AUTHORS: Jiang Jian-Feng, Zhu Bin-Ruo, Yang Xi-Jun, Zhao Wei-Na, Tang Hou-Jun

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ABSTRACT: As for high air power plasma cutting power supply, the employed chopper type DC-DC converter needs to adopt two kinds of magnetic components, including mains frequency step-down phase-shifting transformer (Hereinafter referred to as power transformer ) and smoothing reactors. Because the power level is larger and the design needs to be optimized, the resultant total losses of magnetic components will reach several kilowatts or more, which seriously affects the overall conversion efficiency. The countermeasure is to select efficient magnetic materials and optimize design and manufacturing technique. Meanwhile, it is needed to modify the topology of chopper type DC-DC converter and design appropriate switching frequency multiplication modulation algorithm. In this paper, at first, a double Buck DC-DC converter in parallel with respective dual power switches is employed, in which by using the trapezoidal carrier phase shift driving, the output current ripple frequency multiplied by four times at any duty ratio can be obtained, thereby as can reduce the size and heating of smoothing reactors, leading to high precision cutting current and improved workpiece cutting quality. Secondly, the core losses of different magnetic materials are compared and analyzed, so are the structure of smoothing reactor and step-down phase-shifting transformer, winding arrangement and conductor type. On the above basis, the optimization design procedure of the power transformer and smoothing reactor is put forward, the optimal design is picked up from lots of candidates, which brings about less size and loss as well as high conversion efficiency of the plasma cutting power supply. Finally, a smoothing reactor with 1mH inductance and 20kHz switching frequency and a power transformer with thin oriented silicon steel sheet and flat aluminum wire and fabricated and tested within the production-ready plasma cutting machine, which features maximum output current of 270A, the output voltage of 150V and the maximum input power of 45kW. The measured results are satisfactory, including the quality of cutting current and the quality of the cut workpiece approaching the minimum level of laser cutting. it is proved that the proposed switching frequency multiplication modulation algorithm and magnetic design optimization method utilized in chopper type plasma cutting power supply are effective and feasible.

KEYWORDS: Plasma cutting power supply, Buck DC-DC converter, Trapezoidal carrier phase-shifting driving, switching frequency multiplication modulation, Step-down transformer, Smoothing reactor

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