Analysis of a multiresonant forward converter based on nonideal coupling of the transformer

There are many transformer applications where tight coupling is difficult to achieve. Therefore, an analysis of a resonant converter with a nonideal coupling is required. In this paper, a forward resonant converter with a coupling ranging from zero to one was analyzed. It was found that the traditional quasi-resonant converter becomes a multiresonant converter when the coupling is less than one. This is because the finite switching time of the rectifying diode reduces the input inductance of the transformer to effectively give a converter that uses two different inductances during any one switching period. It was also found that, in general, the converter has seven topological states. The design equations are derived analytically to provide a fundamental understanding of the converter. The peak voltage and current stress of the power switch as a function of the coupling were investigated. It was found that as the coupling is reduced, the peak voltage reduces and the peak current increases. A 48-5 V 20-W forward converter with a coupling of 0.9 was designed and tested. A coupling of 0.9 was chosen as it gives a duty cycle of 50%, has zero voltage switching for all loads, and has a peak switch voltage of 3.3 times the input voltage. The experiment verified the analysis and the practicality of a reduced coupling transformer, and the measured efficiency of the converter was 80%.