G. Ortiz

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.

Optimized design of medium frequency transformers with high isolation requirements

For future DC electric power systems, high-power DC-DC converters will play a major role as they will substitute todays bulky 50/60Hz transformers. One key component within this DC-DC converters is the medium frequency transformer that provides the isolation level and the step up/down of the different voltage levels. As a consequence, an optimized design methodology that considers this high isolation requirements is needed. This paper presents a step-by-step design for medium frequency transformers with high isolation requirements. Each step in the design is carefully discussed and the required design considerations, such as flux density limits, isolation and thermal management, are explained in detail. The proposed design procedure is applied to a core-type transformer analyzing the outcome of the optimization process.

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.

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.

10kV SiC-based isolated DC-DC converter for medium voltage-connected Solid-State Transformers

Silicon-carbide semiconductor technology offers the possibility to synthesize power devices with unprecedented blocking voltage capabilities while achieving outstanding switching and conduction performances. Accordingly, this new semiconductor technology is especially interesting for Solid-State Transformer concepts and is utilized in this paper for designing a 25 kW/50 kHz prototype based on 10 kV SiC devices, featuring a 400V DC output. The focus is on the DC-DC converter stage while special attention is placed on the large step-down medium frequency transformer, whereby the impact of the rather high operating frequency and high number of turns with respect to the transformers resonance frequency is analyzed This leads to useful scaling laws for the resonance frequency of transformers in dependence of the operating frequency and construction parameters. Finally, a transformer prototype and efficiency and power density values for the DC-DC stage are presented.

SiC-based unidirectional solid-state transformer concepts for directly interfacing 400V DC to Medium-Voltage AC distribution systems

400 V DC distribution networks present a promising solution for supplying high-power DC loads such as information processing systems, transportation battery charging facilities and DC micro grids, among others. For these applications, high transmission efficiency, reliability and controllability are mandatory. With the current technology, these loads are fed from PWM rectifiers which are connected to the three-phase Low-Voltage (LV) distribution grid (400 V AC in Europe). The LV grid itself is supplied via Low-Frequency Transformers (LFT) from the Medium-Voltage (MV) grid, providing galvanic isolation and the required voltage step down. This paper presents three unidirectional AC/DC SiC-based Solid-State Transformer (SST) topologies with direct connection to the MV grid, which avoid the utilization of the aforementioned LFT by integrating a Medium-Frequency (MF) conversion stage, thus increasing the efficiency and power density of this supply system. The SST topologies are compared by means of a chip area-based comparative evaluation. Finally, the most suited among the presented topologies is Pareto-optimized, achieving a total MV AC to 400 V DC efficiency of 98.3 %. It is shown that the optimized SST features 40 % less overall losses compared to state-of-the-art solutions.

Soft-switching techniques for medium-voltage isolated bidirectional DC/DC converters in solid state transformers

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 semiconductors, giving a clear base to understand the previously proposed zero-current-switching techniques for IGBT-based resonant dual-active-bridges. From these previous approaches, the two main concepts that allow switching loss reduction in high-voltage 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 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 NPC bridge. Additionally, the impact of the modified currents in the converters performance are quantified in order to determine the benefits of the introduced concepts in the overall converter.

Mixed MOSFET-IGBT bridge for high-efficient Medium-Frequency Dual-Active-Bridge converter in Solid State Transformers

High power DC-DC conversion is a key element within the Solid-State-Transformer concept. In order to reduce the switching losses of the Medium-Voltage side semiconductors, a Triangular-Current-Mode modulation scheme presents an attractive option. This modulation scheme, however introduces considerable challenges in the design of the low-voltage side power electronic bridges, which need to deal with high conducted and high switched currents. In order to increase the converters efficiency, a combination of IGBTs and MOSFETs in a full-bridge configuration is considered. Practical hardware realizations are utilized in order to quantify the improvements introduced by the combination of these switches. Furthermore, the MOSFETs current conduction phase is supported by parallel connected IGBTs, which are used in order to further increase the full-bridges efficiency, as shown by the provided experimental verification.

Optimal Design of a 3.5-kV/11-kW DC–DC Converter for Charging Capacitor Banks of Power Modulators

For the generation of short high-power pulses in many applications, power modulators based on capacitor discharge are used, where the peak power is drawn from the input capacitor bank. In order to continuously recharge the energy buffer during operation at a lower average power, usually, power supplies connected to the mains are used. Due to the worldwide variation in mains voltages and the desired ability to adapt the capacitor voltage of the modulator, the power supply has to support a wide input and output voltage range, whereby the supply should draw a sinusoidal current from the mains due to EMI regulations. Additionally, depending on the modulator concept, a galvanic isolation also has to be provided. In order to achieve the mentioned specifications for the considered power supply, a combination of an ac-dc and a dc-dc converter is proposed, whereas the mains voltage is rectified by a three-phase buck-boost converter to 400 Vdc, and thereafter, an isolated dc-dc converter charges the input capacitor bank of the power modulator up to 3.5 kV. This paper focuses on the basic operation and the design of the 3.5-kV/11-kW isolated dc-dc converter, which includes transformer design, efficiency-volume optimization, and component selection. In this paper, compared with the well-known flyback converter, the proposed full-bridge-based topology results in a much higher efficiency and power density.

Flux Balancing of Isolation Transformers and Application of “The Magnetic Ear” for Closed-Loop Volt–Second Compensation

Semiconductor switches possess nonideal behavior which, in case of isolated dc-dc converters, can generate dc-voltage components which are then applied to the isolation transformer. This dc-voltage component is translated into a dc flux density component in the transformer core, increasing the risk of driving the core into saturation. In this paper, a novel noninvasive flux density measurement principle, called “The Magnetic Ear,” based on sharing of magnetic path between the main and an auxiliary core is proposed. The active compensation of the transformers dc magnetization level using this transducer is experimentally verified. Additionally, a classification of the previously reported magnetic flux measurement and balancing concepts is performed.