The LLC resonant converter can operate at frequency
below or above the resonance frequency (fo), as
illustrated in Figure 10. Figure 11 shows the waveforms
of the currents in the transformer primary side and
secondary side for each operation mode. Operation below
the resonant frequency (case I) allows the soft
commutation of the rectifier diodes in the secondary side,
while the circulating current is relatively large. The
circulating current increases more as the operation
frequency moves downward from the resonant frequency.
Meanwhile, operation above the resonant frequency (case
II) allows the circulating current to be minimized, but the
rectifier diodes are not softly commutated. Below
resonance operation is preferred for high output voltage
applications, such as Plasma Display Panel (PDP) TV
where the reverse recovery loss in the rectifier diode is
severe. Below resonance operation also has a narrow
frequency range with respect to the load variation since
the frequency is limited below the resonance frequency
even at no load condition.