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

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.