COMPREHENSIVE STUDY ON MODULATION OF A DUAL ACTIVE BRIDGE CONVERTER
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In recent years, the single-phase Dual Active Bridge (DAB) converter has gained significant importance as it has simple power circuit, bi-directional power flow, and zero voltage switching (ZVS) operation. Although, numerous researchers have worked on this converter and proposed many modulation techniques, a thorough mathematical treatment of this converter has been missing from the literature. This thesis presents a thorough mathematical treatment of the DAB converter. All the possible theoretical modulation patterns for the DAB converter are derived from the mathematical analysis. It is mathematically proven that there are up to 140 distinct switching patterns that can be applied to the converter. It is shown in the thesis that there are only two modulation patterns those result in the optimal performance of the converter in terms of minimum rms current and guaranteed zero-voltage-switching under wide range of operating conditions. An analytical solution is developed and a step-by-step algorithm to obtain optimal time ratios for the two selected modulation patterns (denoted by Pattern (z) and Pattern (q) in this thesis) is given. The concept of idle time interval is introduced in this thesis. Then it is shown that the optimal modulation is based on pattern (z) with no idle time interval and pattern (q) with idle time interval. An optimal pattern modulation technique based on variable frequency has been introduced. It has been shown that a frequency variation of about 50% around the optimal power level corresponding to the maximum pattern efficiency can further reduce the conduction losses by 10%. It is also shown how the optimal modulation can be extended to the scenario with reverse power flow as well as with an inverse ratio of voltages. The modulation pattern presented here is the optimal solution for every DAB converter with arbitrary parameters.