Florian Krismer

Efficiency-Optimized High-Current Dual Active Bridge Converter for Automotive Applications

An efficiency-optimized modulation scheme and design method are developed for an existing hardware prototype of a bidirectional dual active bridge (DAB) dc/dc converter. The DAB being considered is used for an automotive application and is made up of a high-voltage port with port voltage V₁, 240 V ≤ V₁ ≤ 450 V, and a low-voltage port with port voltage V₂, 11 V ≤ V₂ ≤ 16 V; the rated output power is 2 kW. A much increased converter efficiency is achieved with the methods detailed in this paper: The average efficiency, calculated for different voltages V₁ and V₂, different power levels, and both directions of power transfer, rises from 89.6% (conventional phase shift modulation) to 93.5% (proposed modulation scheme). Measured efficiency values, obtained from the DAB hardware prototype, are used to verify the theoretical results.

Accurate Power Loss Model Derivation of a High-Current Dual Active Bridge Converter for an Automotive Application

An accurate power loss model for a high-efficiency dual active bridge converter, which provides a bidirectional electrical interface between a 12-V battery and a high-voltage (HV) dc bus in a fuel cell car, is derived. The nominal power is 2 kW, the HV dc bus varies between 240 and 450 V, and the battery voltage range is between 11 and 16 V. Consequently, battery currents of up to 200 A occur at nominal power. In automotive applications, high converter efficiency and high power densities are required. Thus, it is necessary to accurately predict the dissipated power for each power component in order to identify and to properly design the heavily loaded parts of the converter. In combination with measured efficiency values, it is shown that conventional converter analysis predicts substantially inaccurate efficiencies for the given converter. This paper describes the main reasons why the conventional method fails and documents the different steps required to predict the power losses more accurately. With the presented converter prototype, an efficiency of more than 92% is achieved at an output power of 2 kW in a wide input/output voltage range.

Closed Form Solution for Minimum Conduction Loss Modulation of DAB Converters

An optimal modulation scheme that enables minimum conduction and copper losses is presented for a bidirectional dual active bridge (DAB) dc-dc converter. The considered converter system is employed for an automotive application and comprises of a high voltage (HV) port with port voltage <i>V</i>₁, 240V ≤ <i>V</i>₁ ≤ 450 V, and a low voltage (LV) port with port voltage <i>V</i>₂, 11 V ≤ <i>V</i>₂ ≤ 16 V; the rated output power is 2 kW. The closed-form expressions for the optimal control parameters are derived and implementation details are presented in order to facilitate the direct application to a given DAB converter. The paper further details the properties of the presented modulation scheme with respect to switching losses. Experimental results confirm a considerable increase of the converter efficiency achieved with the proposed optimal modulation scheme, compared to the efficiency obtained with conventional phase shift modulation. The efficiency increase is most distinct at <i>V</i>₁=450V and <i>V</i>₂ = 11V with an increase from 78.6% to 90.6% at 1 kW output power and from 85.9% to 90.7% at rated output power as compared to conventional phase shift modulation.

Performance Optimization of a High Current Dual Active Bridge with a Wide Operating Voltage Range

The main aim of this paper is to improve the performance of high current dual active bridge converters when operated over a wide voltage range. A typical application is for fuel cell vehicles where a bi-directional interface between a 12V battery and a high voltage DC bus is required. The battery side voltage ranges from 11V to 16V while the fuel cell is operated between 220V and 447V and the required power is typically 1kW. Careful analysis shows that the high currents on the battery side cause significant design issues in order to obtain a high efficiency. The standard phase shift modulation method can result in high conduction and switching losses. This paper proposes a combined triangular and trapezoidal modulation method to reduce losses over the wide operating range. Approximately, a 2% improvement in efficiency can be expected. An experimental system is used to verify the improved performance of the dual active bridge using the proposed advanced modulation method.

A comparative evaluation of isolated bi-directional DC/DC converters with wide input and output voltage range

The working principles and design equations of four different isolated, bi-directional DC to DC converter topologies (a dual active bridge converter, a series resonant converter and two multiple stage topologies) for a 2 kW bi-directional battery charger that can be operated in a wide input and output voltage range are presented in this paper. The results of a detailed mathematical analysis of the converter topologies as well as digital simulation results are used to select the most efficient topology for this specific converter application, where the two-stage series resonant converter is identified to be the most promising, with up to 90% efficiency at rated power.

Accurate Small-Signal Model for the Digital Control of an Automotive Bidirectional Dual Active Bridge

The derivation of an accurate small-signal model for a galvanically isolated, bidirectional dc-dc converter and the implementation of a corresponding controller on a DSP as well as key methods and functions required for the digital implementation are detailed in this paper. The investigated dc-dc converter, an automotive dual active bridge (DAB) system, enables power transfer between a low-voltage port (ranging from 11 to 16 V) and an HV port (240 to 450 V). The nominal power rating is 2 kW. The developed small-signal model yields highly accurate results for the DAB system, but the proposed modeling procedure could also be applied to arbitrary resonant power converters with unidirectional or bidirectional power transfer.

Optimal ZVS Modulation of Single-Phase Single-Stage Bidirectional DAB AC–DC Converters

A comprehensive procedure for the derivation of optimal, full-operating-range zero voltage switching (ZVS) modulation schemes for single-phase, single-stage, bidirectional and isolated dual active bridge (DAB) ac-dc converters is presented. The converter topology consists of a DAB dc-dc converter, receiving a rectified ac line voltage via a synchronous rectifier. The DAB comprises primary and secondary side full bridges, linked by a high-frequency isolation transformer and a series inductor. ZVS modulation schemes previously proposed in the literature are either based on current-based or energy-based ZVS analyses. The procedure outlined in this paper for the calculation of optimal DAB modulation schemes (i.e., combined phase-shift, duty-cycle, and switching frequency modulation) relies on a novel, more accurate, current-dependent charge-based ZVS analysis, taking into account the amount of charge that is required to charge the nonlinear parasitic output capacitances of the switches during commutation. Thereby, the concept of “commutation inductance(s)” is shown to be an essential element in achieving full-operating-range ZVS. The proposed methods are applied to a 3.7 kW, bidirectional, and unity power factor electric vehicle battery charger which interfaces a 400 V dc-bus with the 230 Vac, 50-Hz utility grid. Experimental results obtained from a high-power-density, high-efficiency converter prototype are given to validate the theoretical analysis and practical feasibility of the proposed strategy.

4.7 A sub-ns response on-chip switched-capacitor DC-DC voltage regulator delivering 3.7W/mm 2 at 90% efficiency using deep-trench capacitors in 32nm SOI CMOS

For an on-chip or fully integrated microprocessor power-delivery system, the on-chip power converter must 1) be designed using the same technology as the microprocessor, 2) deliver high power density to supply a microprocessor core with small area overhead, 3) achieve high efficiency, and 4) perform fast regulation over a wide voltage range for dynamic voltage and frequency scaling (DVFS). On-chip switched-capacitor (SC) converters have gained increasing popularity for this application due to their ease of integration using only transistors and capacitors readily available in the chosen technologies [1-6].

A 4.6W/mm 2 power density 86% efficiency on-chip switched capacitor DC-DC converter in 32 nm SOI CMOS

The future trends in microprocessor supply current requirements represent a bottleneck for next generation high-performance microprocessors since the number of supply pins will constitute an increasingly larger fraction of the total number of package pins available. This leaves few pins available for signaling. On-chip power conversion is a means to overcome this limitation by increasing the input voltage - thereby reducing the input current - and performing the final power conversion on the chip itself. This paper details the design and implementation of on-chip switched capacitor converters in deep submicron technologies. High capacitance density deep trench capacitors with a low parasitic bottom plate capacitor ratio available in the technology facilitate high power density and efficiency in on-chip switched capacitor converter implementations. The measured performance of a 2 : 1 voltage conversion ratio on-chip switched capacitor converter implemented in 32nm SOI CMOS technology with 1.8V input voltage results in a power density of 4.6W/mm2 at 86% efficiency when operated at a switching frequency of 100MHz.

Conceptualization and Multiobjective Optimization of the Electric System of an Airborne Wind Turbine

Airborne wind turbines (AWTs) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power kite flying at high speed perpendicular to the wind. On the kite are mounted a turbine, an electrical generator, and a power electronics converter. The electric power generated is transmitted via a medium voltage cable to the ground. Because of the high flight speed of the power kite, several times the actual wind speed, only a very small swept area of the turbine is required according to Betzs Law and/or a turbine of low weight for the generation of a given electric power. Moreover, because of the high turbine rotational speed, no gear transmission is necessary and the size of the generator is also reduced. For takeoff and landing of the power kite, the turbines act as propellers and the generators as motors, i.e., electric power is supplied so that the system can be maneuvered like a helicopter. In the present work, the configuration of power electronics converters for the implementation of a 100 kW AWT is considered. The major aspect here is the trade-off between power-to-weight ratio (W/kg) and efficiency. The dependence of cable weight and cable losses on the voltage level of power transmission is investigated, and a comparison is made between low voltage (LV) and medium voltage (MV) versions of generators. Furthermore, the interdependence of the weight and efficiency of a bidirectional dual active bridge dc-dc converter for coupling the rectified output voltage of a LV generator to the MV cable is discussed. On the basis of this discussion, the concept offering the best possible compromise of weight and efficiency in the power electronics system is selected and a model of the control behavior is derived for both the power flow directions. A control structure is then proposed and dimensioned. Furthermore, questions of electromagnetic compatibility and electrical safety are treated. In conclusion, the essential results of this paper are summarized, and an outlook on future research is given. To enable the reader to make simplified calculations and a comparison of a CWT with an AWT, the aerodynamic fundamentals of both the systems are summarized in highly simplified form in an Appendix, and numerical values are given for the 100 kW system discussed in this paper.