Dead-Time influence on fast switching pulsed power converters design - A high current application for accelerator's magnets

Abstract

Dead-time in IGBT-based DC-DC power converters is a well-known issue that causes a limitation of the output voltage and the distortion of the output voltage and current. These distortions are more critical when the converter switching frequency increases and in applications requiring high di/dt. This is especially true for power converters used to supply pulsed electro-magnets in particle accelerators where output waveforms with high slew rates are needed. Several solutions exist to compensate the distortions caused by dead-time; however, in fast and precise applications, the compensation method should be analysed in conjunction with design choices, such as the selection of switching frequency, output filter's inductances and topology. At CERN, hybrid capacitor discharge/switch mode power converters have been developed to supply electro-magnets used for PS beam injection. For standardisation and cost reduction. Because of this design option, dead-time represents 16 % of the switching period thus widely affecting the converter performances. This paper illustrates the effects of dead-time on various design aspects of these hybrid power converters able to produce precise 1ms long current pulses with maximal output current up to 2,5 kA. The converter control loops that have been designed using Energetic Macroscopic Representation approach can partly compensate for the dead-times effects. However, a dead-time compensation technique based on the inductor current sign measurement is nevertheless required to improve the converter performances. It is experimentally demonstrated that this classical compensation method is effective, even in interleaved switching module.