X-Ray tube bias supply

Apparatus for controlling the bias voltage on the control grid of an X-ray tube in connection with the operation of producing alternating X-ray beams having nominally low and high energies where higher X-ray tube current flows during the low energy beams than during the high energy beams, comprising:

• an X-ray tube including an anode, a control grid and an electron emissive filament comprising a cathode,
• means for causing a predetermined current flow through the filament to thereby set the temperature and emissivity of the filament,
• power supply means for applying alternatingly to said anode the lower of a selected dc kilovoltage and the higher of a selected dc kilovoltage to produce the nominally low and high energy beams, respectively,
• inverter means operative to output alternating current (ac) signals having a frequency depending on the frequency of switching signals that are input to said inverter means,
• voltage-to-frequency converter means responsive to input of a variable control voltage signal that is proportional to the desired bias voltage by supplying said switching signals to said inverter means at a frequency corresponding to the value of said control voltage signal,
• one transformer having a primary winding for being energized with said ac signals and having a secondary winding, said transformer having parasitic capacitance and inductance that results in resonance and maximum voltage output from said secondary winding at one ac signal frequency and results in lower voltage output at frequencies above or below resonant frequency,
• rectifier means having input terminals for the ac output of said secondary winding and having negative and positive bias voltage output terminals connected respectively to said control grid and filament, and
• means for controlling said power supply means to apply said low kilovoltage to said X-ray tube anode while said ac frequency is zero or away from the resonant frequency so said bias voltage is less negative and tube current is high and to apply said high voltage to said anode while said ac frequency is at or near resonant frequency so the bias voltage is more negative and said X-ray tube current is lower.