This example shows the operation of a resonant LLC converter.
The LLC converter is a DC/DC converter based on a resonant circuit which allows soft-switching operation. The LLC resonant circuit reduces switching loss through zero-voltage switching (ZVS). Unlike the SLR converter, the LLC converter can keep the output voltage regulated even under light load condition.
The converter consists of a switching bridge (half-bridge MOSFET), an LLC resonant circuit, a full-bridge rectifier and an output filter (Co). The resonant circuit consists of the primary leakage inductance of a transformer (Llkp), its secondary leakage inductance (Llks), its magnetization inductance (Lm) and a capacitor Cr. The resonant frequency of this LLC circuit is given by:
where and is the transformer winding ratio.
For a given load (3 or 30 ohms) and/or input voltage (340V or 390V), run several simulations using the appropriate switching frequency (Fs) values in order to maintain the output voltage at 24V (signal Vo on Scope1).
Also, observing waveforms on Scope2, you will see that the current lags the voltage applied to the resonant circuit which allows the MOSFETs to be turned on with zero voltage.
H. Choi; 'Half-Bridge LLC Resonant Converter Design Using FSFR-Series Fairchild Power Switch', Fairchild Semiconductors, AN-4151 S. Abdel-Rahman; "Resonant LLC Converter: Operation and Design", Infineon AN 2012-09