DE102011005446A1 - Circuit arrangement with inverter for powering an X-ray tube and associated method - Google Patents

Abstract

The invention specifies a circuit arrangement and an associated method for generating an X-ray tube voltage (Uhv). The arrangement comprises a mains input circuit (1) for generating an intermediate circuit voltage (Udc), an inverter circuit (2) for converting the intermediate circuit voltage (Udc) into a high-frequency alternating voltage (Uac) and a high-voltage generator (6) which converts the high-frequency alternating voltage (Uac) into converts a high voltage (Uhv) for the X-ray tube (5), the high-voltage generator (6) having a transformer (3) and a high-voltage capacitor (7) connected in parallel to the secondary winding of the transformer (3), which together with the inductance of the secondary winding forms an oscillating circuit forms, which has its resonance frequency in the frequency range between 10 kHz and 500 kHz. The invention offers the advantage that the existing parasitic winding capacitance of the transformer is, if necessary, increased by an additional capacitor to such an extent that it can be used usefully as a component for increasing the voltage of a high-voltage source.

Description

The invention relates to a circuit arrangement for generating an X-ray tube voltage with an input circuit for generating a DC link voltage, with an inverter circuit for converting the DC link voltage into a high-frequency AC voltage and with a high-voltage generator, the high-frequency. AC voltage with a certain gear ratio converted into a high voltage for the X-ray tube. Moreover, the invention relates to an X-ray generator with such a circuit arrangement and to a corresponding method for generating an X-ray tube voltage.

Modern generators often have circuit arrangements of the type mentioned for generating an X-ray tube voltage. Since the mains frequency is first rectified and then converted back into a high-frequency AC voltage, which is finally transformed to the desired voltage, such generators are also referred to as high-frequency generators. Compared with conventional generators, in which the high voltage is first transformed with the present network frequency, then rectified and finally fed to the X-ray tube, such a circuit arrangement has the advantage that they are made almost independent by a fast control loop of changes in both the mains voltage and the tube current can and therefore the tube voltage is very well reproducible and can be kept constant. Compared to the well-known so-called DC voltage generators, in which a transformed with mains frequency and rectified high voltage is finely controlled by means of triodes, the high-frequency generators have the advantage of a relatively small volume and lower manufacturing costs. These advantages are the reason for the preferred use of such circuitry in today's X-ray generators.

The circuit arrangements must in this case be dimensioned such that they provide a constant power over a very large voltage range. The voltage range used in medical diagnostics for x-ray tubes usually ranges from 40 kV to 150 kV. In order to deliver constant power over this range, the current at the lowest voltage level of 40 kV must be nearly four times as high as at the upper limit of 150 kV.

wobei U_dc die gleichgerichtete Zwischenkreisspannung und U_t die an der Röntgenröhre anliegende, gleichgerichtete Hochspannung ist.The high voltage generator must be chosen in its transmission ratio so that the highest required tube voltage is achieved at a given by the available mains voltage DC link voltage. The transmission ratio of the high voltage generator is usually defined in such X-ray generators as the ratio of the output voltage to the input voltage of the high voltage generator. In general, the high voltage generator consists essentially of a transformer and a downstream rectifying and smoothing circuit. If it is assumed that the effective voltage close to the inverter circuit corresponds approximately to the intermediate circuit voltage and that the high voltage generator is designed such that a voltage doubler is achieved by a suitable rectifier circuit connected downstream of the transformer, the transmission ratio ü of the transformer of the high voltage generator results:

where U _{dc is} the rectified _{DC link} voltage and U _{t is} the rectified high voltage applied to the X-ray tube.

_{Consequently,} for a desired tube current I _t , the average current load I _{avgWR is} for the primary side of the high-voltage _generator and for the inverter circuit connected upstream I _avgWR = I _t · 2 · ü (2)

It can be seen that at a constant output power P _t = U _t · I _{t of} the circuit arrangement of the inverter is loaded with a high current, in particular when the X-ray tube voltage U _{t is set} relatively low. As a result, the inverter with its passive components and power semiconductors as well as the intermediate circuit and the high voltage generator must be designed for these currents. In addition to higher costs for the components, this leads to the fact that in addition to the active power and much reactive power in the inverter and transformer is processed.

It is therefore an object of the present invention to provide an alternative to the cited prior art, which avoids these disadvantages.

According to the invention, the stated object is achieved with the circuit arrangement and the method of the independent claims.

The invention claims a circuit arrangement for generating an X-ray tube voltage with a mains input circuit for generating a DC link voltage, with an inverter circuit for converting the DC link voltage into a high-frequency AC voltage and with a high voltage generator, which converts the high-frequency AC voltage into a high voltage for the X-ray tube, wherein the high voltage generator is a transformer and has a parallel to the secondary winding of the transformer high voltage capacitor, which forms a resonant circuit together with the inductance of the secondary winding, which has its resonant frequency in the frequency range between 10 kHz and 50 kHz. The invention has the advantage that the existing parasitic winding capacitance of the transformer is optionally increased by an additional capacitor so far that it can be used as a useful element for increasing the voltage of a high voltage source. This allows the inverter to be designed for smaller currents. Also, the transmission power between the inverter and transformer are relieved.

In one development of the invention, the capacitance of the high-voltage capacitor can be selected such that at a predeterminable operating point of the x-ray tube at the resonant frequency of the alternating voltage, the voltage at the x-ray tube at least doubles.

The invention also claims a circuit arrangement for generating an X-ray tube voltage with a mains input circuit for generating a DC link voltage, an inverter circuit for converting the DC link voltage into a high-frequency AC voltage and a high voltage generator, which converts the high-frequency AC voltage into a high voltage for the X-ray tube, wherein the high voltage generator comprises a transformer and a resonant circuit capacitor connected in parallel with the primary winding of the transformer and a resonant circuit coil connected in series with the primary winding and the resonant circuit capacitor, which forms a resonant circuit having its resonant frequency in the frequency range between 10 kHz and 500 kHz.

In a further embodiment, the capacitance of the resonant circuit capacitor and the inductance of the resonant circuit coil can be selected such that at a predeterminable operating point of the x-ray tube at the resonant frequency of the AC voltage, the voltage at the x-ray tube at least doubles.

The invention also claims an X-ray generator with a circuit arrangement according to the invention.

The invention also claims an X-ray device with an X-ray generator according to the invention.

The invention also claims a method for generating an X-ray tube voltage in which generates a DC link voltage from a mains input voltage and from the DC link voltage, a high-frequency AC voltage is converted with a high voltage generator with a transformer in a high voltage for the X-ray tube, one to the secondary winding of the Transformer parallel-connected high voltage capacitor is brought together with the inductance of the secondary winding in the frequency range between 10 kHz and 500 kHz in resonance.

The invention further claims a method for generating an X-ray tube voltage in which generates a DC link voltage from a mains input voltage and from the DC link voltage, a high-frequency AC voltage is converted with a high voltage generator with a transformer in a high voltage for the X-ray tube, wherein a Primary winding of the transformer connected in parallel resonant circuit capacitor together with a primary coil and the resonant circuit capacitor in series resonant circuit coil in the frequency range between 10 kHz and 500 kHz are brought into resonance.

In a further embodiment of the method, at a predeterminable operating point of the x-ray tube at the resonance frequency, the voltage at the x-ray tube can be at least doubled.

Other features and advantages of the invention will become apparent from the following explanations of several embodiments with reference to schematic drawings.

Show it:

1 FIG. 1 is a block diagram of a high voltage generating circuit according to the prior art;

2 FIG. 2 is a block diagram of a high voltage generating circuit including a secondary high voltage capacitor.

3 FIG. 2 is a block diagram of a circuit arrangement for generating a high voltage with a primary-side resonant circuit capacitor and a primary-side resonant circuit coil;

4 FIG. 2: a diagram of the tube voltage as a function of the inverter frequency for different tube currents, FIG.

5 a block diagram of a high voltage generator and

6 : a block diagram of another high voltage generator.

1 shows in a block diagram the typical components and their interconnection of a high-frequency X-ray generator for generating the high voltage for an X-ray tube according to the prior art, such as in DE 10227841 A1 specified.

For this purpose, a network input circuit belongs on the input side 1 , which generates a rectified intermediate circuit voltage U _dc from the two- or three-phase input voltage. This rectified _{DC link voltage} U _dc is then applied to the input of an inverter circuit 2 given, which converts the intermediate circuit voltage U _dc into a high-frequency AC voltage. In the in 1 illustrated embodiment, a rectangular inverter is shown. Instead of such a rectangular inverter, resonant circuit inverters are often used.

The at the output of the inverter circuit 2 present high-frequency AC voltage is then to a high voltage generator 6 relayed, essentially from a transformer 3 and one to the transformer 3 secondary rectifier and rectifier 4 consists. The rectifying and smoothing device 4 is often designed as a doubler circuit. The at the output of the high voltage generator 6 adjacent, rectified high voltage U _hv is then applied to the x-ray tube 5 created.

A control is usually such that the at the X-ray tube 5 applied value of the high voltage U _hv as an actual value to a controller (not shown) is supplied there compared with a predetermined by the operator of the X-ray voltage setpoint and correspondingly a correction value is generated, which is given for example as a control variable to the inverter circuit, so that there according to the frequency and thus ultimately the high voltage is changed in the desired manner.

The transformer must be selected in its transmission ratio ü so that the highest required tube voltage U _{hv is} achieved at a given by the network intermediate _{circuit voltage} U _dc . Unless the rectifying and smoothing device 4 is formed as a doubler circuit, the necessary gear ratio ü by the above-mentioned equation (1) is given.

In the following calculation example it is assumed that the power supply of the X-ray tube 5 over a voltage range between 40 and 160 kV must provide a constant power of 40 kW. At 40 kV then the maximum current I _tmax , which must be available at the X-ray tube 1 A.

If it is further assumed that U _dc = 500 V are available as a rectified intermediate _{circuit voltage} , equation (1) results for the required transformation ratio u of the transformer 3 :

Only with this transmission ratio ü can be achieved at the given input voltage, the maximum X-ray tube voltage of 160 kV.

According to equation (2), at the transmission ratio u = 160 and at a maximum current I _tmax = 1 A at the X-ray tube for the maximum current load I _avgWR on the primary side of the transformer and the inverter: I _avgWRmax = I _tmax - 2 · ü = 1 A · 2 · 160 = 320 A (4) Equation (4) shows that at the maximum tube current of the inverter circuit 2 a very high current must be taken. In order to reduce the maximum Stormbedarf, therefore, the transmission ratio ü must be reduced. This is achieved by the arrangement according to the invention as in 2 shown.

2 shows a block diagram as in 1 described. In addition, in the high voltage generator 6 to the secondary winding of the transformer 3 a high voltage capacitor 7 parallel and in series with the primary winding a resonant circuit capacitor 8th connected. The high voltage capacitor 7 and the resonant circuit capacitor 8th form with the stray inductance of the transformer 3 a series resonant circuit. The capacity of the high voltage capacitor 7 is chosen so that the resonant frequency of the oscillating tube for tube currents between 0.5 and 1 A in the range 10 kHz to 500 kHz. Alternatively, the parasitic capacitance of the transformer 3 be enlarged by structural measures.

When the resonant circuit is operated at or near its resonant frequency, it behaves like a constant power source. By appropriate dimensioning of the oscillating circuit, the tube voltage U _hv higher than the secondary voltage of the transformer 3 be. This can now be used to achieve a higher tube voltage U _hv at lower tube _currents . This allows a smaller transmission ratio ü of the transformer 3 be selected, reducing the current in the inverter circuit 2 is significantly reduced. How out 4 can be read, a doubling of the voltage can be achieved, whereby the Übersetzungsugsverhältnis u can be halved. This means according to equation (4) halving the maximum current load I _{avgWR of} the inverter 2 , The required capacity of the high voltage capacitor is about 100 pF. It is an always existing stray capacitance of the transformer 3 connected in parallel.

3 shows a block diagram of an alternative embodiment 2 , Instead of parallel to the secondary winding of the transformer 3 switched high-voltage capacitor 7 in 2 will be on the primary side of the transformer 3 a resonant circuit is formed, which has its resonant frequency for tube currents between 0.5 A and 1 A in the range 10 kHz to 500 kHz. For this purpose, a resonant circuit capacitor 8th parallel to the primary winding of the transformer 3 connected. In addition, in series with the primary winding and the resonant circuit capacitor 8th a resonant circuit coil 9 inserted. This embodiment achieves the same inventive effect as the embodiment of 2 ,

4 shows in the form of a diagram through the resonant circuit according to the invention in the circuits according to the 2 and 3 caused voltage overshoot as a function of the AC frequency f (in kHz) of the inverter for three different operating points (tube currents). The size of the tube current is determined in x-ray tubes by the heating power supplied to the cathode. In y-direction is in 4 the ratio s of the tube voltage applied to the secondary voltage of the transformer.

The load characteristic A shows the voltage ratio s for a tube current of 0.5 A. The resonant frequency of the resonant circuit damped by the tube resistance is about 35 kHz. The voltage overshoot at the resonant frequency is more than twice. Via a variation of the drive frequency of the inverter, the tube voltage along the load characteristic A can be set to the desired value. It is also possible to control the inverter with the resonance frequency and to realize the voltage regulation via a pulse width modulation or phase shift.

The load curve B in the 4 shows the course of the voltage ratio s for a tube current of 0.75 A and the load curve C the course for 1 A tube current.

5 shows a further embodiment of a high voltage generator 6 similar to the circuit arrangement in 2 , where only no primary-side resonant circuit capacitance is present.

6 shows a further embodiment of a high voltage generator 6 similar to the circuit arrangement in 3 , wherein the series connection of the primary-side coil and capacitor by a parallel circuit of a resonant circuit capacitor 8th and a resonant circuit coil 9 have been replaced.

The invention is preferably used in X-ray generators and X-ray devices.

LIST OF REFERENCE NUMBERS

1

Power input circuit

2

Inverter circuit

3

transformer

4

Rectification and smoothing device

5

X-ray tube

6

High voltage generator

7

High voltage capacitor

8th

Resonant circuit capacitor

9

Resonant circuit coil

A

Load line

B

Load line

C

Load line

f

AC frequency

s

tension

U _ac

High-frequency AC voltage

U _dc

Intermediate circuit voltage

U _hv

tube voltage

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10227841 A1 [0026]

Claims (9)

Circuit arrangement for generating an x-ray tube voltage (U _hv ) with - a network input circuit ( 1 ) for generating an intermediate _{circuit voltage} (U _dc ), - an inverter circuit ( 2 ) for converting the intermediate circuit voltage (U _dc ) into a high-frequency alternating voltage (U _ac ), - and a high-voltage generator ( 6 ), which converts the high-frequency _ac voltage (U _ac ) into a high voltage (U _hv ) for the x-ray tube ( 5 ), characterized in that the high voltage generator ( 6 ) a transformer ( 3 ) and one to the secondary winding of the transformer ( 3 ) parallel-connected high-voltage capacitor ( 7 ), which together with the inductance of the secondary winding forms a resonant circuit which has its resonance frequency in the frequency range between 10 kHz and 500 kHz. Circuit arrangement according to Claim 1, characterized in that the capacitance of the high-voltage capacitor ( 7 ) is such that at a predeterminable operating point of the x-ray tube ( 5 ) at the resonant frequency of the AC voltage, the voltage (U _hv ) at the X-ray tube ( 5 ) At least doubled. Circuit arrangement for generating an x-ray tube voltage (U _hv ) with - a network input circuit ( 1 ) for generating an intermediate _{circuit voltage} (U _dc ), - an inverter circuit ( 2 ) for converting the intermediate circuit voltage (U _dc ) into a high-frequency alternating voltage (U _ac ), - and a high-voltage generator ( 6 ), which converts the high-frequency _ac voltage (U _ac ) into a high voltage (U _hv ) for the x-ray tube ( 5 ), characterized in that the high voltage generator ( 6 ) a transformer ( 3 ) and one to the primary winding of the transformer ( 3 ) resonant circuit capacitor connected in parallel ( 8th ) and one to the primary winding and the resonant circuit capacitor ( 8th ) in series resonant circuit coil ( 9 ), which forms a resonant circuit, which has its resonance frequency in the frequency range between 10 kHz and 500 kHz. Circuit arrangement according to Claim 3, characterized in that the capacitance of the resonant circuit capacitor ( 8th ) and the inductance of the oscillating circuit coil ( 9 ) are such that at a predeterminable operating point of the x-ray tube ( 5 ) at the resonant frequency of the AC voltage, the voltage (U _hv ) at the X-ray tube ( 5 ) At least doubled. X-ray generator with a circuit arrangement according to one of claims 1 to 4. X-ray device with an X-ray generator according to claim 5. Method for generating an x-ray tube voltage (U _hv ) in which a DC link voltage (U _dc ) is generated from a mains input voltage and a high-frequency AC voltage (U _ac ) is generated from the intermediate circuit voltage (U _dc ) which is connected to a high-voltage generator ( 6 ) with a transformer ( 3 ) into a high voltage (U _hv ) for the X-ray tube ( 5 ), characterized in that one to the secondary winding of the transformer ( 3 ) parallel-connected high-voltage capacitor ( 7 ) is resonated together with the inductance of the secondary winding in the frequency range between 10 kHz and 500 kHz. Method for generating an x-ray tube voltage (U _hv ) in which a DC link voltage (U _dc ) is generated from a mains input voltage and a high-frequency AC voltage (U _ac ) is generated from the intermediate circuit voltage (U _dc ) which is connected to a high-voltage generator ( 6 ) with a transformer ( 3 ) into a high voltage (U _hv ) for the X-ray tube ( 5 ), characterized in that one to the primary winding of the transformer ( 3 ) parallel resonant circuit capacitor ( 8th ) together with one to the primary winding and the resonant circuit capacitor ( 8th ) in series resonant circuit coil ( 9 ) are resonated in the frequency range between 10 kHz and 500 kHz. A method according to claim 7 or 8, characterized in that at a predeterminable operating point of the x-ray tube ( 5 ) at the resonant frequency the voltage (U _hv ) at the X-ray tube ( 5 ) is at least doubled.

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