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Control of surge currents in inverter fed loads Venalainen, Jarmo

Abstract

Switch mode power converters have been used in high power applications such as DC links and variable frequency motor drives for a number of decades. In these applications the load is fixed and well known, and therefore the nature of the power demand placed on the inverter is also well known. In more recent years, high frequency switch mode inverters have become available at smaller power levels as consumer products. These inverters are marketed as capable of providing AC power to ANY AC load which the customer may have. In this case the inverter is called upon to supply power to a diverse variety of loads, presenting it with very challenging operational issues, especially with regards start up surge currents. This thesis investigates the characteristics of the input current drawn by consumer type AC loads, when driven by a standard sinusoidal 60 Hz 110VAC source, with particular emphasis on the inrush current transients found during start up. It then goes on to look at how the surge currents differ when the driving voltage is a non-sinusoidal single step waveform as produced by the vast majority of small inverters. This is followed by a discussion on the benefit of having multiple step waveforms and trapezoidal waveforms, in terms of their effect on the surge current transients. From the discussion on multi step and trapezoidal waveforms, it is shown that a current regulated output is the optimum solution for small modified sinewave inverters when they are used to drive typical consumer type loads. This approach is demonstrated in a 300W inverter through the implementation of hysteretic output current control. Finally, AC output bridge switching losses are investigated, and the benefit of a small amount of series output inductance is demonstrated through simulations and circuit trials.

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