MMC with Parallel-connected MOSFETs for LVDC Distribution Networks

Abstract

A highly efficient DC-AC converter is key to the success of low-voltage DC (LVDC) distribution networks. Calculated power losses in a conventional IGBT 2-level converter, a SiC MOSFET 2-level converter, a Si MOSFET modular multilevel converter (MMC) and a GaN HEMT MMC are compared. Calculations suggest that the parallel-connected Si MOSFET MMC may be the most efficient topology for this LVDC application. In this paper, the current unbalance limits for the parallel-connected MOSFETs and the optimal number of parallel-connected MOSFETs for MMC are investigated. Experimental results are presented for current sharing in parallel-connected MOSFETs and for the verification of power loss in a single Si MMC module.

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.…”

“…Heatsink thermal resistance was calibrated by applying DC voltage and current to the Si MOSFET switches, and used to correlate temperature rise with dissipated power during switching. The resulting loss is compared with the standard loss model [11][12][13] The standard loss model for Si MOSFET switching is given by equations (1)- (4) [11] .…”

“…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.…”

“…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. MMC switching loss is negligible compared with conduction loss [11][12][13] allowing the possibility of slowed switching, and initial experiments demonstrate reduced dv/dt, di/dt and ringing [11][12][13] as switching slows. Reduced switching transients, when combined with smaller ringing amplitude and lower switching frequency, are all expected to reduce EMI generated by converters.…”