1. Field of the Invention
The present invention relates to a switching amplifier operating a switching amplifier, of the type having a cascade of cascade elements of the same type numbering three or more, whereby each cascade element being a single-phase, fully controlled power inverter bridge with switch elements, and to a method for operating such a switching amplifier. The invention is suitable for use in all types of switching amplifiers. In particular, for medical devices, particularly MR tomography devices. In the latter case, the switching amplifier is used as a gradient amplifier for purposes of supplying gradient coils with current.
2. Description of the Prior Art
In the operation of MR tomography devices, among other things, rapidly switched magnetic fields with a constant gradient are generated by gradient coils. Each gradient coil has a current therein which attains values of up to 300 A or more in an exactly specified curve profile, for example. The current curve often has steep edges. To achieve the necessary high current rates of change, it is necessary to apply voltages of up to 2 kV or more at the gradient coils.
Typical gradient coil amplifiers which are in use today have switch regulators whose output-stage transistors are operated directly with the required voltages and switching frequencies. However, with current technologies, this is possible only for voltages up to approximately 600 V and switching frequencies up to approximately 25 kHz and requires a high outlay and spatial demand even within these limits. Power semiconductor switches that can switch more than 800 V with the required steepness, given the arising current strengths, are currently unavailable.
U.S. Pat. No. 5,546,299 teaches connecting at least two controllable voltage sources in series at the output side of a power supply for predominantly inductive loads for generating pulse-like high output currents with a steep controlled pulse rise. Each voltage source has at least one voltage-setting unit, such as a single-phase inverter bridge, and a voltage-divided direct voltage source. The switched mode of the voltage-setting units is controlled by control units, for instance by a pulsewidth modulation. A voltage source for basic load operation for delivering a basic load output voltage, and at least one voltage source for peak-load operation for delivering peakload output voltages, which exceed the basic load output voltage, are connected in series. In another embodiment, two voltage sources for the peak-load operation are driven with chronologically staggered switching signals, so that the effective switching frequency for the total output voltage is increased and the ripple of the load current at the output side is smaller than given non-staggered driving. Moreover, the control dynamic is correspondingly larger.
German OS 40 17 207 teaches a mechanism for supplying a consumer dipole with direct current that is predominantly free of harmonics but which can still be varied rapidly. To this end, n direct-voltage step-down setters of the same type are cascaded. The direct-voltage step-down setters are driven according to the principle of pulsewidth modulation, thereby shifting the uniformly high cycle frequency from step-down setter to step-down setter by 1/n times the period duration, respectively.
PCT Application WO 95/10881 teaches a switching amplifier wherein a number of output stages in the form of controllable voltage sources are likewise connected in series. The switch cycles of the output stages here are likewise staggered relative to one another such that the effective switching frequency of the output voltage is increased.
These three cited references respectively describe a staggered driving of the individual cascade elements of a switching amplifier so that the effective switching frequency at the cascade output is higher than the switching frequency of individual cascade elements, or switch elements. This means an increased control dynamic and, given a constant load inductance, a lower ripple of the load current, or of the cascade output voltage. It is also possible to shift undesirable harmonic portions of the load current to higher frequencies by switched mode operation thereby facilitating their filtering effect.
Particularly in MR tomography devices, it is important that the gradient currents follow the prescribed curves (i.e., set amplitude values at set times) exactly. Deviations from the preset curves, such as due to undesirable harmonic portions that are not filtered out, lead to a significant degradation of the MR image quality.
Besides the harmonics described above, circulating currents, what are known as common-mode charging currents, are caused by charge reversals of conductor-to-ground and coupling capacities occur during switched mode operation of a switching amplifier, also are superimposed on the charging current.