German G. Oggier

Switching Control Strategy to Minimize Dual Active Bridge Converter Losses

A switching control strategy to control the power flow and minimize the total power losses of the dual active bridge converter topology is proposed in this paper. The control strategy consists of driving the bridge with the largest DC voltage to generate a three-level pulsewidth-modulated (PWM) voltage waveform. This PWM is ruled by two manipulated variables: the phase shift between the primary and secondary transformer voltages and the modulation index. These variables are calculated using an algorithm that is deduced on the basis of particular calculation and analysis of converter losses, which are also presented in this paper. An experimental prototype was implemented to validate the theoretical analysis and feasibility of the proposal. The experimental results revealed that the overall efficiency of this converter can be improved up to 10% using the control strategy instead of the conventional one.

Modulation strategy to operate the dual active bridge DC-DC converter under soft switching in the whole operating range

A new modulation strategy that allows operating the dual active bridge (DAB) dc-dc converter under soft switching in the whole operating range is proposed. This strategy is ruled by imposing a certain modulation index in one of the two bridges and a phase shift between the transformer primary and secondary voltages. Moreover, the proposed algorithm reduces the reactive power and thus reducing the converter conduction losses. An experimental prototype was implemented and some experimental results are presented to validate the theoretical analysis. The experimental results reveal that the overall efficiency of the DAB topology can be improved up to 20% by implementing the proposed modulation strategy instead of the conventional one.

Extending the ZVS Operating Range of Dual Active Bridge High-Power DC-DC Converters

A switching control strategy to extend the soft-switching operating range of the dual active bridge (DAB) dc-dc converter under the zero-voltage-switching (ZVS) operating mode is proposed. The converter topology consists of two active bridges linked by a high-frequency transformer. One drawback of this strategy is that soft-switching is only possible in a restricted converter operating region. A novel pulse width modulation strategy to extend the conventional soft-switching operating mode region and its analysis are presented in this paper. Experimental results are given in order to validate the theoretical analysis and practical feasibility of the proposed strategy.

Fast Transient Boundary Control and Steady-State Operation of the Dual Active Bridge Converter Using the Natural Switching Surface

This paper presents a boundary control scheme for dual active bridge (DAB) converters using the natural switching surface (NSS). The implementation of a curved switching surface for DAB converters is a new area of research undertaken in this paper. The proposed technique brings the benefit of unprecedented dynamic performance, already developed for nonisolated topologies (e.g., buck and boost), to this more complex isolated topology. The analysis provides insight into the natural trajectories of the DAB converters and creates an accurate framework in the normalized geometrical domain. As a result, the physical limits of the converter under study become evident. Those physical limits are exploited by employing the NSS to obtain fast transient response under start-up, sudden load transients, and reference change. In addition, fixed-frequency operation is one of the key features of the proposed control scheme, which allows optimizing the design of the high-frequency transformer. Experimental results are presented to validate the NSS for DAB converters and illustrate the benefits of the normalization technique.

Soft-switching analysis for three-port bidirectional DC-DC converters

The behavior of three-port bidirectional DC-DC converters (TPC) under soft-switching mode for hybrid electric vehicles is analyzed in this paper. The principle of power flow control and the operation under soft-switching mode are studied to establish the converter operation range. All this allows to establish design criteria to operate the TPC under soft-switching within a wide operation region. The parameters considered are the transformer turns ratio and the leakage inductance of the transformer, which allows determine the maximum power that can be transferred. Simulation results that validate the analysis are also presented.

Boundary Control of Full-Bridge ZVS: Natural Switching Surface for Transient and Steady-State Operation

This paper presents the use of a high-performance boundary controller for the full-bridge zero-voltage-switching topology. An enhanced dynamic response is obtained by employing the natural switching surface (SS), which is thoroughly derived in the normalized geometrical domain. The advantages of the normalization are the simple graphical representation, the generality for any combination of parameters, and the mathematical simplicity. Recently, nonisolated basic topologies have benefited from advancements in boundary control. The analysis and derivation in this work bring the benefit of outstanding dynamic performance to this isolated topology. As demonstrated in this work, the relationship between the leakage and output filter inductances makes the formulation of the natural trajectories for isolated converters possible. The resulting SS provides an excellent dynamic response during start-up, reference change, and sudden output loading conditions. Experimental results are presented to illustrate the characteristics and advantages of the control scheme and the converter operation with fixed switching frequency.

High efficiency switching sequence and enhanced dynamic regulation for DAB converters in solid-state transformers

This paper presents an advanced switching sequence based on boundary control for the Dual Active Bridge (DAB) DC-DC converter applied in a Solid-State Transformer (SST). The implementation of the switching sequence follows the recent advancements in curved switching surfaces for DAB converters - the Natural Switching Surface (NSS) - and contributes to higher efficiency and transient regulation. The proposed switching sequence improves the overall efficiency of the converter by ensuring soft switching transitions in the full operating range. Furthermore, it incorporates a fully controlled burst-mode switching sequence for light loading conditions to further extend the efficiency gains. Dynamic regulation, an important requirement in SST, is also addressed by the proposed boundary-based sequence. As a result, excellent dynamic response is obtained by taking advantage of all the possible switching structures of the converter. The analysis provides insight into the natural trajectories of the converter, which produce soft-switching transitions and enable the converter structures to achieve the target operating point directly. Simulation and experimental results are presented to validate the benefits of the switching sequence and illustrate the dynamic regulation.

Bidirectional power flow with constant power load in electric vehicles: A non-linear strategy for Buck+Boost cascade converters

The constant power behavior of tight-speed controllers in the vehicles traction system and tightly regulated DC-DC converters connected to the HV-DC bus produces instability effects. This paper proposes a simple and practical non-linear control, using Circular Switching Surfaces, to address constant power load instability in electric vehicles power systems. The proposed control technique provides a solution in the geometrical domain to constant power loading conditions while achieving outstanding dynamic response. The controller is implemented in a bidirectional Buck+Boost cascade converter as a battery charge/discharge unit and ensures reliable system operation. The predictable and consistent behavior of the converter with constant power load is presented by analyzing the system curves in the normalized phase plane with the switching surfaces employed. Simulation and experimental results on a scaled 2kW Buck+Boost cascade converter validate the proposed switching surfaces and predictions regarding the converters behavior under constant power loading conditions.

Analysis and evaluation of power switch losses for three-port bidirectional DC-DC converter

An analysis and evaluation of power switch losses of three-port bidirectional DC-DC converters to be used in hybrid electric systems is presented in this work. The analysis is carried out considering all the different converter operation modes. The evaluation considers different voltage levels, different switches (IGBT and MOSFET) and different high-frequency transformer leakage inductance. As a result, design considerations for a converter, specially the high-frequency transformer are obtained in order to operate the converter with low losses. Simulation results are presented in order to validate the theoretical analysis.

Open transistors and diodes fault diagnosis strategy for Dual Active Bridge DC-DC converter

The operation of a Dual Active Bridge (DAB) DCDC converter when one of its power semiconductors presents an open circuit fault is analyzed in this paper. In order to be able to detect this sort of failure, a new diagnosis strategy is proposed. The strategy consists basically of two stages: 1) measure the voltage drop in each power semiconductor in on-state by a proper modification in the driver circuits and 2) feedback the fault signal generated in the drive circuits to the microcontroller to decide the next action. Then, the voltage measured is compared with a reference value to determine if a semiconductor presents an open-circuit fault. The proposed strategy has the advantage that it can be implemented without including additional sensors. Simulation and experimental results are included to validate the theory.