Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2000-11-13
2004-05-18
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S021020
Reexamination Certificate
active
06738275
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for powering X-ray tubes, and more particularly, to using a stack of medium-voltage high-frequency inverters.
SUMMARY OF THE INVENTION
Typical X-ray generators are devices that supply regulated, high-voltage, DC power to X-ray producing vacuum tubes, as well as power to the tube filament. The high-voltage is applied between the anode and the cathode of the tube. In a conventional X-ray tube, X-rays are produced be generating electrons by thermionic emission from a tungsten filament (cathode). The electrons are then accelerated to an anode (which may be rotated for wear-averaging purposes) to generate the X-rays. The X-ray emissions are controlled by the applied voltage or potential between the anode and the cathode, as well as by the anode current. X-ray energy is controlled by the applied voltage, typically between 40 kV and 150 kV for medical applications, but sometimes as low as 20 kV as in mammography.
X-ray intensity is determined by anode current, which is controlled by varying filament power. Varying filament power changes the filament temperature, thus varying the electron emission of the cathode. Most electrons emitted by the cathode reach the anode and constitute anode current. Filament power ranges from a few tens to a few hundred watts. Typically anode currents range from as low as 500 microamperes, as in lower power continuous fluoroscopy, to as high as 1 ampere, as in conventional radiography or during cine-radiography runs and computed-tomography (CT) scans.
The high-voltage is either applied continuously, though at low power levels, or as medium to high power pulses or pulse trains. In continuous mode, typical power levels are on the order of 1 kilowatt (100 kV×1 ma); in pulsed mode, instantaneous power levels are on the order of 150 kW (150 kV×1A). X-ray generators used in medial applications have power ratings in the 10 to 100 kilowatt range.
X-ray generators pypically employ one of two fundamental methods to produce the required high-voltage, DC power. In one method, line frequency generators use a step-up transformer to raise the AC line voltage to the desired level, and then rectify and filter the high AC voltage to obtain DC voltage. Due to the low line frequency and high power levels involved, and due to the high amount of insulation required, the transformer and filter capacitors are very bulky and very expensive. The use of dielectric insulated oil is mandatory to achieve the level of insulation required and to assist in dissipating the heat lost in the transformer windings and other components. The insulating transformer oil creates a large space requirement, creates very heavy equipment, and requires seals which often allow the transformer oil to leak and create an environment hazard as well as degrade the line frequency generator. The second method of X-ray generator involves a high-frequency generator using a high-frequency inverter typically made up of a high-frequency oscillator, a high-frequency high-voltage transformer, a high-frequency high-voltage rectifier, and a high-frequency high-voltage filter to obtain the high DC voltage required. The inverter is powered directly from a low voltage DC source such as a battery bank or from the rectified and filtered AC line. Although many inverter topologies exist, high frequency generators typically use a resonant-inverter topology. In this configuration, the high frequency oscillator drives the primary winding of the transformer through a damped resonant circuit. This resonant circuit is generally composed of an inductor, a capacitor, and an equivalent resistance due to the external load connected to the secondary winding of the transformer, and reflected to the primary. The resonant circuit can be configured with the inductor, capacitor, and resistor in parallel or series. Power transferred to the load, thus voltage across the load, can be varied by changing the oscillator frequency. Power is maximum when the circuit is at resonance, that is, when the inductive reactance is exactly cancelled by the capacitive reactance of the circuit. Power drops when the oscillator frequency is either lower or higher than resonant frequency. High-frequency generators are much smaller and lighter than comparable line frequency generators, due to the reduced size of the transformers, capacitors, and inductors; however, typical high-frequency generators still require use of dielectric insulating oil to insulate and dissipate heat in the transformer windings and other components.
All X-ray generators use a high voltage divider to measure accurately the high-voltage outputs. The high voltage divider is made up of a string of equal value multimegaohm resistors, the top of which is connected to the high voltage output, the bottom of it going to a voltage sampling resistor, that in turn is connected to the high voltage return which is grounded. Typical divider ratio is 1V: 10 kV and divider current is on the order of 1 milliampere (ma). High voltage dividers have to be frequency-compensated by connecting a small capacitor in parallel with each resistor, such as to maintain divider accuracy and pulse shape integrity when the high voltage is pulsed. Capacitor values must be many times larger than the stray capacitances that exist between the divider sections and the surroundings. High voltage capacitors are costly and large, so a typical capacitor-compensated high voltage divider is a bulky and expensive device.
Instead of using compensating capacitors, high voltage dividers can also be guarded by enclosing each resistor in a cylindrical shield section that is maintained at about the same potential as the enclosed resistor, as disclosed in U.S. Pat. Nos. 5,023,769 and 5,391,977. This potential is obtained through a second resistor string that is not used for measurement. This ensures that essentially no current flows through the inevitable stray capacitances since there is very little potential difference between any resistor of the precision divider and its own guard section.
The high voltage output of the X-ray generator is connected to the X-ray tube anode and cathode by means of a pair of high voltage coaxial cables. Cable lengths range from a few feet to about 50 feet. The inner conductor carries the high tension and is thoroughly insulated from the outer coaxial conductor, which is solidly grounded for safety purposes. Because of their coaxial construction, high voltage cables behave as transmission lines; characteristic impedance is normally 50 ohms and capacitance is on the order of 50 picofarads per foot. Tube arcing between anode and cathode, or between either tube electrode and ground, is a rather frequent occurrence. It is equivalent to a momentary short circuit across the tube end of the high voltage cable. Since the high voltage cable acts as a transmission line, the short circuit typically reflects back all of the energy received from the line. The reflected energy adds to the incoming energy and provokes a very large voltage spike at the generator end of the line. The sum of the high voltage output from the generator and the spike will oscillate between twice the normal high voltage output and some negative value, inverting in fact the polarity of the output, until all of the reflected energy has been damped. Due to the large spike, output components of an unprotected X-ray generator will catastrophically and irreversibly fail when the X-ray tube arcs. Nevertheless, many cost-conscience X-ray high-frequency generators are not protected against tube arcing. Tube arc protection is typically implemented with a specially designed lossy inductor, where the inductance of the device slows the rise time of the fault current, and the resistance of the device damps the reflected energy, as disclosed in U.S. Pat. Nos. 5,241,260 and 5,495,165. Slowing the rise time of the fault current allows time for other protective devices, such as fuses and shutdown circuitry, to take over and limit the value of the fault current to tolerable levels
Electromed Internationale Ltee.
Han Jessica
Zitkovsky Ivan David
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