CA1149076A - Constant potential high voltage generator - Google Patents

Abstract

19 RCA 75,934 CONSTANT POTENTIAL HIGH VOLTAGE GENERATOR

Abstract A circuit, which produces voltages suitable, for example, for application to anode and cathode, respectively, of an x-ray tube, includes an alternating voltage source.
The source includes: means for producing first and second varying direct voltages, each of which has an average potential which is positive and negative, respectively, relative to ground, means responsive to the first and second voltages for producing a control signal which is proportional to the difference between those voltages; a reference signal generator for producing direct reference signal at a level proportional to a desired voltage difference between the first and second direct voltages; means for comparing the reference signal with the control signal for producing an error signal when they are unequal; and feedback means responsive to the error signal for changing the values of the first and second d.c. voltages in a sense to reduce said error signal to zero with this arrangement, the source maintains the difference between the first and second direct voltages at a substantially constant value.

Description

1 1 RCA 75,934 CONSTANT POTENTIAL HIGH VOLTAGE GENERATOR

This invention relates to an improved high voltage generator such as may be used for an x-ray tube. More particularly, the invention relates to a circuit for stabilizing the high voltage for reducing fluctuations in x~ray energy production.
In order to perform computed tomography (CT) scanning, an x-ray tube, which includes an anode and a cathode, is rotated about and directs x-radiation through a patient to an array of x-ray intensity detectors. The signals produced by the detectors are translated to binary form and the latter are processed to provide the data employed to produce a density mapping of a cross-sectional area of the patient. To generate the x-rays, electrons from the cathode are accelerated to the anode by the force exerted on them by an electric field between these two electrodes. ThiS field is proportional to the voltage between those electrodes. Fluctuations in the voltage result in fluctuations of the electric field and therefore fluctuations in the speed with which the electrons strike the anode. Electrons striking the anode transfer their kinetic energy to x-radiation energy, and to heat energy which is dissipated by the anode. Variations in electron kinetic energy result in variations of x-ray energy emitted by the x-rày tube and this, of course, is highly undesirable because it results in an inaccurate mapping of the patient cross section.
A prior art constant potential generation apparatus for x-ray apparatus is disclosed in the United States Patent No. 3,325,645 entitled "X~~ay Tube System with Voltage and Current Control Means." It includes an alternating high voltage source whose output is coarsely adjusted then rectified, and then smoothed or filtered with a capacitor. The filtered voltage is transmitted to the x-ray tube cathode and operates as the accelerating potential for electrons in the x-ray tube. In that apparatus, fine adjustment in the accelerating ~otential is achieved by a feedback control circuit which modifies the unfiltered high ' , 1 2 ~CA 75,934 tension voltage in response to the filtered voltage appearing at the x-ray tube. Any tendency of the filtered voltage to increase results in a change in the impedance of the feedback control circuit in a sense which tends to decrease the unfiltered voltage and vice versa.
Ilhile achieving substantial commercial success, especially in the application for which it was designed, namely x-ray diffraction, the prior art apparatus disclosed in U.S. Patent 3,325,645 has some disadvantages when apPlied to CT. It employs a filtering capacitor to smooth a pulsating signal. Such a capacitor charges to many thousands of volts and can retain a substantial amount of stored energy. In CT applications this stored energy can present a safety hazard to both service personnel and to the x-ray tube. Another disadvant~ge is the limited range of feedback control. If the unfiltered voltage from a transformer secondary in the prior art circuit is outside of a certain range, the feedback control circuit no longer stabilizes the accelerating potential. For stabilization beyond this range, a primary transformer control circuit is used to return to the proper range the output from the secondary transformer. This adds complexity to the stabiliza~ion circuit.
In a system embodying the invention which is suitable, for example, for~producing the high voltage needed for an x-ray tube, means are provided for producing first and second varying d.c. voltages, the first such voltage having an average potential which is relatively positive with respect to a point of reference potential and the second haviny an average potential which is relatively negative with respect to said point of reference potential.
The difference between these voltages is sensed to produce ~5 a control voltage proportional to the difference. This control voltage is compared with a reference voltage level indicative of a desired constant difference in potential between the first and second vaxying d~c. voltages~ Means are provided, responsive to the error signal, for changing the values of the first and second voltages in a sense to 1 3 RCA 75,934 reduce the error signal to zero and thereby to maintain the difference between the first and second voltages at a constant value.
In the drawing:
FIGURE l is a schematic showing of a CT scanning unit;
FIGURE 2 is a rear elevational view of a CT
machine wlth its housing removed;
FIGURE 3 is a sectional view of the CT scanner of FIGURE 3 along the line 3-3 of FIGURE 2;
FIGURE 4 is a diagram mostly in block form of an improved x-ray potential generator embodying the invention;
FIGURE 5 is a more detailed sehematic diagram of FIGURE 4.
FIGURE 1 shows a eomputed tomography system 10 used for examining the internal strueture of a patient.
The unit eomprises a scanning unit 12, couch 16, signal processor 20 and imager 22. The seanning unit 12 is mounted to the floor and remains stationary relative to the patient.
The seanning unit includes a housing 13 which covers x-ray apparatus including an x~ray tube which rotates for CT
scanninq. The housing 13 is formed with an aperture 14.
The couch 16 is movably mounted and is operative to position the patient ~ithin the aperture 14 for x-ray scanning. The - signal processor 20 and the imager 22 are electrically connected to the scanning unit. The scanning unit obtains x-ray intensity da~a and sends this intensity data to the signal processor. The x-ray intensity data is then proeessed by the signal processor to obtain information concerning the relative densities of a patient eross-section of interest. This density data is transferred to the imager where the doctor can view the relative density information on a viewing screen.
Referring to FIGURES 2 and 3, an x-ray tube support and manipulating assembly comprising a stationary detector scanning unit is shown generally at 50. The assembly 50 includes a housing and frame structure 51. A
pair of spindle bearings 52 are carried bv the housing and . .. i :
,, :

~-L~ 37~
1 4 RCA 75,934 frame structure 51, (see FIGURE 2). A tubular spindle 54 is journalèd in the bearings 52. The spindle 54 delineates a patient receiving opening 55 (corresponding to 14 of FIG. 1).
An x-ray tube assembly 58 (see FIGURE 3) is fixed to the tubular spindle for orbital rotation about an axis 56 of the spindle 54 and the opening 55. The x-ray tube assembly includes an x^-ray tube indicated by the dotted line 60, a collimator shown diagrammatically at 61, and other known and conventional components of an x-ray tube assembly of the type used in CT studies.
The tube support and manipulating assembly 50 shown in FIGURES 2 and 3 is of a maehine of the stationary detector type. For clarity of illustration, and because the detector array is now known in the art, the annular deteetor array which is around the orbital path of the x-ray tube assembly 58 is not shown except in a fragmentary schematic way at 62 in FIGU~E 3.
In use, the x-ray tube is orbited about the axis 56 over a range of orbital motion over a path of suffieient length to accelerate the tube to its full speed for a 360 scan, and to decelerate through an additional orbital path which is long enough to permit the tube to be smoothly brought to a stop. The orbital motion is first in one direction and then the other. Expressed another way, the tube may be moved in a clockwise direction to perform one study and then counterclockwise during a following scan to perform the next study.
A drive for this orbital motion is shown schematically and it includes an annular motor 64 which is conneeted to the spindle 54. The drive shown is for schematic illustration only. Any of the known and com~ercially accepted drive systems ean be employed.
3~ Four flexible eonduits or eable 68 are eonnected to the x-ray tube assembly 58. These cables include conductors for supplying electron accelerating potential for the x-ray tube, for collimator and filter adjustment, and sueh other power requirements the tube assembly may have.
The cables 68 extend from the x-ray tube through the opening 7~
1 5 RCA 75,934 65 where they are adjacent the spindle 54 and into a cable delivery opening 69 (see FIGU~E 3).
In a CAT system embodying the invention, the accelerating potential is supplied by a voltage generator embodying a stabili~ation circuit. The stabilization circuit shown in block form in FIG. 4 and in schematic form in FIG. 5 maintains the potential at a highly constant level thereby insuring the x~radiation is of a constant mean energy level.
~ eferring now to FIGURE 4, a high voltage generator for an x-ray tube 112 (corresponding to tube 60 of FIG. 3) is shown schematically at 110. The x-ray tube 112 has an anode 114 and cathode 116 electrically connected to the generator 110. An input 120 transmits a potential to the anode 114, and a second input 122 to the cathode 116.
The potential difference between these two inputs causes electrons emitted by the cathode to accelerate toward the anode. I~hen the accelerated electrons strike the anode their kinetic energy of motion is transformed into heat and x-radiation energy 118.
To provide a voltage differential to the anode and cathode, the generator 110 includes a source of alternating voltage which in the embodiment shown comprises two high voltage transformers secondary circuits 130, 132. One transformer secondary 130 produces a signal which is rectified and sent to the x-ray tube anode, the second transformer secondary 132 produces a second signal which is also rectified and sent to the x-ray tube cathode. Although the outputs from these secondaries are rectified, they are not filtered and therefore comprise pulsating d.c. signals~
In the preferred embodiment the output from a first secondary 130 is rectified to provide a voltage above ground potential and the other output is rectified to produce an output below ground potential. Although the secondary transformers are driven by the same primary, the existence of other components within the circuitry produces a phase shift between the two pulsating signals and therefore the ; 40 signals are not symmetric above and below ground. Due to 1 6 ~CA 75,93 this factor, the voltage waveform representing the anode-to-cathode potential is an irregularly shaped, pulsating waveform. It is the function of the remainder of the circuitry within the generator 110 to smooth and stabilize this waveform so that while the voltage at the anode and that at the cathode may each vary relative to ground, their potential difference remains constant so that the electrons accelerated in the x-ray -tube have a stable average kinetic energy and therefore the x-radiation produced has a stabilized mean value.
To achieve this stabilized energy, the generator 110 includes a reference voltage generator 134, an error voltage genera-tor 136, and a feedback circuit responsive to the output 13B from the error generator~ The reference generator when it is on, produces a constant reference voltage level at 135 which is proportional to a desired high voltage potential difference between cathode and anode of the x-ray tube. This reference signal is sent to an error generator 136 which compares the reference signal 135 with a control signal input 137 produced by differential circuit 221. The control signal 137 is proportional to the potential difference appearing across the anode and cathode ~5 of the x-ray tube. But for the advantageous operation of the feedback loop system of the generator 110 this control signal 137 would comprise a pulsating potential proportional to the voltage difference supplied by the transformer secondaries. Due to the advantageous operation, however, of the feedback technique embodied by the invention, the pulsating output from these secondaries is modified and the accelerating potential across the tube anode and cathode is stabilized at the reference value.
In an embodiment where two transformers' secondaries provide the accelerating potential difference to the x-ray tube, two feedback portions 140, 142 are required in the generator 110. Each portion includes a control tube (triodesl44 and 146, respectively) with a control grid 148, 150. The error signal 138 is sent through each of the . 40 feedback portions and modifies the control tube grid voltage ~1.¢~76 1 7 RCA 75,934 in such a way that the signal sent to the x-ray tube electrodes 114, 116 from the transformer secondaries is modified and the potential difference appearing across the tube is stabilized at the constant reference value.
As seen in FIGURE 4, each control tube 144, 146 comprises a portion of an electrical path between the secondary transformers 130, 132 and electrical ground 152.
By modifying the voltage on the grids 148, 150 of the control tubes the impedance these control tubes presents is altered in such a way that the error signal 138 will be minimized. The change in control tube impedance is transmitted as a signal having two components to the x-ray tube electrodes. Two shunt paths 154, 156 (shown in FIG. 5 as ~C networks) bypass the transformer secondaries and transmit the a.c. component resulting from the change in control tube i~pedance to the x-ray tube cathode/anode 114, 116. These shunt paths represent a low impedance path for alternating current signals generated through control of the control grids 148, 150 and allow these a.c. signals to moderate fluctuations in accelerating potential. There is - also a d.c. component corresponding to the voltage drop across the triodes 144 and 146 which affects the voltages applied to the anode 114 and cathode 116. Thus, the voltage voltages at the anode and cathode are a function both of the output of the transformer secondaries and of the a.c. and d.c. components of the voltage developed in the feedback paths.
As an illustration, assume that the voltage difference between the cathode and anode of the x-ray tube is smaller than an optimum value. That is, the desired accelerating potential is greater than the instantaneous actual accelerating potential appearing across that tube.
3~ i~hen this condition exists the x-ray beams emitted by the tube have an average energy less than an optimum desired value. Under these circumstances control signal 137 will tend to decrease in value to level lower than that of reference signal 134 and the error generator 136 will cause an error output signal 138 to be sent to the grids 148, 150.

1 8 RC~ 75,~34 In actual operation, it is not the error signal 138 but an amplified signal which is used to control the voltage on the two grids. This amplification is achieved by sending the signal through two compensating amplifiers 158, 160 and then through two grid drivers 162, 164. (The signal sent to grid driver 162 passes through an isolation circuit which includes means for translating the output of 158 to light, and means for translating the light signal back to a voltage, as will be discussed later.) The voltage output on the grid drivers will modify the voltage on the grids 148, 150 to increase the x-ray tube anode/cathode voltage differential to its desired constant value to thereby increase the control signal 137 so that it again equals the reference signal 135 to thereby reduce the error signal 137 to zero. If as was postulated the anode to cathode voltage differential is to be increased, the potential drop across the control tubes 144, 146 must be decreased through modification or adjustment of the grid potentials. The decrease in the voltage drop across tube 144, ~lhich is a negative voltage drop between the secondary 130 and ground, results in the anode 114 becoming more positive relative to ground. The decreased voltage drop across tube 146, which is a positive voltage drop between the secondary 132 and ground, resul-ts in the cathode 122 becoming more negative relative to ground.
It should be appreciated that the grid potential does not stabilize at an optimum value and that instead the system operates in a dynamic feedback mode. The rectified outputs from the transformer secondaries are pulsating voltages so that the grid drivers 162, 164 must continually adjust as the error signal generated in the error generator changes. The feedback circuitry responds quickly enough to the pulsating d.c. voltage to achieve voltage stabilization (substantially constant anode 114-to--cathode 116 voltage).
This stabilization requires no filtering capacitors and is satisfactory for accurate computed tomography x-ray generation.
The tl,~o control tubes 144, 146 included in the ~J~7Ç~
1 9 RCA 75,934 feedback portions 140, 142 perform similar functions yet due to the opposite polarity of the cathode and anode x-ray tube potentials, the two tubes are configured differently. One control tube 14~ has its anode essentially grounded and the second 146 tube has its cathode very close to ground. The opposed (non-grounded) electrodes are many thousand volts removed from ground with the grounded anode tube having its filament well below ground and the grounded filament tube having its anode well above ground.
To control the flow of electrons in the control tubes, the control grid voltage must be held in a range near the filament voltage. For the grounded filament tube 146 this constraint presents no problem. Its grid 150 potential is maintained slightly below ground and may be increased to a value of approximately 150 volts negative.
Modification of this voltage by the grid driver 146 modifies the flow of electrons in that -tube 146 and therefore modifies the impedance between ground and the cathode of the x-ray tube.
The constraint on the other grid 148 presents control problems since that grid must be maintained at a potential on the order of the non-ground filament potential which is approximately 10,000 volts below ground. The problem presented is to send a control function proportional to the error signal to a control grid 148 which is maintained at a potential of approximately 10,000 volts.
Electrical coupling between the high voltage grid and the low voltage error siynal would result in voltage spikes, arcing, and current flows of unsuitable magnitude.
Avoidance of these problems has been achieved by the inclusion of an electrical isolation circuit portion 166 interposed between the error signal compensation amplifier 158 and the high potential grid driver 162.
The isolation portion 166 comprises a frequency modulated receiver 169 interconnected through a light pipe to a frequency modulated driver 167. The error signal is sent to the frequency modulation driver 167 which transforms the voltage signal into a frequency modulated signal. The frequency modulated signal is transmitted through the light pipe to the frequency modulated receiver which decodes the frequency modulated information and returns it into the form of an electrical voltage signal. The light pipe is, of course, an electrical insulator and therefore the high potential on the grid 148 does not affect the low potential portions of the generator 110. The coding and decoding of information through the electrical isolation portion 166 is achieved by amplitude modulating with a 160 kilocycle frequency modulated subcarrier, a light beam signal. Techniques for modifying this signal in such a way as to carry the error signal information are known. One optical coupling system capable of performing such functions is commercially available under the tradename or trademark Burr-Brown. That system comprises a model 3712T transmitter, a model 3712R receiver and fiber optic coupling.
Connected to the non-grounded electrodes of each control tube are two voltage dividers 168, 170.
These dividers function in helping maintain the two control tubes 144, 146 within a dynamic range of operation. Two outputs 172, 174 from the dividers 168, 170 are transmitted to a summing or balancing amplifier 176. This amplifier 176 receives these two signals and produces an output proportional to their algebraic sum. As appreciated by those skilled in the art, the output 172 from one voltage divider 168 is a signal proportional to the output on the non-grounded filament of the grounded anode control tube 144. The output 174 from the other divider is propor-tional to the voltage appearing at the anode of the grounded filament control tube 146. To maintain the differential in voltage across the cathode and anode of the x-ray tube these values need not be equal, but to insure control tube operation is maintained in a dynamic range of operation (i.e. neither tube goes into saturation or cutoff) an output 178 from the SUmMing amplifier 176 is used to modify the error signal 138 emitted by the error generator. This modification maintains each nn-grounded control tube electrode at approximately the same absolute voltage from " .

1 11 RCA 75,934 ground and -thereby maintains the control tube in an effective operating range to dynarnically control x-ray tube potential differences. sut for the utilization of this balancing or su~ming amplifier 176 it is possible that while the potential difference across the x-ray tube cathode and anode 114, 116 would be maintained at a relatively stable value, one control tube voltage drop would be substantially less than the other and at some time the feedback stabilization circuitry would fail due to either cutoff or saturation of one or the other of the control tubes.
Operation of the summing or balancing amplifier in the feedback loop maintains the voltage drop across each control tube at ap~roximately equal absolute values. Modification of the voltage on the tube control grids 148, 150 continues to maintain the difference in potential across the x-ray tube at a constant level.
Figure 5 is a detailed schematic of the system shown in Figure 4. As noted above a voltage from two inputs 120, 122 appears across the anode 114 and cathode 116 of an x-ray tube 112. Electrons are emitted from the cathode 116 in response to a current flow generated by a filament supply 210. They accelerate across the x-ray tube, strike the anode 114, and x-radiation is emitted.
The high voltage is provided by two secondary transformers 130, 132. One secondary 130 is configured in a wye format and the second secondary 132 is configured in a delta format. Outputs from the delta and the wye secondary windings are rectified by a number of diodes. Diodes 212a-c and 214a-c serve to rectify the output from the wye transformer secondary and a second set of diodes 216a-c and 218a-c serve to rectify the output from the delta transformer. Were it not for the feedback operation of the present generator, the outputs from these rectified transformer secondaries would be pulsating DC potentials and would provide an irregular pulsating accelerating potential to the x-ray tube.
The feedback correction circuit includes a high voltage divider 220 which reduces in magnitude the high ~/ `

1 12 ~CA 75,934 voltages appearing at the cathode and anode of the x-ray tube. These smaller magnitude voltages are suitable for use in the feedback POrtiOnS of the x-ray stabilization generator. The high voltage divider 200 comprises a first 222 and a second 224 voltage divider which reduce the high input from the anode and cathode by a factor of 10,000.
The output from these two voltage dividers is transmitted to two power amplifiers 226, 228.
Two outputs 230, 232 leave the high voltage divider 220 and are transferred through a second pair of power amplifiers 234, 236. These two outputs form inputs to a differential amplifier 238. The output 137 from the differential error amplifier is a signal proportional to the absolute voltage difference between the high voltage appearing between the cathode and anode. In the preferred embodiment shown in Figure 5 a voltage separation of 20 kv produces an output 137 from the differential amplifier 238 of one volt.
As noted previously, a reference generator 134 provides a reference voltage level at output 135 proportional to a desired accelerating potential. In the preferred embodiment of the invention a one volt signal appears at the output 135 for each 20,000 volts of desired accelerating potential. The output 135 is generated by a reference input 233 and two amplifiers 235, 237. The input 233 is transmitted to the output 135 only during a patient exposure.
A switch 239 completes the circuit during e~posure and at other times completes a connection to a -15 volt power supply.
The two outputs 135, 137 from the reference generator 134 and the differential amplifier 238 respectively are sumrned by a summing or error generator 136. IE the instantaneous voltage appearing between the cathode and anode of the x-ray tube is equal to the desired accelerating potential, the output from the error generator will be zero volts. A difference between the actual instantaneous accelerating potential and the desired or reference signal produces either a positive or negative voltage output from '76 1 13 ~CA 75,~34 the error generator 136 which is used to modify the grid potentials on the two control tubes.
A plurality of operational amplifiers 240-242 are included which transmit the error signal to a cathode grid driver 164 which modifies voltages appearing upon the grounded filament tube control grid. Other amplifiers 243, 244 transmit the error signal to a frequency modulated driver 167 which in turn transmits the error signal to the isolated portion of the circuit 166. These operational amplifiers 240-244 are inserted to maintain the proper power transfer and also to maintain circuit stability. Without these amplifiers it is possible that under varying feedback conditions the circuitry might go into oscillation and disrupt functioning of the system.
An output 2 4 6 f rom the amplifier 244 forms an input to the frequency modulated driver 167. This driver converts the error signal which has been in the form of a voltage into a frequency modulated signal which can be conveniently transferred to an isolated portion 166 by optically coupled circuitry such as a light pipe. ThQ
frequency modulated signal is received by a receiver 169 which reconverts the frequency modulated signal into a voltage signal and transmits it through two amplifiers to the anode grid driver 162. Both anode and cathode grid drivers comprise amplifiers with gains of approximately 150 and a dynamic range of approximately 170 volts. By modulating the voltage output from the two grid drivers it is possible to change the control tube impedances and therefore the voltage drop across these two control tubes.
This modification in control tube impedance results in a voltage signal appearing at two outputs 248, 250 on the nongrounded electrodes of the two control tubes. Due to the presence of a shunt path 154, 156 between these points and the x-ray tube anode and cathode respectively this modulated signal appearing on the nongrounded electrodes of the two control tubes is transmitted to the cathode and anode of the x-ray tube. In this way modifications in the control voltage on the control tube grids directly modifies the 7~
1 14 RCA 75,934 voltage separation appearing between the cathode and the anode of the x-ray tube and by proper modulation of this control voltage the voltage separation appearing across these two electrodes is maintained at a steady or constant value.
Since it is desirable not only to maintain constant the voltage separation of the x-ray tuke but also to maintain each control tube in a dynamic range of operation, the nongrounded electrode voltages are also adjusted to insure that they are always at approximately the same absolute voltage level. It is important to fix the control tubes in a dynamic range of operation so that the maximum possible control over x-ray tube accelerating potentials is achieved. The voltage appearing at the nongrounded electrodes of the two tubes is monitored and a nonequality in their absolute value (note: one is approximately 10,000 volts above ground and one 10,000 volts below ground) results in a control signal modifying the error signal transmitted to the cathode and anode grid drivers.
Two voltage dividers 168, 170 sample the voltage at the nongrounded electrodes of the two control tubes and send a signal proportional to these voltages to a summing amplifier 176. If the two voltages are equal tof the same absolute magnitude) then the output 178 from the summing amplifier is zero volts and the error signal appearing at a junction 180 within the feedback circuit is unmodified.
If, however, the two voltages appearing at the nongrounded electrodes are unequal, a signal 178 modifies the error signal sent to the cathode grid driver in sense to cause the - anode voltage of triode 146 again to become equal, in absolute value, to the cathode voltage of triode 144. Thus, a type of double feedback circuit is arranged to maintain the voltage or accelerating potential across the cathode and anode of the x-ray tube at a constant value, and to maintain the control tubes in a dynamic range of operation to achieve maximum control over the accelerating potential.
The balance portion of the circuit includes two amplifiers 260, 262. These are buffer amplifiers and '7~
1 15 RCA 75,934 transmit the signal from the voltage dividers 16~, 170 and transmit those signals to a summing junction 264. Also connected to the output of the voltage dividers 168, 170 are two zener diodes 266, 268. These protect the amplifiers 260, 262 from large voltage spikes should either divider 168, 170 have an open circuit in its 25 kQ resistors.
The balance portion also includes a switch 270 which disables the balance signal 178. I~hen disabled, the constant potential between x-ray tube cathode and anode is maintained but the control tubes' nongrounded electrodes are no longer maintained at the same potential relative to ground. This switch is used for testing and aligning purposes.
Each control tube circuit further includes a large resistor 270, 272 connected between an x-ray tube electrode and ground. This resistor helps bias the control tubes even at low x-ray tube currents. With the resistor 270, 272 in the circuit, the current passing through the control tubes is equal to the current passing through these large biasing resistors plus the current flowing through the x-ray tube.
Other circuits within the system 10 monitor x-ray tube current and modify that current as changes are made in the desired current selection. To accurately monitor the x-ray tube current, two outputs 256, 258 are transmitted to other circuitry not shown in the diagrams. These outputs are combined into one signal proportional to tube current and used to control the output of the filament supply 210.
In the detailed schematic (Figure 5) preferred values for capacitors and resistors have been given but the high voltage stabilization could be achieved using other component values. A model #6423F control tube is utilizea in the preferred circuit.
Although a preferred embodiment has been d~scribed, it should be appreciated that design modifications could be incorporated without departing from the spirit or scope of the invention as set forth in the appended claims.
A system utilizing the present invention maintains the electrical potential difference between anode and '76 1 16 ~CA 75,g34 cathode of an x-ray tube at a stabilized value over a large range without the need for filtering capacitors. Further, no primary transformer control is needed during a CT
exposure. A primary transformer control may be employed prior to the exposure. Stabilized high voltage is then maintained by the dynamic control which is flexible enough -to take into account normal power line fluctuations.
In the embodiment of the invention described, two three-phase transformer secondaries with full wave rectification are employed for providing two pulsating d.c.
voltages. One of these voltages is above ground and the other below ground and it is the difference in these voltages which appears across the cathode and anode of the x-ray tube. Both secondary transformers may be energized by the same primary but their outputs need not be symmetric abGut ground otential. In one embodiment, one pulsating d.c. signal leads the other so that the voltage difference between the two signals has a periodicity of 12 cycles per one primary energization cycle. As noted above this periodicity would adversely affect x-ray generation but for operation of the differential feedback control featured in the preferred embodiment of the invention.

Claims (6)

17 RCA 75,934 CLAIMS:

1. In a circuit for producing voltages suitable, for example, for application to the anode and cathode, respectively, of an x-ray tube, said circuit including an alternating voltage source;
means connected to the source for producing first and second varying d.c. voltages, said first voltage having an average potential which is relatively positive with respect to a reference potential and said second having an average potential which is relatively negative with respect to said reference potential;
means responsive to said first and second voltages for producing a control signal proportional to the difference between them;
a reference signal generator for producing a reference signal at a level proportional to a desired voltage difference between said first and second d.c.
voltages;
means for comparing said reference signal with said control signal for producing an error signal when they are unequal; and feedback means responsive to said error signal for changing the values of said first and second voltages in a sense to reduce said error signal to zero and thereby to maintain the difference between said first and second voltages at a substantially constant value.

2. In a circuit as set forth in claim 1, said feedback circuit means including first and second controllable impedance means, said first connected between the means producing said first varying d.c. voltage and ground, and the second connected between the means producing said second varying d.c. voltage and ground.

3. In a circuit as set forth in claim 2, each controllable impedance means comprising a triode, the first triode connected to said means for producing said first d.c.
voltage being connected at its cathode to said means and at its anode to a point of reference potential and the second triode connected to said means for producing said second d.c. voltage being connected at its anode to said means and at its cathode to said point of reference potential, the impedance of the respective triodes being controlled by respective signals applied to their control grids, said signals being derived from said error signal.

4. In a circuit as set forth in claim 3, further including means for maintaining the cathode of the first triode and the anode of the second triode at approximately the same absolute voltage level.

5. In a circuit as set forth in claim 1, said means for producing said varying d.c. voltages comprising first and second three-phase transformer secondary windings, one said transformer winding being in wye configuration and the other in delta configuration and further including diodes connected to each such winding for rectifying the a.c. vol-tage produced thereby.

6. In a circuit as set forth in claim 1, said feedback means including means for adding to each varying voltage a d.c. component and an a.c. component.