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