An LLC-Type Resonant Forward Converter With Adjustable Turning-Off Time Control

This paper proposes a LLC type resonant forward converter using adjustable turning-off time control for low power and voltage applications. The new circuit is derived from the conventional LLC converter, and it has simple structure while maintaining decent soft-switching characteristics. The switching losses are reduced by ZVS (zero voltage switching) and ZCS (zero current switching). Also, regulation of the output voltage does not require a small ratio between magnetizing inductor and resonant inductor, so the leakage inductor of transformer can be utilized instead of an external inductor to realize resonance, which makes it possible to use only one magnetic component in the resonant topology. Besides, the adjustable turning-off time control strategy is adopted in the scheme, which optimizes the switching condition while ensuring the wide range operation. In comparison to the traditional PWM (pulse width modulation) control, the proposed control strategy can further reduce the switching loss and improve power efficiency. This is also suitable for the utilization of GaN (gallium nitride) devices due to the requirement of higher switching frequencies at wide input applications. In addition to the detailed theoretical analysis, the validity of the proposed concept is confirmed by the experimental results of a prototype.