Abstract
Due to several factors, including the increasing utilization of renewable energy and the trend towards electrification of mobility, there is a strong demand of electrical power conversion solutions for the interfacing between different systems, such as energy storage devices, renewable sources, local and wide grids and specific loads. In this scenario, bidirectional isolated DC-DC converters are attracting more and more attention, because of their versatile application areas. Despite the strong commitment and investments in research by industry and academia, many challenges remain open and many aspects would benefit even from small improvements in several aspects, especially efficiency, power density, reliability and cost. As in almost all applications of power electronics, there is a strong focus on efficiency, since thermal issues are crucial and ultimately significantly affect the cost and density of converters.
Among many applications, medium and large-power energy storage systems are expected to become very common in a few years, especially in the automotive market, given the growth trend of electric mobility. Converters operating as interfaces to batteries, for example, typically require isolation between input and output, operation across a wide range of input or output voltage and current, ensuring reliability, efficiency and controlled (smooth) operation. Considering these requirements, the Dual Active Bridge (DAB) has been selected in this work, because of its flexibility and potential efficiency. Before this choice, advantages and disadvantages of the most promising topologies of bidirectional isolated DC-DC converters have been compared. The single-phase DAB converter topology is considered most suitable with respect to the achievable converter efficiency, its ease of bidirectional operation, modular structure, and power density (due to the low number of inductors and due to the employed capacitive filters on the HV side and on the LV side), with respect to other competing topologies. Although this topology has been investigated quite extensively, the control of DAB still poses some challenges, due to the multiple control variables that affect the complex behavior of the converter. Moreover, in the design of hardware and control there are several goals, e.g. regulating power flow in the two directions and in the whole range, achieving soft-switching (ZVS) and minimizing current stresses.
In the analysis of the Dual Active Bridge, as for other converters, different points of view and abstraction levels need to be considered, e.g. the ideal behavior (steady state, dynamics), analysis of loss mechanisms, optimization of parameters and control. Following this need, in this work a theoretical model for the DAB converter is proposed. The model, which considers ideal (non-dissipative) components, is very general, i.e. it can be applied to any modulation technique. In fact, all the possible combinations of phase-shifts between the converter legs (i.e. all the “degrees of freedom” of the system) can be incorporated and studied with the same model. The analytical developments presented, based on the superposition principle, allow obtaining the waveforms of inductor and output current using a simple and fast closed-form procedure. This approach can thus replace dynamical simulations, when only steady-state behavior is to be analyzed, with much faster execution. This allows to apply optimization methods for the selection of the operating point or during the design stage (e.g. for selecting inductance and frequency values). Moreover, a novel fully analytical model describes the output current vs. phase shifts relationship. The results obtained describe the average output current (cycle-by-cycle) as a function of input voltage and phase-shift values only, i.e. independent of output voltage. Such a model is very suitable for characterizing the dynamical behavior (e.g. for control purposes), since the converter can be considered a controlled (average) current source, as typically applied to with other converters.
Based on the analytical results, which link the desired output current with the corresponding phaseshifts, a novel control loop is proposed, which adopts a “fictitious” (i.e. open-loop) inner current regulation loop. The main advantage of this control scheme is the ability to decouple the simple dynamics of output voltage vs. average output current (dominated by the output capacitor) with respect to the complicated relationship linking phase-shifts to the output current. This control approach becomes straightforward in the case of Single Phase Shift (SPS), but can be applied to any other modulation scheme. In fact, by leveraging the DAB behavioral model developed and mentioned above, an optimization procedure is set up, which results in the choice of optimal operating points, given input voltage and desired output average current. The performance is evaluated and compared under both controls methods, with variable output voltage. Moreover, a Finite Control Set method is proposed, which selects the optimal operating points for each operating condition and control request, ensuring full Zero Voltage Switching in all cases.
The analytical developments and proposed control methods have been verified through simulations (using PLECS models) and experimentally, considering in particular the average output current at different operating points. Experimental tests have been carried out on a single-phase DAB prototype board, showing good agreement with theoretical findings and confirming the validity of the proposed closed-loop SPS control with “fictitious” (open-loop) current control. Tests with arbitrary (optimized) phase-shift triplets were also performed, validating the proposed optimization method, especially with respect to the extension of ZVS operation range. In addition, by analyzing the converter performance, some interesting second-order effects (related to dead-times) were highlighted, which require further investigation.