximity effect when two conductors carrying currents running in the same direction are proximally positioned together; 
     FIG. 7 is a perspective view which shows a first embodiment of the invention; 
     FIG. 8 is a cross-sectional view taken along the line  8 — 8  of FIG. 7; 
     FIG. 9 is a schematic drawing of the first embodiment used in conjunction with a power supply and a load; 
     FIG. 10 is a perspective view which shows a second embodiment of the invention; 
     FIG. 11 is a cross-sectional view taken along the line  11 — 11  of FIG. 10; 
     FIG. 12 is a schematic view of the first embodiment used in conjunction with a power supply and a load; 
     FIG. 13 is a cross-sectional view showing a variation of busbar arrangement in accordance with the invention; and 
     FIG. 14 is cross-sectional view showing busbars arranged in accordance with the invention in an array configuration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to describing the embodiments of the invention, there is another phenomenon called the “proximity effect” that needs first be explained. FIG. 5 diagrammatically depicts the proximity effect in which two conductors  20  and  22  are brought adjacent to but not in contact with each other. Assuming the conductors  20  and  22  each has a cross-sectional area A and a surface S. Further assuming the conductor  20  carries an AC current i in a direction  24  which is perpendicular to and out of FIG.  5 . In a similar manner, the conductor  22  carries another AC current i in a direction  26  which is perpendicular to but points into FIG.  5 . Depending on the physical separation of the conductors  20  and  22 , the current distribution i in each conductor  20  and  22  is distorted. The closer the conductors  20  and  22  are brought near each other, the more distorted will be the current distribution. The proximity effect arises when the inductance of one conductor is effected by the mutual inductance of another conductor. For example, as shown in FIG. 5, the current i flowing through the conductor  20  in the proximal presence of the conductor  22  experiences not only the self-inductance of the conductor  20  but also the mutual inductance excerted by the conductor  22 . As a consequence, the overall inductance on the right side  20 R of the conductor  20  is lower than that at the left side  20 L of the conductor  20 . AC current always seeks a path with the lowest inductance to pass through. In this case, the current i passing through the conductor  20  tends to crowd itself at the right side  20 R of the conductor  20 . Under the principle of reciprocity, the current distribution in the conductor  22  can likewise be explained. 
     FIG. 5 shows the direction of current flows in the conductors  20  and  22  opposite to each other. In FIG. 6, the conductors  20  and  22  are shown as each carrying a current i flowing in the same direction  26 . This time, the current distribution is substantially opposite to that as shown in FIG.  5 . Specifically, for the conductor  20 , the current i crowds itself on the left side  20 L of the conductor  20 . Similarly, for the conductor  22 , the current i clusters itself on the right side  20 R of the conductor  22 . The explanation is substantially the same as above except the current polarity is reversed in the conductor  20 . For the sake of conciseness, explanation of the current distribution shown in FIG. 6 is not further elaborated. 
     Reference is now directed to FIGS. 7-9 which show the first embodiment of the invention signified by the reference numeral  30 . FIG. 7 is a perspective view showing the assembly of this embodiment having a section of a pair of busbars  32  and  34 . FIG. 8 is a cross-sectional side view taken along the line  8 — 8  of FIG.  7 . FIG. 9 is a schematic view of this embodiment illustrating the electrical disposition of the busbars  32  and  34  between a power supply  36  and a load  38 . 
     In this embodiment, the busbar  32  has a cross-sectional area A 1  and four longitudinal surfaces  32 A- 32 D. Likewise, the busbar  34  has a cross-sectional area A 1  and four longitudinal surfaces  34 A- 34 D. 
     As shown in FIGS. 7-9, the busbars  32  and  34  are spaced by a distance d. There is also a plurality of noise filters  40  disposed between the busbars  32  and  34 . In this embodiment, the noise filters  40  are capacitors with low equivalent series resistance (ESR). 
     In accordance with the invention, the value of d is chosen so as to induce the proximity effect as mentioned above once the proximity effect sets in, noise, which can be expressed as high-frequency harmonics under the Fourier analysis, mostly adheres to the closely positioned busbar surfaces  32 D and  34 D of the busbars  32  and  34 , respectively. Because the noise filters  40 , low ESR capacitors  42  in this case, are disposed between the surfaces  32 D and  34 D, the filters  40  can efficiently and effectively remove the noise. This is in contrast with most prior art arrangement in which the high frequency noise are located at other surfaces, such as the surfaces  32 A- 32 C and  34 A- 34 C respectively on the busbars  32  and  34 , which are beyond the reach of the noise filters  40 . 
     A exemplary design for the first embodiment  30  can have a width W of each busbar  32  or  34  to be 0.5 inch wide. The height H for each busbar  32  or  34  can be set at 0.2 inch high. The material for the busbars  32  and  34  can be copper (Cu) plated with tin (Sn). The distance d between the busbars  32  and  34  can be from 0.05 inch to 0.2 inch, preferably at 0.1 inch apart. The capacitors used in this embodiment are ceramic chip capacitors, part number: C0805C224K5RAC, manufactured by KEMET Electronics Corporation of Greenville, S.C. 
     FIGS. 10-12 shows a second embodiment of the invention signified by the reference numeral  50 . FIG. 10 is a perspective view depicting a section of this embodiment  50  having four busbars  52 ,  54 ,  56  and  58 . Each busbar  52 ,  54   56  or  58  is disposed between two of the four busbars  52 ,  54 ,  56  and  58 . In this embodiment, the 4 busbars  52 ,  54 ,  56  and  58  are adjacently disposed longitudinally side-by-side and somewhat forms a closed boundary  60  and enclosing a volume of space  62 . 
     FIG. 11 is a cross-sectional side view taken along the line  11 — 11  of FIG.  10 . FIG. 12 is a schematic view of this embodiment  50  illustrating the electrical disposition of the busbars  52 ,  54 ,  56  and  58  between a power supply  36  and a load  38 . 
     In this embodiment, the busbar  52  has an angular-shaped cross-sectional area A 2  and six longitudinal surfaces  52 A- 52 F. Likewise, each of the other busbars  54 ,  56  and  58  has a cross-sectional area A 2  and six surfaces  54 A- 54 F,  56 A- 56 F, and  58 A- 58 F, respectively. 
     During normal operation, the directional flow of current of each busbar is different from that of the other two adjacent busbars. Thus, for example, the direction of current flow  66  in the busbar  54  is different and opposite to the directions of current flow  68  for the two adjacent busbars  52  and  56 . 
     As shown in FIGS. 10-11, each busbar is spaced from the other busbar by a distance d. There is also a plurality of noise filters  40  disposed between any two of the busbars. For example, between the busbars  54  and  56 , there is a plurality of noise filters  40  in the form of low ESR capacitors  42  distributed along the longitudinal directions of the busbars  54  and  56 . The same holds true for the separations between the other busbars. 
     As with the previous embodiment, in accordance with the invention, the value of d is chosen to be as small as possible so as to induce the proximity effect, Once the proximity effect sets in, for each busbar  52 ,  54 ,  56  or  58 , the high-frequency noise which can be expressed as a multiplicity of harmonics under the Fourier analysis, will migrate to the surfaces which are closely disposed adjacent to the other busbars. For instance, for the busbar  52 , most of the noise harmonics will be found near the surfaces  52 C and  52 F, which are respectively adjacent to the busbars  54  and  58 . Similarly, for the busbars  54 ,  56  and  58 , most of the noise will be found near the surfaces  54 C and  54 F,  56 C and  56 F,  58 C and  58 F, respectively. As explained before, because the noise filters  40  are low ESR capacitors  42  disposed between the aforementioned closely adjacent surfaces, noise can be efficiently removed. This is in contrast with most prior art designs in which the high frequency noise are located at other surfaces, which are beyond the reach of the noise filters  40 . The geometrical dimensions of this embodiment can be made comparable to the corresponding dimensions of the previous embodiment. The advantage of this embodiment are the edge surfaces  52 C and  52 F of the busbar  52 ,  54 C and  54 F of the busbar  54 ,  56 C and  56 F of the busbar  56 ,  58 C and  58 F of the busbar  58 , are relatively narrow thereby allowing noise to be more controllably confined for removal. 
     Finally, other changes are possible within the scope of the invention. For all the embodiments as described, the adjacent busbars are described as arranged in even numbers. It certainly is possible to have an odd number of busbars in the assembly as shown in FIG. 13 in which a ground return busbar  62  is disposed between two other busbars  64  and  66 , for example. Furthermore, the busbars can be arranged in the form of an array as shown in FIG. 14 which is a cross-sectional view of a part of an array  68  with a multiplicity of noise filters  40  disposed between the busbars  72 . It will be understood by those skilled in the art that these and other changes in form and detail may be made therein without departing from the scope and spirit of the invention.