Harmonics in DC to AC converters of single phase uninterruptible power supplies

Single phase uninterruptible power supplies (UPS) employ an inverter for DC to AC conversion. Advanced inverters consist of a full bridge transistor topology and an LC filter. An optimal inverter performance can be achieved if the output voltage is controlled by unipolar pulse width modulation (PWM). The paper describes the influence of the unipolar PWM on the voltages and the current within the inverter. Both frequency domain and time domain are used to evaluate the most important harmonics as well as peak and RMS values of the current and voltage ripple. This results in general equations suitable for the inverter design. In addition, two PWM schemes are compared with respect to EMI problems. Finally, measurements are presented which confirm the theoretical work.