Future work

In order to achieve higher efficiency and power density figures future research of the multiple port dc/dc transformers could be related to:

 Synchronizing of the driving signals with tank currents to assure

load independent ZCS operation. The expected operation of the

converter’s power stage was verified in the open loop. In a closed- loop test the switching frequency and duty cycles of the switches automatically adapt to the resonant capacitors tolerances and the magnetizing current effect.

Multiple Input-Output Bidirectional Solid State Transformer Based on a Series Resonant Converter

152

 Utilizing GaN transistors. The comparison of the Si and enhancement-mode GaN (eGaN) devices has already been demonstrated in [34] on an unregulated, fixed ratio dual active bridge dc/dc converter. eGaN FETs from [35] have significantly improved gate drive (QG x RDS(on)) and output charge (Qoss x RDS(on))

figure of merits when compared to Si MOSFETs. Lower gate charge of the eGaN FETs together with the required lower gate drive voltages lead to significantly reduced gate drive losses which are the only switching related losses in the converter. Reduction of the energy stored in the output capacitors results in lower magnetizing current needed to discharge them which has a great beneficial impact on both the high frequency transformer design or lower dead time interval providing higher power delivery duty cycle and reduced rms currents. Another advantage of the eGaN FETs over Si MOSFETs is their linear grid array (LGA) package with interleaved drain and source pads which minimizes package parasitic inductance and resistance.

 Transformer optimization. The resonant nature of the proposed multiple port dc/dc transformer allows for increasing the switching frequency of the active power devices. However, as the frequency increases, the core and winding losses of the high frequency transformer also increase which may lead to the excessive temperature rise in the magnetic component and limit power density increase and foot print reduction. The optimization of the high frequency transformer design, therefore, requires special attention since it is the most bulky component in the converter. Additional improvements are required in order to achieve higher efficiency conversion and reliable converter operation. In order to spread the dissipated power and drop the temperature rise of the transformer a two transformer structure can be considered with the primary sides connected in series and the secondary sides connected in parallel, [36]. By dividing the transformer primary side voltage the number of the primary side turns or the core size can be reduced. This would lead to lower magnetizing inductance requiring smaller gap to be added which is highly beneficial since

Conclusions and Future Work

the introduction of the gap induces additional winding losses due to the fringing flux. Future work should also include investigation for a low permeability magnetic material suitable for high frequency operation to achieve lower magnetizing inductance and to need smaller gap to adjust its value for proper ZVS operation.

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