Datasheet

AN3303 SRK2000 main characteristics
Doc ID 18164 Rev 2 9/27
Figure 6. Effect of parasitic elements on power MOSFET turn-on
1.3 Blanking time
One peculiarity of resonant converters and in particular of LLCs which differentiate them
from hard switching topologies, like flyback or forward, is that secondary currents have a
sinusoidal shape. This requires handling of the SR with a dedicated logic scheme: as shown
in Figure 7, the secondary current is not monotonic and consequently the same current level
is crossed twice each half-cycle and the DVS voltage also crosses the turn-off threshold
twice.
Figure 7. Blanking time
To avoid switch-off after the first crossing, a blanking time is introduced. It is important to
point out that this blanking time must be related to the switching period, because the
operating frequency can change considerably depending on the converter design and the
operating point. In the SRK2000, this blanking time is set to 50 % of each half-cycle.
There is a peculiar condition that needs highlighting. In many cases, except for some
unusual designs operating well below resonance, the secondary current peak value is
achieved after 50 % of the half-cycle. Depending on the converter design and on power
MOSFET R
DS(on)
, at light load the turn-off threshold may happen to be very close to the
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