Novel hybrid modulation based bidirectional electrolytic capacitor-less three-phase inverter for fuel cell vehicles

This paper proposes a novel hybrid modulation technique for 3-phase dual-stage bidirectional inverter to achieve electrolytic capacitor-less design for the fuel cell vehicles (FCVs) propelling system. The 3-phase bidirectional inverter consists of front-end naturally clamped current-fed dual-active-bridge dc/dc converter and back-end voltage-fed six switches 3-phase inverter. The front-end converter is modulated by the modified secondary modulation at high frequency (HF) range and 6-pulse modulation at low frequency (LF) range. Soft-switching of all the switches of front-end converter have been guaranteed by the proposed modified secondary modulation in both power transferring directions. The 6-pulse modulation controls the high voltage dc-link bus to pulsate with 6-pulse envelope, from which 3-phase sinusoidal waveform for the FCVs propulsion can be generated. The following back-end inverter is modulated by 33% pulse width modulation (33% PWM) at HF range. At any time, only two switches (one bridge leg) are operated at HF and other four switches are maintained in either turned-on or turned-off state. Therefore the 3-phase dual-stage bidirectional inverter's switching loss can be significantly reduced through the combination of the front-end soft-switching and back-end partial switching characteristics. In addition, the use of large electrolytic dc-link capacitor can be eliminated owing to the proposed LF pulsating high voltage bus design. Simulation and experimental results have been provided in this paper to validate the proposed hybrid modulation technique.