In certain applications, there is a need to transfer alternating current (AC) power from an AC power source to a load in a circuit in which there is a very large direct current (DC) voltage differential between the AC power source and the load. A transformer is often used in such applications for isolating the AC power source from the load.
For example, in an x-ray tube, a cathode is electrically isolated from an anode. A power supply can provide a DC voltage differential between the cathode and the anode of typically about 4-150 kilovolts (kV). This very large voltage differential between the cathode and the anode provides an electric field for accelerating electrons from the cathode to the anode. The cathode can include a cathode element for producing electrons. The cathode element is a load in the circuit. A power supply can also provide an alternating current to the cathode element in order to heat the cathode element for electron emission from the cathode element. For instance, the alternating current may be supplied by a separate power supply or an AC power source embedded with the DC power supply.
There is a very large DC voltage differential between the AC power source and the cathode element, such as about 4-150 kilovolts (kV). The AC power source can be part of a low voltage side of the circuit and the cathode element can be part of a high voltage side of the circuit. A transformer is normally used to isolate the AC power source from the cathode element, or in other words the transformer can isolate the low voltage side of the circuit from the high DC voltage side of the circuit.
Due to the very high DC voltage differential between the AC power source and the load, arcing can occur at the transformer between the wires on the low voltage side of the transformer and the wires on the high voltage side of the transformer. Such arcing can reduce or destroy the DC voltage differential and thus reduce or destroy cathode electron emission and electron acceleration between the cathode and the anode. Although increased wire insulation can help to reduce this problem, defects in the wiring insulation can result in arcing. Also, due to space constraints, especially in miniature x-ray tubes, increased wiring insulation may not be feasible.
It has been recognized that it would be advantageous to transfer AC power from an AC power source to a load in a circuit in which there is a very large DC voltage differential between the AC power source and the load without the use of a transformer and without problems of arcing between the two sides of the circuit.
The present invention is directed to a circuit for supplying AC power to a load in a circuit in which there is a large DC voltage differential between an AC power source and the load. Capacitors are used to provide voltage isolation while providing efficient transfer of AC power from the AC power source to the load. The DC voltage differential can be at least about 1 kV. This invention satisfies the need for reliably and efficiently transferring AC power across a large DC voltage differential.
The present invention can be used in an x-ray tube in which (1) the load can be a cathode element which is electrically isolated from an anode, and (2) there exists a very large DC voltage differential between the cathode element and the anode. AC power supplied to the cathode element can heat the cathode and due to such heating, and the large DC voltage differential between the cathode element and the anode, electrons can be emitted from the cathode element and propelled towards the anode.
As used in this description and in the appended claims, the following terms are defined
Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one skilled in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.
As illustrated in
The circuit 10 for supplying AC power to a load further comprises the load 14 having a first connection 14a and a second connection 14b. The second connection of the first capacitor 11b is connected to the first connection on the load 14a and the second connection of the second capacitor 12b is connected to the second connection on the load 14b. The load 14, the first and second connections on the load 14a-b, the second connection on the first capacitor 11b, and the second connection on the second capacitor 12b comprise a second voltage side 23 of the circuit.
The first and second capacitors 11, 12 provide voltage isolation between the first and second voltage sides 21, 23 of the circuit, respectively. A high voltage DC source can provide at least 1 kV DC voltage differential between the first 21 and second 23 voltage sides of the circuit.
As shown in
The DC voltage differential between the first 21 and second 23 voltage sides of the circuit can be substantially greater than 1 kV. For example the DC voltage differential between the first and second voltage sides of the circuit can be greater than about 4 kV, greater than about 10 kV, greater than about 20 kV, greater than about 40 kV, or greater than about 60 kV.
The AC power source 13 can transfer at least about 0.1 watt, at least about 0.5 watt, at least about 1 watt, or at least about 10 watts of power to the load 14.
Sometimes a circuit such as the example circuit displayed in
In selected embodiments of the present invention, the capacitance of the first and second capacitors can be greater than about 10 pF or in the range of about 10 pF to about 1 μF. In selected embodiments of the present invention the alternating current may be supplied to the circuit 10 at a frequency f of at least about 1 MHz, at least about 500 MHz, or at least about 1 GHz.
For example, if the capacitance C is 50 pF and the frequency f is 1 GHz, then the capacitive reactance X, is about 3.2. In selected embodiments of the present invention, the capacitive reactance X, of the first capacitor 11 can be in the range of 0.2 to 12 ohms and the capacitive reactance Xc of the second capacitor 12 can be in the range of 0.2 to 12 ohms.
It may be desirable, especially in very high voltage applications, to use more than one capacitor in series. In deciding the number of capacitors in series, manufacturing cost, capacitor cost, and physical size constraints of the circuit may be considered. Accordingly, the first capacitor 11 can comprise at least 2 capacitors connected in series and the second capacitor 12 can comprise at least 2 capacitors connected in series.
In one embodiment, the load 14 in the circuit 10 can be a cathode element such as a filament in an x-ray tube.
As shown in
A power supply 46 can be electrically coupled to the anode 44, the cathode 42, and the cathode element 43. The power supply 46 can include an AC power source for supplying AC power to the cathode element 43 in order to heat the cathode element, as described above and shown in
In accordance with another embodiment of the present invention, a method 500 for providing AC power to a load is disclosed, as depicted in the flow chart of
The DC power supply can provide a DC voltage differential between the load and the AC power supply that is substantially higher than 1 kV. For example the DC voltage differential can be greater than about 4 kV, greater than about 20 kV, greater than about 40 kV, or greater than about 60 kV.
In various embodiments of the present invention, the power transferred to the load can be at least about 0.1 watt, at least about 0.5 watt, at least about 1 watt, or at least about 10 watts. In various embodiments of the present invention, the AC power supply can be capacitively coupled to the load with single capacitors or capacitors in series. The capacitance of the capacitors, or capacitors in series, can be greater than about 10 pF or in the range of about 10 pF to about 1 μF. In embodiments of the present invention the selected frequency may be at least about 1 MHz, at least about 500 MHz, or at least about 1 GHz.
In the above described methods, the AC power coupled to the load can be used to heat the load. The load can be an x-ray tube cathode element, such as a filament.
It is to be understood that the above-referenced arrangements are only illustrative of the application for the principles of the present invention. Numerous modifications and alternative arrangements can be devised without departing from the spirit and scope of the present invention. While the present invention has been shown in the drawings and fully described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiment(s) of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications can be made without departing from the principles and concepts of the invention as set forth herein.