Daniel Aggeler

A compact, high voltage 25 kW, 50 kHz DC-DC converter based on SiC JFETs

In the area of power electronics there is a general trend to higher power densities and efficiency. In order to continue this trend new devices, which enable high switching frequencies at higher power levels or show reduced losses at moderate switching frequencies are required. High voltage switches based on a series connection of SiC JFETs and one MOSFET in cascode connection meet these demands. For investigating the performance of the SiC based switch and its influence on the power density/efficiency a dual active bridge, which could transfer 25 kW bidirectionally between a 5 kV and a 700 V dc bus at a switching frequency of 50 kHz, is presented in this paper. There, especially the design of the high voltage/high frequency transformer and the switching as well as the static behaviour of the SiC switch is investigated in detail by simulations and experimental results in this paper.

Bi-Directional Isolated DC-DC Converter for Next-Generation Power Distribution - Comparison of Converters using Si and SiC Devices

In this paper two bi-directional DC-DC converters for a 1MW next-generation BTB system of a distribution system, as it is applied in Japan, are presented and compared with respect to design, efficiency and power density. One DC-DC converter applies commercially available Si-devices and the other one high voltage SiC switch, which consists of a SiC JFET cascode (MOSFET+1 JFET) in series with five SiC JFETs. In the comparison also the high frequency, high voltage transformer, which ensures galvanic isolation and which is a core element of the DC-DC converter, is examined in detail by analytic calculations and FEM simulations. For validating the analytical considerations a 20kW SiC DC-DC converter has been designed in detail. Measurement results for the switching and conduction losses have been acquired from the SiC and also for a Si system for calculating the losses of the scaled 1MW system.

D

Switching devices based on SiC offer outstanding performance with respect to operating frequency, junction temperature, and conduction losses enabling significant improvement of the performance of converter systems. There, the cascode consisting of a MOSFET and a JFET has additionally the advantage of being a normally off device and offering a simple control via the gate of the MOSFET. Without dv /dt-control, however, the transients for hard commutation reach values of up to 45kV/μs, which could lead to electromagnetic interference problems. Especially in drive systems, problems could occur, which are related to earth currents (bearing currents) due to parasitic capacitances. Therefore, new dv/d t-control methods for the SiC JFET/Si MOSFET cascode as well as measurement results are presented in this paper. Based on this new concepts, the outstanding performance of the SiC devices can be fully utilized without impairing electromagnetic compatibility.

v

Switching devices based on SiC offer outstanding performance with respect to operating frequency, junction temperature and conduction losses and enable a significant improvement of the system performance. There, the cascode consisting of a MOSFET and a JFET additionally has the advantage of being a normally off device and offering a simple control via the gate of the MOSFET. Without dv/dt control, however, the transients with hard commutation reach values of up to 45kV/µs, which could lead to EMC problems and especially in drive systems to problems related to earth currents (bearing currents) due to parasitic capacitances. Therefore, new dυ/dt control methods for the SiC MOSFET/JFET cascode as well as measurement results are presented in this paper. Based on this new concepts the outstanding performance of the SiC devices can be fully utilised without impairing EMC.

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In many pulse power applications there is a trend to modulators based on semiconductor technology. For these modulators high voltage and high current semiconductor switches are required in order to achieve a high pulsed power. Therefore, often high power IGBT modules or IGCT devices are used. Since these devices are based on bipolar technology the switching speed is limited and the switching losses are higher. In contrast to bipolar devices unipolar ones (e.g. SiC JFETs) basically offer a better switching performance. Moreover, these devices enable high blocking voltages in case large bandgap materials as SiC are used. At the moment SiC JFET devices with a blocking voltage of 1.5 kV per JFET are available. Alternatively, the operating voltage could be increased by connecting N JFETs and a low voltage MOSFET in series resulting in a Super Cascode switch with a blocking voltage N-times higher than the blocking voltage of a single JFET. In order to evaluate the achievable switching speed of the Super Cascode and its applicability in solid state modulators, the performance of such a SiC switch is examined in this paper. Furthermore, the performance of the Super Cascode is compared with 4.5 kV IGBTs made by Powerex, which are mounted in a special low inductive housing for minimising the rise and fall times.

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This paper discusses three-phase bidirectional high-power factor mains interfaces for application in smart-houses featuring a local DC distribution grid. The DC grid demanded power can be supplied by local DC generators, such as renewable power sources, and/or by the public three-phase AC mains, which gives the option of feeding back power into the mains in case of a low local power consumption. In order to generate a local 400V DC bus, bidirectionally connected to the European three-phase low voltage AC mains rated at 400V line-to-line, buck-type converter topologies are required. Several possible converter concepts are initially presented and further comparatively evaluated based on the following performance indices: total required semiconductor chip area, overall efficiency, overall passive components volume, and required EMI filter damping. As result of the comprehensive evaluation, the Bidirectional 3rd Harmonic Injection Active Filter Type Rectifier with DC/DC Output Stage is identified as most advantageous topology for the realization of a bidirectional buck-type PFC rectifier in the considered power range of 5 to 10 kW.

-Control Methods for the SiC JFET/Si MOSFET Cascode

In many pulsed-power applications, there is a trend to modulators based on semiconductor technology. For these modulators, high-voltage and high-current semiconductor switches are required in order to achieve a high pulsed power. Therefore, often, high-power IGBT modules or IGCT devices are used. Since these devices are based on bipolar technology, the switching speed is limited, and the switching losses are higher. In contrast to bipolar devices, unipolar ones (e.g., SiC JFETs) basically offer a better switching performance. Moreover, these devices enable high blocking voltages in the case where wide-band-gap materials, for example, SiC, are used. At the moment, SiC JFET devices with a blocking voltage of 1.2 kV per JFET are available. Alternatively, the operating voltage could be increased by connecting N JFETs and a low-voltage MOSFET in series, resulting in a super cascode switch with a blocking voltage N times higher than the blocking voltage of a single JFET. For the super cascode, auxiliary elements are required for achieving a statically and dynamically balanced voltage distribution in the cascode. In this paper, a new balancing circuit, which results in faster switching transients and higher possible operating pulse currents, is presented and validated by measurement results.

Controllable dυ/dt behaviour of the SiC MOSFET/JFET cascode an alternative hard commutated switch for telecom applications

In many pulse power applications there is a trend to modulators based on semiconductor technology. For these modulators high voltage and high current semiconductor switches are required in order to achieve a high pulsed power. Therefore, often high power IGBT modules or IGCT devices are used.

5kV/200ns Pulsed Power Switch based on a SiC-JFET Super Cascode

This paper presents a galvanic isolated multi-level dc-dc converter using different modulation strategies for wide voltage range operation. The dc gain of the converter can be changed in three steps by the proposed modulation schemes and thus achieve the change in output voltage among Vout, 0.5×Vout and 0.25×Vout. The operating frequency of the resonant tank is the same in all three cases. Therefore, the converter can be designed to operate at the optimal operating point to maintain high efficiency for a wide voltage range operation. The output voltage can then be adjusted within the three output voltages depending on the application by either varying the switching frequency of the converter or cascading a second stage dc-dc converter. The operating principle and the dc characteristics of the proposed modulation scheme are discussed and demonstrated with a 17kW five-level half-bridge LLC resonant dc-dc converter prototype operating at 150 kHz. The input and output voltage of the converters are 750V voltage and, 650V, 325V and 163V output voltages, respectively. The experimental results are closely matched with the theoretical predictions.

Comparative evaluation of bidirectional buck-type PFC converter systems for interfacing residential DC distribution systems to the smart grid

This paper presents a three-level isolated LLC resonant converter using a novel modulation strategy for wide-voltage range operation. With the proposed modulation strategy, the DC gain of the converter can be changed between Vout and ½Vout. The output voltage can be then adjusted within the two resulting voltage ranges by either varying the switching frequency of the converter or utilizing a post regulator. In the last option, the three-level converter works as an electronic transformer with variable turns ratio. Therefore, the converter can be designed to operate with a narrow operation condition maintaining high efficiency despite of wide-voltage range variations. A hybrid gate driver which simplifies the implementation of the proposed modulation method is also presented. The novel modulation method is analysed and verified on a 20kW, LLC three-level resonant prototype operating at 110 kHz.