Abstract
The Dual Active Bridge converter is an isolated bidirectional DC-DC converter with the ability to operate efficiently over a wide voltage and load range which can be controlled via simple means. This topology has become widely prevalent in battery charging systems of domestic energy storage systems and electric vehicles. The Dual Active Bridge is also well known for its versatility for having galvanic isolation, high frequency capability as well as a reduced and compact size.In this thesis, the author presents a thorough mathematical analysis which relates the modulation strategies being investigated, i.e., Single Phase Shift (SPS) and Dual Phase Shift (DPS) to the Zero Voltage Switching concept exhibited by the Dual Active Bridge. ZVS is a soft switching feature possessed by the DAB converter for operation at high frequency with zero turn-on switching losses. The mathematical analysis involves formulating the switching interval inductor current and turn-on transition current equations. Zero Voltage Switching is determined by identifying the polarity of the turn-on instant current equations corresponding to the designated switching devices.
Dual Phase Shift (DPS) control offers the advantage of an additional degree of freedom of inner phase shift angle, α to the pre-existing outer phase shift, φ with the Single-Phase Shift modulation. Extensive mathematical analysis of the Dual Phase Shift modulation was carried out based on its four conditions depending on the relationship between α and φ. The relationship between Zero Voltage Switching and voltage conversion ratio, m was investigated through the two methods, i.e., SPS and DPS by which the Dual Active Bridge could be modulated. The inductor current and voltage expressions exist as functions of input voltage, output voltage, turns ratio, switching frequency, the phase shifts and inductor.
The next two chapters explore the concept of Variable Transformer and how its integration benefits the Dual Active Bridge in terms of Zero Voltage Switching and power output. A cantilever model of the Variable Transformer was derived containing the corresponding parameters of a realistic transformer, i.e., series leakage inductance, magnetising inductance and equivalent turns ratio. The VT integration with the DAB was simulated for output voltage fluctuations, i.e., 130V-180V. Adjustment to the VT equivalent turns ratio puts the DAB back into ZVS operating range by restoring its voltage conversion ratio, m to unity from a non-unity value.
| Date of Award | 10 Dec 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | David Drury (Supervisor) & Ian Laird (Supervisor) |