WO2015062950A1

WO2015062950A1 - Prediction of zero-crossing for a load current of a resonant converter

Info

Publication number: WO2015062950A1
Application number: PCT/EP2014/072706
Authority: WO
Inventors: Peter Lürkens
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2013-10-30
Filing date: 2014-10-23
Publication date: 2015-05-07
Anticipated expiration: 2016-04-30

Abstract

The invention relates to a system and a method for prediction of zero-crossing for a load current of a resonant converter. To achieve an in-time prediction of the zero crossing without increase of the phase shifter property, the expected change of the slope by switching of the inverter is considered in advance. This is achieved by combining or superimposing a signal with the phase shifter output or adjusting the reference input of a comparator by a signal which equals the integral of the difference of the slopes before and after the switching event over the period from the switching moment until the actual zero crossing of the current.

Description

Prediction of zero-crossing for a load current of a resonant converter

FIELD OF THE INVENTION

The invention relates to a method and a system for prediction of zero-crossing for a load current of a resonant converter a predefined time before the actual zero crossing occurs. The invention relates further to a high voltage generator and an X-ray tube.

BACKGROUND OF THE INVENTION

For the control of the switching frequency of a resonant converter, a phase- shift-means is used, which detects the moment of the zero crossing of the load current a short time in advance. By this it is achieved, that the switching of the inverter output voltage happens at moment short, in terms of the switching period, before the actual zero crossing and hard switching is avoided. By means of such a detection-in-advance mechanism, signal propagation delays and reaction time of components of the resonant converter can be accounted for.

For example, WO 2006/114719 Al describes a resonant DC/DC converter for supplying an output power comprises a switching device for supplying a switched voltage to a resonant circuit having a transformer. The switched voltage of the described converter is derived from an intermediate circuit voltage having substantially a fixed pulse width and frequency so that the zero crossings of the resonant current generated in the resonant current are defined. The switching configuration of a described inverter circuit is selected by a control device to either increase, decrease or maintain at a substantially constant level the resonant current according to the polarity of the switched voltage, so as to control the output power as required. However, improved switching could reduce the energy consumption and losses in the switching means of the converter.

US2012/200273A1 describes a switch controller and a converter including the same.

HO-SUNG KIM ET AL: "Protection circuit to prevent a transition from ZVS mode to ZCS mode under variable frequency operation", INTELEC 2009, IEEE, PISCATAWAY, NJ, USA, 18 October 2009, pages 1-4, XP031579393 describes a protection circuit for resonant power converters using soft switching topologies. SUMMARY OF THE INVENTION

There may be a need to improve the accuracy of the prediction of the zero- crossing for the load current in resonant converters. There may be also a need for a prediction of the zero-crossing for the load current at low load.

These needs are met by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following

description.

An aspect of the invention relates to a system for prediction of zero-crossing for a load current of a resonant converter, the system comprising:

a processing unit (110), configured to receive a direction and a step height of a switching event;

a differentiator unit (140) configured to determine a polarity of a gradient of the load current;

a phase shifter unit (130), configured to provide a zero-crossing prediction for a sinusoidal current waveform; and

a comparator unit (120), configured to evaluate an expected change of a slope of the load current at the moment of the switching event by superimposing a signal with a phase shifter output of the resonant converter based on the received direction and the step height and based on an effective inductance of the resonant converter; the signal

superimposed with the phase shifter output depending on the polarity of the gradient of the load current and further depending on a difference-integral between a first tangent of the load current before a switching event and a second tangent of the load current after the switching event.

A further aspect of the invention relates to a high voltage generator comprising the system for prediction of zero-crossing for a load current, an electrical resonant circuit, excitable by a converter on a low voltage side of the high voltage generator, and a

transformer, generating a high ac- voltage for feeding a high voltage rectifier.

A further aspect of the invention relates to an X-ray tube comprising an anode, a cathode, and a high voltage generator, wherein the high voltage generator is connected to either the anode or the cathode of the X-ray tube, or different output levels of the generator to each electrode.

To achieve an in-time prediction of the zero crossing without increase of the phase shifter property, the expected change of the slope by switching of the inverter in advance is considered. This is achieved by combining or superimposing a signal with the phase shifter output which equals the integral of the difference of the slopes before and after the switching event over the period from the switching moment until the actual zero crossing of the current:

The superimposing can be achieved by adding the signal to the output signal using an adder or by adjusting a threshold value of a comparator. A switching event is defined as a switching of the driver circuit characterized by the step height, time and further characteristics. Resonant techniques are used to allow the switching devices to turn-on and turn-off at zero current or zero voltage, hence the switching loss becomes low.

A further advantage of the invention may be seen that by evaluation of the sign of the current slope instable conditions may not occur, during which the zero-crossing detector would generate persistently inverter switching signals with the maximum possible frequency. Therefore, it is proposed to derive the sign of the correction from the current gradient. This changes in the middle of a switching cycle, so that the corrected phase shift signal becomes insensitive to the switching actions of the inverter itself.

According to an exemplary embodiment of the invention, a differentiator unit is further provided with the system, wherein the differentiator unit is designed to determine a polarity of a gradient of the load current. This allows significant operational benefits in terms of more precise prediction of the zero current.

According to an exemplary embodiment of the invention, an effective inductance of the resonant converter is considered and used to evaluate the expected change of the slope of the load current. This advantageously provides that the phase shifter produces a waveform which is subject to the upcoming switching of the inverter output.

According to an exemplary embodiment of the invention, the method further comprises the step of determining a polarity of a gradient of the load current. This allows adjusting the prediction of the zero-crossing of the load current to the present polarity of the gradient of the load current.

According to an exemplary embodiment of the invention, the signal superimposed with the phase shifter output depends on the polarity of the gradient of the load current. This allows an improved prediction of the zero-crossing.

According to an exemplary embodiment of the invention, the signal superimposed with the phase shifter output further depends on a difference-integral between a first tangent of the load current before a switching event and a second tangent of the load current after the switching event. Thereby the quality of the zero-crossing predictor is increased.

According to an exemplary embodiment of the invention, a time window is considered, during which further switching events are suppressed for a predetermined time period. This minimizes the influence of unwanted switching events with an excessive frequency.

According to an exemplary embodiment of the invention, an amplitude of an output voltage of the resonant converter corresponds to a bus voltage.

A further aspect of the invention relates to a system comprising a processing unit as a digital circuit and a differentiator unit and a comparator unit as analog circuits, both circuits building up the hybrid system.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and the attendant advantages thereof will be more clearly understood by reference to the following schematic drawings, which are not to scale, wherein:

Fig. 1 shows a circuit diagram of a resonant LCC converter for explaining the invention;

Fig. 2 shows a circuit diagram of a resonant LLC converter for explaining the invention;

Fig. 3 shows a flow diagram of a method for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention;

Fig. 4 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention;

Fig. 5 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention;

Fig. 6 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention;

Fig. 7 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention; Fig. 8 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention;

Fig. 9 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention;

Fig. 10 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention;

Fig. 11 shows a three dimensional parameter map for explaining the invention; and

Fig. 12 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention;

Fig. 13 shows a schematic diagram of a high voltage generator according to an exemplary embodiment of the invention; and

Fig. 14 shows a schematic diagram of an X-ray tube according to an exemplary embodiment of the invention.

Generally, identical parts, units, entities or steps are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a circuit diagram of a resonant LCC circuit 10 for explaining the invention. Figure 1 shows a resonant LCC circuit 10, e.g. used in a resonant LCC converter, comprising a first inductor LI and a first capacitor CI and a second capacitor C2. The load is connected in parallel to the second capacitor C2. The term "Load current" is understood so far as the inverter output current. For simplicity, the load of the converter is not depicted in detail. It can be a simple resistor, a gas discharge lamp, but also a rectifier with an output smoothing capacitor, or a more complex load circuit, e.g. a high voltage multiplier.

This configuration represents the resonant circuit in an LCC resonant converter commonly used in electronic lamp ballast for gas-discharge lamps. Resonant converters can achieve very low switching loss thus enable resonant topologies to operate at high switching frequency. The actual switching of the resonant converter may happen at a moment, when the load current is still high enough to allow for a complete commutation of the output voltage of the resonant converter by recharging of capacitors before the load current reverses and soft-switching becomes impossible. These capacitors are usually in parallel to the inverter switches or formed by their parasitic drain-source capacitance.

For the case of a pure sinusoidal current waveform, the phase-shifter means can be realized with an amplifier and a differentiator component, in which the output signal ps, depending on t and τ is formed according the following rule:

At sinusoidal current waveform, this phase shift signal produces a new sinusoidal signal which zero crossings are by a time τ in advance of the zero-crossings of the original current waveform.

Figure 2 shows a circuit diagram of a resonant circuit in an LLC converter for explaining the invention. A resonant circuit of an LLC converter is depicted in Figure 2, using two inductors and one capacitor, a resonant circuit for an LLC converter 11 , comprising a first inductor LI, a second inductor L2 and a first capacitor CI, with the load connected in parallel to the second inductor L2. This configuration represents resonant circuit in an LLC inverter. Also here the load is considered to be constituted from the items given in the description of Figure 1.

Figure 3 shows a flow diagram of a method for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention. The method comprises the following steps:

At first, providing SI a zero-crossing prediction for a sinusoidal current waveform by means of a phase shifter unit 130 is performed.

As a second procedural step of the method, considering S2 a direction and a step height of an inverter voltage V-inv at the intended moment of switching is performed. The inverter voltage V-inv is applied to the resonant circuit 10, 11.

As a third procedural step, evaluating S3 an expected change of a slope of the load current I-inv at the moment of the switching event by superimposing a signal with a phase shifter output of the resonant circuit 10, 11 based on the considered direction and the considered step height is performed, whereby the zero-crossing of the load current I-inv is predicted S4.

The method may further comprise the step of determining a polarity of a gradient of the load current, wherein evaluating the expected change is further based on the determined polarity of the gradient of the load current. It is to be understood that the sequence of the steps outlined above is merely exemplary. The described procedural steps of the method could be iteratively or recursively repeated.

Figure 4 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention. On the x-axis, a time period of approximately 1.7 x 10^"5 seconds is depicted. The first y-axis on the left shows the inverter voltage, the second y-axis on the right depicts the load current.

The inverter voltage V-inv follows a step function comprising three steps, i.e. three switching events occur during the depicted time period. Two output signals of the phase shifter are depicted comprising different τ-property, i.e. τ =0.47 and τ =1.3 μβ.

Figure 4 shows the prediction for a real circuit, in which current waveforms are not sinusoidal, but deviate the more, the lower the effective quality factor of the resonant system is. It is actually advantageous, to operate at a low quality factor, because it reduces the reactive power in the reactive components of the resonant converter circuit, which otherwise would lead to increased losses.

In Figure 4, a representative situation is depicted. As desired, the inverter switching should occur 0.47 in advance of the zero crossing of the load current. The first signal curve 23 of the phase-shifter, which is designed with the same τ-property, does not exhibit a zero crossing yet at this moment. In order to achieve a real advancement of 0.47 before the actual zero-crossing, the τ-property of the phase shifter may be increased to 1.3 μβ, i.e. 2.8-fold. As one can easily see by the significant deviation from the original signal waveform, the amplitude of higher- frequency signal component in the waveform is increased by the same factor and such the sensitivity to higher frequency components in the current waveform, as depicted by a second signal curve 24.

Figure 5 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage V-inv for explaining the invention. The same physical quantities are plotted on the two y-axes and the x-axis as in Figure 4. Figure 5 shows the aspects of the prediction in detail. For the prediction of the error of the zero crossing, the phase shifter produces a waveform which is subject to the switching of the resonant converter output voltage. In addition to the inverter load current I-inv, the diagram of Figure 5 shows also the tangents both immediately before, e.g. first tangent 33 and second tangent 34, and after, e.g. third tangent 35 and fourth tangent 36, the switching event of the inverter. It becomes visible from that, that the slope of the waveform has a sudden increase after the switching event leading to reaching the zero crossing earlier than predicted from the preceding part of the waveform and assuming a sinusoidal extrapolation. To compensate this effect, the phase shifter above was retuned to a higher τ-property to advance the zero-crossing prediction to the desired moment.

While analyzing the relationship between the switching states of the circuit and the change of the tangent lines to the current waveforms, one can recognize that the difference between the steepness of the slopes just before and after the switching event can be described as: i \ t=t_s+s ^~ i \ t=t_s-s ^{= ~~}j

i.e. the change of the slopes of the current waveforms is defined by the change of the voltage and the effective circuit (resonant) inductance.

Figure 6 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention. The same physical quantities are plotted on the two y-axes and the x-axis as in Figure 4.

The effective inductance includes also the leakage of the transformer, if present. The change of the voltage is practically solely determined by the change of the inverter output voltage. Therefore, a relevant back-electromagnetic force curve EMF of the system is determined by the voltage of the resonant series and parallel capacitors. However, these cannot change in the switching instance by principle.

Moreover, during in proximity to the current zero crossing, the voltage waveforms at these components are in the flat top (or bottom) part of a sinusoidal waveform, and change only very little. A fifth tangent 44 and sixth tangent 45 are set to the load current curve. A differential integral curve 46 denotes the value of the difference-integral between the fifth tangent 44 and the sixth tangent 45 before and after the switching event.

Figure 7 shows a multiple y-axes diagram comprising a current-time diagram of the load current and a voltage-time diagram of the inverter voltage for explaining the invention. The same physical quantities are plotted on the two y-axes and the x-axis as in Figure 4. In Figure 7, a phase shifter output signal 52 is shown which is calculated based on the formula: ps₂ (_t, T, s, s₊₁) = Si Ct) + sgn(i' t ) ^■ (s + s₊₁) - τ

r Where s denotes the current inverter switching state, s+ 1 denotes the subsequent switching event state, Lr is the physical inductance, and VDC denotes the bus voltage. The phase shifter output signal 52 advantageously predicts the zero-crossing of the load current. It is proposed to derive the sign of the correction from the current gradient. This changes in the middle of a switching cycle, so that the corrected phase shift signal becomes insensitive to the switching actions of the inverter itself. Other lock mechanisms can also be considered, e. g. a time window, in which further switching actions are suppressed for a certain time after a switching action. The amplitude of the inverter output voltage usually resembles the DC bus voltage. If this is sufficiently constant, a precise measurement may not be necessary and the correction may be applied using constant quantities, which are only depending on the switching states, as depicted in the formula stated above.

Figure 8 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention. The system 100 comprises a differentiator unit 140, a processing unit 1 10 and a comparator unit 120. The differentiator unit 140, consisting of a differentiation 143 and a polarity detector 142, may be designed to determine the sign of the slope of the inverter current.

The processing unit 1 10 might be designed to consider a direction and a step height of the switching event. The comparator unit 120 may be designed to evaluate the integral over the advance time of an expected change of a slope of the load current at the moment of the switching event by superimposing a signal with a phase shifter output of the resonant converter based on the sign of the slope of the inverter current, the considered direction, or the considered step height, or the effective inductance of the circuit, whereby the zero-crossing of the load current is predicted.

The system 100 further comprises a phase shifter 130. The differentiator unit 140 may comprise a differentiator circuit 142 and a polarity detector unit 143. A potential analogue electronics implementation of the system 100 comprises a zero-biased comparator 122 and four additional comparators 125 to which the original phase shifter signal are supplied. For the four additional comparators 125 zero is not used as the threshold level, but instead the DC bus voltage, multiplied with a factor (-2, - 1 , 1 , 2)XTXV_DC/L_I, representing the influence of the switching state (-2, - 1 , 1 , 2), the DC voltage V_DC, and the effective inductance L_r. The parameter τ indicates the desired time shift.

The system 100 may further comprise a frequency generator circuit 160. The differentiator unit 140 produces the sign of the current gradient i'. The selection of the appropriate comparator is realized in a digital way by means of the sign of the current gradient and the combination of the initial and subsequent switching state (s, s+1).

In Figure 8, an exemplary structure of the system 100 is depicted. The inverter current signal is differentiated and supplied to a polarity detector unit 143 of the differentiator unit 140. Additionally, the inverter current signal is supplied to the phase shifter 130. The output signal of the phase shifter 130 is connected to the inputs of a number of the comparators 125 of the comparator unit 120. One comparator, i.e. the zero-biased

comparator 122, compares the signal with a zero threshold. A signal is generated by a signal generator circuit 121 by scaling the DC bus voltage with the factor: T_p/(L_r+L_si_g). Thereby, the term L_r+L_si_g represents the effective inductance of the resonant circuit 10, 11, and Tp the desired advancement of the zero-crossing detection. This signal is supplied to a number of amplifiers 124 of the comparator unit 120 or gains, which have gain factors of (-2, -1, 1, 2). The outputs of the amplifiers 124 serve as threshold levels for the additional

comparators 125. The number of additional amplifiers 124 and comparators 122, 125 and the required gain factors result from the number and the amplitude of the different possible voltage transitions at the output terminal of the inverter.

E.g. a neutral point clamped, NPC, -inverter with three voltage levels at each branch would require eight additional comparators 125 in the comparator unit 120 and amplifiers with the gain factors of -2, -1.5, -1, -0.5, 0.5, 1.0, 1.5, 2.0. The polarity signal is submitted to a selection block circuit 113 together with the current switching state and the upcoming switching state. The switching states are received from a system control unit under consideration of various system states and reference quantities.

The system control unit is not subject to this invention. The selection block circuit 1 13 of the processing unit may receive input value concerning the current voltage amplitude 111 and the subsequent voltage amplitude 112, e. g. after the next switching event. The selection block circuit 113 for example controls a selector switch 114 of the processing unit 110. The selector switch 114 selects one of the comparator outputs to synchronize the frequency generator 160 which produces the reference for the inverter switching signals. Each time, when the frequency generator produces a pulse, the new switching state becomes active in the inverter, and supersedes the current switching state. By synchronizing the frequency generator with the advanced zero-crossing detection, it can be achieved, that a new switching state is activated sufficiently before the zero-crossing of the current, so that soft switching can be achieved. Figure 9 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention. Figure 9 shows a simplified solution of the system, e. g. with a superposition of a fixed correction quantity, which only depends on the DC bus voltage, however neglecting correct considerations of transitions starting with a voltage o zero. If only considering the extreme transitions of +1-2 the switching would happen somewhat too early in unfavorable cases, but never critically too late.

In Figure 9, such a simplified implementation is shown. Potential instabilities of the switching frequency are suppressed by locking the frequency generator during a minimum time window after each polarity change. The further reference signs as depicted in Figure 9 were already described in Figure 8 and are, therefore, not explained in detail.

Figure 10 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention. Figure 10 shows an even further simplified implementation achieving the compensation by direct superposition of the comparator with a fixed correction quantity which is derived from the actual current direction assuming a constant DC bus voltage.

A polarity detector determines the current direction and produces a polarity signal which is amplified with the factor and which serves as a threshold for the comparator. This represents a further simplified implementation of the system with an analogue signal processing and fixed compensation amplitude. The further reference signs as depicted in Figure 10 were already described in Figure 8 and are, therefore, not explained in detail.

Figure 1 1 shows a three dimensional parameter map for explaining the invention. The three axes are defined by the DC bus voltage (z-axis), the transition (x-axis), and the phase advance (y-axis). For different voltages, VDC 1 , VDC2, and VDC3, the measured phase shifts are plotted as dots the curve is calculated by the measured phase shifts and represents a fitted line.

Figure 12 shows a circuit diagram of a system for prediction of zero-crossing for a load current of a resonant converter according to an exemplary embodiment of the invention. Figure 12 shows an analog implementation option of the preceding circuit with consideration of the actual DC bus voltage. Derivation of the compensation signal from the actual output voltage is usually not recommended, because there will be no correction for output voltage levels of zero leading to a too late detection of the zero crossing of the current. The further reference signs as depicted in Figure 12 were already described in Figure 8 and are, therefore, not explained in detail. Figure 13 shows a schematic diagram of a high voltage generator according to an exemplary embodiment of the invention.

The high voltage generator 200 may comprise a system 100, an electrical inverter 210 on a low voltage side 220 with a resonant circuit 10,11, wherein the system 100 is used for the prediction of zero-crossing for the load current of a resonant circuit 10,11 implemented in the low voltage side 220. The high voltage generator 200 may further comprise a transformer 230 in a high voltage unit 240. Depending on the purpose, the high voltage unit may have a rectifier 250. The high voltage generator 200 may be constructed for an X-ray tube or for lighting applications or for further purpose.

Figure 14 shows a schematic diagram of an X-ray tube according to an exemplary embodiment of the invention.

The X-ray tube 300 may comprise an anode 310, a cathode 320, and a high voltage generator 200, wherein the high voltage generator 200 is connected to the anode 310 and the cathode 320 of the X-ray tube in order to generate a direct voltage on the X-ray tube 300.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SIGNS:

10 LCC resonant circuit

11 LLC resonant circuit

23 first signal curve

24 second signal curve

33 first tangent

34 second tangent

35 third tangent

36 fourth tangent

44 fifth tangent

45 sixth tangent

46 differential integral curve

52 phase shifter output signal

100 system

110 processing unit

111 current voltage amplitude

112 subsequent voltage amplitude

113 selection block circuit

114 selector switch

120 comparator unit

121 signal generation circuit

122 zero-biased comparator

124 amplifier

125 comparators

130 phase shifter unit

140 differentiator unit

142 differentiator

143 polarity detector unit

160 frequency generator circuit

200 high voltage generator

210 inverter

220 low voltage side

230 inductive coupler 240 high voltage side

250 rectifier

300 X-ray tube

310 anode

320 cathode

CI first capacitor

C2 second capacitor

EMF back-electromagnetic force curve

I-inv load current

LI first inductor

L2 second inductor

SI providing

S2 considering

S3 evaluating determining

S4 predicting

V-inv inverter voltage

X Axis

Y Axis

z Axis

Claims

CLAIMS:

1. A system (100) for prediction of zero-crossing for a load current of a resonant converter, the system (100) comprising:

- a differentiator unit (140) configured to determine a polarity of a gradient of the load current;

2. A high voltage generator (200), comprising the system (100) for prediction of zero-crossing for a load current according to claim 1, an electrical resonant circuit (10,11), excitable by an inverter (210) on a low voltage side (220) of the high voltage generator, and a transformer (230) on a high voltage side (240).

3. An X-ray tube (300) comprising an anode (310), a cathode (320), and a high voltage generator (200) according to the claim 2, wherein the high voltage generator is connected to the anode and/or to the cathode of the X-ray tube.

4. A method for prediction of zero-crossing for a load current (I-inv) of a resonant circuit (10, 11) in a resonant converter, the method characterised by the steps of: a) receiving and processing a direction and a step height of an inverter voltage (V-inv), which is applied to the resonant circuit (10, 11);

b) determining a polarity of a gradient of the load current by means of a differentiator (140);

c) providing a zero-crossing prediction for a sinusoidal current waveform by means of a phase shifter unit (130);

d) evaluating an expected change of a slope of the load current at the moment of the switching event by by means of a comparator (120) by superimposing a signal with a phase shifter output of the resonant converter based on the received direction and the step height and based on an effective inductance of the resonant converter; the signal

5. The method of claim 4,

wherein an effective inductance of the resonant converter is considered and used to evaluate the expected change of the slope of the load current.

6. The method of claim 4 or 5,

wherein the method further comprises the step of determining a polarity of a gradient of the load current.

7. The method of one of the preceding claims 4 to 6,

wherein the signal superimposed with the phase shifter output depends on the polarity of the gradient of the load current.

8. The method of one of the preceding claims 4 to 7,

wherein the signal superimposed with the phase shifter output further depends on a difference-integral between a first tangent of the load current before a switching event and a second tangent of the load current after the switching event.

9. The method of one of the preceding claims 4 to 8,

wherein a time window is considered, during which further switching events are suppressed for a predetermined time period.

10. The method of one of the preceding claims 4 to 9,

wherein an amplitude of an output voltage of the inverter (210) corresponds to a bus voltage.

11. A computer program for processing data of a resonant circuit (10, 11) in a resonant converter, which when executed on a processor is adapted for performing the steps of the method of one of the preceding claims 5 to 11.

12. A computer-readable medium, on which a computer program according to claim 11 is stored.