Modulation-Enhanced Nearest-Level Quantization for a Wide Output Bandwidth

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 rate limit of the transistors, with high quality, e.g., to actively stabilize and dampen a dc grid or specifically excite certain molecules or neural circuits in medical applications. 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, 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%.

I. Introduction

With overwhelming scalability and flexibility, cascaded bridge (CBC) and modular multilevel converters (MMC) have become increasingly popular in power delivery and conversion applications, such as HVdc [1], [2], [3], [4], [5], medium-voltage motor drives [6], [7], [8], [9], [10], [11], as well as applications outside the conventional energy field, including medical devices, pulse generators, and neuroscientific instrumentation [12], [13], [14], [15], [16]. Cascaded circuit structures, such as cascaded chopper cells, cascaded H bridges, or cascaded double-H bridges (CHB), allow splitting larger power levels, i.e., voltages and/or currents into smaller and more manageable units. Since they can further also spread out switching load across modules, they can circumvent the power–bandwidth product limitation of conventional switched power electronics. Thus, such cascaded circuits allow high-power and high-signal bandwidth at the same time.