Current resonance type of LLC converter is widely used owing to their low switching losses; however, the problem is that they have a large transformer loss. We examine the reduction of AC resistance of the transformer winding and high coupling between the primary and secondary windings of the transformer, as a method for reducing the copper loss. In this case, it is necessary to consider the effects of the increase in stray capacitance between the primary and secondary windings of the transformer. This paper describes the influence of the loss due to the capacitance generated between the transformer windings when a noise filter is connected to the LLC converter. Furthermore, we propose a new method for reducing loss by connecting a bridge-capacitor between the primary and secondary sides of the transformer. The results of the new method are shown, and compared with those of the simulations to demonstrate effectiveness.

A new circuit and a transformer structure is described for a high-input-voltage converter operating at a high switching frequency. The two-MOSFET forward converter is suitable for a high-input-voltage converter. To increase the switching frequency, the reset period of the transformer core flux must be reduced. There are a few methods for decreasing the reset period. Increasing the transformer flyback voltage and reducing its stray capacitance are effective in decreasing the reset period without increasing power loss. A new two-MOSFET forward converter is proposed which uset the increased flyback voltage and a transformer structure to reduce the stray capacitance. The new converter using this transformer provides the basis for a 48-V, 100-W output, 270-V input converter operating at 200kHz with high efficiency (above 95%).