Three-level Frequency-Doubling LLC Resonant Converter with High Step-down Ratio for High Input Voltage Applications

In this paper, a frequency-doubling LLC resonant converter with high step-down voltage gain for high input voltage applications is proposed. The three-level structure consisting two series half bridges is adopted in the converter to reduce the voltage stresses of the primary-side switches to only half of the input voltage. Thus the low-voltage rated power devices could be used to improve the converter performance. On the other hand, a single conventional resonant tank is shared by both the half bridges, which reduces circuit complexity and eliminates the negative effect brought by parameter inconsistency. Moreover, by applying symmetrical driving signals to the bridges, the input capacitor voltage unbalance is suppressed effectively without extra components or control overhead. By adopting the proposed control scheme, the resonant tank operating frequency is twice as the device switching frequency, which largely reduces the switching loss and increases the converter power density. In addition, the converter voltage gain is only 0.25 exclusive of the transformer, which makes suitable for high voltage step-down applications. Finally, a 600V-input 48V-output 800W prototype is built to demonstrate the effectiveness of the proposed converter and analysis.