FPGA based pulse width modulation drive for underwater low frequency magnetic field generation

Abstract

Main focus is to design an electronic circuit which drives a coil for producing underwater magnetic fields at aimed distances. This circuit should handle AC, DC drive individually and both at the same time. After some trials of quite lossy analogue circuitry, the switching converter & inverter structure is determined to be the framework. Although that brings extra complexity of controlling switches digitally with a processor circuitry; it is quite flexible for modes of operations. H-bridge with MOSFETs as switches, is the drive circuitry and with proper software selection, the hardware serves a DC-DC converter or a DC-AC inverter. In addition, because switches do operate at "saturation", it is much more efficient than the previous design. Besides EMI handicap and some switching losses of the structure, controlling the switches is also somewhat cumbersome. The asynchronous double-edge natural sampling pulse width modulation is employed as the switching scheme of DC-AC inverter mode. With this technique, the output waveform has none of fundamental frequency harmonics which are of prime concern. However, digital application of such analog circuit compatible method will be problematic. There may occur "glitches" at PWM drive signals, because of the mismatch between sampling rates of fixed carrier and variable modulating signal frequencies. In addition, the finite switching duration also is another issue to be considered. For both inverter and converter mode of operation, some precautions must be taken to prevent DC supply from being shoot-through. For removing the previously mentioned "glitch", re-adjusting the sampling rate of the comparator to a suitable value seems practical. For preventing the shorting of the DC supply, an idle time along switching duration, “dead-time” is generated. For EMI and carrier harmonics in the output waveforms, no snubber or such switching-aid network is planned to be used because these high frequency components of the load current pass through the self-capacitance of the load coil. The magnetic field generating inductive part only has negligible ripple at the switching frequency. Even this situation is ignored; such high frequency alternating fields cannot penetrate through much distance underwater. They are attenuated sufficiently according to underwater characteristics of the magnetic fields propagations. Thus, magnetic field of the fundamental frequency at the target will not be distorted.