Resonant converters do, though, have several disadvantages.
Performance can be optimized at a single operating point, but
not across a wide range of input-voltage and load-power
variations. Also, the quasi-sinusoidal waveforms found in a
resonant converter exhibit higher peak values than their
equivalent rectangular waveforms. In addition, current can
circulate through the tank elements even when the load is
disconnected, leading to poor efficiency at light load. A
similar switching technology is today often employed in low-
power LED drivers in which a quasi-resonant or valley-switching
topology is implemented. To start, current IQ1 ramps up until
the desired energy level is charged in to coil L - see figure 5
. Then switch Q1 is turned off. When the switching transient is
complete and the coil current equals zero, the drain voltage
starts to oscillate around the input voltage level VDC. The
amplitude equals V0. Circuitry connected to the Q1 drain pin
senses when the voltage on the drain of the switch has rea