DC-DC Converter For Fuel Cell (Control Strategy, Voltage Curve, Optimization, Converter Type). Current-fed Full-bridge Isolated DC-DC Converter With An Active Clamp Circuit For Fuel Cell Application

Abstract

This thesis investigates one of the most favorable converters for integrating fuel cells into a DC microgrid. A current-fed full-bridge isolated DC-DC converter has advantages such as high voltage conversion ratio, low switches’ losses, and low input current ripple which make it one of the best choices for Fuel cells (FC). This converter transfer power to the load through the leakage inductance which can be controlled by regulating the duty cycle of switches. Also, the transformer in the converter can solve the lack of galvanic isolation in FC to some extent by creating a barrier between the FC and loads. The simulation of the converter is done while assuming the minimum voltage of FC is 800V and the maximum voltage is 1000V. The switching frequency of the converter is assumed to be 20kHz and the DC voltage link is 1000V. The simulation’s result of the converter shows huge voltage overshoots across the switches which may damage the transistors and other components. This surge of voltage across the switches is one of the disadvantages of current-fed converters and can be removed considerably by adding a clamp circuit. As the output of an FC stack changes from no load to full load, the controller is designed for the converter to provide the constant DC voltage link. The small-signal analysis of the converter leads to the transfer function of steady-states and the calculation of the controller parameters. The designed controller consists of an outer voltage and an inner current loop. The measured output voltage and boost inductor current are input to the controller and the duty cycle of switches is the output. The simulation in MATLAB and Simscape verifies the results and performance of the converter and controller. The whole system is redesigned with scaled values to implement the circuit in the laboratory. The new assumptions are minimum and maximum output voltage of FC, 12V, and 20V respectively, and DC voltage link of 20V (due to a high current passing through the circuit, the circuit is designed for quite low voltages). Intelligent Power Modules (IPM) are kinds of power integrated circuits and applications that can benefit from the outstanding performance and the efficiency of these devices. IPMs are integrated the Insulated-Gate Bipolar Transistor (IGBT) with high speed, low power drive circuit, and protection circuit together and mostly need an external evaluation board to drive them. The IPM50RSA120 contains 7 IGBTs, 4 of them are used as the primary side of the converter (with its evaluation board). Also, another IPM is used as an auxiliary clamp switch. This module is designed for 1200V, 50A, and frequency operation range from 5kHz to 20kHz. The built transformer, diode bridge, clamp switch, and the other components are used for implementing the converter. The physical model is controlled by the dSPACE 1202 and fulfills the expectation to some extent. One problem here is the performance of the circuit. The efficiency of the designed circuit is about 97% in simulation while efficiency in the laboratory, in minimum voltage of FC, is less than 50% and maximum voltage is around 70%. The circuit is tested under different input voltages, frequencies, and loads to realize the causes of this relatively poor performance. It is observed that the performance of the system is improved with higher voltage, lower frequency, and higher loads.