Design of low inductance, long life capacitor for linear transformer drivers

Abstract

In the linear transformer driver system, the capacitor needs to satisfy the requirement of long life time and low inherent inductance. The inherent inductance of the capacitor plays an important role in limiting the rise time and peak current of the LTD output pulse, therefore, the volume of the capacitor should be as small as possible to reduce the inductance. Since the paper’s permittivity is larger than the film’s and the breakdown strength of film is larger than that of the paper, we adopt oil immersed paper-film mixed dielectric as the capacitor dielectric structure to take advantage of the combined properties of the two different dielectric films. In this paper, based on the result of the simulation, the electrode structure was optimized and a capacitor was designed with a capacitance of 100 nF and an inherent inductance of less than 140 nH. According to results of the experiment, the capacitor inherent inductance satisfies the expected requirement. The typical failure can be classified into two main modes: (1) insulation erosion at the pressure-contact taps and (2) foil edge breakdown, both resulting in short circuit failure. Based on the typical failure modes found from the dissection of failed capacitors, an improved design is presented.