The global demand for fossil fuels in the transportation sector is increasing rapidly due to the continuous growth of internal combustion engine vehicles. This leads to severe environmental problems, including greenhouse gas emissions and air-quality deterioration. Thus, it is necessary to increase the use of renewable energy sources in the transportation sector as well as other off-grid applications. Battery and fuel cells are promising alternatives owing to high efficiency and low (even zero) local emissions. However, they are limited by either the low capacity or sluggish dynamic response. These shortcomings can be overcome by the hybridization of battery and fuel cells, which have been the focus of leading international automotive and shipbuilding companies. This paper presents a comprehensive evaluation and comparison of different hybrid systems of Proton Exchange Membrane Fuel Cell with battery and Solid Oxide Fuel Cell with battery for mobility and other off-grid applications from perspectives of system configurations, technical specifications, energy management strategies, and experimental validation. With the existing issues and corresponding solving strategies highlighted, the suggestions for designing high-performance fuel cell hybrid power systems are concluded accordingly. This review can serve as a reference and guide to advance the development of the fuel cell and battery hybrid power systems for mobility and off-grid applications.

Fuel cell-battery hybrid systems for mobility and off-grid applications: A review

Energy management of heavy-duty fuel cell vehicles in real-world driving scenarios Robust design of strategies to maximize the hydrogen economy and system lifetime

Optimization and cutting-edge design of fuel-cell hybrid electric vehicles

Automotive on-board dc–dc converters are required to operate over a wide input and output voltage range depending on the state of charge of the input and the output battery. Conventionally, the unidirectional power transfer between these batteries is enabled by a two-stage converter concept. A first-stage nonisolated dc–dc converter regulates the input voltage of the galvanically isolated second-stage dc–dc converter such that the second-stage converter operates in its optimum operating point. This article presents a single-stage interleaved LLC resonant converter of 3.6 kW for this purpose. While LLC converters are usually not suitable to cover such a wide voltage range (the input voltage between 240 and 420 V and the output voltage between 8 and 16 V), this LLC converter is operated in a full-bridge mode for large gains and in a half-bridge mode for low gains. For intermediate gains and loads, the LLC employs the phase-shift mode. To operate the interleaved LLC s at equal switching frequencies enabling output current ripple cancellation to reduce the output capacitor, again, the phase-shift mode is utilized to balance the power transfer during the full-bridge mode while the asymmetrical duty-cycle mode is proposed for power balancing during the half-bridge mode. This article analyzes the converter design for these modes of operation and provides a comprehensive design procedure allowing the designer to simultaneously analyze all stress values for various resonant tank designs. A 3.6-kW prototype employing Si superjunction MOSFETs achieves a power density of 2.1 kW/L. The maximum efficiency reaches 96.5%, while for most operating points, it is kept well above 90%. The experimental measurement results validate the analysis and show that phase-shift operation and asymmetrical duty-cycle modulation can be utilized for power balancing for full-bridge and half-bridge configurations, respectively, such that a much smaller output capacitor can be employed.

Interleaved Single-Stage LLC Converter Design Utilizing Half- and Full-Bridge Configurations for Wide Voltage Transfer Ratio Applications

Experimental and numerical study of energy and thermal management system for a hydrogen fuel cell-battery hybrid electric vehicle

Phase-shift modulated full bridge converters suffer from thermal imbalances of the inverter switches. The lagging leg switches are subject to larger commutation currents compared to those of the leading leg as the transformer current reduces in the freewheeling interval. Furthermore, after this interval, the energy in the series inductance may not be large enough to achieve zero-voltage switching (ZVS) for the leading leg. Both effects result in thermal imbalances. This paper analyzes the alternating-asymmetrical phase-shift modulation to achieve balanced conduction and switching losses for all four switches while showing that this modulation is easily implemented on standard DSPs. The modulation has been implemented to LLC converters where experimental measurement results proved its effectiveness for LLC converters by reducing the temperature deviation from 6.3 K to only 0.2 K such that the peak temperature is reduced from 95 °C to 92 °C. The paper also proves that the modulation can be utilized to improve the efficiency of LLC converters operated at very low gains while simultaneously reducing the junction temperature of all four switches compared to the conventional complementary modulation. Finally, EMI implications are analyzed, which show that the modulation may be beneficial for reducing the common-mode emissions around the operating frequency.

Alternating Asymmetrical Phase-Shift Modulation for Full-Bridge Converters with Balanced Switching Losses to Reduce Thermal Imbalances

On-board DC-DC converters are required to operate over a wide input and output voltage range depending on the state-of-charge of the input and output battery. Conventionally, the power transfer between these batteries is enabled by a two-stage converter concept where a galvanically-coupled DC-DC converter regulates the input voltage of the second-stage galvanically-isolated DC-DC converter. This paper presents a single-stage interleaved 3.6 kW LLC converter for this purpose. While LLC converters are usually not suitable for such a wide voltage range, this LLC converter is operated in full-bridge mode for large gains and in half-bridge mode for low gains. For intermediate gains and loads, the LLC makes use of phase-shift mode. To operate the interleaved LLCs at an equal switching frequency enabling output current ripple cancellation, again phase-shift mode is utilized to balance the output currents during full-bridge mode while asymmetrical duty-cycle mode is proposed for current balancing during half-bridge mode. This paper analyzes the converter design for these modes of operation. A 3.6 kW-prototype employing Si-superjunction MOSFETs achieves a power density of 2.1 kW/1. The maximum efficiency reaches 96.5 % while for most operating points it is kept well above 90 %.

Dual Interleaved 3.6 kW LLC Converter Operating in Half-Bridge, Full-Bridge and Phase-Shift Mode as a Single-Stage Architecture of an Automotive On-Board DC-DC Converter

In an electric vehicle (EV), two major voltage levels have to be coupled by an on-board DC-DC-converter - the high voltage traction battery of about 400 V and the auxiliary battery of 12 V. Due to the varying state of charge (SOC) of the batteries, the converter has to cover very wide input and output voltage ranges. Considering recent developments in semiconductors, the active clamp forward converter is an interesting topology since only a very small energy is required in the series inductance for soft turn-on of the main switch. In the design process, however, common methods are not suitable because the clamp capacitor voltage cannot be calculated accurately. This paper presents a novel analytical model, which can be used for accurate calculation of the shape parameters. Results show a maximum deviation of 1.8 % from detailed numerical simulations, which proves that the proposed model is an excellent basis for the converter design procedure. A component stress analysis built on this model demonstrates the potential of this topology for the described application.

Analytical Modeling and Design of an Active Clamp Forward Converter Applied as a Single-Stage On-Board DC-DC Converter for EVs

DC-DC converters connecting the traction battery with the conventional auxiliary battery in EVs are characterized by a major challenge: The required voltage ratio (gain range) depends on the input and output voltages, both varying considerably with the state of charge (SOC) of their batteries. However, the properties of new wide-bandgap semiconductors lead to modified design requirements of the converter, offering some additional design freedom. This paper presents an analytical converter comparison based on a modified component stress factor analysis. Five converters are compared: The phase-shifted full bridge (PSFB), the LLC resonant converter (half bridge and full bridge with varying operating modes), and the active clamp forward converter (ACFC) (non-interleaved and interleaved). The comparison shows that the ACFC and the LLC with varying operating modes are attractive topologies while the half-bridge LLC converter and the PSFB have generally higher stress values.

Philipp Rehlaender

Papers

Interleaved Single-Stage LLC Converter Design Utilizing Half- and Full-Bridge Configurations for Wide Voltage Transfer Ratio Applications

Alternating Asymmetrical Phase-Shift Modulation for Full-Bridge Converters with Balanced Switching Losses to Reduce Thermal Imbalances

Dual Interleaved 3.6 kW LLC Converter Operating in Half-Bridge, Full-Bridge and Phase-Shift Mode as a Single-Stage Architecture of an Automotive On-Board DC-DC Converter

Analytical Modeling and Design of an Active Clamp Forward Converter Applied as a Single-Stage On-Board DC-DC Converter for EVs

Analytical Topology Comparison for a Single Stage On-Board EV-Battery Converter