SRK2000 main characteristics

AN3303

10/27

Doc ID 18164 Rev 2

peak current and, consequently, the turn-off point may jump from one half-cycle to the next

between the 50 % limit imposed by the blanking time and the desired turn-off point. The

result is a sort of sub-harmonic oscillation in the duty cycle of the SR power MOSFET. This

has no significant effect as long as the oscillation amplitude is limited but could cause

instability in the case of wider oscillation. For this reason the SRK2000 sets the blanking

time in track with the switching period, therefore, keep it as close as possible to the peak

current.

Figure 8.

1.4

Light load operation and sleep mode

A unique feature of the SRK2000 is its intelligent automatic sleep mode. The internal logic

circuitry is able to detect a light load condition of the converter and stop gate driving,

reducing also IC’s quiescent consumption. This improves converter efficiency at light load,

where the power losses on the rectification body diodes become lower than the power

losses in the power MOSFETs and those related to their driving.

The IC is also able to detect an increase of the converter's load and automatically restart

gate driving.

The automatic sleep mode detection is performed by comparing the duration of a half-cycle

to the SR power MOSFET conduction time. The duration of a half-cycle is measured by

generating a clock signal each time the half-bridge voltage reverts. The IC enters sleep

mode when the conduction time of at least one of the two SR power MOSFETs falls below

40 % of the measured half-period. Figure 9 shows details of time measuring for sleep mode

entering. To avoid erroneous decisions, this sleep mode condition must be confirmed for 16

consecutive resonant converter switching cycles.