Hybrid Nearest-Level Switching Modulation for a Wide Output Bandwidth at
Low Switching Frequency and High Output Quality
Abstract
Multilevel converters have enabled various applications that are not
possible with conventional two-level converters. Many of these
applications, however, need a high output bandwidth, often approaching
the switching or tolerated loss limit of the transistors, and still high
quality, e.g., to actively stabilize and dampen a DC grid or
specifically excite certain molecules or neural circuits in medical
applications. Modulation in multilevel converters has two dimensions for
improving the output quality, namely temporal switching modulation and
amplitude quantization. A high bandwidth approaching the switching rate
challenges existing modulation methods: carrier-based switching
modulation is fine at low frequencies but experiences interaction
between the carrier and the signal at the upper end of the spectrum;
fundamental-frequency switching, such as nearest-level modulation (NLM),
perform well at high frequencies but cause intolerable distortion for
low frequency contents. We propose a hybrid modulation concept that can
combine any methods from these two classes. It passes the error of a
fundamental frequency method through a filtered switching modulator to
combine the high output quality of the latter with the high bandwidth of
the former. We optimize the filter to avoid under-modulation of the
signal with the carrier of the modulator and to achieve the minimum
overall distortion throughout a wide output bandwidth. We demonstrate
the performance experimentally with a cascaded-bridge converter and
compare it with the best prior arts. This technique ensures a usable
output bandwidth up to 100% of the switching rate and maintains a total
distortion level below 3%.Â