Dominik Bortis

Classification and Comparative Evaluation of PV Panel-Integrated DC–DC Converter Concepts

Strings of photovoltaic panels have a significantly reduced power output when mismatch between the panels, such as partial shading, occurs since integrated diodes are then partly bypassing the shaded panels. With the implementation of DC-DC converters on panel level, the maximum available power can be extracted from each panel regardless of any shading. In this paper, different concepts of PV panel integrated DC-DC converters are presented, comparative evaluation is given and the converter design process is shown for the buck-boost converter which is identified as the best suited concept. Furthermore, the results of high precision efficiency measurements of an experimental prototype are presented and compared to a commercial MIC.

1 Megawatt, 20 kHz, isolated, bidirectional 12kV to 1.2kV DC-DC converter for renewable energy applications

The design of a 1 MW, 20 kHz, isolated, bidirectional 12kV to 1.2kV DC-DC converter for renewable energy applications is presented. The main topics addressed are: High-Voltage (HV) side switch, topology & modulation and Medium Frequency (MF) transformer. A study of the possible HV side switches, considering 4.5kV IGBTs is performed, fixing the requirements from the topology and modulation side in order to reach a highly efficient system. The studied topologies are the Dual Active Bridge (DAB) with triangular modulation and the Series Resonant Converter (SRC) with constant frequency operation. Both topologies are able to achieve Zero Current Switching (ZCS) in the HV side switches, reducing the switching losses in these devices, which contribute to a large share to the system losses. Efficiency curves are presented for different semiconductor technologies for the Low-Voltage (LV) side switch in order to study the trade-offs between the selected topologies. Three MF transformer concepts, namely core-type, shell-type and matrix transformer, are presented and compared in respect of winding arrangement, isolation mechanisms and thermal management. Power losses and volume are calculated in each case and used to compare the different transformer concepts.

Active Gate Control for Current Balancing of Parallel-Connected IGBT Modules in Solid-State Modulators

In modern pulsed power systems often fast solid state switches like MOSFETs and IGBT modules are used to generate short high power pulses. In order to increase the pulsed power, solid state switches have to be connected in series or in parallel. Depending on the interconnection of the switches, parameter variations in the switches and in the system can lead to an unbalanced voltage or current. Therefore, the switches are generally derated, which results in an increased number of required devices and volume. With an active gate control, derating and preselection of the switching devices can be avoided. In this paper an active gate control of paralleled IGBT modules, which has been developed for converters with inductive load, is explained in detail and adapted to a solid state modulator. There, the paper focuses on the low inductance IGBT current measurement, the control unit implementation with a FPGA and DSP, as well as the balancing of the pulse currents.

A 120 °C Ambient Temperature Forced Air-Cooled Normally-off SiC JFET Automotive Inverter System

The degree of integration of power electronic converters in current hybrid electric vehicles can be increased by mitigation of special requirements of these converters, especially those regarding ambient air and cooling fluid temperature levels. Today, converters have their own cooling circuit or are placed far away from hot spots caused by the internal combustion engine and its peripheral components. In this paper, it is shown, how the use of SiC power semiconductors and active control electronics cooling employing a Peltier cooler can help to build an air-cooled inverter system for 120 °C ambient temperature. First, a detailed analysis shows, how the optimum junction of this high-temperature system can be calculated. Then, the operating temperature ranges of power semiconductors, thermal interface materials, capacitors, and control electronics are investigated, leading to a comprehensive analysis of mechanical concepts for the inverter system in order to show new ways to solve electrical and thermal tradeoffs. In particular, the operation of the signal electronics and the gate driver for power semiconductors with a junction temperature of 250 °C within the specified operating temperature range is ensured by appropriate placement and cooling methods, while taking the electrical requirements for limits on the wiring inductances and symmetry requirements into account. The analysis includes an accurate thermal model of the converter and an optimized active cooling of the signal electronics using a Peltier cooler. Finally, a hardware prototype with discrete power semiconductor devices and thus with a junction temperature limit of 175 °C driving high-speed electrical machines is shown to validate the theoretical considerations in a custom-designed high-temperature test environment.

Modeling of Soft-Switching Losses of IGBTs in High-Power High-Efficiency Dual-Active-Bridge DC/DC Converters

Soft-switching techniques are very attractive and often mandatory requirements in medium-voltage and medium-frequency applications such as solid-state transformers. The effectiveness of these soft-switching techniques is tightly related to the dynamic behavior of the internal stored charge in the utilized semiconductor devices. For this reason, this paper analyzes the behavior of the internal charge dynamics in high-voltage (HV) semiconductors, giving a clear base to perform overall converter optimizations and to understand the previously proposed zero-current-switching techniques for insulated-gate bipolar-transistor (IGBT)-based resonant dual active bridges. From these previous approaches, the two main concepts that allow switching loss reduction in HV semiconductors are identified: 1) shaping of the conducted current in order to achieve a high recombination time in the previously conducting semiconductors; and 2) achieving zero-voltage-switching (ZVS) in the turning-on device. The means to implement these techniques in a triangular-current-mode dual-active-bridge converter, together with the benefits of the proposed approaches, are analyzed and experimentally verified with a 1.7-kV IGBT-based neutral-point-clamped (NPC) bridge. Additionally, the impact of the modified currents in the converters performance is quantified in order to determine the benefits of the introduced concepts in the overall converter.

Novel AC-Coupled Gate Driver for Ultrafast Switching of Normally Off SiC JFETs

Over the last years, more and more SiC power semiconductor switches have become available in order to prove their superior behavior. A very promising device is the 1200 V 30 A JFET manufactured by SemiSouth. It features a very low on-resistance per die area (2.8 mΩ-cm2), switching within 20 ns, normally off characteristic, high-temperature operation and has already been commercialized in contrast to many other SiC switches. To fully exploit the potential of the SiC normally off JFET, conventional gate drivers for unipolar devices must be adapted to this device due to its special requirements. During on-state, the gate voltage must not exceed 3 V, while a current of around 300 mA (depending on the desired on-resistance) must be fed into the gate; during switching operation, the transient gate-source voltage should be around ±15 V and the low threshold voltage of less than 0.7 V requires a high noise immunity which is a severe challenge as the device has a comparably low gate-source but high gate-drain capacitance. To meet these requirements, several concepts have been published recently. They deal with the challenges mentioned, but they still show certain limitations (e.g., frequency and duty cycle limitations or need for additional cooling due to high gate driver losses). In this paper, a novel gate driver consisting of only one standard gate driver IC, resistors, capacitors, and diodes is designed and experimentally validated. It supplies enough gate current for minimum on-resistance, allows fast switching operation, features a high noise immunity, and can be used for any duty cycle and typical switching frequencies without significant self-heating.

A 120°C ambient temperature forced air-cooled normally-off SiC JFET automotive inverter system

The degree of integration of power electronic converters in current hybrid electric vehicles can be increased by mitigation of special requirements of these converters, especially those regarding ambient air and cooling fluid temperature levels. Today, converters have their own cooling circuit or are placed far away from hot spots caused by the internal combustion engine and its peripheral components. In this paper, it is shown, how the use of SiC power semiconductors operated at a junction temperature of 250 °C and active control electronics cooling employing a Peltier element can help to build an air-cooled inverter system for 120 °C ambient temperature. First, a detailed analysis of the operating temperature ranges of power semiconductors, thermal interface materials, capacitors and control electronics is conducted. Then, concepts for deriving a converter design that takes the electrical, mechanical and thermal requirements of the components and their interaction into account are shown. The inverter and the active Peltier cooling of the control electronics are dimensioned. Finally, a hardware prototype with discrete power semiconductor devices and thus with a junction temperature limit of 175 °C is shown to validate the theoretical considerations.

Active gate control for current balancing in parallel connected IGBT modules in solid state modulators

In modern pulsed power systems, often, fast solid-state switches like MOSFETs and insulated gate bipolar transistor (IGBT) modules are used to generate short high power pulses. In order to increase the pulsed power, solid-state switches have to be connected in series or in parallel. Depending on the interconnection of the switches, parameter variations in the switches and in the system can lead to an unbalanced voltage or current. Therefore, the switches are generally derated, which results in an increased number of required devices, cost, and volume. With an active gate control, derating and preselection of the switching devices can be avoided. In this paper, an active gate control of paralleled IGBT modules, which has been developed for converters with inductive load, is explained in detail and adapted to a solid-state modulator. This paper focuses on achieving a low-inductance IGBT current measurement, the control unit implementation with a field-programmable gate array and a digital signal processor, as well as the balancing of the pulse currents.

Novel AC coupled gate driver for ultra fast switching of normally-off SiC JFETs

Over the last years, more and more SiC power semiconductor switches became available in low production volumes in order to prove their superior behavior with respect to fast switching speed, low on-resistance per chip area, high voltage range and high temperature operation. A very promising device among those introduced in numerous publications over the last years is the 1200 V 30 A JFET introduced by SemiSouth. It features a very low on-resistance (2.8m Ω cm2), switching operation within 20 ns, a normally-off characteristic and has already been commercialized in contrast to many other SiC switches. To fully exploit the potential of the SiC normally-off JFET, conventional gate drivers for unipolar devices must be adapted to this device due to its special requirements: During on-state the gate voltage must not exceed 3 V, while a current of around 300 mA must be fed into the gate, during switching operation the transient gate voltage should be around ±15 V and the low threshold voltage of 0.7 V requires a high noise immunity which is a severe challenge as the device has a comparably low gate-source but high gate-drain capacitance. To meet these requirements, several concepts have been published recently. They deal with the challenges mentioned, but they also note certain limitations (e. g. frequency and duty cycle limitations or need for additional cooling). In this paper, a novel gate driver consisting only of one standard gate driver IC, resistors, capacitors and diodes is designed and experimentally validated. It supplies enough gate current for minimum on-resistance, allows fast switching operation, features a high noise immunity and can be used for any duty cycle and usual switching frequencies without significant self-heating.

Design procedure for compact pulse transformers with rectangular pulse shape and fast rise times

Microseconds range pulse modulators based on solid state technology often utilize a pulse transformer, since it could offer an inherent current balancing for parallel connected switches and with the turns ratio the modulator design could be adapted to the available semiconductor switch technology. In many applications as e.g. radar systems, linear accelerators or klystron/magnetron modulators a rectangular pulse shape with a fast rise time and a as small as possible overshoot is required. In reality, however, parasitic elements of the pulse transformer as leakage inductance and capacitances limit the achievable rise time and result in overshoot. Thus, the design of the pulse transformer is crucial for the modulator performance. In this paper, a step by step design procedure of a pulse transformer for rectangular pulse shape with fast rise time is presented. Different transformer topologies are compared with respect of the parasitic elements, which are then calculated analytically depending on the mechanical dimensions of the transformer. Additionally, the influence of the core material, the limited switching speed of semiconductors and the nonlinear impedance characteristic of a klystron are analyzed.