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 lossless forward resonant converter with a coupling ranging from 0 to 1 was studied. The peak voltage and current stress of the power switch were compared as a function of the coupling. It was found that as the coupling decreases from 1 to 0.9, the peak switch voltage reduces dramatically whilst the peak current increases slowly. Therefore, at the expense of a small increase in peak current, a coupling of 0.9 decreases the peak voltage to approximately three times the input voltage. It was also found that the traditional quasiresonant 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. An experimental 48-5 V 20 W forward converter with a coupling of 0.9 was designed and tested. It verified the practicality of a reduced coupling transformer and yielded an extremely simple converter with a minimum parts count.