Kasunaidu Vechalapu

High voltage dual active bridge with series connected high voltage silicon carbide (SiC) devices

Using 6.5 kV Silicon (Si) IGBTs, high voltage high power DC to DC converters are realized either by multi-level converters or series connected devices based two level converters or modular multi-level converter using series connected devices two level converter as building blocks. The introduction of high voltage SiC devices (10 kV to 20 kV) reduces the component count significantly while improving the efficiency and power density of the converter. To explore further, this paper investigates the state of the art high voltage (>10 kV) SiC devices for high voltage dual active bridge (DAB) with series connected devices for high power applications. Experimental results for static and dynamic voltage balancing of 15 kV, 20 A SiC IGBT devices are given to validate the feasibility of series connection and also the experimental characterization of 10kV SiC MOSFET and 15 kV SiC IGBT with RC snubber are reported. By using energy loss data from the experimental characterization, HV side switching loss of a 1 MVA 16 kV/2 kV DAB topology has been evaluated for two independent cases with 10 kV, 10 A SiC MOSFET and 15 kV, 20 A SiC IGBT on the HV side of the converter, while using 1.7 kV Si IGBTs on LV side of the converter.

Comparative Evaluation of 15-kV SiC MOSFET and 15-kV SiC IGBT for Medium-Voltage Converter Under the Same dv/dt Conditions

The 15 kV SiC MOSFET and 15 kV SiC IGBT are two state-of-the-art high voltage SiC devices. These high voltage SiC devices enable simple two level converters for medium voltage source converter compared to the complex three level and multilevel topologies with Silicon devices. This paper presents the detailed experimental results for the characterization of 15 kV SiC MOSFET module at 10 kV and 12 kV DC bus for two different configuration of device under test. This paper also presents the switching loss comparison of 15 kV SiC MOSFET with 15 kV SiC IGBT for the same dv/dt. Based on loss data obtained from experiments, this paper finally reports the switching frequency limits of 15 kV SiC MOSFET for 10 kV DC bus, 3-Phase two level converter and Bi-directional DC-DC phase leg converter with 10 kV output voltage and comparative evaluation of 15 kV SiC MOSFET and 15 kV SiC IGBT for the same dv/dt in a unidirectional DC-DC boost hard switching converter for 10 kV output voltage.

A MV intelligent gate driver for 15kV SiC IGBT and 10kV SiC MOSFET

This paper presents an Intelligent Medium-voltage Gate Driver (IMGD) for 15kV SiC IGBT and 10kV SiC MOSFET devices. The high voltage-magnitude and high dv/dt(> 30kV/μs) of these MV SiC devices, pose design challenge in form of isolation and EMI. This problem is solved by development of a <; 1pF isolation capacitance power-supply. But due to applied high stress, smaller short-circuit withstand time and the criticality of the application, these devices need to be monitored, well protected, active gate-driven and safely shut-down. This paper presents an EMI hardened IMGD built around a CPLD, sensing and optical interfacing unit. It provides advanced gate-driving, added protection and optically isolated state-monitoring features. The device operating conditions such as module temperature and Vds(on) can be data-logged. They can be used for diagnosis/prognosis purposes such as to predict failure and safely shut-down the system. This paper describes the functionality of different building blocks. The 15kV SiC IGBT has higher second switching slope above its punch-through level which is moderated without increasing losses by using digitally controlled active gate-driving. The shoot-through protection time can be reduced below withstand time by advanced gate driving. Soft turn-on and over-current triggered gate-voltage reduction helps reducing blanking time and quick turn-off reduces the protection response time. In this paper, the IMGD is high side tested at 5kV with device state monitoring on. The active gate-driving is tested at 6kV.

Medium voltage (≥ 2.3 kV) high frequency three-phase two-level converter design and demonstration using 10 kV SiC MOSFETs for high speed motor drive applications

High speed variable frequency motor drives are required for marine applications, compressors for oil and gas industries, wind energy generation systems etc. Traditionally, low voltage high speed motor drives are used in such applications. This results in large currents at high power levels leading to large copper loss in the motor winding. Therefore, medium voltage (MV) drives are being considered. The silicon (Si) based MV drives need gears to increase the speed due to low switching frequency operation of Si devices in the converter. Gears reduce both efficiency and power density. With the development of 10 kV SiC MOSFET, high switching frequency at MV is possible, which has enabled the scope of high power density MV direct drive variable speed controlled motors. In this paper, the design of a three-phase, 2-level, ≥ 2.3 kV MV, high frequency converter based on 10 kV SiC MOSEFT is explained. Performance analysis is presented along with experimental demonstration.

Design and evaluation of isolated gate driver power supply for medium voltage converter applications

The commercial gate drivers are available upto 6.5 kV IGBTs. With the advances in the SiC, power devices rated beyond 10 kV are being researched. These devices will have use on medium voltage power converters. Commercial gate drivers rated for such high voltages are not available. These power devices have very high dv/dts (30-100 kV/μs) at switching transitions. Such high dv/dts bring in challenges in the gate driver design. The isolation stage of the gate power supply needs to have very low coupling capacitance to limit the high frequency circulating currents from reaching the gate driver control circuits. Also, the isolation stage has to be designed with insulation several times higher than the peak system voltage level. In this paper, design, development and evaluation of the gate power supply for medium voltage level applications have been investigated. Several isolation transformer designs have been investigated and optimum design, with very low coupling capacitance ≈ 0.5 pF, has been identified and used in the gate driver design. Experimental characterization of the transformer has been done. The performance of the gate driver power supply has been evaluated in several MV power converters, using 10 kV SiC MOSFETs.

Soft switching characterization of 15 kV SiC n-IGBT and performance evaluation for high power converter applications

The 15 kV SiC IGBT with 2 μm and 5 μm field-stop buffer layer thicknesses are two state of the art HV SiC devices. These 15 kV SiC IGBTs generate high dv/dt with two slopes in punch through and non-punch through regions. To design 15 kV SiC IGBT with reduced dv/dt and single slope dv/dt similar to 10-15 kV SiC MOSFET, requires significantly larger drift epitaxial layer thickness and it increases the size and cost of the 15 kV SiC IGBT. This paper presents the zero voltage switching (ZVS) characteristics of 15 kV SiC N-IGBTs to reduce the dv/dt at switching pole along with reduction in the switching losses and increase in the switching frequency limits with external snubber capacitor. The ZVS characteristics are reported up to 9 kV dc bus voltage at 25°C and 150°C for both IGBTs. This paper also reports continuous mode experimental demonstration of zero voltage switching (ZVS) of 5 μm 15 kV IGBT in a medium voltage half bridge converter up to 7 kV dc bus voltage and calculation of power dissipation per IGBT module and its comparison of switching frequency limits with hard switching of half bridge converter.

Performance evaluation of series connected 15 kV SiC IGBT devices for MV power conversion systems

The 15kV SiC IGBT (2 μm buffer layer) with chip area of 8.4 × 8.4 mm2 is the state of the art high voltage device designed by Cree Inc. This device is expected to increase the power density of converters and the demonstration of the device in applications like Solid State Transformers has been published. Therefore, it is interesting to investigate the performance of the device in very high voltage (HV) application, where the series connection of devices is required. This paper addresses design considerations of the series connection of 15kV SiC IGBT devices for high voltage converter applications. A simple RC snubber has been used to control both ‘dv/dt’ and dynamic voltage balancing during turn-off. The experimental results show that there is a significant difference in the static and dynamic voltage sharing between two unmatched 15kV SiC IGBTs without active compensation method. With external RC snubber at total DC bus voltage of 10 kV, the difference in dynamic voltage between the two 15 kV SiC IGBT devices during turn-off transition nearly negligible. Also with external snubber, the total turn-off dv/dt of less than ‘5 kV/ μs’ is achieved across each device of two series connected 15kV SiC IGBTs. Furthermore, optimization of RC snubber to minimize semiconductor switching losses and total losses per device including the snubber resistor losses in series connection has been presented.

Modular multilevel converter based medium voltage DC amplifier for ship board power system

In this paper, the Modular Multilevel Cascaded Converter based on Double-Star Bridge Cells(MMCC-DSBC) for Medium Voltage DC (MVDC) amplifier is proposed. The medium voltage DC (MVDC) amplifier system is required to validate new technologies, new high power non-linear loads based on power electronics, in all electric ships as part of the proposed medium voltage (MVDC) ship power system. Thyristor based medium voltage DC amplifier (6 kV to 24 kV DC from 4.2 kV (L-L) AC three phase system) with Series Active Inverter (SAI) and DC side transformer in series with DC bus has been reported in the literature, where the series active Inverter compensates the slow dynamics of the Thyristor converter and facilitates fast dynamic response for step changing loads. However, this system has disadvantages as shown in Table III. This paper first shows a control method for AC to DC operation of MMCC-DSBC converter to control the wide DC output voltage in both buck and boost modes from fixed AC source. The simulation results show that the MMCC-DSBC converter provides a variable DC voltage from 6 kV to 24 kV DC and also the extended wide range of output DC voltage - 1 kV to 24 kV - from fixed 4.2 kV (L-L) AC three phase system. It is also shown that this topology will provide fast dynamic response compared to Thyristor based Line Commutated Converter (LCC) without the need of Series Active Inverter, DC side transformer in series with DC bus. This paper also shown the the control method to operate the MMCC-DSBC converter in STATCOM mode-to provide reactive power support to the AC side grid-during DC side pole to pole fault. Simulation results are presented on EMTDC/PSCAD platform to validate the controls.

Comparative performance evaluation of series connected 15 kV SiC IGBT devices and 15 kV SiC MOSFET devices for MV power conversion systems

The 10–15kV SiC MOSFET and 15kV SiC IGBT (2 μm and 5 μm buffer layer) are the state of the art high voltage devices designed by Cree Inc. These devices are expected to increase the power density of converters and the demonstration of these devices in applications like Solid State Transformers (SST) have been reported up to 4.16 kV–13.2 kV grid connection. It is interesting to investigate the performance of the devices in very high voltage (≥13.2 kV) application, where the series connection of devices is required. Therefore, this paper addresses design considerations of the series connection of 15 kV Silicon Carbide (SiC) IGBT devices and a series connection of 10 kV/15 kV Silicon Carbide (SiC) MOSFET devices in two separate independent cases and their experimental comparison.

Series injection enabled full ZVS light load operation of a 15kV SiC IGBT based dual active half bridge converter

The 15kV SiC IGBT has second higher dv/dt turn-off slope above the punch-through level resulting in EMI. Increasing gate-resistance also slows the first dv/dt causing increased switching loss. A snubber capacitor assisted turn-off solves these issues for a high power dual active bridge (DAB) converter based on this device, but the light load turn-on ZVS becomes hard to achieve. This paper proposes a series injection enabled triangular current shaping at the light load turn-off instant in the DAB to create enough current for smooth free-wheeling transition of device voltage during the dead-time period for ZVS turn-on. The proposed technique is validated through simulations followed by experiments on a medium voltage DAB hardware implementation of this technique.