A novel light load solution for LLC series resonant converter

LLC series resonant topologies has many unique characteristics and improvements over PWM topologies. However, at light load or open load, the unstable output voltage is one of critical issues of LLC-SRC which uses diodes as rectifiers. The traditional solution employs dummy load, nevertheless it results in high dummy load loss and low system efficiency. This paper analyzed the root cause that result in the open load issue and proposed a simple and practical solution, which can stabilize the output voltage at no dummy load and make LLC series resonant converter work in a narrow frequency range. At last, the solution is employed in a 580 kHz, 54 V/50 A telecom power supply and the LLC circuit operating frequency is 748 kHz with 15 W total losses, rather than 1 MHz with 200 W total losses in an unstable case.