JPS60169116A - High voltage transformer for x-ray generator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Background of the Invention] The present invention relates generally to an X-ray generator, and more particularly to a high voltage, high frequency transformer applied to an inverter of an X-ray generator.

In the field of X-ray imaging, an X-ray tube is constructed to generate X-rays that are directed to pass through an X-ray object, and these penetrating X-rays are applied to an imaging device to image the object. image or density distribution. The resolution and clarity of the images obtained depends not only on the quality of the x-ray tube and imaging device, but also on the quality of the power supply.

The most common way to make an exposure for a given time is
The idea is to pulse the x-ray tube during that time. For this purpose, the output supplied from the power supply to the x-ray tube is typically a square wave output with very fast rise and fall times and very low ripple during the steady-state exposure time. desirable. It is also preferred that the square wave be repeatable with minimal distortion over a wide range of x-ray tube operating currents. This high reproducibility is achieved by the Gauge Digital
This is particularly important in dual energy applications such as those used in radiographic subtraction or computed tomography.

A typical power supply for an X-ray generator operates at a low frequency (i.e., 50 or 601-1z) and usually includes an adjustable autotransformer to vary the voltage applied to a step-up transformer. . A rectifier converts high voltage alternating current into low frequency, high amplitude, high ripple direct current. The ripple is then smoothed using a filter. The output waveform thus obtained has relatively slow rise and fall times, large ripple in steady state, and poor reproducibility and distortion when used over a wide range of x-ray tube currents. The high voltage transformers used in such equipment are typically mineral oil insulated and cooled and are relatively large and heavy (on the order of 350 Bond).

It has been found that it is preferable to increase the frequency of the transformer in order to improve the ripple of the waveform and its rise and fall times. However, increasing the frequency causes undesirable effects on both the transformer core and coil.

In a magnetic core, as the frequency increases, hysteresis loss rapidly increases as the frequency increases, and as a result, the humidity of the magnetic core rapidly increases. Therefore, it is necessary to change the material of the magnetic core, and it is also necessary to increase the cooling capacity of the transformer.

In transformer coils, leakage reactance, distributed capacitance current and skin effect increase proportionally as frequency increases. Each of these characteristics also tends to degrade the performance of the x-ray generator transformer. That is, a high leakage inductance represents a high level of electromagnetic energy that must be overcome, which tends to slow the rise and fall times of the output voltage waveform. This is particularly undesirable in systems that include static contactors with forced commutation circuits that inherently have high levels of leakage inductance.

The distributed capacitance of the high voltage secondary winding directly affects the reflected peak primary current under light load conditions, while the high input current and low output power during light loads such as in X-ray fluoroscopy. voltage), thereby forcing the inverter into current limit.

A third frequency-dependent feature is the skin effect.

This tends to increase the AC resistance of the windings, thereby causing high losses during full load conditions.

The three undesirable conditions mentioned above are exacerbated by increasing the operating frequency, but they can also be worsened by reducing the size of the transformer (i.e. by reducing the cross-sectional area of the magnetic core or by reducing the number of turns in the coil). It has been recognized that it is possible to reduce the Referring to the basic transformer equation, as the frequency increases, the cross-sectional area of the magnetic core can be reduced and the number of turns of the coil can be reduced.

However, it is recognized that this reduction in transformer size results in a reduction in efficiency as well as iron and copper losses. This will of course generate more heat,
When this is combined with the heat generated with the initial increase in frequency, significant cooling problems arise. For example, if current methods using mineral oil were applied to provide the required additional cooling, the size of the transformer tank would become prohibitive. Furthermore, in the field of X-ray generators, there has been no prior art that achieves the cooling function using anything other than a mineral oil tank.

In addition to the increased heat problem mentioned above, reducing the size of the transformer reduces the distance between the coil layers, which increases the distributed capacitance of the high voltage windings and reduces the output waveform, especially at very light loads. There is a tendency for the phenomenon that distorts to occur again.

For the reasons mentioned above, the use of medium or high frequency power in the transformer of an X-ray generating device has not hitherto been considered.

OBJECTS OF THE INVENTION Accordingly, it is an object of the present invention to provide an X-ray generator with improved performance.

Another object of the present invention is to provide an X-ray generator that provides a voltage output waveform with short rise time and fall time and low ripple during steady state.

Still another object of the present invention is to provide a high voltage transformer for an X-ray generator with low leakage intuftance, low distributed capacitance and low skin effect.

Yet another object of the present invention is to provide an x-ray generator that produces an improved output waveform while limiting the size of the transformer cooling tank.

Yet another object of the invention is to provide a high voltage transformer whose performance characteristics are well matched to those of a pulse width modulated square wave inverter of an X-ray generator.

These objects and other features and advantages will become more readily apparent from the following description taken in conjunction with the accompanying drawings.

[Summary of the Invention 1 According to one aspect of the present invention, a pulse width modulated inverter operates with a tightly coupled high voltage transformer and associated filters to provide very fast rise and fall times and low ripple. , generates low distortion and highly reproducible voltage output waveforms. The inverter constitutes a high-frequency variable voltage source, which is supplied to a high-frequency high-voltage step-up transformer with a smaller magnetic core and smaller coil dimensions than the transformer for conventional X-ray generators, and the performance characteristics of this transformer The cover matches very well the performance characteristics of the inverter. The smaller phase modulus reduces the leakage reactance and distributed capacitance of the transformer, thus improving the output waveform of the device. Improved cooling systems are provided to address the higher heat losses caused by using smaller transformers and operating at higher frequencies.

In accordance with another aspect of the invention, transformer coils are cooled by an evaporative cooling fJI device that has significantly improved heat transfer characteristics than conventional mineral oil based methods. The coolant used is liquid Freon 113, which achieves a strong cooling 7,11 effect due to its ability to cycle between liquid and gas states.

These high heat transfer properties allow the coil windings to operate at much higher current densities and magnetic flux densities than would normally be possible using conventional mineral oil cooling.

In addition to improving the output waveform due to reduced leakage reactance and distributed capacitance, the transformer itself is substantially smaller in size and weight. For example, the weight of the transformer can be reduced to about one-fifteenth for the same output power.

Preferred embodiments are shown in the drawings described below. However, without departing from the true scope and spirit of the invention,
Various other modifications are possible.

[Description of a Preferred Embodiment] FIG. 1 shows an X-ray generator. This X-ray generator receives three-phase AC power from the main power line and supplies high-voltage controlled DC power to the X-ray tube 11. The three-phase power is rectified by a bridge-type full-wave silicon rectifier 12. Its output is smoothed by a filter 13 consisting of a conventional capacitor and an inductor.The smoothed DC output is passed through a pulse width modulated transistor inverter 14.
The inverter 14 converts the DC output into high frequency AC output. This inverter and its drive system are described in detail in, for example, US Patent Application Nos. 564,538 and 564,612, filed December 22, 1983. For purposes of this specification, it is sufficient to understand that the output voltage of the inverter 14 is varied by adjusting the pulse width of each half cycle such that a high frequency square wave variable voltage output is generated.

The output of the inverter 14 is supplied to the primary winding of a high voltage, high frequency step-up transformer 16, which is connected to the inverter 14.
at relatively high frequencies (i.e. 5-15kHz)
2) and high voltage (i.e. 150kV)
(up to) generates an AC output. Transformer 16 is configured to have desired performance characteristics that are tightly coupled to the specific operating characteristics of inverter 14. For example, the square wave output from the inverter needs to have minimal distortion to maintain a desired level of x-ray reproducibility. This is achieved in the invention by, among other things, providing a high voltage transformer 16 with very low leakage reactance and low distributed capacitance. Tight coupling is further achieved by using evaporative cooling techniques to obtain improved performance characteristics from the transformer. These specific features of transformer 16 will be described in further detail below.

The output of transformer 16 is fed to a high voltage full wave silicon rectifier 17 whose DC output is fed to a smoothing capacitor filter and voltage feedback network, generally designated by reference numeral 18. These components and their interaction with the x-ray generating device are described in more detail in the above-mentioned US patent applications.

Referring now to FIG. 2, a high voltage transformer 16 is shown having a magnetic core 21, a primary coil 22, and a secondary coil 23. In a preferred embodiment of the invention, the magnetic core 21 is constructed of a nickel-iron alloy commonly known as Megaperm 4 PL, which has a temperature of 310°C.

When operating in accordance with the present invention, the magnetic flux density of magnetic core 21 is in the range of 0.6 to 1.2 Tesla.

The primary coil 22 is constructed of a single winding to minimize the thickness indicated by the dimension raJ to minimize the leakage inductance of the transformer.

The secondary coil 23 is a multilayer pyramidal winding forming a graded capacitance characteristic. Furthermore, to minimize leakage inductance, the secondary and primary coils are tightly coupled with minimal gap, as indicated by rcJ. The radius of gap C is indicated by the letters rPJ. The secondary winding thickness rbJ is minimized to minimize leakage inductance, and the interlayer gap rdJ is maximized to minimize distributed capacitance. The radius of the center of the secondary coil is designated by the letter "R".

In one embodiment of the invention in which transformer 16 is configured and adapted to operate with the X-ray generator described above, the features are as follows.

Primary winding thickness a: 4mm Secondary winding thickness b: 30mm Gap between primary and secondary windings c: 6.8mm Interlayer spacing d: 6.81111R For each secondary winding layer Length 1: 140mm to 62mm (
Pyramid shape〉 Coil radius 8156111111 Secondary coil length [: average 100IIun maximum 140m
m (pyramid shape) Number of coil turns N: 6 to 11 on the primary side. 2 coils per transformer on the secondary side) Area of the magnetic core: 33600 + n1112 Cross-sectional area of the magnetic core:
16001111 The transformer described above is designed to operate in the frequency range 5 to 15 kl-11, the magnetic flux density range 0.6 to 1.2 Tesla, and the current density range 4.65 to 20.4 Amps/ml12. ing.

When constructed as described above and operated with an inverter arrangement and with the load characteristics described above, the transformer 16
When operating at 5kl-1z, the leakage reactance is 2.
It is about 5%. This should be compared to the 30 to 40 percent leakage reactance in a typical 12-pulse conventional transformer operating at 60 Hz. When operated under these conditions, the distributed capacitance of the transformer described above was found to be 36 picofarads per coil.

As previously mentioned, increasing the operating frequency concomitantly increases the hysteresis losses within the magnetic core 21, which significantly increases the temperature of the magnetic core. FIG. 3 shows an evaporative cooling system which absorbs the temperature increase associated with the nucleate boiling process. Transformer 16 is attached to the bottom of tank 28 by conventional means. tank 28
is partially filled with a coolant 29 having relatively high heat transfer properties d3 and a relatively low boiling point. A suitable coolant was found to be commercially available Freon 113, which has a boiling point of 47° C. at 1 atmosphere and a heat transfer ratio of 0.15 W/mm 2 . The air within the tank 28 is preferably evacuated to enhance heat transfer properties and operates in the range of 11 atmospheres to +1 atmospheres between cold and warm conditions, respectively.

As can be seen in FIG. 3, the coolant 29 is always maintained at a sufficiently high level to completely cover the transformer 16. When the temperature of the transformer core 21 tends to rise, heat is transferred to the coolant 29. When the coolant 29 reaches its boiling point, it evaporates and rises to the top 30 of the tank 28. here,
The coolant is cooled, condensed, and returned to the bottom of the tank 28 for further cooling of the magnetic core 21. In this manner, the use of an evaporative cooling process cools the transformer core 21 in a more efficient and effective manner than conventional oil-filled transformer tanks. This increased heat transfer characteristic counteracts the increased hysteresis losses inherent in the transformer due to the reduction in magnitude (j). Reducing the size of the transformer allows the design of the transformer to reduce the distributed volume ω and leakage reactance,
The operating frequency can be increased. By combining these characteristics, better waveform control of the output of the device can be achieved.

Depending on the particular operating conditions and requirements of the device, it may be desirable to augment the evaporative cooling process with additional cooling means. One method is to provide a plurality of hollow fins 31 at the top of the upper portion 29 of the tank 28 and to direct air against the fins by means of a fan 32 to accelerate the condensation process.

By using the nucleate boiling process described above, it is much more efficient than the conventional air radiative conduction process (1 W/in.
0 times) can dissipate heat.

Therefore, the magnetic core has a higher flux density than recommended by material manufacturing considerations (i.e., 0.15 at conventional 6 kHz).
1.2 Tesla at 6kHz) and can operate at higher frequencies. This allows the cross-sectional area and volume of the magnetic core to be reduced, reducing losses that would otherwise occur.

Furthermore, due to the higher frequency and magnetic flux density within the magnetic core,
The number of coil turns can be reduced, thereby reducing leakage reactance, distributed capacitance and resistance. Moreover, the increased cooling capacity allows the coil to operate at higher current densities (i.e. 20A/mm2 versus conventional 2.6A/nun2), which also depends on the winding diameter, coil length. This makes it possible to reduce the height and radius. The cumulative effect of these improved properties is that for a 50 Hz core and coil assembly rated at 100 kW, a weight-to-power ratio of 30
: Reduce to 1.

Referring now to FIG. 4A, there is shown an output waveform 32 representative of the output waveform from a conventional x-ray generator. As can be seen, the rise time T1 and fall time T3 are relatively long compared to the "useful time" or "T on" time. As a result, the useful time is short compared to the total elapsed time, thereby increasing the radiation dose during the rise and fall times. More importantly, these relatively long rise and fall times tend to substantially reduce the ability to repeatedly reproduce the same or similar output in successive exposures.

In addition, the characteristics of the conventional output waveform include a steady state or "
The “useful time” part contains a relatively large ripple. On the negative side, a 12-pulse conventional transformer produces an output with ripple in the 10 percent range.

In contrast, the waveform output of the present invention, as shown in FIG. 4B, has relatively short rise times T1 and T3. The "T-on" time is substantially reduced over conventional devices, and the "useful time" is substantially the same. Furthermore, the ripple during the useful time T2 is substantially reduced compared to the ripple of conventional devices. In the present invention, waveforms with ripples on the order of 5 percent are consistently obtained.

Due to the improved output waveform characteristics obtainable with the transformer according to the invention, its repeatability has been substantially increased. In this context, the device according to the invention has a
．． It can be consistently performed with a coefficient of variation in the range of 0.03.

Although the invention has been described in detail with reference to preferred embodiments, it will be understood that various modifications may be made within the scope of the invention.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of an X-ray generator according to a preferred embodiment of the present invention, FIG. 2 is a schematic configuration diagram of the transformer portion of FIG. 1, and FIG. 4A and 4B are schematic structural diagrams illustrating a transformer and associated cooling tank in accordance with a preferred embodiment; FIGS. 4A and 4B are waveforms illustrating the output waveforms of a conventional device and the improved output waveforms of the present invention, respectively; FIG. 12... Rectifier, 13... Filter, 14... Inverter, 16... Transformer, 17... Rectifier, 18
... Smoothing capacitor filter and voltage feedback circuit network, 21 ... Magnetic core, 22 ... Primary coil, 23 ... 2
Next coil, 28...tank, 29...coolant, 31
...Hollow fin, 33...Fan. patent applicant

Claims (1)

[Claims] 1. A high-frequency X-ray generator used in an inverter type X-ray generator having a plurality of switching elements configured in a full bridge type for supplying high frequency AC output to the primary side of a transformer. An apparatus for cooling a voltage transformer, comprising: a tank capable of containing a liquid; means for mounting a transformer within the tank; a liquid refrigerant having a heat transfer ratio and a boiling point such that it evaporates in said tank under normal operating conditions; a cooling device comprising: condensing means for returning to the bottom of the tank; 2. The cooling device according to claim 1, wherein the coolant is composed of Freon 113. 3. The cooling device according to claim 1, wherein the condensing means extends from the tank and communicates with an upper part of the tank so that vapor of the coolant can enter therein. A cooling device consisting of multiple hollow fins. 4. A high-voltage transformer for use in an X-ray generator having an inverter with a plurality of switching elements configured in a full-bridge configuration to supply high-voltage direct current output to an X-ray tube, the related A magnetic core that interlinks magnetic flux to the coil, at least one primary coil and one secondary coil wound around the magnetic core so as to interlink with the magnetic flux, and a current flowing through the primary coil causes the secondary coil to be coil means configured to cause current flow at a higher voltage in the coil and to have a leakage inductance of less than 5 percent under normal operating conditions; A high voltage transformer having means for preventing; 5. The high voltage transformer according to claim 4, wherein the coil means comprises a coil in which the primary coil and the secondary coil are arranged concentrically. 6. A high voltage transformer as claimed in claim 4, wherein said coil means has a distributed capacitance of less than 50 mF per coil. 7. The magnetic flux high voltage transformer according to claim 4, wherein the magnetic core is made of a nickel-iron alloy. 8. A high voltage transformer according to claim 7, wherein the magnetic core operates at a magnetic flux density in the range of 0.6 to 1.2 Tesla. 9. The high voltage transformer according to claim 4, wherein the cooling means surrounds the magnetic core and the coil means, and the cooling means has a relatively low boiling point and a temperature below the Curie point. A high voltage transformer comprising a liquid coolant that can be evaporated by heat from the core and means for condensing the evaporated coolant. 10. The high voltage transformer according to claim 9, wherein the coolant is composed of Freon 113. 11. A relatively low voltage DC source; a square wave inverter that receives the output of the DC source and generates a high frequency output; and a square wave inverter that receives the output of the inverter and generates a high voltage output and operates in a medium frequency range. a high frequency transformer configured to have a leakage inductance of less than 5 percent when operated at 5Kl-1z; An X-ray generator comprising: a filter for smoothing the output from the rectifier for supply to a ray tube. 12. The X-ray generator according to claim 11, wherein the inverter comprises a pulse width modulated transistor.
X-ray generator that is an inverter. 13. The X-ray generator according to claim 11, wherein the high frequency transformer is configured to operate with a distributed capacitance of less than 50 picofarads per coil. 14. The X-ray generator according to claim 11, wherein the high-frequency transformer has primary and secondary coils arranged in a concentric relationship. 15. The X-ray generator according to claim 11, wherein a liquid having a relatively low boiling point surrounds the high-frequency transformer and boils to form steam due to heat transfer from the high-frequency transformer. An X-ray generating device comprising evaporative cooling means having a coolant and means for condensing the vapor of the coolant back to a liquid. 16. The X-ray generator according to claim 15, wherein the coolant is composed of Freon 113.
Line generator.