Resonant switched mode power supply system

Electric power conversion systems – Current conversion – With starting arrangement

Reexamination Certificate

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Details

C363S017000, C363S098000

Reexamination Certificate

active

06683800

ABSTRACT:

BACKGROUND
The invention relates to a power supply system and is particularly related to an X-ray unit having a power supply system and a method for determining operating parameters of a power supply system.
Switched-mode power supplies are frequently used for the power or voltage supply of electrical consumers. Such switched-mode power supplies usually convert the mains voltage into such voltages as are necessary for the operation of the respective consumers. Known switched-mode power supplies comprise an inverter or converter, which generates a switched a-c voltage from a d-c voltage. For this purpose such a converter has controllable switches. This switched a-c voltage (converter output voltage) is converted by a transformer into an a-c voltage having a suitable amplitude for the respective consumer, that is to say, it is stepped up or stepped down. If the consumer requires a d-c voltage supply, the secondary-side transformer-a-c voltage is rectified and generally stabilized by means of a smoothing capacitor.
In order to optimize the operation, switched-mode power supplies are operated as resonant arrangements. In these, a resonant capacitance together with the leakage inductance of the transformer (which can be generated by a discrete inductance) forms a series resonant circuit, which where necessary is extended by the secondary-side winding capacitance of the transformer to form a series-parallel resonant circuit. In operation, the working frequency of the converter is selected so that this is close to the intrinsic resonant frequency of the load circuit. As a result the voltage drop at the impedance of the resonant circuit is minimal.
One example of embodiment of a switched-mode power supply is the voltage supply of an X-ray tube. X-ray tubes are supplied with voltages ranging from approx. 40 kV to 150 kV and currents of up to 1.3 A. A controller adjusts the tube voltage to the required set value. As a control variable, the controller uses the activation of the converter, i.e. the switching frequency and where applicable the duty cycle. The resonant circuits provided in this application are generally of a very high quality. As a result the transient response is largely dependent on the frequency. Tolerances of the components used, such as different capacitance values of the resonant capacitor, or different values for the leakage inductance of the transformer, due also to aging, affect the resonant frequency.
For control purposes, therefore, the ability to proceed from known values in the case of determinant variables of the controlled system—such as the capacitance of the resonant capacitance and the leakage inductance of the transformer—is of some interest. This can be ensured by the use of high-precision components, which is, however, correspondingly expensive. In addition, there is the problem of deviation in component values due to aging.
A power supply system, which is formed from a switched-mode power supply with an additional control device has already been described in the article “Fast estimation of unknown resonant frequencies by means of the VeCon chip set”, EPE'97 Trondheim, 1997, Vol. 3, pp. 353-357. The control device has a current sensor for measuring the current on the primary side of the transformer and a control output for controlling the converter. The control device comprises a measuring device for determining the resonant frequency of the controlled resonant circuit. In the operation of the power supply system, in which the converter is operated at a switching frequency f, the path of the primary-side current is monitored by means of sample values. The resonant frequency is estimated from the sample values and the converter is adjusted according to the estimate, so that the power supply system is operated precisely at the resonant frequency.
With the control method indicated, stable operation is possible in the event of sudden fluctuations. For this purpose, however, monitoring of the primary-side current is necessary in routine operation, which gives rise to difficulties in measurement. Moreover, the “on-line” method used is expensive, so that only very simple adjustments of the activation—in this case the excitation frequency—are possible.
SUMMARY OF THE INVENTION
An object of the invention, therefore, is to propose a power supply system and a method for determining operating parameters thereof, and in particular an X-ray unit having such a power supply system, by means of which the necessary operating parameters can be determined with particular ease.
This object is achieved by a power supply system in accordance with one embodiment of the present invention includes a converter for the generation of a switched converter output voltage (U
w
) and a transformer. The primary side of the transformer is fed by the converter and has a load output connected to its secondary side. The power supply having at least one resonant capacitance (C
r
), which forms a resonant arrangement with a leakage inductance (L
r
) of the transformer and/or an external inductance. A measuring device is included for determining operating parameters of the power supply system. The measuring device having a current sensor for measuring the current (i
r
) on the primary side of the transformer. The power supply includes means for controlling the converter wherein the measuring device is designed so that it activates the converter so that with initially unenergized components of the power supply system, a predetermined converter output voltage (U
w
) is generated. One or more parameters (f
0
, Z
0
, C
r
, L
r
) characteristic of the resonant arrangement are determined from the measurement of the current (i
r
) on the primary side of the transformer.
In accordance with a method applying principles of the present invention for determining operating parameters of a power supply system having a converter for generating a converter output voltage (U
w
), a transformer supplied thereby, a leakage inductance (L
r
) and/or an external inductance in combination with a resonant capacitance (C
r
) form a resonant arrangement, the system having a load output which is connected to the secondary side of the transformer wherein the method includes the steps of ensuring that the components of the power supply system (
10
) are unenergized, activating the converter (
12
) so that a predetermined converter output voltage (U
w
) is generated at least for a brief excitation interval, measuring the current (i
r
) appearing on the primary side of the transformer at least during the excitation interval and determining one or more parameters characteristic of the resonant arrangement from the measurement of the current (i
r
).
The power supply system has a converter and a transformer supplied by the latter. The converter in turn has a voltage supply, usually a d-c voltage supply (intermediate circuit voltage) from which, by clocked switching, it generates an a-c voltage of a frequency predetermined by the activation. A converter may have two such switches, for example, converter topologies with four power switches also being usual. A further variant of a converter topology is specified, for example, in EP 884 830. In the context of the invention the precise design of the converter does not matter.
The converter output voltage feeds a transformer, which may likewise be of any construction and may have different numbers of turns, transformation ratios, numbers, tappings etc—depending on the design of the system. The transformer generally has a certain winding capacitance, which can play a part in the behavior of the controlled system. On the secondary side, the output voltage of the transformer is often rectified and smoothed by means of a smoothing capacitor. A resonant capacitance, which is usually connected in series on the primary side, forms a resonant arrangement together with the leakage inductance of the transformer.
Also forming part of the power supply system is a measuring device, which has a current sensor and means for activating the converter. The measuring device controls the measurin

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