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Physics, Techniques and Procedures

Medium-frequency generator

a state of the art generator design, also named high-frequency generator and inverter generator, which uses a high-frequency current to produce nearly constant potential voltage to the X ray tube with a transformer of much smaller size than found in ordinary X-ray generators. The basic principle involved in achieving constant potential high voltage and reduced size of the generator, is the fact that the voltage induced in the secondary coil of a transformer is proportional to the rate of change of the current in the primary coil:

V º fnA

where V is the output voltage, f is the frequency of the primary (and secondary) coil current, n is the number of windings in the secondary coil, and A is the cross-sectional area of the core of the secondary winding. To a certain extent, the same output voltage may be obtained with fewer windings and reduced cross-sectional area if the frequency of the current is increased.

The incoming power supply to a medium frequency generator may be an ordinary 60 Hz (110 V) single phase current (Fig.1). This current is rectified and smoothed and then fed to a chopper and inverter circuit which transforms the smooth, direct current (DC) into a high-frequency (5 - 100 kHz) alternating current (AC). (The chopper "chops" the continuous DC into high-frequency DC pulses and the inverter transforms this into AC.) A transformer converts this high-frequency low-voltage AC into high-voltage AC, which then is rectified by half wave rectification and smoothed to provide a nearly constant potential high voltage to the X-ray tube. The voltage is controlled by varying the frequency of the chopper/inverter circuit, which determines the frequency of the current delivered to the transformer. Fast exposure switching, in the order of 1 ms, is easily obtained with the medium frequency generator.

One of the great benefits of this generator design, is the reduced weight and size. The main components of the generator may be placed within the same enclosure as the X-ray tube, or in e.g. the C-arm of the equipment. This generator principle was previously used only in small mobile and/or battery-powered generators with low power rating, but today it is applied to all modern high-voltage generators up to the highest needed power ratings above 100 kW.

HJS

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Fig.1

Schematic, simplified illustration of the various steps in a medium frequency generator.
Medium-frequency generator, Fig.1