A Half-Bridge LLC Resonant Converter Adopting Boost PWM Control Scheme for Hold-Up State Operation

This paper presents a half-bridge LLC resonant converter having a boost pulse width modulation (PWM) converter characteristic for hold-up state operation. The proposed converter is based on a half-bridge LLC resonant converter structure and a single auxiliary switch is added at the primary side. The converter has two different operational characteristics. It shows the same operational characteristic with the conventional LLC resonant converters during nominal state, which is frequency modulation (FM) method. However, when ac line lost and the converter enters into the hold-up time state, which requires wide voltage gain changes, the control method of the proposed converter is changed to the PWM method using the auxiliary switch. Since the proposed converter compensates wide voltage gain variation with PWM method of the auxiliary switch rather than adopting the FM method of main switches, the frequency variation range for the LLC resonant converter is highly reduced in the proposed converter. Therefore, the transformer in the proposed converter can be designed at the optimal operating point and it results in decreased conduction loss of the magnetizing inductor current. Furthermore, the maximum voltage gain of the proposed converter is easily increased by extending the duty ratio of the auxiliary switch. It helps to decrease the link capacitance. To verify the effectiveness of the proposed circuit, operational principle will be explained and experimental results will be presented with following specification. 100 kHz of switching frequency, 250-400 V of input voltage range, 250 V of output voltage, and 75 W output power.