IGBT and it’s Use in X-Ray Machines

The role of X-ray machines is indispensable in the field of medical diagnostics. Electromagnetic radiations with a wavelength of 0.1 to 100 angstroms are called.X-rays. Hard X-rays with wavelengths of 0.1 to 1 angstroms have penetration abilities that make them competent for producing images of the inner parts of human bodies. Today X-rays are usually generated by using the X-ray tube developed by William Coolidge, although X-rays were first discovered by Wilhelm Roentgen in 1895. In the Coolidge tube, a high voltage is used to accelerate electrons emitted from a filament to gain high energy. The electron beam is guided to a target, usually tungsten, to excite the X-rays. The X-ray machine power supply must operate at very high voltages (20-150 kV) while supplying sufficient current to generate the X-ray beam needed to produce images.

The input AC voltage is corrected and fed to a high-frequency PWM resonant inverter that is based upon IGBTs. This allows using a high-frequency transformer to create the very high voltages needed for the X-ray tube operation while keeping the size and weight small. The output of the high-frequency transformer is rectified to generate the desired DC voltage for the X-ray tube. The dynamic response of the X-ray power supply must be fast and its DC output voltage must reach steady-state in a short time to prevent noise and defects in the X-ray image. These requirements can be met by using an IGBT based PWM resonant inverter.

A 48kW resonated converter involves four power modules, per module contains two paralleled IGBTs and antiparallel diodes. They are arranged in a half-bridge or push-pull configuration depending on the input, at 400Vac or 200Vac. At highest power, peak load current is 550A at 50 kHz, or 275A per module for 48kW out. The generator is zero-voltage switched to create a continuous series resonant output current that's transformer-isolated —stepped up and rectified to the desired output level. The output voltage is regulated by a DSP-based frequency-modulated controller, with dual loop feedback on resonant current and load kV.

For an extent of output power, the system operates from 48 kHz up to 68 kHz with a resonant LC shunt across the load transformer. With fundamental series resonance at 48 kHz, the shunt resonates at 68 kHz. At low frequencies, the generator operates close to resonance, with high power throughput. As frequency rises, the impedance increases —the load being shorted by the resonant shunt. The power is zero at 68 kHz.

Least size is significant for ultra-modern X-ray equipments. In this variant of the converter, the four ZVS modules with their compactly packed IGBT and FRED chips need just ¼ of the surface area previously used. Consolidation and separation of the drivers, and ZVS logic circuitry farther curtail the footprint. Mechanical integrity & noise immunity are improved because control signals have less distance to travel.

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