TRANSFORMER WITH HIGHLY RESISTIVE CORE |
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申请号 | US14916307 | 申请日 | 2014-09-04 | 公开(公告)号 | US20160217901A1 | 公开(公告)日 | 2016-07-28 |
申请人 | NEWTON SCIENTIFIC, INC; | 发明人 | Robert E. Klinkowstein; | ||||
摘要 | An electrical transformer is provided. The transformer may include a first winding, a second winding, and a highly resistive magnetic core. The highly resistive magnetic core may provide galvanic isolation between the core material and both the first and second windings. | ||||||
权利要求 | |||||||
说明书全文 | The present invention relates to electrical transformers, in particular transformers utilizing highly resistive magnetic core materials. An improved electrical transformer is provided in this disclosure. The transformer may include a first winding, a second winding, and a highly resistive magnetic core. The highly resistive magnetic core may provide galvanic isolation between the core material and both the first and second windings. Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. It is often a requirement of a transformer to provide galvanic isolation between the windings of the transformer for the purpose of transferring power or a signal between two circuits. The two circuits may be at substantially different reference voltages, several kilovolts or higher. For example, an isolation transformer is often used in combination with high voltage power supplies to provide power to an x-ray tube or cathode ray tube device. The primary winding of the isolation transformer is driven by an AC source and maintained at a potential close to ground while the secondary winding is maintained at a high voltage potential. The purpose of the isolation transformer is to provide isolated AC power to the powered device. In the case of the x-ray tube, the secondary winding is connected to the cathode of the x-ray tube and provides power to heat a thermionic cathode. The isolation transformer can also be used to provide an isolated control signal to the device, such as signals to a control grid or electrode. Some terms commonly used when specifying or characterizing the galvanic isolation property of a transformer are: isolation voltage, dielectric strength, standoff voltage, breakdown voltage, hold off voltage, insulating voltage. In some implementations, the means for providing galvanic isolation in transformers is provided by electrical insulation surrounding or applied directly to the windings and/or surrounding or applied to the magnetic core. These materials are necessary to prevent current flow or high voltage breakdown between the primary and secondary windings, between the windings and the magnetic core, or between the windings and earth. In other implementations, the magnetic core material is a conducting material or material having a resistivity too low to provide sufficient galvanic isolation, for example laminated steel and many ferrites. Some previous systems rely on insulating materials such as plastics, silicone rubbers, oils, varnish, air, insulating gasses, or other insulating liquids to insulate the windings and the magnetic core and to provide galvanic isolation. Some previous systems also use insulating gaps in the magnetic core to achieve galvanic isolation and to prevent high voltage breakdown between transformer windings. These gaps may be filled with air, or dielectric insulators such as plastics, ceramics or insulating oils or gasses. In one exemplary implementation, the magnetic core material provides substantially all or at least a substantial portion of the insulation necessary for achieving galvanic isolation of the transformer. The highly resistive insulating properties of the magnetic core are used to achieve high voltage isolation between the primary and secondary windings of the transformer. The isolation voltage may be applied directly to the magnetic core and a small DC current is permitted to flow in the core in response to the applied voltage. The resulting voltage drop along the length of the core may provide a smooth voltage distribution which enhances the standoff voltage of the transformer. The smooth voltage distribution provides unique benefits over older implementations for certain applications. The physical design parameters in combination with the resistivity of the core may also be chosen to allow a high voltage potential to be maintained between the windings of the transformer while insuring that the leakage current is maintained at an acceptable level. The acceptable leakage current level depends on the application. In some implementations, the leakage current should be less than the load current. However, there could be implementations in which this is not a requirement. The dielectric properties of the core material may be sufficient to avoid breakdown. This may require the DC bulk resistivity to be sufficiently high to limit the current flow in the core. It also may require good dielectric strength of the core material to prevent breakdown. In addition, the described implementation need not rely on gaps in the magnetic core to achieve an insulating magnetic core assembly. The presence of gaps in the core can have a detrimental effect on the performance of the transformer since these gaps cause flux leakage and reduce coupling between the windings of the transformer. The described implementation would find use in high voltage power supplies and in particular power supplies that are designed to minimize size, weight and cost. Examples of applications are x-ray generators, portable, handheld or miniature x-ray equipment including XRF analyzers, XRD analyzers, medical imaging devices, security imaging devices, miniature x-ray tube modules, monoblocks, or power supplies. X-ray techniques can also be combined with other portable or handheld analytical techniques such as Raman scattering, Laser induced breakdown spectroscopy, or optical emission spectroscopy. These combined analytical instruments place additional constraints on the size, weight and form factor of the high voltage power supply or x-ray system. The described transformer may provide advantages for these instruments and techniques. One exemplary implementation is provided in In another implementation, shown in The features of transformer 210 may be combined with features of the other transformers described in each of the other implementations and as shown in each of the other figures. Another implementation is shown in The features of transformer 310 may be combined with features of the other transformers described in each of the other implementations and as shown in each of the other figures. Another implementation is shown in In another implementation the transformers of The features of transformer 410 may be combined with features of the other transformers described in each of the other implementations and as shown in each of the other figures. In yet another implementation, multiple nodes on the highly resistive core are used to establish potentials along the core that are advantageous to the performance of the transformer. These nodes may, for example, be connected to the winding terminations as exemplified in The features of transformer 610 may be combined with features of the other transformers described in each of the other implementations and as shown in each of the other figures. In another implementation, the magnetic core may be divided into regions of relatively high resistivity and lower resistivity magnetic material, as shown in The high resistivity material may have a ρ greater than 1E10 ohm-cm and the high core resistance may be achieved using a high resistivity Ni-Zn ferrite. In particular, it has been determined that a suitable core can be fabricated using fully machined CMD5005 ferrite. For example, if ρ=1E10 ohm-cm, L=1 cm, A=0.1 cm2, then R=5E10 ohms. However, in many implementations ρ may be greater than 1E10 ohm-cm, L may be less than 5 cm, A may be less than 1 cm2, an the isolation voltage may be greater than 1 kV. The low resistivity material may have a ρ less than 1 E4 ohm-cm and the low core resistance may be achieved using a MnZn ferrite. The primary winding 712 may be positioned over (e.g. wound around) or in close proximity to the low resistivity region 730. The low resistivity region 730 may be connected (e.g. in physical contact and/or electrical connection) with the high resistivity region 740 on one end and connected with the high resistivity region 742 on the other end. Similarly, the secondary winding 714 may be positioned over (e.g. wound around) or in close proximity to the low resistivity region 732. The low resistivity region 732 may be connected (e.g. in physical contact and/or electrical connection) with the high resistivity region 740 on one end and connected with the high resistivity region 742 on the other end. Further, the low resistivity region 732 may have a different size (e.g. length and/or thickness), shape, or resistivity than the low resistivity region 730. In the same manner, the high resistivity region 740 may have a different shape, size, or resistivity than the high resistivity region 732. The features of transformer 710 may be combined with features of the other transformers described in each of the other implementations and as shown in each of the other figures. A sample lot of five prototype transformers were constructed using high resistivity ferrite to fabricate the cores. Toroidal cores measuring OD=2.3 cm, ID=1.47 cm, Height=0.77 cm were used for the transformers. The cross sectional area of the cores was A=0.32 cm̂2. The DC bulk resistivity and resistance of the ferrite used in each of the finished cores was measured by applying a 50 kV potential across the core. The measured range of DC resistivity was between 1E11 to 1E12 ohm-cm at 50 kV. The range of resistance, measured across the diameter of each toroid, was approximately 5.4E10 to 5.4E11 ohms at 50 kV. CMD5005 Ni—Zn ferrite material was used to fabricate the cores. All surfaces of the cores were machined to avoid anomalies in material properties at the surface. Each of the transformers had a primary winding and a secondary winding made of 27 AWG magnet wire with HPN film insulation, 0.0016 inches thick. The center tap of each winding was electrically connected to the ferrite core using silver epoxy. The assemblies are schematically represented by The prototype transformers were tested for galvanic isolation properties by applying a high voltage potential between the primary winding and the secondary winding. The leakage current flowing from primary to secondary winding through the magnetic core material was measured, and the transformers were observed for any breakdown phenomena. Each of the transformers was immersed in Fluorinert dielectric liquid for during the test. With a voltage of 50 kV was applied between the primary and secondary windings, the measured range of leakage current for the sample lot of transformers was 0.09 to 0.9 microamps. All of the transformers sustained the 50 kV isolation voltage without failure or any signs of high voltage breakdown. The prototype transformers were cleaned and then individually potted in RTV potting material. The transformers were then retested for galvanic isolation as described in the preceding paragraph. Leakage current from primary to secondary windings was measured. The results were in good agreement with the measurements made in Fluorinert. All of the transformers sustained the 50 kV isolation voltage without failure or any signs of high voltage breakdown. The features of the implementations described herein may be used in conjunction and/or combined as would be understood from this disclosure. Further, the features of the implementations or combinations thereof may be combined with the features of various X-ray sources including, but not limited to those described in U.S. Pat. No. 7,448,801 and U.S. Pat. No. 7,448,802, each of which are hereby incorporated by reference. The descriptions and illustrations given in this disclosure are illustrative of the principals and applications of the inventive transformer. It will be recognized by one skilled in the art that many other configurations, variations and modifications are possible. As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims. |