Physical Investigation into Effective Voltage Balancing by Temporary Clamp Technique for the Series Connection of IGBTs

The series connection of IGBTs is essential for high-voltage applications where fast switching performances need to be maintained. However, unbalanced voltage sharing is a major resistance to the converter application of this structure. There are a number of causes leading to voltage unbalance, such as different signal delays, parasitic parameters, tail currents, and so on. A temporary clamp scheme performed by active voltage control (AVC) has been proven to be effective in solving the unbalanced voltage-sharing issue. However, the basic physics has not been investigated. In this paper, the physical principle of voltage unbalance within IGBTs series operation is discussed. The carrier storage region differences are concluded to be the intrinsic cause of unbalanced voltage sharing. By using an accurate Fourier-series-based IGBT simulation model with appropriate assumptions, a physical explanation for temporary clamp is provided in detail. At the end of the tail current period when the excess carrier concentration becomes close to the intrinsic doping density, the temporary clamp is able to achieve satisfactory equal voltage sharing

Voltage sharing scheme for series-connected power semiconductors

Dynamic voltage-sharing schemes have been investigated which allow high-voltage power-semiconductor devices, such as thyristors, IGCTs, IGBTs or power MOSFETs, to be series connected in strings and switched, as simply in high-voltage applications, as when used as single devices. The circuits have many of the advantages of simply using RC or RCD snubbers, including being easily applicable to both low- and high-side switches. However, because the snubber capacitors are not fully discharged their associated reset current and power-losses are minimized. To illustrate the principle of operation experimentally, a string of three series-connected power MOSFETs switching 100 A from 330 V has been used to obtain practical waveforms. The schemes are discussed and illustrated, using SPICE simulation results. The new, relatively simple voltage-sharing schemes are much easier to design and optimize than recently reported active gate-control and regenerative-snubber methods, allow very rapid turn-on and turn-off switching, and give composite-device switches a usable voltage rating similar to the aggregated voltage ratings of the string

Devices Selection and Topology Comparison for Medium Voltage DC Solid State Circuit Breakers

DC grid has been treated as a viable solution to solve green-house effect and reduce the cost of fossil fuels. Compared with AC grid, DC grid shows many inherent advantages, such as high efficiency power delivery, less expensive to deploy and no need for phase and frequency synchronization. However, over current protection for DC grid require fast response. To solve this problem, solid state DC circuit breakers need to be promoted. In this thesis, thermal property was taken into consideration to find the most suitable semiconductor devices. Per unitized thermal resistance of the heat sink to the ambient was used to find the reasonable conducting current. Module MOV topology and one MOV topology were compared to see the voltage and current stress ANSYS Simplorer is used to assist in the process. Simulation is used to see the waveform of the current stress and the voltage stress under ideal and non-ideal condition. According to the results, during the ideal condition the current stress and the voltage stress are almost the same. However, under the non-ideal condition, the current stress and voltage stress are different, and the module MOV mode topology is much more reliable and has less current and voltage stress

Electrothermal simulation and characterisation of series connected power devices and converter applications

Power electronics is undergoing significant changes both at the device and at the converter level. Wide bandgap power devices like SiC MOSFETs are increasingly implemented in automotive, grid and industrial drive applications with voltage ratings as high as 1.7kV now commercially available although much higher voltages have been demonstrated as research prototypes. In high power applications where high DC bus voltages are used, as is the case in voltage source converters for industrial drives, marine propulsion and grid connected energy conversion systems, it may be necessary to series connect power devices for OFF-state voltage sharing. In high power applications, before the advent of multi-level converters, series connection of IGBT power modules was commonplace especially for HVDC-voltage source converter applications. However, with the advent of the modular multi-level converter, where the AC voltage waveform is synthesized by discrete voltage steps, the need for series connected is obviated. Most HVDC-VSC applications are now implemented by modular-multi-level converters. However, in some applications like VSCs for distribution network power conversion, there can be a combination between series connection of power devices and multi-level converter. Traditionally, voltage balancing in series connected power devices was achieved using snubber capacitors for dynamic voltage sharing and resistors for static voltage sharing. However, the use of snubber capacitors reduces the switching speed of the converter thereby defeating the purpose of using SiC power devices especially in power converters with high switching frequencies. To avoid this, active gate driving techniques that avoid the use of snubber capacitors during switching are under intensive research focus. This involves intelligent gate drivers capable of dynamically adjusting the gate pulse during switching. To use these gate drivers, it is necessary to explore the boundaries of static and dynamic voltage imbalance in series connected power devices. For example, it is necessary to understand how differences in device junction temperature and gate driver switching rates affects voltage divergence between series connected devices and how this differs between silicon IGBTs and SiC MOSFETs. This is similarly the case between series connected silicon PiN diodes and SiC Schottky diodes. Since silicon IGBTs and PiN diodes respectively exhibit tails currents and reverse recovery during turn-OFF, the dynamics of voltage divergence between series devices will differ from unipolar SiC power devices. Furthermore, the leakage current mechanisms determine the OFF-state voltage balancing dynamics and since Si IGBTs have different leakage current mechanisms from SiC devices, OFF-state voltage balancing in series connected devices will be different between the technologies. The contribution of this thesis is using finite element and compact device models backed by experimental measurements to investigate static and dynamic voltage imbalance in series connected power devices. Starting from the fundamental physics behind device operation, this thesis explores how the leakage currents and tail currents affects voltage divergence in series silicon bipolar devices compared to SiC power devices. This analysis is compared with how the switching dynamics peculiar to fast switching SiC devices affects voltage balancing in series connected SiC devices. Simulations and measurements show that series connected SiC power devices are less prone of excessive voltage divergence due to the absence of tail currents compared to series connected silicon bipolar devices where voltage divergence due to tail currents is evident. Reduced leakage currents due to the wide bandgap in SiC also ensures that it is less prone to voltage divergence (compared to silicon bipolar devices) under static OFF-state conditions. This means the snubber resistances can be increased thereby reducing the OFF-state power dissipation in series connected SiC devices. In the analysis of voltage sharing of series connected devices during the static ON-state and OFF-state it was shown that the zero-temperature coefficient of the power devices determines the voltage sharing and loss distribution in the ON-state while the leakage current and switching synchronization is critical in the OFF-state. Simulations and measurements in this thesis show that the higher ZTC points in silicon bipolar devices compared to SiC unipolar devices means that ON-state voltage divergence depends on the load current. The dominant failure mode for series connected power devices is failure under dynamic avalanche which occurs in cases of extreme uncontrolled voltage divergence. In the investigations of the switching transient behaviour of series connected IGBT and SiC MOSFETs during turn-OFF, it was shown that the voltage imbalance for Si IGBT is highly dependent on the carrier concentration in the drift region during switching while for SiC MOSFET it depends on the switching time constant of the gate voltage and the rate that the MOS-channel cuts the current. The thesis also explores the limits of power device performance under dynamic avalanche conditions for both series silicon bipolar and SiC unipolar devices. In the analysis of SOA of series connected devices it was discussed that the SOA is reduced by increased switching rates and DC link voltages. Finally, the thesis explores the 3L-NPC converter and how the power factor of the load on the AC side of the converter alters the power dissipation sharing between the devices. The results show that loss distribution between the devices in the converter is not just affected by the load power factor but also by the switching frequency

Asociación en serie de transistores IGBT para conmutación de alta tensión con bajas pérdidas

El surgimiento de aplicaciones de conversión en alta tensión ha creado la tendencia de uso de asociaciones en serie de dispositivos semiconductores. Las asociaciones en serie permiten operar con tensiones de bloqueo superiores a la tensión nominal de cada elemento semiconductor. El principal reto en estas topologías es garantizar el balance de tensión entre cada dispositivo tanto en bloqueo como en conmutación. La mayoría de los métodos propuestos para mitigar los desbalances de tensión estáticos y dinámicos incrementan las pérdidas en el dispositivo. En esta tesis se presenta una nueva topología para asociación en serie de IGBTs en la cual se reducen los desbalances de tensión, disminuyendo las pérdidas por conmutación. La topología consta de un circuito que asegura una conmutación suavizada en cada IGBT y un circuito auxiliar que permite recuperar energía desde el lado de alta tensión hacia la fuente de suministro de los controladores de compuerta. El principio de funcionamiento de la topología es expuesto y la validación mediante simulación y con prototipo experimental para tres módulos es realizada. La topología muestra un excelente desempeño tanto en la conmutación al encendido como en la conmutación al apagado en cada dispositivo. Los desbalances estáticos y dinámicos de tensión y las pérdidas por conmutación obtenidos son reducidos. Los desbalances estáticos de tensión se limitaron al 1 % y se presentan diferencias muy bajas entre pendientes de tensión colector-emisor en las conmutaciones. Las pérdidas por conmutación se reducen en un 40 % en comparación con una configuración que presenta conmutación forzadaAbstract: The emergence of high voltage conversion applications has created a trend of using semiconductor device series associations. Series associations allow for operation at blocking voltages, which are higher than the nominal voltage for each semiconductor device. The main challenge with these topologies is finding a way to guarantee voltage balance between devices in both blocking and switching transients. Most methods that have been proposed to mitigate static and dynamic voltage unbalances result in increased losses within the device. This thesis introduces a new series stack topology, in which voltage unbalances are reduced, thus mitigating switching losses. The topology consists of a circuit that ensures the soft switching of each device, and one auxiliary circuit that allows switching energy recovery. The principle for topology operation is presented and an experimental test is performed for three modules. The topology performs excellently for switching transients on each device. Voltage static unbalances were limited to 1%, and very low differences between collector-emitter voltages are obtained in the switching. Switching losses are thus reduced by 40%, as compared to hard switching configurationsDoctorad