The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

In recent years, SiC MOSFET s have gained strong attention in medium-voltage power conversion applications. To increase the blocking voltage level, series-connection of SiC MOSFET s is an attractive solution but may suffer a severe voltage unbalance issue. To gain insights into the voltage unbalance issue, this article presents a detailed study of the impact of parasitic capacitors on the voltage sharing of series-connected SiC MOSFET s and body-diodes. The parasitic capacitors are categorized into two groups for analysis: 1) parasitic capacitors from gate terminal; 2) parasitic capacitors from drain/source terminals. The study reveals that gate parasitic capacitors affect gate miller-plateau voltage and ultimately the d v /d t during the turn- off . The voltage sharing will be worse under higher turn- off current or larger gate resistor. The drain/source parasitic capacitors will introduce additional capacitance across device drain-source terminals which results in an unbalanced voltage sharing. The position of switching unit and the heatsink connection schemes will affect the distribution of drain/source parasitic capacitors to cause different voltage sharing results. The drain/source parasitic capacitors will also cause voltage unbalance of series-connected body-diodes under different conditions. To verify the analysis, the voltage sharing between two series-connected 10 kV SiC MOSFET s is tested under different parasitic capacitors conditions.

Analysis of Voltage Sharing of Series-Connected SiC MOSFETs and Body-Diodes

High-voltage and fast-switching silicon carbide (SiC) power modules are needed to construct high-voltage and high power-density converters. Stacking multiple low-voltage SiC devices to increase the equivalent blocking voltage shows advantages in on -state resistance, current capacity, and cost over one single high-voltage device. An indirect series-connected SiC MOSFET power module using quasi-two-level hybrid-clamp topology is proposed in this article. The voltages across the devices are automatically balanced with an open-loop modulation strategy to avoid sensors and control algorithm. Moreover, only ceramic capacitors are needed besides the MOSFETs and diodes in the topology. Thanks to this, the topology was highly integrated into a power module using SiC dies, which is equivalent to a general-purpose two-level medium-voltage SiC MOSFET power module. The switching losses of the power module show advantages over other stacking methods and even a single high-voltage switch and it can be evaluated with a two-level half-bridge (HB). To achieve this, the automatic voltage-balancing performance is analyzed in detail and the parasitic output capacitance in the topology is investigated, based on which the design considerations are presented. The simulation results from LTspice confirm the voltage-balancing and a 3600V/20A HB power module is built in the lab and the experimental results verify the design.

A Quasi-Two-Level Medium-Voltage SiC MOSFET Power Module With Low Loss and Voltage Self-Balance

This paper focuses on voltage balancing control of series-connected 10 kV SiC MOSFETs for medium-voltage dc-ac power conversion applications. Under ac load current conditions, gate delay time control is used as an example to analyze the limitation of active voltage balancing control methods: 1) active voltage balancing control has a limitation to adjust delay time accurately under a wide-range of load current; 2) the voltage unbalance of body diodes cannot be solved by active voltage balancing control for MOSFETs. For two series-connected 10 kV SiC MOSFETs realized by one half bridge module, the analysis indicates that the unbalance parasitic capacitors inside the power module is the dominant factor causing the voltage unbalance. To achieve voltage balancing control of series-connected SiC MOSFETs and body-diodes, a hybrid approach is proposed in this paper: 1) one small compensation capacitor is applied to balance the non-uniform distribution of package parasitic capacitors inside the power module ; 2) close-loop gate signal turn-off time adjustment is applied to compensate the gate signal error. To verify the proposed balancing approach, a single phase pump-back test is conducted to show the improvement of voltage sharing of both MOSFETs and body-diodes.

Hybrid Voltage Balancing Approach for Series-Connected 10 kV SiC MOSFETs for DC-AC Medium-Voltage Power Conversion Applications

Series-connection of silicon carbide (SiC)-MOSFETs is an attractive method to achieve a high-voltage and fast-switching power semiconductor switch. In order to deal with the unequal voltage sharing problem in such series connection method, a novel active voltage clamping circuit topology is proposed in this letter. The maximum drain–source voltages of the series-connected SiC-MOSFETs are clamped by individual clamping capacitors; therefore, high reliable switching of the power semiconductors is guaranteed. Besides, the accumulated energy in the clamping capacitors can be actively transferred back into the power supply through a simple gate drive signal adjustment algorithm; thus, the induced clamping circuit loss can be effectively suppressed. The proposed active clamping topology has been experimentally verified in a half-bridge inverter with four SiC-MOSFETs connected in series.

A Novel Active Voltage Clamping Circuit Topology for Series-Connection of SiC-MOSFET s

This paper presents a medium voltage (1.5 kV) flying capacitor DC/DC converter rated at 30 kW. The fast switching 1.2 kV/11 mΩ SiC MOSFETs converter operates in quasi-2-level (Q2L) mode at high frequency (100 kHz) to obtain a low size of the flying capacitor (330 nF). Moreover, a novel switching pattern is introduced and ZVS at turn-on is also enabled. Such control method leads to lower switching losses, higher switching frequency and minimized inductor volume compared to more conventional topologies. Presented experimental results prove correct operation of the proposed converter and control method.

High-Frequency SiC-Based Medium Voltage Quasi-2-Level Flying Capacitor DC/DC Converter With Zero Voltage Switching

This paper presents various issues related to a medium voltage power switch designed with series-connected transistors of the 1700 V/300 A SiC MOSFET power module. The focus of the paper are the voltage imbalance compensation methods: passive snubber circuits and active gate driving. Both methods are simulated in Saber software with the use of detailed models of the transistors and diodes of the module. At first, elements of the DRC snubber were determined and simulated during the double pulse procedure. Then, new active gate driver is proposed and verified via series of simulations. Finally, the test setup was designed with two series connected SiC MOSFETs inside CAS300M17BM2 power modules and experimental tests were performed at 1.5 kV DC to compare two compensation methods.

Medium voltage power switch based on 1.7 kV SiC MOSFETs connected in series inside power modules

This article presents a medium voltage half-bridge circuit where 3.3 kV power switches are built with two 1.7 kV/325 A SiC MOSFETs series-connected inside power modules. Essence of this work is an active balancing method eliminating the voltage imbalances between the devices by means of gate signals modification. The advantage over commonly-used passive snubbers is low cost and limited amount of additional losses. Performance of the half-bridge is illustrated with a model-based Saber simulation at first, then, an experimental full-scale model is designed with on-the-shelf power modules capable operating at power level above 150 kVA. A series of laboratory tests at 1.5 kV DC confirmed reduction of voltage differences across devices in the stack to an acceptable level – the designed system is working correctly despite EMI caused by fast switching transients. Moreover, the observed current and voltage waveforms prove the positive impact of the method as not only voltages but also switching losses are better balanced among the devices. Experimental comparison to a 3.3 kV counterpart shows two times lower turn-off energy, therefore, the presented design with two 1.7 kV SiC MOSFET power modules can beneficently compete not only in terms of lower cost but also switching performance.

/pdf/active-voltage-balancing-of-series-connected-1-7-kv-325-a-588tcoh4k0.pdf

Active Voltage Balancing of Series-Connected 1.7 kV/325 A SiC MOSFETs Enabling Continuous Operation at Medium Voltage

This paper presents an original method of power loss validation in medium-voltage SiC MOSFET (metal–oxide–semiconductor field-effect transistor) modules of a three-phase inverter. The base of this method is a correct description of the on-state performance of the diodes and the transistors in a PWM (pulse width modulation)-controlled inverter phase leg. Combined electro-thermal calculations are applied to precisely estimate the losses in the power devices and then, to find the suitable circuit parameters of a test circuit to emulate these conditions. A simple square-wave-controlled half-bridge with an inductive load enables the electrical and thermal stresses comparable to these in the inverter, and moreover, provided equations that confirmed the possibility of balancing the load between the diodes and the transistors. The circuit with 3.3 kV SiC MOSFETs was tested to verify the impact of selected parameters on power losses with the main focus on duty ratio. The same module was applied, in addition to an inductive load (3 × 112 μH) and two sets of DC-link capacitors (750 μF), to validate a phase leg of a 220 kVA inverter. In spite of a significantly apparent power, the active power delivered from the DC supply settled around 1 kW, which was enough to emulate 390 W of losses in two transistors and diodes.

https://www.mdpi.com/1996-1073/13/18/4773/pdf

A Simple Method to Validate Power Loss in Medium Voltage SiC MOSFETs and Schottky Diodes Operating in a Three-Phase Inverter

This article discusses an active gate driver for a 1.7 kV/325 A SiC MOSFET module. The main purpose of the driver is to adjust the gate voltage in specified moments to speed up the turn-on cycle and reduce the amount of dissipated energy. Moreover, an adequate manipulation of the gate voltage is necessary as the gate current should be reduced during the rise of the drain current to avoid overshoots and oscillations. The gate voltage is switched at the right moments on the basis of the feedback signal provided from a measurement of the voltage across the parasitic source inductance of the module. This approach simplifies the circuit and provides no additional power losses in the measuring circuit. The paper contains the theoretical background and detailed description of the active gate driver design. The model of the parasitic-based active gate driver was verified using the double-pulse procedure both in Saber simulations and laboratory experiments. The active gate driver decreases the turn-on energy of a 1.7 kV/325 A SiC MOSFET by 7% comparing to a conventional gate driver (VDS = 900 V, ID = 270 A, RG = 20 Ω). Furthermore, the proposed active gate driver lowered the turn-on cycle time from 478 to 390 ns without any serious oscillations in the main circuit.

Przemyslaw Trochimiuk

Papers

High-Frequency SiC-Based Medium Voltage Quasi-2-Level Flying Capacitor DC/DC Converter With Zero Voltage Switching

Medium voltage power switch based on 1.7 kV SiC MOSFETs connected in series inside power modules

Active Voltage Balancing of Series-Connected 1.7 kV/325 A SiC MOSFETs Enabling Continuous Operation at Medium Voltage

A Simple Method to Validate Power Loss in Medium Voltage SiC MOSFETs and Schottky Diodes Operating in a Three-Phase Inverter

Parasitic-Based Active Gate Driver Improving the Turn-On Process of 1.7 kV SiC Power MOSFET