Optimal design of a modular 11kW serial parallel resonant converter for a solid state 115-kV long pulse modulator

Modern accelerator driven experiments like linear colliders or spallation sources are supplied by RF amplifiers using klystrons. The cathode voltage for these klystrons can be generated by long pulse modulators generating highly accurate voltage pulses in the length of milliseconds. Common designs like Bouncer Modulator topologies using pulse transformers become huge for long pulses. The series parallel resonant converter (SPRC) is a modular topology which avoids this drawback as the transformer is operated at high frequencies. In the considered application, the required nominal pulse voltage amplitude is 115kV with a pulse power of 2.88MW and a pulse length of 2.8ms. The pulse to pulse reproducibility of 0.02% and a voltage ripple at top of less than 0.05% are highly demanding. Additionally, the energy delivered to the load in case of an arcing klystron should not exceed 10J and the time of the converter should exceed 10⁹ pulses. In order to meet these highly demanding specifications, the modulator is based on interleaved SPRC modules. Because of the high number of degrees of freedom as geometric parameters of the transformer, number of parallel semiconductors, design of the resonant tank the optimization procedure presented in [1] has been developed for designing the modulator. A SPRC module contains a full bridge connected to a series parallel circuit followed by a transformer a rectifier and a filter capacitor. In this paper, an optimal design of a single module according to the design considerations in [1] which are based on an electrical model of the inverter, a magnetic, a thermal and an isolation model of the transformer is presented and the design trade-offs/alternatives are discussed in detail. For validation of the models and the optimization procedure, a prototype of a single module has been built and is tested under full load conditions. There the focus is on evaluating the thermal behaviour of the transformer and the isolation of the transf- rmer, which is especially crucial for a series connection of the modules. In order to meet the highly demanding requirements on the ripple and the reproducibility, a comprehensive small signal model based on [2] and control strategy for optimal interleaving of the series connected modules is presented in the paper. This enables to minimize the output voltage ripple and reducing the filtering effort at the same time.