Analysis and Design of a Single Ended Resonant Reset Forward Converter

Abstract

A resonant reset forward converter was developed as one of the topologies of the forward converter because it offered the simplest technique of transformer core resetting. The resonant reset topology does not require any external passive components or any tertiary winding to facilitate the core reset but it utilizes the magnetizing inductance of the transformer and the parasitic capacitances of the devices to bring down the core flux to its initial position and hence avoids core saturation of the transformer, which is the principle working device in the converter from the point of view of electrical isolation and reproduction of good quality dc voltage at the output (or the load). The saturation of the transformer would have otherwise led to heavy inrush current resulting in huge losses and degradation in efficiency of the converter. Thus in a low power dc-dc converter this resonant reset topology of the forward converter offers significant advantages. The small signal model of the converter which was proposed using the switch averaging method was validated using the software PSPICE and it gave perfect ripple free dc waveform at the output. The closed loop stage involved the development of a type IIIB controller to generate a sufficient phase margin and hence improve stability. The Optocoupler was also used in the feedback for the purpose of maintaining the electrical isolation and the pole which it generated (due to Miller’s capacitance) was compensated by manually placing a combination of a parallel resistor and capacitor at the output of the collector terminals of the transistor of the Optocoupler. Finally the hardware developments were done for the voltage mode controlled feedback loop using IC UC 3525A which gave satisfactory results for various load tests and desired input voltage fluctuations.