Design and Implementation of 24V output and 1kW High-Efficiency LLC Converter : With Unbalanced Magnetic Flux and Synchronous Rectification

Sammanfattning

The demand for high-efficiency, high-power-density DC-DC converters continues to grow in modern applications such as telecommunications and industrial systems. This master's thesis presents the design, implementation, and experimental validation of a 1 kW LLC resonant converter delivering 24 V output, combining an unbalanced-flux magnetic core with synchronous rectification. The work builds upon a previous 48 V, 1 kW diode-based design and addresses the challenge of maintaining high efficiency at twice the output current by replacing the secondary-side diode bridge with Synchronous Rectifier (SR) MOSFETs. The converter utilizes a slightly enlarged unbalanced-flux core (volume approximately 4.79cm³) specifically calculated for the 24 V, 1 kW application. The secondary winding is implemented on a dual 4-layer PCB structure using BSC037N08NS5 MOSFETs with low controlled by IR1168S gate drivers. The primary-side inverter, primary windings, and resonant tank are reused from the previous design, with an existing Texas Instruments C2000 microcontroller configured to generate the 340 kHz switching frequency. An air gap was introduced to lower the magnetizing inductance and raise the ZVS current, improving efficiency across all power levels and reducing primary-MOSFET stress below resonance. Experimental results demonstrate a peak efficiency of 96.09% at 400 W, with efficiency remaining above 95% from 300 W to 1000 W and measuring 95.30% at full load. The converter operates reliably up to 1100 W with stable thermal performance, reaching a core temperature of 88.5°C at this power level. The core temperature at peak efficiency (400 W) is 55°C. The achieved power density is approximately 230 W/cm³ at 1100 W, which is about three times higher than balanced-flux designs constrained to MHz frequency ranges. Cooling configuration tests revealed that adequate fan capacity is essential for full-power operation, with larger fans enabling sustained 1100 W operation compared to smaller fan configurations limited to 750-800 W. The findings validate the effectiveness of combining unbalanced-flux magnetics with synchronous rectification for low-voltage, high-current applications. The research question has been successfully answered, demonstrating that the existing 48 V design can be redesigned for 24 V output with significantly higher efficiency while maintaining high power density using synchronous rectification, an air gap, and a slightly enlarged core of the same fundamental architecture.