Abstract:
An energy storage coil comprises a core having an electrical conductor wound thereabout in a plurality of turns. The turns define a main zone and at least one first auxiliary zone extending along the core. The main zone has a first end and a second end. The turns in the main zone overlie one another. The first auxiliary zone is arranged adjacent to the first end of the main zone. The turns in the first auxiliary zone are arranged to provide the first auxiliary zone with lower parasitic capacitance from turn to turn than the main zone.

Description:
FIELD OF THE INVENTION 
   This invention relates to an energy storage coil comprising a core having wound thereabout a plurality of turns of an electrical conductor and, more particularly, to an energy storage coil for minimizing unintentional electromagnetic interference (EMI) generated when a current through the coil changes rapidly. 
   BACKGROUND OF THE INVENTION 
   The subject of electromagnetic compliance (EMC) has in recent years become very significant, especially in the field of electronics. Many environments now include large numbers of electronic devices, especially personal desktop computers and similar apparatuses. It is known that the power supplies that form a part of such devices switch at exceedingly high frequencies. Such switching causes EMI. Stray EMI can interfere with the correct operation of neighboring apparatuses. The effects of such interference can range from mere inconvenience to users to catastrophic or even life-threatening consequences. 
   As a result, many governments have passed legislation requiring manufacturers of electronic equipment to limit the amplitude of EMI created by its products and/or to filter the EMI so that it is attenuated in frequency ranges that would otherwise hamper or affect the operation of other devices. In view of these requirements, it has become commonplace to include EMI filters connected in printed circuit boards of power supplies of devices such as desktop computers. It is generally more beneficial, however, to avoid generating EMC than to filter it by additional circuitry. Typically, power supplies use a semiconductor device to switch current through an inductor. At the moment of switch off, the current flowing in the inductor is interrupted, and the voltage across the inductor changes rapidly. The limit on the rate of change of voltage is usually imposed by parasitic capacitances, typically between the wire forming one turn and that forming an adjacent turn, which turns form a resonant circuit with the inductor or part of the inductor. The net effect is to cause energy to be emitted from the circuit at one or more pseudo-resonant frequencies. This spurious energy is in addition to the wanted energy that is transferred to the load. Often the spurious energy is in a frequency band which is controlled by legislative limits. 
   It is known in the art to replace the regularly spaced windings of a conventional EMI filter toroid with “piled” windings, i.e. windings that overlie one another in a substantially irregular manner, over a major part of the toroid. However, this solution leads to a very large number of small resonant circuits. Thus, the undesirable self-resonances are reduced in energy, but increased in number. It is therefore necessary to apply further filtering or other suppression measures in order to reduce this energy to acceptable levels. 
   Such windings are commercially available, for example, for power factor correction circuits. An increasing proportion of electronic devices is equipped with power factor correction circuitry. Older devices typically use a rectifier and capacitor combination as an alternating current (AC) to direct current (DC) converter to provide a DC supply for the AC to DC converter that actually powers the device. Despite their simplicity, such AC to DC converters draw large peak currents from the AC supply when the AC voltage is at or near its peak, and little current elsewhere in the cycle. The resulting distortion of the current waveform from an ideal sinusoidal shape causes higher root mean square (RMS) currents in the supply wiring than would be expected from the electrical power drawn by an electronic device. 
   This effect may not be significant when considering a single device such as a personal computer. On the other hand, it is now commonplace for entire buildings, on completion, to be equipped with large numbers of identical apparatuses, such as a bulk order of identical personal computers. The power factor effects of the plurality of AC to DC converters that such an installation represents are cumulative. Consequently the opening of ego a new call or data centre may for example cause significant supply current distortion, purely as a result of a large number of AC to DC converters being connected to an alternating mains supply. 
   Electricity companies have for many years sought to eliminate the inefficiency of transmission that this represents. In the case of personal computers, however, it is not readily possible to use the kinds of power factor correction apparatus, such as capacitive shunts, that are suitable for ego electric motors. It follows therefore that there is a need for an improved means of reducing the supply current distortion. Typically this need is met by a switched mode power factor correction circuit that makes the shape of the current waveform substantially the same as, and in phase with, the voltage waveform. As well as its beneficial effects, the power factor correction circuit often gives rise to significant EMI. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention is an energy storage coil comprising a core having an electrical conductor wound thereabout in a plurality of turns. The turns define a main zone and at least one first auxiliary zone extending along the core. The main zone has a first end and a second end. The turns in the main zone overlie one another. The first auxiliary zone is arranged adjacent to the first end of the main zone. The turns in the first auxiliary zone are arranged to provide the first auxiliary zone with lower parasitic capacitance from turn to turn than the main zone. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a conventional energy storage coil having essentially uniform conductor windings; 
       FIG. 2  is a perspective view of another conventional energy storage coil having irregularly piled windings; 
       FIG. 3  is a perspective view of an energy storage coil according to the invention; 
       FIG. 4  is a chart showing a frequency response of the coil of  FIG. 2 ; and 
       FIG. 5  is a chart showing the frequency response of the coil of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a conventional energy storage coil  10 . The coil  10  includes a toroidal core  11  having an electrical conductor, such as a copper wire or a metallic alloy wire, coiled through a plurality of turns  12  wound over almost its entire length. The core  11  may be, for example, a solid, ferrite material, such as a sintered ferrite material or a laminated ferrite material. The core  11  may also be in the form of a hollow, toroidal former (manufactured ego from a polymer) that is packed with a powdered, ferritic material. The turns  12  are substantially uniformly and evenly spaced. The turns  12  of the electrical conductor terminate, for example, at a single interrupted zone  13  where terminal ends  14 ,  16  of the electrical conductor lead out from the coil  10  to permit operative connection to a switched mode power supply of a device, such as a personal computer. The coil  10  of  FIG. 1  suffers from the disadvantages noted hereinabove. 
     FIG. 2  shows another conventional energy storage coil  20 . The coil  20  is substantially similar to the coil  10  shown in  FIG. 1 . The coil  20  includes a toroidal core  21  having an electrical conductor coiled through a plurality of turns  22  wound over almost its entire length. The turns  22  of the electrical conductor terminate, for example, at a single interrupted zone  23  where terminal ends  24 ,  26  of the electrical conductor lead out from the coil  20 . The coil  20  of  FIG. 2  differs from the coil  10  of  FIG. 1  the turns  22  are overlain one on another in a substantially irregular fashion, as shown in  FIG. 2 . A protective cover  27  encloses components of the coil  20  except for the terminal ends  24 ,  26 . As noted hereinabove, the average energy of resonance of each resonant circuit defined in the coil  20  is reduced compared with the coil  10  of  FIG. 1 . On the other hand, the number of resonators is dramatically increased in the coil  20  of  FIG. 2 , as compared with the coil  10  of  FIG. 1 . As explained hereinabove, this leads to a requirement for additional filtering and suppression apparatuses. 
     FIG. 3  shows an energy storage coil  30  in accordance with the invention. The coil  30  includes a toroidal core  31  having an electrical conductor, such as a copper wire or a metallic alloy wire, coiled through a plurality of turns  32  wound over almost its entire length. The core  11  may be, for example, a solid, ferrite material, such as a sintered ferrite material or a laminated ferrite material. The core  31  may also be in the form of a hollow, toroidal former (manufactured ego from a polymer) that is packed with a powdered, ferritic material. The turns  32  of the electrical conductor terminate, for example, at a single interrupted zone  33  where terminal ends  34 ,  36  of the electrical conductor lead out from the coil  30  to permit operative connection to a switched mode power supply circuit or any of a wide range of other applications. 
   The turns  32  of the coil  30  are divided into at least two types of zones. The zones include a main zone  37  and first and second auxiliary zones  38 ,  39 , respectively. In the illustrated embodiment, the main zone  37  is arranged on a side of the coil  30  opposite to the interrupted zone  33 . At the main zone  37 , the turns  32  overlie one another in a substantially irregular fashion, as shown in  FIG. 3 . Immediately to either side of the main zone  37  are arranged the first auxiliary zones  38 . Immediately to either side of the first auxiliary zones  38  are arranged the second auxiliary zones  39 . At the first and second auxiliary zones  37 ,  38 , the turns  32  are substantially uniform and substantially equally spaced from one another. The spacings of the turns  32  in the first auxiliary zones  38  are narrower on average than the spacings in the second auxiliary zones  39 . In the second auxiliary zones  39  the turns  32  are spaced from each other by typically one or two electrical conductor diameters. 
   In the coil  30  of  FIG. 3 , the main zone  37  accounts for approximately 70% of the total number of the turns  32  wound over the core  31 , and the main zone  37  occupies approximately 20% of the circumference of the coil  30 . Each of the first auxiliary zones  38  accounts for approximately 10% of the total number of the turns  32 , and each of the second auxiliary zones  39  accounts for approximately 5% of the total number of the turns  32 . Considerable variations of the above-indicated proportions of the coil  30  are possible within the scope of the invention. It will be apparent to the worker of skill in the art how to embody such variants. 
   In the coil  30  of  FIG. 3 , the main zone  37  is flanked immediately on either side by the first auxiliary zones  38  of regular, relatively narrowly-spaced turns  32  that are serially connected to the second auxiliary zones  39  of regular, relatively broadly-spaced turns  32 . In alternative arrangements, however, the first and second auxiliary zones  38 ,  39  may be reversed on each side of the main zone  37 , such that the relatively broadly-spaced second auxiliary zones  39  lie immediately adjacent to the main zone  37 . In yet a further variant, if the electrical conductor to one side of the main zone  37  is grounded, there may be reduced benefit in providing the first and second auxiliary zones  38 ,  39  on that side of the main zone  37 . Thus, the principles of the invention extend to asymmetric patterns of the main and first and second auxiliary zones  37 ,  38 ,  39 , as well as the symmetric pattern shown in  FIG. 3  and the alterative symmetric pattern described hereinabove. 
   In the coil  30  according to the invention, high-frequency spurious oscillation energy generated in the main zone  37  is attenuated by the combination of the first and second auxiliary zones  38 ,  39  before it reaches the remainder of the circuit in which the coil  30  is connected, as illustrated by the data in  FIGS. 4 and 5 . This arrangement is particularly advantageous in the case of the core  31  of the type used for power factor correction circuits of less than about 1 kW rating, since it allows for attenuation of the spurious resonances on either side of the main zone  37 . The invention offers a marked improvement over the spurious EMI generating properties of irregularly wound coils by combining several coils onto the same magnetic core. The invention can also be applied in other areas of switched mode power supplies, or indeed more widely where spurious resonant frequencies are generated by virtue of rapidly changing currents in inductors. 
   In the illustrated embodiment, the first auxiliary zones  38  lie closer than the second auxiliary zones  39  to the main zone  37 . However, in an alternative arrangement the or each of the first auxiliary zones  38  may lie further than the second auxiliary zones  39  away from the main zone  37 . It has been found that coils  30  manufactured in accordance with the principles of the invention are effective at attenuating undesirable resonances, regardless of whether the first auxiliary zones  38  or the second auxiliary zones  39  lie closest to the main zone  37 . 
     FIG. 4  is a chart showing the frequency response of the coil  20  of  FIG. 2 . Shaded zone  41  illustrates the frequency response in the 1 MHz–30 MHz frequency range. The zone  41  shows that the peak excitation of the coil  20  around the 20 MHz frequency achieves an amplitude exceeding a limit line  42  (i.e. the upper, dark line). Thus, the oscillation exceeds a regulatory or design limit. 
     FIG. 5  is a chart showing the frequency response of the coil  30  of  FIG. 3 . In contrast to the coil  20  of  FIG. 1 , shaded zone  41 ′ corresponding to the frequency response of the coil  30  of  FIG. 3  shows on average noticeably lower amplitude peaks. Moreover, in the approximately 20 MHz range, the maximum amplitude is considerably below the limit line  42 , which is set at the same level as in  FIG. 4 . In fact, at all the harmonic frequencies of the coil  30 , the resonance peaks are approximately 10 dB below the limit line  42  which in practical terms represents a very significant improvement over the conventional coil  20  of  FIG. 2 . 
   The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. For example, the core  31  is not limited to the toroidal shape illustrated herein and may alternatively be a different shape, such as a straight, elongated, cylindrical rod shape, dumbbell shape, E—E shape, etc. Additionally, the core  31  may be solid, sintered, laminated, or hollow and powder-filled. Further, the coil  31  may just have either the first or second auxiliary zones  38 ,  39  and the turns  32  in such zone do not have to be overlain one on another. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.