Based upon the Pulse Step Modulator technology, during the last decade a number of different high voltage modulators and power supplies have been built and are now in service world-wide. They have proven to be highly reliable systems which are well suited for special applications. Custom made versions are used for NBI systems as well as for Gyrotrons for ECRH. The maximum voltage being realised up to now is 85 kV with a system for 120 kV in planning. Due to higher demands on beam energy, several institutes are working on negative ion neutral beam injection. The required voltage for the accelerating grid power supply has to be raised to 300 kV and more. To build a power supply at this voltage with the required voltage regulation and load protection is an engineering challenge. This paper describes a study about a 280 kV / 40 A power supply in PSM technology.

Due to the increase of voltage by more than a factor of 3 compared with existing PSM systems, the voltage insulation in the system is the most difficult task. For achieving a proper insulation at an affordable price innovative solutions are demanded.

Another key issue is the energy delivered into a short circuit. The system must be designed to keep the stored energy to a minimum and under short circuit conditions, the energy dissipated in the load must remain below the specified limits. Due to the high output voltage, low stray capacitances are asked for as well as means to discharge this energy somewhere else than in the load. It is still an important target not to use a crowbar for this protection.

One potential solution for these requirements is presented in the paper together with calculations and simulations about the system behavior under normal and short circuit conditions. It will be shown that such a system can be realised and that it has advantages in terms of cost, size, efficiency and reliability compared with conventional solutions.