Abstract:
An inductive power transfer circuit comprises an inductive rotary coupling with a primary side rotatably arranged a secondary side. The primary side has a primary winding and the secondary side has at least two secondary windings. The secondary windings deliver a signal with the same phase and are connected at one end to a pair of capacitors, being further connected to a positive output and a negative output. The other ends of the secondary windings each are separately connected to a pair of rectifiers connected in forward direction to the positive output and in reverse direction to the negative output. 
     By paralleling multiple secondary windings and rectifier circuits, the stray inductances and capacitances can be reduced which further leads to a reduced base load which helps to reduce total energy consumption of the circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from and benefit of European Patent Application No. 14198916.0 filed on Dec. 18, 2014. The disclosure of this European Patent Application is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an inductive power transfer device configured to couple electrical power between two units that are rotatable with respect to each other, and, specifically to power couplers for use in computer tomography scanners. Such power couplers are also known as rotary joints. 
         [0004]    2. Description of Relevant Art 
         [0005]    In a computer tomography (CT) scanner and other related machines high-power in the range from 10 kW up to more than 100 kW is transferred from a stationary side to a rotating side of the scanner. There, a high voltage at levels above hundred kilovolts is generated to produce x-ray radiation. 
         [0006]    In U.S. Pat. No. 7,054,411 a multiple-channel inductive rotary joint is disclosed. It has inductive channels for transferring power from the stationary side to the rotating side. There is an auxiliary power and a main power circuit. Furthermore a capacitive feedback link for power control is provided. 
       SUMMARY 
       [0007]    The embodiments of the invention provide an inductive power transfer device or rotary joint, which has a large dynamic range and requires a low base load. 
         [0008]    In an embodiment, an inductive rotary joint is built like a power transformer, where one side is rotating against another side. For example, in CT scanners, power has to be transferred from the stationary side of a scanner to its rotating side. Therefore, the power coupler is a transformer having a stationary primary winding and a rotating secondary winding. For simplicity, the following explanations and embodiments refer to such a CT-scanner rotary joint. The same type of implementation can be applied to any rotary joint in general and furthermore not only to a rotary joint for transferring power from a stationary side to a rotating side, but also to a rotary joint configured to transfer power from a rotating side to a stationary side. 
         [0009]    As a transformer can only transfer AC (alternating current), it is either fed by an AC line voltage or by an inverter, generating an AC voltage of a higher frequency which can better be transferred via a rotating transformer. Therefore, it is preferred to have an inverter at the primary or input side. At the secondary or output side, in most cases this AC voltage is converted to a DC voltage to provide a DC output. This may be done by a bridge rectifier, followed by a filtering capacitor to generate a smooth DC voltage. 
         [0010]    In certain environments, like a CT scanner, the load range of the inductive power transfer device is broad. Under full load conditions, an X-ray tube may be supplied with power in an order of 100 kW or more. During idling, when most of the systems like the X-ray tube and the detector are off, only a small number of low power consuming devices must be supplied with electrical power. In the past few years, the power consumption of computers, memories, and other electronic components has continuously decreased, so that the idling power of CT scanners also decreased significantly, whereas the required full load power of the X-ray tubes has increased due to higher X-ray power requirements. A rotary joint as used in a CT scanner is comparatively large. A typical CT scanner may have a diameter of between 1 m and 1.5 m. Therefore, the primary winding and the secondary winding are also comparatively large. Furthermore, it is impossible to have an extremely small air gap between the rotating and stationary parts, as there are mechanical tolerances. This causes a comparatively large stray inductance of the transformer. The large diameter further results in large windings and therefore in comparatively large parasitic capacitances within the windings, between the windings and between individual turns of the windings. The stray inductance or parasitic inductance in conjunction with the parasitic winding capacitances causes a parasitic resonant circuit which leads to an increase of the output voltage under high Q conditions of the parasitic resonant circuit which are met when the load at the output of the power coupler has a high resistance. Therefore, a high output resistance or low load drawing a low power at the output causes the output voltage to increase. This may lead to damaging of electronic components at the rotating side. To prevent an unwanted increase of output voltage, there may be a base load or dummy load which decreases the Q factor of the resonant circuit, and therefore decreases the output voltage in an idle state. A dummy load is not economical and requires further components like a high power resistor and cooling means like fans. Therefore it is desirable to design the inductive power coupler, such that no dummy load is required or at least the power dissipated in a dummy load is reduced. 
         [0011]    To reduce the required dummy load, an increase of the output voltage is reduced in an inductive power coupler. This is done by reducing the stray inductance and the total parasitic capacitance in the inductive power coupler and by providing a new rectifier circuit. 
         [0012]    In a first embodiment, there are at least two secondary windings on the secondary side, which preferably is the rotating side. These secondary windings are wound in the same direction, have the same number of turns, and are connected together either at the beginning of the windings or at the end of the windings at a common connection point. This common connection point is connected to a pair of capacitors, wherein the first capacitor is connected to a positive output, and the second capacitor is connected to the negative output. The other ends of the windings are each connected to a pair of rectifiers. For each secondary winding, an individual pair of rectifiers is provided. If there are multiple secondary windings connected to the same pair of rectifiers, this works like a single secondary winding in the sense of this embodiment. 
         [0013]    Each pair of rectifiers includes a first rectifier connected in a forward direction to the positive output, and a second rectifier in reverse direction connected to a negative output. Preferably, at least one of the rectifiers comprises a diode, but it may also comprise any equivalent element, like an actively controlled switch which may be a FET. 
         [0014]    By this circuit, a plurality of windings are connected in parallel in the same direction by means of rectifiers. 
         [0015]    Compared to a full-bridge rectifier, which simply delivers an output voltage roughly corresponding to one half of the peak to peak AC voltage coming from a secondary winding, this circuit acts as a voltage doubler and delivers approximately the full peak-to-peak output voltage of a secondary winding. For this reason, each secondary winding requires only half the number of turns compared to a circuit supplying a full-bridge rectifier. As a result, the energy stored in the parasitic capacitance is significantly lower and, therefore, a lower dummy load or higher dummy load resistance is required, which further leads to lower heat dissipation during idling. 
         [0016]    In a further embodiment, at least two secondary windings are wound parallel. This means that the wires of the windings are guided in parallel, preferably in close contact to each other. This leads to a significantly reduced parasitic capacitance between the turns. At the areas where the turns are in close proximity to each other, there is a comparatively large parasitic capacitance between the neighbored turns. As the windings are wound in the same direction, there is no voltage difference between the neighbored turns, and therefore the parasitic capacitance does not lead to a movement of charges, and therefore a parasitic current flow. As a result, this component of the parasitic capacitance has no effect. Therefore, the total parasitic capacitance can be significantly reduced by aligning the at least two windings in parallel. 
         [0017]    It is required that there are at least two secondary windings, but there may be any higher number of secondary windings. 
         [0018]    In a further preferred embodiment, there is a DC/DC converter between the positive output and negative outputs and the load. This DC/DC converter may be an up-converter a down converter or a combination thereof. It also may be switchable between up- and down-conversion. Alternatively, there may also be a DC/AC converter. 
         [0019]    In a further embodiment, the first capacitor and the second capacitor dimensioned as part of the total resonance capacitance of a series resonance converter. 
         [0020]    Furthermore, it is preferred, if the total resonance capacitance of a series resonance converter is formed by at least one primary resonance capacitor ( 131 ) and on the secondary side a first capacitor ( 231 ) and a second capacitor ( 232 ). 
         [0021]    According to another embodiment, the total resonance capacitance of a series resonance converter is formed by at least one primary resonance capacitor ( 131 ) and on the secondary side a first capacitor ( 231 ) and a second capacitor ( 232 ) being approximately evenly distributed between primary and secondary side. Therefore, the total capacitance of the at least one primary resonance capacitor ( 131 ) and the total capacitance of the secondary side capacitors ( 231 ,  232 ) are approximately equal. 
         [0022]    Furthermore, the winding ratio primary to secondary winding may be approximately 2. When dimensioning the capacitors the winding ratio needs to be considered. For example, with the capacitance evenly distributed between primary and secondary side and a winding ratio of 2 of primary to secondary winding the secondary capacitors ( 231 ,  232 ) capacitance each should have approximately 2 times the value of the primary capacitor&#39;s ( 131 ) capacitance which has approximately half the value of the total resonance capacitance necessary. 
         [0023]    In a further embodiment, the ratio of the total capacitance of the at least one primary resonance capacitor and the total capacitance of the secondary side capacitors is approximately equal to the square of the winding number ratio of the at least one secondary winding to the at least one primary winding. 
         [0024]    It is most efficient to use the embodiments disclosed herein in large rotating power transformers, as in such large units there such use provides a significant operational improvement, although it may be beneficial to apply the embodiments to smaller units. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    In the following, the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment and with reference to the drawings. 
           [0026]      FIG. 1  shows a preferred embodiment. 
           [0027]      FIG. 2  shows a first current diagram. 
           [0028]      FIG. 3  shows a second current diagram. 
           [0029]      FIG. 4  shows a circuit diagram with parasitic components. 
           [0030]      FIG. 5  shows a cross-section through a conventional secondary winding. 
           [0031]      FIG. 6  shows the parasitic capacitances between neighbored windings 
           [0032]      FIG. 7  shows a scheme of a preferred embodiment. 
           [0033]      FIG. 8  shows the parasitic capacitances and voltages of a preferred embodiment. 
           [0034]      FIG. 9  shows schematically a CT (Computed Tomography) scanner gantry. 
           [0035]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    In  FIG. 1 , a preferred embodiment with two separate primary windings is shown. It is required that there are at least two secondary windings, but may be any higher number of secondary windings can be employed. 
         [0037]    An inductive power coupler has a primary side  100  and a secondary side  200 , which are rotatable against each other. 
         [0038]    At the primary side  100 , there is a primary winding  110 , which preferably is supplied by an inverter  120  with an AC signal, preferably having a frequency significantly higher than the standard line frequency. Preferably, there is a primary resonance capacitor  131  between the inverter and the primary winding. It is preferred to have a frequency in the range from 1 kHz to 100 kHz, preferably between 20 and 40 kHz. At the secondary side  200 , there are at least two secondary windings with a first secondary winding  211  and a second secondary winding  212  as shown in this figure. The dots marked on the top of the windings show the beginnings of the windings or the ends of the windings. It is not essential which end of a winding is meant, but it is essential that the windings are oriented in the same direction and have the same number of turns. The orientation of the secondary windings with respect to the primary winding is not critical, and may be changed without any negative effect. Furthermore, there may be any number of primary windings. The first secondary winding  211  has a first output  213  and a second output  214 . The second secondary winding  212  has a first output  215  and a second output  216 . The second outputs  214  and  216  are connected together and are further connected to a first capacitor  231 , and a second capacitor  232 . The first capacitor  231  being connected to a positive output  251  and the second capacitor  232  being connected to a negative output  252 . The first capacitor  231  and the second capacitor  232  are dimensioned as resonance capacitors. It is preferred, if the primary resonance capacitor  131  and the secondary resonance capacitors ( 231 ,  232 ) have approximately the same values, which results in a further lowering of the output voltage at underload. Due to the same orientation of the first and the second secondary windings, the output signals at the first winding outputs  213  and  215  are the same. Each of the first winding outputs is connected to a pair of rectifiers  222 ,  224  and  221 ,  223 . The rectifiers are oriented in a forward direction towards the positive output  251  and in a negative direction to the negative output  252 . Preferably, there is a secondary filter capacitor  233  between the positive output  251  and the negative output  252  parallel to the load. Basically, there are two parallel voltage doubler circuits. Due to the voltage doubling, the output voltage of each winding is twice the voltage compared to a bridge rectifier circuit. As a consequence, the number of required turns may also be half the number of a bridge rectifier circuit. Consequently, the inductance of the windings is ¼, resulting in a reduced stray inductance about Vt. This leads to a significantly reduced energy in the inductive circuit, and therefore, the minimum load at the load component  240  can be reduced significantly. Tests have shown on large inductive couplers like these used for CT scanners, that the minimum load is about half the minimum load of a full-bridge circuit or a regular half bridge circuit. Therefore, a dummy load, which may be part of the load  240 , can be reduced. 
         [0039]    In  FIG. 2 , a first simplified current diagram is shown with the voltages at the first winding output  213 ,  215  being positive with respect to the voltages at the winding outputs  214 ,  216 . There is a first current path  261  from the first winding output  213  of the first secondary winding, propagating through rectifier  222  to the positive output  251  and via capacitor  231  to the second winding output  214  of the first primary winding. From the second secondary winding  212 , current propagates beginning from its first winding output  215  via a second current path  262  through rectifier  221  to the positive output  251  and via capacitor  231  back to the second winding output  216  of the second secondary winding  212 . 
         [0040]    In  FIG. 3 , a second simplified current diagram is shown with the voltages at the first winding output  213 ,  215  being negative with respect to the voltages at the winding outputs  214 ,  216 . Current through the first secondary winding  211  is propagating from the second winding output  214  via current path  263  through capacitor  232  to the negative output  252  and through rectifier  224  back to the first winding output  213 . Current through the second secondary winding  212  is propagating from second winding output  216  via current path  264  through capacitor  232  to the negative output  252  and through rectifier  223  to the first winding output  215 . It can be seen in these current diagrams that the current through both secondary windings is flowing at the same time through different rectifiers oriented in the same direction into the same capacitor. As shown in this embodiment, during a positive half wave, the current may charge the first capacitor  231  and during a negative half wave, the current may charge the second capacitor  232 . 
         [0041]    In  FIG. 4 , a circuit diagram with parasitic components is shown. In addition to the previously shown circuit components, a parasitic series inductance  311  of the first secondary winding  211  and a second stray inductance  312  of the second secondary winding  212  are shown. Furthermore, a first parasitic capacitance  321  of the first secondary winding  211  and a second parasitic capacitance  322  of the second secondary winding  212  are shown. There may be parallel resonances with the mutual inductance of the secondary winding and the parasitic capacitance as well as series resonances with the stray inductances of the secondary winding together with the parasitic capacitances. 
         [0042]    In  FIG. 5 , a cross-section through a conventional secondary winding is shown. There may be one piece of wire wound several times around a bobbin or within a magnetic core. The winding may have a plurality of layers to use the available winding space. Usually, the turns of the windings are wound layer for layer. This figure shows a schematic diagram with a winding starting with a first turn  601  and ending with a tenth turn  610 . The turns are wound in sequence. Each turn has a parasitic capacitance as will be shown later, to its neighbored turns. As there are ten turns, the voltage difference between the beginnings of neighbored turns in the same layer is 1/10 of the total voltage. The voltage difference between turns of different layers is determined by the number of turns electrically connected inbetween these turns. For example, between the turns  605  and  606 , the voltage is only 1/10 of the total voltage. Between the first turn  601  of the winding and the last turn  610 , the voltage is 9/10 of the total voltage. It is noted that the last 1/10 of the voltage is between the beginning and the end of the last turn  610 . 
         [0043]    In  FIG. 6 , the parasitic capacitances between neighbored turns are shown in a schematic of a section of the previous Figure. The beginning of turn  608  has parasitic capacitances  611  and  612  between the neighbored turns  607  and  609 . The voltages  621  and  622  there-between are 1/10 of the total voltage. The parasitic capacitance  613  between turn  608  and  603  is the same as the parasitic capacitances  621  and  622 , but the voltage  623  between turn  608  and  603  is 5/9 of the total voltage, as there are four other turns in-between. Therefore, the current flowing through the parasitic capacitance  613  is 5-times as high as the current through parasitic capacitances  611  and  612 . As a consequence, the parasitic capacitance  613  has a significant higher contribution to the total parasitic capacitance the capacitances  611  and  612 . 
         [0044]    In  FIG. 7 , a schematic arrangement of a preferred embodiment of the secondary transformer windings is shown. There may be a first secondary winding comprising the turns  651  to  655  and a second secondary winding comprising the turns  661  to  665 . The turns marked with a dot in the schematic diagrams may be the turns  651  and  661 . As these turns are wound in the same direction, there is the same voltage at these turns, and therefore there is no voltage between the neighbored turns which may be in different layers, like turns  651  and  661 . This applies to all successive turns ending with turns  655  and  665  which also have the same voltage. 
         [0045]    In  FIG. 8 , the parasitic capacitances and voltages of a preferred embodiment are shown. Again, as previously shown, there are parasitic capacitances  671 ,  672  and  673  between neighbored windings which have approximately the same capacitance. The voltages  682 ,  681  between turns  663  and  662 , as well as turns  664  and turn  663  are about the same as the voltages between the turns  609 ,  608  and  607  shown in the previous example. It should be noted that the total voltage at the winding in this embodiment is only half the total voltage compared to the previous example for driving a full-bridge rectifier. Therefore, the voltage difference at five turns is the same as at ten turns in the previous example. As the second secondary winding&#39;s turn  663  has the same voltage as the first secondary winding&#39;s turn  653 , the voltage  683  between these windings is zero, and therefore no current is flowing through the parasitic capacitance  673 . As a consequence, there is no contribution of this parasitic capacitance  673  to the total parasitic capacitance. Therefore, by using this winding arrangement, the total effective parasitic capacitance ( 321 ,  322 ) can be reduced, which further leads to a reduction of energy in the resonant circuit, and therefore the base load and a dummy load can further be decreased. 
         [0046]      FIG. 9  shows schematically a CT (Computed Tomography) scanner gantry. The stationary part is suspended within a massive frame  810 . The rotating part  809  of the gantry is rotatably mounted with respect to the stationary part and rotates along the rotation direction  808 . The rotating part may be a metal disk which supports an X-ray tube  801 , a detector  803  and further electronic and mechanic components. This disk may define a secondary ground. The X-ray tube is used for generating an X-ray beam  802  that radiates through a patient  804  lying on a table  807  and which is intercepted by a detector  803  and converted to electrical signals and imaging data thereof. The data obtained by the detector  803  are transmitted via a contactless rotary joint (not shown) to an evaluation unit  806  by means of a data bus or network  805 . Electrical power from a stationary power supply unit  811  may be transmitted by an inductive power coupler  800  to the rotating part. Other scanners like baggage scanners work in a similar way. 
         [0047]    It will be appreciated to those skilled in the art having the benefit of this disclosure that this invention provides an inductive power transfer device or rotary joint. Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           100  primary side 
           110  primary winding 
           120  inverter 
           131  primary resonance capacitor 
           200  secondary side 
           211 ,  212  secondary windings 
           213 - 216  winding outputs 
           221 - 224  rectifiers 
           231 ,  232  capacitors 
           233  filter capacitor 
           240  load 
           251  positive output 
           252  negative output 
           261 - 264  current paths 
           311 ,  312  stray inductances 
           321 ,  322  parasitic capacitances 
           601 - 610  prior art secondary winding turns 
           611 - 613  prior art parasitic capacitances 
           621 - 623  prior art voltages 
           651 - 655  first secondary winding turns 
           661 - 665  second secondary winding turns 
           671 - 673  prior art parasitic capacitances 
           681 - 683  prior art voltages 
           800  inductive power coupler 
           801  x-ray tube 
           802  x-ray beam 
           803  x-ray detector 
           804  patient 
           805  network 
           806  evaluation unit 
           807  patient table 
           808  rotation direction 
           809  rotating part 
           810  frame 
           811  power supply unit