On frequency domain analysis of dual active bridge dc-dc converters
Creators: name: Riedel, Jan
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400801
subject: Circuits and systems
classificationCode: 400805
subject: Electrical energy transmission, networks and systems
classificationCode: 400907
subject: Industrial electronics

description: Modern society uses electrical energy for a wide range of needs and requirements. Electrical energy is considered high value as it requires a prior conversion step from kinetic/thermal or solar energy. However, electrical power is always defined by certain properties which typically need to be adjusted in multiple stages to satisfy the specifications of electrical loads such as motors, lighting and consumer electronics. For DC (direct current) power systems, switching DC-DC power converters are the state-of-the-art solution to achieve a low-loss modification of the voltage magnitude.<br><br>The Dual Active Bridge (DAB) converter is an attractive DC-DC conversion topology that can widely satisfy the future needs of DC power management and the integration of electro-chemical storage. It offers an unmatched capability to transfer energy in either direction between two DC sources while its inherent Zero Voltage Switching capability offers potential for high conversion efficiency and high power density.<br><br>The current and future research activities on DAB converters mainly focus on maximising the power density through a volume reduction of the embedded passive power devices. This trend is encouraged by the market introduction of wide bandgap fast-switching semiconductor devices using Silicon Carbide (SiC) and Gallium-Nitride (GaN) to replace conventional Silicon material in many applications. The reduced parasitic capacitance and transition time of these devices allow to significantly increase the converter operating frequencies, which is the only way to increase the power density unless the material specifications of passive power devices drastically evolve. However, a higher operating frequency inevitably leads to a stronger influence of practical second-order effects, which for a DAB, particularly address the non-ideality of the switch devices, the parasitic coupling impedances in the high-frequency transformer, the peripheral connecting traces of the AC link network and the DC bus filter. Hence, all these effects have to be accommodated by a universal design framework which is yet to be found in literature.<br><br>A DAB is conventionally designed using time domain analysis of the modulation sequence and device waveforms to evaluate its key performance design criteria such as active power transfer, Zero Voltage Switching (ZVS) and AC link circulating power. This analysis technique typically presumes an idealized single parameter AC link inductance to substitute for the more complex circuit model of a practical high-frequency transformer. This becomes particularly relevant as the operating frequencies increase, causing both active and passive power devices to become less ideal. More than that, advanced multi-level DAB Phase Shifted Square Wave (PSSW) modulation strategies lead to a wide solution space of control parameters that can be used to enhance the performance of a DAB by shaping the AC link current in certain ways. Within the time domain, such volatile modulation strategies require a complicated structure model analysis.<br><br>This thesis now shows how to apply frequency domain harmonic analysis techniques to a DAB DC-DC converter. The approach readily accommodates the influence of complex impedance structures, practical switching effects and advanced multi-level modulation concepts, and leads to generic numerical and analytical solution expressions that significantly enhance the converter design process. The work thus establishes a new analysis strategy in the advancing field of DAB research.<br><br>The thesis begins with the harmonic decomposition of the bridge output voltages and the expression of the DAB coupling network as a generic two-port impedance model. These steps establish the frequency domain analysis (FDA) framework.<br><br>Next, the FDA approach is applied to derive explicit solution terms for the ZVS regions of single and three-phase DAB converters, which are crucial to minimise the power loss of the semiconductor devices during the switching transition. These expressions are used to separately investigate the impact of single impedance parameters, non-ideal switching transitions and PSSW modulation concepts on the ZVS limiting conditions, which determine the preferable operating regions to achieve minimum switching loss operation and best possible controllability of the DAB. From this work, it is shown how a single element high-frequency transformer with a reduced coupling factor is sufficient to ensure continuous and reliable ZVS operation without adding software or auxiliary hardware complexity. More than that, the strategy quantifies the impedance design parameters of the coupling network in correlation with the selected PSSW modulation concept. In this context, the benefit of adaptive 3-level DAB modulation is presented to support continuous ZVS operation and thus allow for high-efficient operation of a single phase DAB across its entire operating range.<br><br>Finally, the established modeling framework is extended to identify the DC bus harmonics injected by the PSSW modulation process, for any particular DAB design and operating context. It is also shown how adaptive modulation can be used to mitigate certain harmonic frequencies that can otherwise cause severe harmonic interferences, DC bus oscillations, and thus impact on the DC side filter design process. To complete the work, experimental results are presented to verify the analytical development, using a laboratory DAB converter that can operate across a wide range of DC voltages at power levels up to 1 kW. Simultaneously, as part of an industry project with BOSCH, a customised 1.2 kW DAB prototype was built using the presented design guidelines, which achieved a conversion efficiency between 96.5% and 98.5% across the operating range.