The Analysis on Hard-switching Phenomenon during Start-up of LLC converter

LLC resonant half bridge converter becomes more and more popular in power supply applications in nowadays given its soft switching and high efficiency characteristics. However, it is revealed recently that its ZVS condition could be lost at start-up due to imbalanced capacitor voltage. The super-junction MOSFETs in LLC converter are not designed for hard-switching, they usually have poor body diode characteristics, thus those MOSFETS are subject to a potential failure. This paper fully analyzed the root cause of this hard switching phenomenon at start-up, and its relationship with imbalanced capacitor voltage mathematically. It also revealed that the imbalanced capacitor voltage at start-up is caused by bootstrap mechanism. The boundary condition of capacitor voltage to maintain soft-switching is provided. Finally, this paper also provided methods to address this hard-switching issue at start up of LLC converter.