X-ray power supply utilizing A.C. frequency conversion to generate a high D.C. voltage

An output voltage of an A.C. power source is input to an frequency converter and the frequency thereof is increased. A plurality of high voltage transformers of small capacity each of which has a secondary winding of a small number of turns and which are connected in parallel with one another are connected to an output terminal of the frequency converter. Outputs of the high voltage transformers are respectively connected to high voltage rectifier circuits. Outputs of the high voltage rectifier circuits are serially coupled, the output voltages thereof are added together, and the addition result is applied to an X-ray tube. Combinations of the high voltage transformers and the high voltage rectifier circuits are molded into units one or a preset number at a time with solid insulating material including gel insulating material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an X-ray generator apparatus having an 
X-ray tube which generates X-rays when applied with a high voltage 
obtained by increasing an input voltage to a high A.C. voltage of a high 
frequency by means of a frequency converter and a high voltage transformer 
or the like and rectifying the high A.C. voltage. 
2. Description of the Related Art 
An example of this type of conventional X-ray generator apparatus is shown 
in FIG. 1 . In order to enhance the performance and make the device small 
and lightweight, a frequency converter 2 for converting the frequency of a 
voltage supplied from an input power source (A.C. power source) into a 
high frequency is connected to the primary winding of a high voltage 
step-up transformer 3. An output voltage of the frequency converter 2 is 
increased by the high voltage transformer 3 and an output voltage of the 
high voltage transformer 3 is rectified by a high voltage rectifier 4. A 
rectified output of the high voltage rectifier 4 is applied between an 
anode and a cathode of an X-ray tube 5 serving as an X-ray source. 
The frequency converter 2 is generally formed of a rectifier for converting 
the input A.C. voltage into a D.C. voltage, a capacitor for filtering the 
D.C. voltage, and an inverter for converting the D.C. voltage from the 
capacitor into an A.C. voltage of a desired frequency. The frequency 
converter 2 converts the frequency fo (which is a commercial frequency and 
is generally 50/60 Hz) of the input A.C. voltage to a frequency f1 which 
is higher than the frequency fo and then applies the voltage to the high 
voltage transformer 3. 
As the output frequency f1 of the frequency converter 2 is set to be 
higher, the size and weight of the frequency converter 2 and high voltage 
transformer 3 can be reduced. Since the impedances of coils and capacitors 
generally vary according to the frequency, the capacitance and inductance 
can be reduced as the frequency is set higher if the impedances are kept 
unchanged. Since the capacitance and inductance vary in proportion to the 
size of the capacitor and coil, the size and weight of the frequency 
converter 2 and high voltage transformer 3 using the coil and capacitor 
can be reduced as the frequency becomes higher. 
However, in the above X-ray generator apparatus, the output frequency f1 of 
the frequency converter 2 cannot be increased limitlessly and the upper 
limit thereof is determined by the characteristic of the high voltage 
transformer 3 for the following reason. 
FIG. 2 shows an equivalent circuit diagram of the device shown in FIG. 1 in 
view of the secondary winding portion of the transformer 3. In FIG. 2, L1, 
L2, and M respectively denote the primary inductance, secondary 
inductance, and mutual inductance of the high voltage transformer 3. N 
denotes the turn ratio (the number of turns of the secondary windings/the 
number of turns of the primary windings) of the transformer 3. In this 
case, in order to obtain a high output voltage, the high voltage 
transformer 3 is so designed that the number of turns of the secondary 
winding must be made very larger than that of the primary winding, and as 
a result, the secondary inductance L2 becomes very larger than the primary 
inductance L1 and mutual inductance M. Therefore, the inductance of the 
secondary portion of the high voltage transformer 3 which is actually 
equal to (L2-M) as shown in FIG. 2 can be regarded as being equal to the 
secondary inductance L2 by neglecting M. Therefore, in the following 
explanation, it is assumed that the inductance of the secondary portion is 
equal to L2. Further, if the equivalent impedance of the X-ray tube 5 is 
Rx, the terminal voltage of the X-ray tube 5 is Ex, and the rectifier 4 is 
omitted from being consideration since it does not relate to the terminal 
voltage Ex, then the secondary inductance L2 is connected in series to the 
impedance Rx. 
If the output frequency of the frequency converter 2 is f1, an impedance Z2 
due to the secondary impedance L2 can be expressed by the following 
equation and it is understood that it varies in proportion to the output 
frequency f1 of the frequency converter 2: 
EQU Z2=2.pi..P.f1.L2 (1) 
Further, the voltage Ex applied to the X-ray tube 5 is expressed as 
follows: 
EQU Ex=E2.Rx/(Rx+Z2) (2) 
Since the turn ratio N is very large and thus the inductance (L1-M)/N.sup.2 
can be neglected, the terminal voltage E2 of a mutual inductance M is 
expressed as follows using the output voltage E1 of the frequency 
converter 2: 
EQU E2=E1. N (3) 
As is clearly understood from the equations (1) and (2), the impedance Z2 
becomes higher as the output frequency f1 of the frequency converter 2 
becomes higher, causing a problem that the voltage Ex applied to the X-ray 
tube 5 is lowered. For this reason, the output frequency f1 of the 
conventional frequency converter 2 has an upper limit of several tens of 
KHz and a higher frequency exceeding this upper limit cannot be attained. 
If the frequency is set to several tens of KHz, it is difficult to reduce 
the size and weight of the transformer and rectifier circuit and audio 
noise may be generated from the transformer 3. 
The reason the output frequency f1 of the frequency converter 2 can be 
increased only to several tens of KHz at most is that the secondary 
inductance L2 of the high voltage transformer 3 is very large. 
In order to solve the above problem, it has been proposed to modify the 
primary portion of the high voltage transformer 3 as shown in FIGS. 3 and 
4. In the circuit of FIG. 3, a capacitor C1 is connected in series to the 
primary winding of the high voltage transformer 3 to form a series 
resonance circuit in the primary portion. In the circuit of FIG. 4, a 
capacitor C2 is connected in parallel with the primary winding of the high 
voltage transformer 3 to form a parallel resonance circuit in the primary 
portion. However, in either circuit, a voltage of the primary portion of 
the high voltage transformer 3 is equivalently increased by the series 
resonance or parallel resonance circuit. The inductance L1 of the primary 
portion is originally small and the resonance voltage is low, and 
therefore, in order to obtain the same voltage applied to the X-ray tube 5 
as that obtained in a case wherein no resonance circuit is connected, it 
is only possible to increase the output frequency of the frequency 
converter 2 to several times the output frequency set in a case wherein no 
resonance circuit is connected. 
Further, in U.S. Pat. No. 4,545,005 (Mudde), the secondary winding of the 
high voltage transformer is divided into a plurality of sub-windings to 
increase the operation frequency of the high voltage transformer. The 
sub-windings are connected in series through bridge rectifier circuits. 
The outputs of the rectifier circuits are coupled in series and applied to 
an X-ray tube. Every other sub-windings have the same sense and the rest 
have the opposite sense. However, the core of the high voltage transformer 
is not divided and thus the high voltage transformer can be regarded as 
being a single transformer. The capacitance of the secondary winding of 
the transformer can be reduced but the inductance thereof cannot be 
reduced by a division of the secondary winding. In this USP, an output of 
one frequency converter is simply connected to a single high voltage 
transformer. Therefore, like the conventional case shown in FIG. 1, it is 
only possible to increase the frequency to several tens of KHz at most. 
Further, in U.S. Pat. No. 4,317,039 (Romandi), a frequency converter is 
formed of a main rectifier for three-phase current, a filter member, and a 
plurality of inverters connected in parallel to the filter member. The 
outputs of the plural inverters are respectively supplied to a plurality 
of transformers. The outputs of the plural transformers are respectively 
supplied to a plurality of rectifiers. The rectified voltages are added 
together and applied to the X-ray tube. The plural inverters are 
controlled by a control circuit in such a manner that the phases of the 
output voltages are chronologically offset relative to one another. The 
phase displacement amounts expediently to 90.degree., so that the ripple 
of the high-voltage at the X-ray tube, as compared with the instance in 
which only a single inverter and a single transformer are provided, is 
reduced by a factor of the number of converters used. In this USP, the 
frequency converter outputs plural different phase voltages to be applied 
to the plural transformers. If there is any capacitance between an output 
rectifier and the X-ray tube, the ripple of the output voltage can be made 
small by increasing the frequency of the output voltage without by 
displacing the phases of the output voltages. A high-voltage rectifier 
means of this USP comprises voltage doubler circuits connected to the 
secondary windings of the transformers. The frequency doubler circuit 
includes capacitors. Moreover, a high-voltage cable transmitting the 
high-voltage from the high-voltage rectifier means to the X-ray tube also 
includes capacitors. The ripple of the output voltage can be smaller by 
increasing the frequency of the output voltage than displacing the phases 
of the output voltages. Therefore, this USP in which the phases of the 
output voltages are displaced in order to make small the ripple does not 
aim to increase the frequency. This USP describes that the feed frequency 
of the high voltage transformers, i.e., the frequency of the inverters 
lies in the medium frequency range and amounts to approximately six to 
seven KHz. In this UPS, the number of the inverters must be plural in 
order to displace the phases of the output voltages. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an X-ray generator 
apparatus in which a power source voltage is converted into an A.C. 
voltage of a desired high frequency, the A.C. voltage is increased by a 
transformer, then the increased voltage is rectified by a rectifier and 
applied to an X-ray tube, and in which the frequency of the A.C. voltage 
is increased and the size and weight of the transformer are reduced. 
An X-ray generator apparatus according to the present invention comprises 
frequency converter means connected to an A.C. power source, for 
increasing the frequency of an A.C. voltage up to a level above 15 KHz; at 
least four transformer means connected to an output of the frequency 
converter means, for increasing the output A.C. voltage from the frequency 
converter means; and rectifier means for converting the output A.C. 
voltages from the plural transformer means to D.C. voltages, serially 
adding all of the D.C. voltages, and applying the result of addition of 
the D.C. voltages to an X-ray tube. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
There will now be described an embodiment of an X-ray generator apparatus 
according to the present invention with reference to the accompanying 
drawings. FIG. 5 is a block diagram showing the construction of a first 
embodiment. An A.C. power source 11 serving as an input power source is 
connected to the input terminal of a frequency converter 12. The frequency 
converter 12 increases the frequency of an A.C. voltage supplied from the 
A.C. power source 11. 
The frequency converter 12 is formed of, as shown in FIG. 6A, a rectifier 
71 for converting the input A.C. voltage from the A.C. power source 11 
into a D.C. voltage, a capacitor circuit 72 for filtering the D.C. voltage 
from the rectifier 71, and an inverter 73 for converting the D.C. voltage 
from the capacitor circuit 72 into an A.C. voltage of a desired frequency. 
The frequency converter 12 of a mobile type X-ray generator apparatus is 
formed of, as shown in FIG. 6B, the rectifier 71, a battery circuit 74, 
and the inverter 73. 
High voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n are connected 
in parallel with one another between output terminals of the frequency 
converter 12. The number of transformers is preferably more than four. If 
the number of transformers is set to four, a reference (ground) potential 
is connected between two transformers connected to the anode side of the 
X-ray tube and two transformers connected to the cathode side of the X-ray 
tube. In FIG. 5, one end of the primary winding of each of the high 
voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n is connected to 
one of the output terminals of the frequency converter 12 and the other 
end of the primary winding of each of the high voltage transformers 
13.sub.1, 13.sub.2, . . . 13.sub.n is connected to the other output 
terminal of the frequency converter 12. It is preferable to form cores 
19.sub.1, 19.sub.2, . . . 19.sub.n of the high voltage transformers 
13.sub.1, 13.sub.2, . . . 13.sub.n by using ferrite or the like, which has 
a good frequency characteristic, in order to attain the high operation 
frequency. 
The secondary windings of the high voltage transformers 13.sub.1, 13.sub.2, 
. . . 13.sub.n are respectively connected to high voltage rectifiers 
14.sub.1, 14.sub.2, . . . 14.sub.n. The output terminals of the high 
voltage rectifiers 14.sub.1, 14.sub.2, . . . 14.sub.n are connected in 
series and the result of serial addition obtained by the series connection 
is applied to an X-ray tube 15. That is, the positive output terminal of 
the high voltage rectifier 14.sub.1 is connected to the anode of the X-ray 
tube 15, the negative output terminals of the high voltage rectifiers 
14.sub.1, 14.sub.2, . . . 14.sub.n-1 are connected to the positive output 
terminals of the high voltage rectifiers 14.sub.2, 14.sub.3, . . . 
14.sub.n, and the negative output terminal of the high voltage rectifier 
14.sub.n is connected to the cathode of the X-ray tube 15. 
In this case, the number of turns of each of the primary windings of the 
high voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n is set to be 
equal to that of the primary winding of the conventional high voltage 
transformer 3 shown in FIG. 1 and the number of turns of each of the 
secondary windings of the high voltage transformers 13.sub.1, 13.sub.2, . 
. . 13.sub.n is set to 1/n (n is the number of the transformers 13) of 
that of the secondary winding of the conventional high voltage transformer 
3 in order to simplify the description. 
Next, the operation of the first embodiment is explained. FIG. 7A is an 
equivalent circuit diagram of a secondary portion (a portion from the 
secondary winding to the X-ray tube with the rectifier being neglected) of 
the conventional transformer 3 of FIG. 1. FIG. 7B is the equivalent 
circuit diagram of the secondary portions of the transformers 13.sub.1, 
13.sub.2, . . . 13.sub.n of the first embodiment shown in FIG. 5. In 
general, the number of turns of the secondary winding of each of the high 
voltage transformers 3, 13.sub.1, 13.sub.2, . . . 13.sub.n is extremely 
larger than that of the primary winding thereof, and the secondary 
inductance L2 is set to a large value. Therefore, the equivalent circuit 
diagrams can be expressed only by the secondary inductance L2 as shown in 
FIGS. 7A and 7B. The frequency converter is generally on/off operated by 
the switching pulse and outputs a pulse signal. Therefore, the output 
voltage E2 of the transformer is also expressed by a pulse. 
If, in FIG. 7A, L2/Rx=.tau.a, then the voltage Ex applied to the X-ray tube 
5 is expressed by using the time constant .tau.a as follows and rises as 
shown by a curve "A" in FIG. 8. The reference time t=0 with respect to 
time t in FIG. 8 is a timing at which the voltage E2 starts to rise. 
EQU Ex=E2 (1-e.sup.-t/.tau.a) (4) 
That is, if it is assumed that the pulse width of the voltage E2 is .tau.a, 
the tube voltage Ex is set to a maximum value (0.63.times.E2) at the time 
of t=.tau.a. 
On the other hand, in the device of the first embodiment shown in FIG. 5, 
the number of turns of the secondary winding of each of the high voltage 
transformers 13.sub.1, 13.sub.2, . . . 13.sub.n is set to 1/n of that of 
the high voltage transformer 3 in the conventional device shown in FIG. 1. 
Since the inductance of a coil varies in proportion to the square of the 
number of turns, the secondary inductance becomes L2/n.sup.2 and the 
secondary voltage becomes E2/n in each of the high voltage transformers 
13.sub.1, 13.sub.2, . . . 13.sub.n. Further, the load of each of the high 
voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n is substantially 
the same as a value obtained by dividing the load Rx of the conventional 
device by n, that is, it becomes Rx/n. As a result, the equivalent circuit 
diagram of the embodiment of FIG. 5 can be expressed as shown in FIG. 7B. 
In secondary portion of each of the high voltage transformers 13.sub.1, 
13.sub.2, . . . 13.sub.n, the time constant .tau.b is expressed as follows 
according to the above description with reference to FIG. 7A: 
##EQU1## 
A voltage E3 applied to the load Rx/n is expressed as follows: 
EQU E3=E2 (1-e.sup.-t/.tau.b)/n (6) 
The voltage Ex applied to the X-ray tube 15 is given as follows by serially 
adding the terminal voltages E3 of the loads: 
##EQU2## 
That is, as shown by a curve B in FIG. 8, at the time of t=.tau.b, the tube 
voltage Ex is set to 0.63.E2 which has been reached at the time of 
t=.tau.a in the conventional device. In this case, since .tau.b=.tau.a/n 
as shown in the equation (5), the time constant of the device according to 
the embodiment shown in FIG. 5 is set to 1/n of that of the conventional 
device shown in FIG. 1, and therefore, it is understood that the frequency 
of each of the transformers 13.sub.1, 13.sub.2, . . . 13.sub.n can be 
increased by "n" times since the same voltage is obtained if the pulse 
width of the output of the frequency converter 12 is set to .tau.b. 
In the conventional high voltage transformer 3 shown in FIG. 7A, even if 
the switching pulse width of the frequency converter 2 is simply changed 
from .tau.a to 1/n times (=.tau.b) to increase the frequency, the peak 
value of the tube voltage Ex expressed by the equation (4) becomes smaller 
as shown by a curve C in FIG. 8 and the power applied to the X-ray tube 
simply becomes small as indicated by a hatched portion. 
As described above, according to the first embodiment, the high voltage 
transformer is divided into a plurality (for example, n) of transformers 
13.sub.1, 13.sub.2, . . . 13.sub.n having a small capacity (the number of 
turns of the primary winding is kept unchanged and the number of turns of 
the secondary winding is reduced to 1/n times the original value), the 
primary windings of the divided transformers 13.sub.1, 13.sub.2, . . . 
13.sub.n are connected in parallel with one another between the output 
terminals of the frequency converter 12, and a voltage obtained by 
serially adding together the results of rectification of the outputs of 
the respective transformers is applied to the X-ray tube 15. Thus, the 
secondary inductance of each of the transformers 13.sub.1, 13.sub.2, . . . 
13.sub.n can be reduced to 1/n.sup.2 times the original value, and as a 
result, the upper limit of the output frequency of the frequency converter 
12 is increased by n times. It is possible to make small the sectional 
area of the core in proportion to the increase of the operation frequency. 
The high voltage transformers and the frequency converter can be made 
small when the operation frequency is increased. Therefore, the apparatus 
including the frequency converter 12 and high voltage transformers 
13.sub.1, 13.sub.2 . . . 13.sub.n can be made small and lightweight. Since 
the output frequency of the frequency converter 12 can be increased up to 
approximately several hundreds of KHz or to a frequency which exceeds the 
audio frequency, generation of audio noise which is a problem in the 
conventional device can be prevented. 
Further, since the output control of the frequency converter 12 can be 
effected at a higher speed as the output frequency thereof increases, a 
high voltage applied to the X-ray tube 15 can be more precisely controlled 
by using the feedback control. Further, since ripples of a waveform of the 
high voltage become smaller as the frequency becomes higher, a flat 
waveform of the high voltage can be obtained. Further, if an A.C. power 
source 11 of a three-phase is used, it is possible to obtain a flat 
waveform of the high voltage due to the high speed feedback of the output 
voltage even if the capacitor 72 of FIG. 6 is omitted. In addition, the 
rising characteristic of the tube voltage can be improved as shown by the 
curve B of FIG. 8, it becomes easy to apply a high voltage in a pulse form 
to the X-ray tube 15 and generate X-rays only at necessary timings, 
thereby making it possible to reduce the amount of X-ray radiation to an 
object. 
Further, it is also possible to connect in series the outputs of the high 
voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n instead of 
connecting the transformers 13.sub.1, 13.sub.2, . . . 13.sub.n to the 
respective rectifiers 14.sub.1, 14.sub.2, . . . 14.sub.n and rectify the 
serially added voltages by a single rectifier. In addition, it is possible 
to connect a resonant capacitor in series or in parallel to the primary 
winding of each of the high voltage transformers 13.sub.1, 13.sub.2, . . . 
13.sub.n. The frequency converter can change the amplitude of the output 
voltage in addition to the output frequency by a pulse width modulation 
(PWM) for changing the pulse width of the switching pulse. In the 
frequency converter 12, the input A.C. voltage is converted into a D.C. 
voltage. The D.C. voltage is on/off controlled by a switching transistor 
to obtain an A.C. voltage having a frequency different from that of the 
input A.C. voltage. The amplitude and the frequency of the voltage output 
from the frequency converter 12 can be changed by controlling the 
switching pulse of the switching transistor, as shown in FIG. 9. 
Next, modifications relating to the improvement of the first embodiment are 
explained. In the conventional X-ray generator apparatus, the high voltage 
transformer and high voltage rectifier are disposed in a container filled 
with insulating oil. Since the container is substantially entirely filled 
with insulating oil, the volume and weight thereof become very large. In 
this case, the maintenance therefor is troublesome and there occurs a 
problem that oil leaks out of the container and stains the surrounding. 
However, in the first embodiment, since the transformer is divided into a 
plurality of transformers of small capacities, the high voltage 
transformer and high voltage rectifier are disposed in a container of 
small capacity and can be molded into one unit with solid insulation 
material including gel insulating material. Mold type insulating material 
such as epoxy and material such as silicone gel which is solidified but 
has a physical property between those of the fluid and solid can be given 
as other examples of the above insulating material. Since silicone gel has 
a good high frequency characteristic, it can be preferably used as the 
insulating material for the device constructed to attain a high frequency. 
Each molding unit may be constructed by a single transformer 13.sub.i and a 
single rectifier 14.sub.i ("i" being 1 to n) as shown in FIG. 10 or by a 
plurality of transformers 13.sub.1 to 13.sub.j and a plurality of 
rectifiers 14.sub.1 to 14.sub.j (j being smaller than n) as shown in FIG. 
11. Further, as shown in FIG. 12, only the secondary winding of the 
transformer 13.sub.i and the rectifier 14.sub.i are molded and it is not 
always necessary to mold the primary winding of the transformer 13.sub.i. 
Although not shown in the drawing, the high voltage transformer and the 
high voltage rectifier may be separately molded and they are connected to 
each other by connectors or cables. Thus, various combinations of the 
molds can be selectively made. 
Unlike the conventional device in which a large-sized high voltage 
transformer and high voltage rectifier are disposed in one container, use 
of the above molded unit makes it unnecessary to fill insulating oil into 
an unnecessary space, so that a small and lightweight X-ray generator 
apparatus can be realized which can be easily assembled by combining the 
units and in which replacement can be effected for each molded unit to 
attain easy maintenance. Further, since the dielectric breakdown voltage 
of solid insulating material is higher than that of insulating oil, a high 
insulation efficiency can be attained and the size and weight can be 
easily reduced. The small and lightweight X-ray generator apparatus 
requires only a small installation space in a hospital or the like and can 
be easily transported. 
Next, a second embodiment is explained. FIG. 13A is a block diagram of the 
second embodiment. Portions which are the same as those of the first 
embodiment are denoted by the same reference numerals and the detail 
description thereof is omitted. In the first embodiment, only one 
frequency converter 12 is provided, but in the second embodiment, a 
frequency converter is also divided into "n" number of frequency 
converters like a transformer. Frequency converters 12.sub.1, 12.sub.2, . 
. . 12.sub.n which are connected in parallel with one another are 
connected to the A.C. power source 11. The switching transistors included 
in the frequency converters 12.sub.1, 12.sub.2, . . . 12.sub.n are 
controlled by a same phase pulse from a pulse generator 36. Therefore, the 
frequency converters 12.sub.1, 12.sub.2 . . . 12.sub.n are operated in the 
same phase. 
Outputs of the frequency converters 12.sub.1, 12.sub.2, . . . 12.sub.n are 
supplied to the rectifiers 14.sub.1, 14.sub.2, . . . 14.sub.n via the high 
voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n. Capacitors 
C.sub.R are respectively connected in series with the secondary windings 
of the high voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n to 
constitute series resonant circuits on the secondary portion of the 
transformers. 
Also, in this embodiment, the same effect as that of the first embodiment 
can be obtained. Further, in a case where some of the frequency converters 
12.sub.1, 12.sub.2, . . . 12.sub.n are set into the rest or nonoperative 
state, outputs of those of the high voltage transformers 13.sub.1, 
13.sub.2, . . . 13.sub.n which are connected to the remaining frequency 
converters bypass the high voltage transformers which are connected to the 
frequency converters set in the rest state and are applied to the X-ray 
tube 15. Therefore, the tube voltage can be roughly controlled by 
controlling the number of frequency converters which are set in the rest 
state. Moreover, if the frequency converters are PWM-controlled by 
changing the pulse width of the switching pulse output from the pulse 
generator 36, the tube voltage can be precisely controlled. 
Further, according to the second embodiment, since a number of frequency 
converters having a small capacity are used, even if a part of the 
frequency converters becomes defective, the defective frequency converters 
are set into the rest state and other frequency converters which are 
otherwise set in the rest or nonoperative state can be used instead of the 
defective frequency converters. Therefore, it becomes possible to prevent 
the whole X-ray generator apparatus from being set into the inoperative 
state. The maximum output is lowered by an amount corresponding to the 
number of defective frequency converters, but it is seldom to use the 
maximum output and the device can be used without receiving practical 
interference while the defective frequency converter is being replaced. 
The resonance capacitor C.sub.R is connected to the secondary winding of 
each of the high voltage transformers 13.sub.1, 13.sub.2, . . . 13.sub.n 
to cause an LC series resonance so as to prevent the voltage applied to 
the X-ray tube 15 from being lowered and to further increase the frequency 
of the frequency converters. It is possible to connect another resonance 
capacitor to the primary winding of the transformer in addition to the 
secondary winding of the transformer. Due to this additional resonance 
capacitor, a multi-resonance circuit is provided. The resonance capacitor 
can be connected in series with (FIG. 13N or in parallel to (FIG. 13C) the 
winding. 
FIG. 13B shows a modification of the second embodiment. Though in the above 
described second embodiment, a plurality of frequency converters 12.sub.1, 
12.sub.2, . . . 12.sub.n are provided, only the inverters 73.sub.1, 
73.sub.2, . . . 73.sub.n are provided in this modification. The A.C. 
voltage from the A.C. power source 11 is supplied to the rectifier 71 and 
is converted into a D.C. voltage. The D.C. voltage from the rectifier 71 
is supplied to the capacitor circuit 72 and is filtered. The output of the 
capacitor circuit 72 is supplied to the inverters 73.sub.1, 73.sub.2, . . 
. 73.sub.n. The inverters 73.sub.1, 73.sub.2, . . . 73.sub.n are connected 
in parallel to the capacitor circuit 72. The outputs from the inverters 
73.sub.1, 73.sub.2, . . . 73.sub.n are respectively supplied to primary 
windings of the high voltage transformers 19.sub.1, 19.sub.2, . . . 
19.sub.n. 
Next, the characteristic of the second embodiment is explained. An 
equivalent circuit diagram of the secondary portion of one of the high 
voltage transformers 13 is shown in FIG. 14. Since the frequency converter 
12 effects a switching operation based on a switching pulse having a 
rectangular waveform, the secondary voltage E2 takes a rectangular 
waveform in the first embodiment as shown in FIG. 7A, but takes 
substantially a sine waveform in the second embodiment in which the 
secondary portion is operated in the resonant condition. If the frequency 
of the sine wave is f and .omega.=2.pi.f, and if the capacitance of the 
capacitor C.sub.R is so determined as to set up the condition of 
.omega.L2=1/.omega.C.sub.R at the frequency f according to the general 
theory of series resonance, then the impedance of the secondary portion 
becomes only Rx. Therefore, even if the frequency f is set at a high 
frequency, influence of the secondary inductance L2 to the tube voltage Ex 
can be neglected. However, voltages across L2 and C.sub.R in FIG. 14 have 
inverted phases and cancel each other but E.sub.L =E2 . .omega.L2/Rx and 
Ec=E2/(.omega.C.sub.R . Rx) are obtained, and in general, they become 
relatively larger than E2. Therefore, in the conventional device shown in 
FIG. 7A, resonance cannot be attained on the secondary portion when the 
withstanding voltage of the transformer and capacitor and the insulating 
measure are considered. 
However, in the second embodiment, since the high voltage transformer is 
divided into "n" number of small transformers and capacitors C.sub.R are 
respectively connected to the secondary windings of the transformers, E2 
and L2 in the respective resonant circuits can be reduced to E2/n and 
L2/n.sup.2 as shown in FIG. 7B as in the first embodiment. In particular, 
L2 decreases inversely with the square of the dividing number "n", it 
becomes extremely small. In this way, since the voltages E.sub.L and 
E.sub.C across L2 and C.sub.R can be suppressed to small values, the 
advantage of the resonance on the secondary portion of the transformer can 
be effectively used. 
As described above, in a case where only the high voltage transformer is 
divided into small transformers as in the first embodiment, the secondary 
inductance L2 becomes smaller, making it possible to attain a high 
frequency operation. However, in a case where the resonance circuit is 
formed on the secondary portion of the transformer as in the second 
embodiment, influence by the secondary inductance L2 can be almost 
neglected, making it possible to attain a higher frequency operation. 
Alternatively, in a case where the device is operated at the same 
frequency as that obtained where no resonance circuit is formed on the 
secondary portion, the dividing number can be reduced within the 
permissible range of the withstanding voltage of the transformer and the 
capacitor. Since the primary voltage becomes a sine wave due to the 
resonance circuit in the secondary portion, it is possible to turn on or 
turn off switching transistors in the frequency converters at the time of 
the current does not flow therethrough. Therefore, the heat radiation of 
the apparatus can be suppressed, thereby increasing the efficiency of the 
apparatus. The secondary resonance is not limited to the series resonance 
described above but may be a parallel resonance attained by connecting a 
capacitor in parallel with the secondary winding of the high voltage 
transformer. 
FIG. 15 shows the characteristic of the voltage applied to the X-ray tube 
15 obtained when the secondary portion is set in the resonant mode. In 
FIG. 15, solid lines indicate Ex, and curves A and B among them 
respectively indicate the case of the conventional device and the case 
wherein the transformer is divided into "n" small transformers like the 
curves A and B of FIG. 8, and a curve D indicates a characteristic 
obtained when the high voltage transformer is divided into "n" small 
transformers as in the second embodiment and the secondary portion thereof 
is set in the resonant state. 
According to the second embodiment, the raising characteristic of the 
curves A and B which is suppressed by the secondary inductance of the 
transformer is improved by means of the resonance as indicated by the 
curve D. Therefore, a higher frequency operation can be attained, and the 
voltage applied to the X-ray tube can be further increased. In FIG. 15, fr 
indicates the resonant frequency. Further, broken line curves indicate the 
voltages obtained by multiplying the terminal voltages E.sub.L and E.sub.C 
of the secondary inductance L2 and the capacitor C.sub.R with the dividing 
number "n". 
As described above, the operation frequency can be further enhanced and the 
dividing number can be reduced by means of the secondary resonance in 
comparison with a case wherein only the high voltage transformer is 
divided into small transformers. 
Further, the modifications explained with reference to the first embodiment 
can also be applied in the second embodiment, and like the first 
embodiment, the transformers and rectifiers can be selectively molded into 
respective units with solid insulation material. It is not necessary to 
respectively connect the transformers to the frequency converters. It is 
possible to connect several transformers to a single frequency converter. 
A third embodiment will now be described with reference to FIG. 16 showing 
the construction thereof in a block diagram. An A.C. power from the A.C. 
power source 11 is converted into a D.C. power by a rectifier 22 and a 
capacitor 24. The D.C. power is applied to plural generator units 
26.sub.1, 26.sub.2, . . . 26.sub.n. Each of the generator units 26.sub.1, 
26.sub.2, . . . 26.sub.n outputs a high voltage which corresponds to the 
output of each of the high voltage rectifiers 14.sub.1, 14.sub.2, . . . 
14.sub.n in the above embodiments. All the outputs from the generator 
units 26.sub.1, 26.sub.2, . . . 26.sub.n are added together in series and 
the result of addition is applied to the X-ray tube 15. 
Each of the generator units 26.sub.1, 26.sub.2, . . . 26.sub.n includes a 
frequency converter card 40 and a molded unit 42. The frequency converter 
card 40 includes a frequency converter for converting the input D.C. 
voltage into an A.C. voltage of a high frequency. The molded unit 42 
includes a high voltage transformer, at least a secondary portion thereof, 
and a rectifier circuit. In order to control a switching operation of the 
frequency converter, a pulse generator 36 generating a pulse with a fixed 
pulse width and a pulse width modulation circuit 34 are provided. The 
output of the pulse width generator 36 is supplied to most of the 
generator units 26, e.g., generator units 26.sub.1 to 26.sub.n-1 via AND 
gates 38.sub.1 to 38.sub.n-1 and to the pulse width modulation circuit 34. 
The output of the pulse width modulation circuit 34 is supplied to the 
other generator units 26, i.e., to the generator unit 26.sub.n. The on/off 
of the AND gates 38.sub.1 to 38.sub.n-1 are controlled by control signals 
from an output voltage controller 32, based on a set value of the output 
voltage. 
The applied voltage of the X-ray tube 15 is detected by a high voltage 
divider 28 which is connected in parallel to the X-ray tube 15. The 
detected voltage is supplied to one input terminal of a differential 
amplifier 30. The other input terminal of the differential amplifier 30 is 
supplied with the set value of the output voltage from the output voltage 
controller 32. The difference between both the inputs is supplied to the 
pulse width modulation circuit 34. The pulse width modulation circuit 34 
varies the pulse width of the switching pulse output from the pulse 
generator 36 in accordance with the difference from amplifier 30. Thus, 
the generator unit 26.sub.n is pulse-width-modulated by a voltage feedback 
method. The switching frequency of the other generator units 26.sub.1 to 
26.sub.n-1 are set to a fixed frequency. Therefore, in order to vary the 
output voltage applied to the X-ray tube 15, some of the AND gates are 
turned on by the output voltage controller 32, the number of which 
corresponds to the output voltage, and the number of the operating 
generator units is controlled. Thus, the output voltage is roughly 
controlled. Then, the output voltage is finely controlled by a feedback 
loop including the pulse width modulation circuit 34. 
Description will be given to the details of the generator unit 26. FIG. 17 
is a detailed block diagram of the generator unit 26 and FIG. 18 shows an 
outer appearance thereof. As shown in FIG. 17, each generator unit 26 
includes the frequency converter card 40 which is formed of two switching 
transistors 46 and 48, and a driver circuit 44 thereof and converts the 
D.C. input voltage from the rectifier circuit 22 into an A.C. voltage. The 
frequency converter card 40 is formed on a printed circuit board. An 
MOSFET is generally used as the switching transistors 46 and 48. However, 
it is possible to use another element, such as an IGBT, a bipolar 
transistor, and a thyristor as the switching transistors 46 and 48. 
The output from the frequency converter card 40 is supplied to primary 
windings of plural, in this case, two high voltage transformers 50.sub.1 
and 50.sub.2. The secondary windings of the high voltage transformers 
50.sub.1 and 50.sub.2 are respectively connected to rectifier circuits 
52.sub.1 and 52.sub.2. The rectifier circuits 52.sub.1 and 52.sub.2 are 
connected in series in order to produce a D.C. voltage corresponding to a 
sum of the outputs from the high voltage transformers 50.sub.1 and 
50.sub.2. 
In the primary circuit of the high voltage transformers 50.sub.1 and 
50.sub.2, a resonance capacitor C.sub.R1 is connected in parallel to the 
primary windings to form a parallel resonance circuit. In the secondary 
circuit of the high voltage transformers 50.sub.1 and 50.sub.2, resonance 
capacitors C.sub.R2 are connected in series to the secondary windings to 
form a series resonance circuit. The efficiency of the frequency converter 
card 40 can be more improved because of the multi-resonance circuit formed 
of the primary resonance circuit and the secondary resonance circuit. It 
is possible to form the multi-resonance circuit by using the serial 
resonance circuit in the primary circuit and the parallel resonance 
circuit in the secondary circuit or by using the same type of the 
resonance circuit both in the primary circuit and the secondary circuit. 
As shown in FIG. 18, the primary circuit of the high voltage transformers 
50.sub.1 and 50.sub.2 is outside the molded unit 42 and the secondary 
circuits of the high voltage transformers 50.sub.1 and 50.sub.2 and the 
rectifier circuits 52.sub.1 and 52.sub.2 are inside the molded unit 42. 
The molded unit 42 is formed of a container of a transparent polycarbonate 
and a molding material of a transparent silicone, as in the above 
embodiments. The other solid insulating material, such as epoxy may be 
used as the molding material. The molded unit 42 is provided with a 
radiator 54 formed of a high thermal conductivity such as ceramics to 
efficiently radiate the heat generated from the rectifier circuits 
52.sub.1 and 52.sub.2. The frequency converter card 40 is also provided 
with a radiator 56 for radiating the heat generated from the switching 
elements and the other. 
According to the third embodiment, the frequency converter for converting 
the input D.C. voltage into an A.C. output voltage is divided into plural 
converters of a small capacity, and high voltage transformers of a small 
capacity are connected to each of the plural converters to make a high 
voltage generator unit. Therefore, the frequency of the A.C. voltage can 
be extremely increased so that the X-ray generator apparatus becomes small 
and lightweight. Further, each of the high voltage generator units is 
molded with the solid insulating material. Therefore, the X-ray generator 
apparatus can be easily assembled by combining the units and easily 
maintained by replacing each inoperable molded unit with a new one. Since 
the resonance capacitor is connected to the high voltage transformer and 
the frequency converter is operated in a resonance frequency, the 
efficiency of the frequency converter can be highly increased. The 
decrease of the efficiency in the fine control of the output voltage can 
be minimized by controlling the frequencies of only one or more frequency 
converters in a feedback method. 
Modifications of the third embodiment are explained. Though the rectified 
voltages from the high voltage generators are serially added in the third 
embodiment shown in FIG. 17, the connection of the rectifying circuits 
cannot be limited to this example. It is possible to first add the 
secondary voltages (including terminal voltages of the resonance 
capacitors C.sub.R2) of all the high voltage transformers and then rectify 
the result of addition. It is also possible to first add the secondary 
voltages of some of the high voltage transformers, then rectify the result 
of addition, and finally add the result of rectifying, as shown in FIG. 
19. 
In FIG. 19, resonance capacitors C.sub.R are respectively connected to the 
high voltage transformers. However, the type of the resonance circuit is 
not limited to this circuit. It is possible to connect the secondary 
windings of the high voltage transformers in series to add the output 
voltages of the transformers and connect a single resonance capacitor to 
the series connection of the secondary windings to rectify the output 
voltage, as shown in FIG. 20. The rectifier circuit can be implemented by 
both of the full-wave rectifier as shown in FIG. 19 and of the multiplier 
type rectifier as shown in FIG. 20. 
The radiator 64 can be shaped as shown in FIG. 21. According to this shape, 
the efficiency of radiation is improved since the bottom of the radiator 
64 is embedded in the molded unit 42 to be contacted to a rectifier 
circuit portion 62 which is a source of heat. 
Moreover, the modifications explained with reference to the first and 
second embodiments can also be applied in the third embodiment. 
As described above, according to the X-ray generator apparatus of the 
present invention, the output frequency of the frequency converter can be 
increased by dividing the transformer for increasing an output A.C. 
voltage of the frequency converter which increases the frequency of an 
A.C. voltage into a plurality of transformers of small capacity in which 
the number of turns of the secondary winding is smaller than that of the 
original transformer, adding outputs of the transformers together, and 
applying the result of addition to the X-ray tube. As a result, the 
apparatus can be made small and lightweight, the control speed of the 
voltage can be enhanced if the frequency is increased, and the output 
voltage can be precisely controlled by a feedback method. Further, the 
assembling and maintenance can be simplified by molding the divided 
transformers and the rectifiers into respective units with solid 
insulating material (including gel insulating material). In addition, 
ripple components included in the output voltage can be easily suppressed 
and stabilized by the high frequency operation and the X-rays can be 
easily generated in a pulse form. When the frequency is increased, the 
frequency of the switching pulse of the frequency converter can be set to 
be higher than the audio frequency so that audio noise can be prevented 
from being generated. Further, if a plurality of transformers are 
respectively connected to a plurality of frequency converters, each 
frequency converter can be easily and independently controlled so that the 
precision of generation of the X-rays can be enhanced, and even if one or 
some frequency converters become defective, the apparatus can be 
continuously operated by using the remaining frequency converters. The 
frequency can be further increased by connecting the capacitor to the 
secondary winding of the transformer to form an LC resonance circuit and 
effect the resonance operation. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details, representative devices, and illustrated examples 
shown and described herein. Accordingly, various modifications may be made 
without departing from the spirit or scope of the general inventive 
concept as defined by the appended claims and their equivalents.