High-efficiency MOSFET-based MMC for LVDC distribution systems

Abstract

Low-voltage DC (LVDC) systems offer a promising means for improving distribution system efficiency and reliability. The DC-AC conversion stage, however, is one of the main challenges for LVDC networks. A low-voltage modular multilevel converter (MMC) for LVDC distribution systems is proposed in this paper. Analysis is presented to show that its efficiency can exceed that of a conventional 2-level converter. The low voltage rating of each MMC submodule enables MOSFETs to be used in place of IGBTs to reduce power losses. The application of synchronous rectification (SR) further reduces conduction losses. It is shown that device switching frequency reduces as the number of MMC levels is increased. MMC power losses, for different numbers of levels, are compared with those of a conventional 2-level converter. Simulation and experimental results are presented to confirm the mathematical analysis.

Modular Multilevel Converters: Control and Applications

“…They can be achieved by MMCs made with silicon carbide metal-oxide semiconductor field-effect transistor (SiC MOSFET) and Si MOSFET [50]. The use of MOSFET instead of IGBT is able to reduce switching losses, while the use of synchronous rectification reduces conduction losses [51]. In [52], an MMC has been used as an interface with the power grid of a Smart Home that includes photovoltaic generation, energy storage, and recharging of electric vehicle, all connected to a DC bus.…”

LV Converters: Improving Efficiency and EMI Using Si MOSFET MMC and Experimentally Exploring Slowed Switching

“…1. Previous investigation of the Si MOSFET MMC [11][12][13][14] has shown that conduction losses dominate switching loss due to low effective cell switching frequency. One phase leg of a 3-phase 5-level Si MOSFET MMC converter is shown in Fig.…”

“…The prevalence of conduction losses in MMC makes parallel connection of MOSFETs attractive in order to reduce on-state resistance and hence reduce conduction loss. However, previous studies [11][12][13][14] did not include measured switching losses as Si MOSFET parallel connection was increased, so measured Si MOSFET switching losses will be explored in this paper. In addition, the possibility of reducing EMI using slowed switching was proposed [11], with modelled losses suggesting that switching could be considerably slowed without impacting overall converter loss.…”

“…(12) By setting the integral of gate current equal to the charge removed from this capacitance, the transition time can be found from (13). (13) Loss is then found by integrating the product of voltage and current as for the previous stage, giving (14), assuming that during this stage V on ≈ 0V. Stage 9: t 8 to t 9 : C ds and C gd are charging with C gs constant, while current remains near the full-load value During this period the gate-source voltage is constant at the Miller voltage.…”

“…MMC decouple harmonic performance and switching frequency. At the same time experimental measurements suggest that MMC suffer considerably lower loss than SiC 2-level converters, particularly when parallel connection is used to reduce conduction loss [11][12][13][14]. Besides achieving good harmonic performance at low switching frequency leading to lower EMI, MMC cells also switch smaller voltages potentially bringing another drop in EMI.…”