An isolated high-voltage DC-to-DC converter with fast turn-off capability for X-ray tube gridding

X-ray tube gridding requires that a high-voltage, in the range of a few kilovolts, be applied and removed very quickly while the lengths of the voltage on- and off-periods are measured in tens of milliseconds. The application requires high-voltage isolation, as the gridding circuit is referenced to the cathode potential, and requires minimal circuit power dissipation, due to cooling limitations in the environment in which this converter is operated. The main challenge, however, is how to turn off the converter and discharge the output capacitor within the required time of approximately 100 μs. The proposed converter provides these features and has a higher reliability, better performance, and lower cost than other known solutions. (The disclosed circuit has been protected by several patents and filed disclosures.) It employs the conventional forward topology, with peak-forward energy transfer. To achieve fast turn-off, a novel crowbar arrangement has been implemented that is controlled via the isolation transformer used for energy transfer. This paper describes the application requirements, the proposed circuit arrangement and its operation, and provides detailed circuit analysis, design guidelines, and experimental results.