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Capacitor-Discharge Power Supplies

DFAD has designed and built several high energy capacitor-discharge power supply units for geotechnical applications..

“Ocean Bottom Profiling” is a sonar application where short, sharp acoustic pulses allow penetration and analysis of the geological structure many meters below the sea floor. The acoustic pulses are created by in-water spark systems, or by magnetic-repulsion transducers, driven by high energy electrical pulses.

Older capacitor-discharge power supply units (CD-PSUs) relied on simple high-voltage transformers to charge the capacitor banks, resulting in large repetitive inrush currents from the AC mains and significant transformer heating. These power supplies also used “triggered spark gaps” to discharge the capacitors into the transducer.

The Challenges:

  • To design CD-PSU charging systems with low repetitive inrush and high efficiency.
  • To develop a solid-state replacement for the noisy, unreliable spark-gap switching.
  • To improve safety and reliability of CD-PSU systems.

The Solution:

DFAD’s first CD-PSU has an energy capacity of 400 joules (46uF charged to 4,200 volts), with a repetition rate of four shots per second. This unit used high inductance iron-core transformers for capacitor charging, and series arrays of low-cost thyristors for discharge switching. Reflected electrical energy from the in-water transducer was clamped by series diode arrays.

This unit was half the weight, and one-third the size, of the spark-gap CD-PSU it replaced. It is much quieter, and has proved to be extremely reliable during three years of marine survey service.

DFAD’s next CD-PSU was more ambitious, and has an energy capacity of 2,000 joules (230uF @ 4200V) with a repetition rate of one shot per second.

This unit uses switch-mode power supplies which provide constant-power charging of the capacitor banks. Inrush from the mains has been virtually eliminated, allowing the size of on-deck petrol generators to be dramatically reduced. Electrical charging efficiency is high; better than 92%.

The proven reliability of the thyristor arrays used in the 400-joule CD-PSU was put to use in the 2,000-joule unit by designing individual 400-joule modules which could be interlinked. As a result, DFAD can now produce CD-PSU’s of any energy rating, by using an appropriate number of modules.

The 2,000-joule CD-PSU was also given a high-accuracy shot timebase generator, high-visibility bargraph meters and several additional safety features. The output connectors were upsized to accommodate current pulses which may reach 10,000 amps.

In addition to field trials, this unit has been successfully subjected to some “weird science” experiments, including hurling heavy aluminium masses several meters into the air by magnetic repulsion.

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