[0001] The present invention relates to an X-ray source, and more particularly to a high-voltage
generating circuitry for generating a high-voltage to be applied to a target of the
X-ray source.
[0002] U.S. Patent No. 5,077,771, U.S. Patent No. 4,646,338, and U.S. Patent No. 4,694,480
describe portable X-ray sources constructed from an X-ray tube and a molded high-voltage
source and its control circuitry.
[0003] When voltage is applied to an X-ray tube, either the cathode is connected to ground,
or the target is connected to ground, or the focus voltage is varied. However, none
of the above-described patents sufficiently describes voltage control of the X-ray
tube in generating microfocused X-rays.
[0004] Voltage is controlled in the high-voltage generation circuitry using pulse width
modulation (PWM) or pulse voltage control. However, the PWM method controls the effective
voltage by changing the pulse width of control pulses. Therefore, the high voltage
generated at the secondary coil of the high-voltage transformer does not follow changes
in pulse width very well, resulting in great fluctuations in strength of the X-rays.
With the pulse voltage control method, a switching transistor controls current supplied
to the primary coil of the high-voltage generating transformer, resulting in great
deal of power loss by heat generation. A cooling fan must therefore be provided to
cool the circuitry. The vibration produced by the cooling fan prevents maintenance
of a microfocus of the X-rays.
[0005] A Cockcroft-Walton multiplier array is often used as the source of high voltage for
portable X-ray sources. The capacitor and diode array of the Cockcroft-Walton multiplier
array are often embedded in an insulating block molded by pouring a silicon or epoxy
resin over the capacitors and diodes. The positions of the capacitors and diodes can
be shifted when this insulating material is poured over these components. When these
components are shifted out of alignment, this can result in a variable and unstable
supply of high voltage.
[0006] To solve the above-described problems, an X-ray source according to the invention
includes an X-ray tube and a target voltage generating circuit. The X-ray tube has
a cathode and a target. The target voltage generating circuit applies a positive voltage
to the target so that a high voltage is developed between the cathode and the target.
Electron beams are generated from the cathode and the target generates X-rays in response
to the electron beam bombardment. The target voltage generating circuit includes a
first voltage generating circuit generating a first voltage in response to an instruction
voltage indicative of a voltage to be applied to the target, and a second voltage
generating circuit connected to the first voltage generating circuit. The second voltage
generating circuit generates an output voltage to be applied to the target. A voltage
divider is provided which is connected to the second voltage generating circuit. The
voltage divider divides the output voltage from the second voltage generating circuit
to produce a detection voltage indicative of a voltage generated by the second voltage
generating circuit. The first voltage generating circuit includes a switching element
that is rendered ON until the detection voltage has reached the instruction voltage
and that is alternately rendered ON and OFF after the detection voltage has reached
the instruction voltage.
[0007] The first voltage generating circuit further includes a capacitive element that is
charged by a current flowing through the switching element when the switching element
is ON, an error amplifier having a first input terminal applied with the instruction
voltage, a second input terminal applied with the detection voltage, and an output
terminal, and a comparator having a first input terminal connected to the output terminal
of the error amplifier, and a second input terminal supplied with the voltage developed
across the capacitive element. The first voltage generating circuit further includes
an inductive element. The capacitive element is charged according to a time constant
determined by the capacitance of the capacitive element and the inductance of the
inductive element.
[0008] The second voltage generating circuit is configured by Cockcroft-Walton multiplier
array which is composed by a plurality of rectifying elements and a plurality of capacitive
elements. A pair of insulation plates mount the rectifying elements and the capacitive
elements which are connected by wiring formed on each of the pair of insulation plates.
The Cockcroft-Walton multiplier array and the pair of insulation plates are embedded
in a mold block made from an insulating material.
[0009] According to the invention, there is virtually no power loss from the first voltage
generating circuit, so that substantially no heat is generated. Therefore, the target
voltage generating circuit can be cooled naturally without provision of a cooling
fan.
[0010] The rectification elements and capacitive elements of a Cockcroft-walton circuitry
are wired and connected on an insulation plate. Because these elements are fixed in
place by the insulation plates, the elements will not shift in position during molding.
Variation in high-voltage output sometimes caused by positional shifting of such elements
will not be produced. As such, the second voltage generating circuit outputs a stable
high voltage.
[0011] A preferred embodiment will now be described with reference to the accompanying drawings,
in which:-
FIG. 1 is a perspective view showing an X-ray source according to an embodiment of
the present invention;
FIGS. 2(a) and 2(b) are cross-sectional views of the X-ray source according to the
embodiment of the present invention;
FIG. 3 is a cross-sectional view showing construction of a microfocus X-ray tube of
the present X-ray source;
FIG. 4 is a perspective view showing external appearance of a mold block of the present
X-ray source;
FIG. 5(a) is a perspective view showing diodes and capacitors making up a Cockcroft-Walton
multiplier array of the present X-ray source and support plates;
FIG. 5(b) is an overhead view of the Cockcroft-Walton multiplier array and support
plates of FIG. 5(a);
Fig 5(c) is a side view showing wiring connecting the Cockcroft-Walton multiplier
array of FIG. 5(a) to the support plates;
FIG. 6 is a block diagram showing components of the present X-ray source;
FIG. 7 is a block diagram showing detailed configuration of an operation block portion
shown schematically in FIG. 6;
FIG. 8 is a graphical representation of the relationship between input voltage and
output voltage of the target high-voltage generating circuit;
FIG. 9 is a circuit diagram showing examples of specific components of a target voltage
circuit serving as the target high-voltage generating circuit;
FIG. 10 is a timing chart for an explanation of principles underlying operation of
the target voltage circuit of FIG. 9;
FIG. 11 shows a conventional target voltage circuit of an X-ray source;
FIG. 12(a) is a graphical representation showing power loss of the target voltage
circuit of FIG. 11;
FIG. 12(b) is a graphical representation showing power loss of the target voltage
circuit of the present X-ray source;
FIG. 13(a) is a graphical representation of output strength of a conventional X-ray
source provided with a PWM type high-voltage generation circuit; and
FIG. 13(b) is a graphical representation of output strength of the present X-ray source;
[0012] FIG. 1 shows a perspective view of the X-ray source of the present embodiment. FIGS.
2(a) is a cross-sectional view of the X-ray source cut along the line A-A' shown in
FIG. 2(b) which is a vertical cross-sectional view of the X-ray source of the present
embodiment. The X-ray source of the present embodiment includes a microfocus X-ray
tube 10 for emitting X-rays; a Cockcroft-Walton multiplier array for applying a high
voltage to the microfocus X-ray tube 10; control circuits 23 and 31 for applying a
high voltage to the microfocus X-ray tube 10; and an external controller unit 40 for
controlling this circuitry. The Cockcroft-Walton multiplier array includes a pair
of support plates 20b made from an insulating material for supporting the array. The
Cockcroft-Walton multiplier array and the support plates 20b are integratedly embedded
in a mold block 21. An insulating oil reservoir cavity 21a is provided at the front
side of the mold block 21. A target high-voltage supply terminal 22 is connected to
the microfocus X-ray tube 10 through the oil reservoir cavity 21a.
[0013] The control circuit 23 and a Cockcroft-Walton circuit 30 are mounted on the upper
surface of the mold block 21. The control circuit 23 includes a step down invertor
circuit and a push-pull invertor circuit for driving the Cockcroft-Walton multiplier
array . The Cockcroft-Walton circuit 30 supplies high voltage to the cathode electrode
of the microfocus X-ray tube 10. A connector 25 for connecting a cable of the controller
unit 40 to the circuitry of the X-ray source is provided at the rear side of the housing
50.
[0014] FIG. 3 is a cross-sectional view showing construction of the microfocus X-ray tube
10. The microfocus X-ray tube 10 includes an assembly of a metal outer envelope 12
and a glass outer envelope 13. A ceramic stem 11 is engaged with one end of the metal
outer envelope 12. An X-ray emission window 14 made from beryllium is formed to the
side surface of the metal outer envelope 12.
[0015] An electron gun 15 is positioned interiorly of the metal outer envelope 12. A target
mounting base 16 made from a material with high thermal conductivity, such as non-oxidized
pure copper, is positioned interiorly of the glass outer envelope 13. The electron
gun 15 includes a heater electrode 15a, a cathode 15b, a grid electrode 15c, and a
focus electrode 15d. A tungsten target 16a is brazed to the tip of the target mounting
base 16 using silver.
[0016] When the cathode 15b is heated up to a prescribed temperature by the heater electrode
15a, electrons are emitted from the surface of the cathode 15b. The emitted electrons
are accelerated by the grid electrode 15c and focused by the focus electrode 15d so
as to be in bombardment with the tungsten target 16a, resulting in the generation
of X-rays and heat. The generated X-rays are emitted outwardly from the X-ray emission
window 14. The generated heat is conducted out of the X-ray source through the target
mounting base 16.
[0017] The tungsten target 16a is positioned at a 25° slant to a plane perpendicular to
the orbit of the electrons fired at the tungsten target 16a. This slant increases
the number of generated X-rays that reach the X-ray emission window 14 and that are
emitted outside the microfocus X-ray tube 10 through the X-ray emission window 14.
[0018] FIG. 4 is a perspective view showing external appearance of the mold block 21. The
Cockcroft-Walton multiplier array and the support plates 20b are embedded in the mold
block 21. The Cockcroft-Walton multiplier array is a circuit often used as a power
source for producing high-voltage of about 70 kV. There is a particular need to mold
the Cockcroft-Walton multiplier array in a mold block 21 made from an insulating material
in order to reduce influence of the ambient environment on positions where voltage
is increased to a high voltage. Conventionally, the positioning of the plurality of
diodes and capacitors that make up the Cockcroft-Walton multiplier array shift during
molding and hardening of the block. High voltage outputted from a Cockcroft-Walton
circuit with position shifts is often unstable so that supplying a stable high voltage
has proven difficult.
[0019] In the present embodiment, as shown in FIGS. 5(a) through 5(c), the plurality of
diodes and capacitors making up the Cockcroft-Walton multiplier array are soldered
to the support plates 20b and also the diodes and capacitors are connected together
by wiring 20
b1 formed on the support plate 20b. This stable structure prevents the diodes and capacitors
from shifting position when the insulation material is poured over the components
of the Cockcroft-Walton multiplier array during molding processes of the mold block
21. Therefore, Cockcroft-Walton multiplier array of the present embodiment can supply
a stable high voltage output.
[0020] Japanese Laid-Open Patent Publication (Kokai) No. SHO-63-186,566 describes a conventional
Cockcroft-Walton circuit support plate in which are formed eyelets for fixing the
diodes and capacitors. However, this conventional Cockcroft-Walton circuit has insufficient
voltage-proof to be used at voltages of between 70 kV and 100 kV. Additionally, the
exposed wiring protruding upward from the plates is a source of potential electrical
discharges. In contrast, according to the present embodiment, because holes are formed
in the print board 20b of Cockcroft-Walton multiplier array in a predetermined pattern
and are mutually connected by the wirings formed on the board 20b, the amount that
wiring protrudes upward from the support plate 20b is reduced to a minimum and the
voltage-proof characteristic of the Cockcroft-Walton multiplier array is greatly improved.
[0021] FIG. 6 is a block diagram showing components provided in association with the X-ray
source of the present embodiment. The block diagram includes an operation block portion
100 for operating the microfocus X-ray tube 10 and a control block portion 200 for
controlling the operation block portion 100.
[0022] The operation block portion 100 includes a target control 110 for controlling target
voltage of the X-ray tube 10; an overcurrent detector 120 for detecting excessive
current of the tungsten target 16a; and a grid control 130 for controlling the grid
voltage of the X-ray tube 10. The operation block portion 100 further includes a cathode
control 140 for controlling the cathode voltage of the X-ray tube 10 and a heater
control 150 for controlling the heater of the X-ray tube 10.
[0023] The control block portion 200 includes a voltage setting D/A converter 210 for applying
target voltage setting voltage to both the target control 110 and the cathode control
140; a current setting D/A converter 220 for applying target current setting voltage
to the grid control 130; and an interlock detector 230 for detecting an interlock.
The control block portion 200 further includes an aging portion 240 for warming up
the control block portion 200; a key switch 250 for stopping generation of X-rays;
and a power source control 260 for controlling change of power source voltage. The
control block portion 200 also includes a ROM 270 for storing control programs; a
RAM 280; a voltage control 290 for setting voltage; a current control 300 for setting
current; and a mode switch 310 for setting an X-ray mode. The control block portion
200 further includes a mode indicator 320 for indicating X-ray mode, target overcurrent,
target voltage, and target current; an overcurrent display 330; a target voltage display
meter 340; a target current display meter 350; and a CPU 360 controlling each component.
[0024] FIG. 7 is a block diagram showing detailed configuration of the operation block portion
100. The target control 110 includes a target voltage control 111 controlling target
voltage according to the target voltage setting voltage applied thereto from the voltage
setting D/A converter 210; and a target high-voltage generator 112 generating a desired
target high-voltage according to a signal from the target voltage control 111. The
target overcurrent detector 120 includes an overcurrent detector 121 for detecting
overcurrent of the target current generated at the target high-voltage generator 112;
and an overvoltage detector 122 detecting overvoltage of the target voltage generated
at the target high-voltage generator 112.
[0025] The grid control 130 includes a target current detector 131 for detecting the target
current; a target current comparator 132 for comparing the target current detected
by the target current detector 131 with the setting voltage signal outputted from
the current setting D/A converter 220; and a cut-off voltage control setting portion
133. The grid control portion 130 further includes a grit voltage control 134 for
controlling grid voltage based on the results of comparisons made at the target current
comparator 132; and a grid voltage generator 135 for generating a desired grid voltage
according to signals received from the grid voltage control 134.
[0026] The cathode control 140 includes a cathode voltage control 141 for controlling cathode
voltage according to target voltage setting voltage received from the voltage setting
D/A converter 210; and a cathode voltage generator 142 for generating a desired cathode
voltage according to signals received from the cathode voltage control 141. The heater
control 150 includes a heater voltage control 151 for controlling the heater voltage;
and a heater voltage generator 152 for generating a desired heater voltage according
to signals received from the heater voltage control 151.
[0027] In a microfocus X-ray tube 10, the X-ray beam diameter is maintained at a small value
even when the target voltage is changed. To precisely control the intensity level
of the X-ray beam, the target high-voltage generator 112 must generate voltage that
changes linearly from low voltage to high voltage. U. S. Patent Nos. 4,694,480 and
4,646,338 describe a conventional target high-voltage generation unit that uses pulse
width modulation (PWM) to control the voltage. The voltage generated in the conventional
target high-voltage generation unit fluctuates, especially at low voltage range, and
linearly changing the voltage level cannot be achieved.
[0028] The target high-voltage generator 112 of the present embodiment uses a double invertor
arrangement to control target voltage. That is, a step down type invertor is provided
for the low-voltage range and a push-pull invertor is provided for the high-voltage
range. With such an arrangement, a variable voltage that changes linearly from low
voltage to high voltage can be obtained. FIG. 8 is a graphical representation of the
relationship between input voltage and output voltage according to results of measurements
of the X-ray source of the present embodiment using a coil of paraffin impregnated
3:600 shunt winding and twenty-stage Cockcroft-Walton voltage multiplier. It can be
seen from this graph that the relationship between the input voltage and the output
voltage changes linearly between an output voltage of about 10 kV to about 100 kV.
[0029] FIG. 9 is a circuit diagram showing a detailed circuit configuration of the target
high-voltage generator 112. The target high-voltage generator 112 of the present embodiment
includes a step-down invertor circuit 410 for the low-voltage range and a push-pull
invertor circuit 420 for the high-voltage range. A stably changing voltage can be
obtained over a broad range from low to high voltage using these two types of different
invertor circuits 410 and 420.
[0030] Next, an explanation of principles underlying operation of the target high-voltage
generator 112 will be provided while referring to the circuit diagram of FIG. 9 and
the waveform diagram of FIG. 10. First, a target voltage setting voltage (Vi) is applied
to the step-down invertor circuit 410, whereupon a setting voltage signal passes through
a buffer IC
1-a and is applied to a comparator IC
1-c through an error amplifier IC
1-b. The error amplifier IC
1-b is also applied with the output from an operational amplifier IC
5-a. Initially a 0 V voltage is developed at the secondary coil of the transformer 430.
A detection voltage representative of the voltage developed at the secondary coil
of the transformer 430 is obtained from the operational amplifier IC
5-a through a voltage divider 440 connected between the output of the Cockcroft-Walton
multiplier array and ground. The detection voltage is also initially a 0 V voltage.
The error amplifier IC
1-b is saturated to a +24 V, and this saturation voltage is applied to the comparator
IC
1-c. The initial voltage of the capacitor C₁ is also a 0 V voltage. The comparator IC
1-c is saturated to a +24 V voltage as represented by level (c) in FIG. 10. As represented
by levels (d) and (e) in FIG. 10, this results in the transistors Q₁, Q₂, and Q₃ being
rendered ON, whereupon the capacitor C₁ is charged through a coil L₁. The transistor
Q₁ repeats switching actions and thus allows the capacitor C₁ to be continuously charged
until the voltage across the capacitor C₁ has reached the target voltage setting voltage
Vi. The coil L₁ is connected between the transistor Q₁ and the capacitor C₁ for determining
a time constant of charging current that flows in the capacitor C₁. As represented
in level (d) of FIG. 10, when the voltage V₂ across the capacitor C₁ exceeds the target
voltage setting voltage V₁, the output from the comparator C
1-c is zeroed so that the transistors Q₁, Q₂, and Q₃ are rendered OFF. As such, the voltage
V₂ substantially equal to the target voltage setting voltage V₁ is stably obtained.
[0031] The voltage at the primary coil of the transformer 430 is subjected to ON-OFF switchings
by virtue of the transistors Q₆ and Q₇ according to oscillation frequency of the oscillator
IC₄. The oscillator IC₄ is rendered ON and begins to oscillate when the comparators
IC
2-a, IC
2-b, and IC
2-c are applied with an ON voltage from the target voltage ON/OFF terminal.
[0032] The voltage developed at the secondary coil of the transformer 430 increases to a
voltage according to the turns ratio and is further increased to a high voltage by
the Cockcroft-Walton multiplier array . The increased output voltage is voltage-divided
by the divider 440. The divided voltage passes through the operation amplifier IC
5-a and is applied to the error amplifier IC
1-b as the detection voltage. This operation is represented by level (g) in FIG. 10.
The error amplifier IC
1-b compares the detection voltage and the target voltage setting voltage Vi and drives
the comparator IC
1-c until the detection voltage and the target voltage setting voltage Vi are in coincidence
with each other.
[0033] In this way, the high-voltage output is controllable by operating the double invertor
including the step down invertor circuit 410 and the push-pull invertor circuit 420.
The characteristic of the circuitry at the primary coil side of the transformer 430,
that is, of the step down invertor circuit 410 and the push-pull invertor circuit
420, is that the ON and OFF operation of the transistors Q₁, Q₂, and Q₃, as driven
by the comparator IC
1-c, controls the charge current of the capacitor C₁. For this reason, drive is accomplished
using only a minimal amount of power and with a circuit configuration that loses very
little power.
[0034] FIG. 11 shows a target voltage circuit 500 of an X-ray source conceived by the present
inventors (not prior art). The target voltage circuit 500 differs from the target
high-voltage generator 112 in that the ON and OFF condition of a transistor Q₅, which
corresponds to the transistor Q₁ of the present embodiment, is controlled by output
from an error amplifier IC₆₋₁, which corresponds to the error amplifier IC
1-b of the present embodiment, and also in that no capacitor, that is, capacitor C₂ of
the present embodiment, is provided for charging by the output current from the transistor
Q₅. Not only is the current flowing from the transistor Q₅ lost power, but also a
source of heat because the lost power is converted into heat. The transistors Q₁ and
Q₂ provided to the invertor circuit are less efficient and produce more heat than
the switching transistors Q₆ and Q₇ for controlling voltage by switching transistors
for controlling the current. For this reason, a cooling fan must be provided for cooling
the circuitry or else operation will become unstable.
[0035] FIG. 12(a) is a graphical representation showing power loss in the target voltage
circuit 500 with respect to the input power thereto. FIG. 12(b) is a graphical representation
showing power loss of the target high-voltage generator 112 of the present embodiment
with respect to the input power thereto. It can be seen from these graphs that less
power loss is present in the target high-voltage generator 112 throughout the high-voltage
output range from 10 to 70 kV. Measurements for these graphs were taken with both
the target voltage circuits 400 and 500 provided with a Cockcroft-Walton multiplier
array having the same configuration.
[0036] FIG. 13(a) is a graphical representation of output intensity of a conventional X-ray
source provided with a PWM type high-voltage generation circuit. FIG. 13(b) is a graphical
representation of output intensity of the X-ray source according to the present embodiment.
It can be seen by comparing these graphs that the X-ray source of the present embodiment
outputs X-rays with more stable intensity than the conventional X-ray source. In measurements
for both of these graphs, both of the X-ray sources had a 40 kV target voltage applied
to the target. Target current was 10 µA.
[0037] While the invention has been described in detail with reference to a specific embodiment
thereof, it would be apparent to those skilled in the art that various changes and
modifications may be made therein without departing from the spirit of the invention,
the scope of which is defined by the attached claims. For example, the microfocus
X-ray tube 10 could be an end window type instead of the side window type shown in
FIG. 3.
[0038] In the X-ray source according to the present invention, the first voltage generation
portion provided with a target voltage generation circuit operates according to ON
and OFF switching of switching elements so that input voltage increases up to a predetermined
set voltage. The switching operations of the switching elements are controlled by
a signal voltage from a second voltage generation portion. For this reason, there
is virtually no power loss from the first voltage generation portion. Also, very little
heat is generated from this circuitry so that natural cooling without the aid of a
cooling fan is sufficient. Because no cooling fan needs to be provided, problems caused
by the vibration of a cooling fan, such as, inability to maintain the focus of the
X-rays, will not occur.
[0039] Also the Cockcroft-Walton circuitry of the second voltage generation portion has
a configuration wherein rectification elements (diodes) and capacitive elements (capacitors)
are wired and connected on an insulation board. Therefore, all the elements are fixed
in place by the insulation board so that the positions of the elements will not shift
during molding to the insulation block. For this reason, no variability in high-voltage
output, which can result from rectification elements and capacitive elements shifting
in position, will not be generated so that a stable high-voltage output can be obtained
from the second voltage generation portion.
1. An X-ray source comprising:
an X-ray tube (10) having a cathode (15) and a target (16); and
a target voltage generating circuit (112) for generating a target voltage applied
to said target (16), said target voltage generating circuit (112) comprising:
a first voltage generating circuit (410) generating a first voltage in response
to an instruction voltage indicative of a voltage to be applied to said target (16),
said first voltage generating circuit (410) including a switching element (Q1); and,
a second voltage generating circuit (420, 21) connected to said first voltage generating
circuit (410), said second voltage generating circuit (420, 21) generating an output
voltage to be applied to said target (16);
characterised by a voltage divider (440) connected to said second voltage generating
circuit (420, 21), the voltage divider (440) dividing the output voltage from said
second voltage generating circuit (420, 21) to produce a detection voltage indicative
of a voltage generated by said second voltage generating circuit (420, 21);
and in that said switching element (Q1) is rendered ON until the detection voltage
has reached the instruction voltage and said switching element (Q1) is alternately
rendered ON and OFF after the detection voltage has reached the instruction voltage.
2. An X-ray source as claimed in claim 1, wherein said first voltage generating circuit
(410) further includes a capacitive element (C1) having a capacitance, said capacitive
element (C1) being charged by a current flowing through said switching element (Q1)
when said switching element (Q1) is ON, said second voltage generating circuit (420,
21) generating the output voltage based on the voltage across said capacitive element
(C1).
3. An X-ray source comprising:
an X-ray tube (10) having a cathode (15) and a target (16); and
a target voltage generating circuit (112) applying a positive voltage to said target
(16) so that a high voltage is developed between said cathode (15) and said target
(16), wherein X-rays are generated from said target (16) resulting from bombardment
of electron beams generated from said cathode (15), said target voltage generating
circuit (112) comprising:
a step down invertor (410) generating a first voltage in response to an instruction
voltage indicative of a voltage to be applied to said target (16), said step down
invertor (410) including a switching element (Q1);
a push-pull invertor (420) generating a second voltage in response to said first
voltage, the second voltage being higher than the first voltage;
a transformer (430) having a primary winding connected to said second voltage generation
circuit, and a secondary winding;
a high voltage generating circuit (20a) connected to said secondary winding of
said transformer (430) and producing an output voltage for applying to said target
(16); and,
a voltage divider (440) connected to said high voltage generating circuit, said
voltage divider dividing the output voltage from said high voltage generating circuit
(20a) to produce a detection voltage indicative of a voltage generated by said high
voltage generating circuit (20a);
wherein said switching element (Q1) is alternately rendered ON and OFF after the
detection voltage has reached the instruction voltage so that the first voltage is
lowered to a first pre-determined level after reaching a second pre-determined level
to thus substantially maintain the first voltage at a constant level corresponding
to the instruction voltage, a current being allowed to flow through said switching
element (Q1) when said switching element (Q1) is rendered ON.
4. An X-ray source as claimed in claim 3, wherein the step down invertor (410) further
includes a capacitive element (C1) having a capacitance, said capacitive element (C1)
being charged by the current flowing through said switching element (Q1), said switching
element being held ON until a voltage developed across said capacitive element (Q1)
reaches the detection voltage, the voltage across said capacitive element (Q1) being
applied to said push-pull invertor (420) as the first voltage.
5. An X-ray source as claimed in claim 2 or 4, wherein said first voltage generating
circuit (410) or said step down invertor (410) further includes an inductive element
(L1) having an inductance, and wherein said capacitive element (C1) is charged according
to a time constant determined by the capacitance of said capacitive element (C1) and
the inductance of said inductive element (L1).
6. An X-ray source as claimed in claim 2, 4 or 5, wherein said first voltage generating
circuit (410) or said step down invertor (410) further includes an error amplifier
(1C1-b) having a first input terminal applied with the instruction voltage, a second input
terminal applied with the detection voltage, and an output terminal, and a comparator
(1C1-c) having a first input terminal connected to the output terminal of said error amplifier
(1C1-b), and a second input terminal supplied with the voltage developed across said capacitive
element (C1).
7. An X-ray source as claimed in any preceding claims, wherein said second voltage generating
circuit (420) or said high voltage generating circuit (21) comprises a Cockcroft-Walton
multiplier array (20a) having a plurality of rectifying elements and a plurality of
capacitive elements, and a pair of insulation plates (20b) for mounting said plurality
of rectifying elements and said plurality of capacitive elements, said plurality of
rectifying elements and said plurality of capacitive elements being connected by wiring
(20b) formed on each of said pair of insulation plates (20b).
8. An X-ray source as claimed in claim 7, wherein said Cockcroft-Walton multiplier array
(20a) and said pair of insulation plates (20b) are embedded in a moulded block made
from an insulating material.