Hybrid filter for a multi-cell switch-mode power amplifier

Abstract

The paper presents a novel filtering concept for multi-cell switch-mode power amplifiers intended to be used for high quality power sources up to the kW-region. Multi-cell switch-mode amplifiers usually are operated in interleaved PWM resulting in effective output voltage harmonics being much higher than the cell´s switching frequency. This major advantage, however, is only given for ideally balanced DC-link voltages; actual systems typically will show low-frequency output voltage noise. For compensating this drawback, hence a closed loop hybrid output voltage filter concept is proposed formed by a combination of a switch-mode power and a linear-mode correction amplifier. The linear amplifier only has to deliver a few percent of the output power, compensating the switch-mode stages output voltage ripple, whereas the switch-mode system delivers nearly the whole output power. An interesting advantage of this system is that the linear amplifier can be designed concerning a comparatively low supply voltage as well as for low output current compared to the total system.