Operation description

SRK2001

12/22

DocID027367 Rev 3

Figure 8. Adaptive turn-off

Figure 8 shows this adaptive algorithm: cycle-by-cycle the conduction time is maximized

allowing in a steady-state the maximum converter efficiency.

During the start-up and on sleep mode exiting, the control circuit turns off the SR MOSFET

at 50% of the clock cycle and progressively adapts this delay in order to maximize the SR

MOSFET conduction time. This helps reducing system perturbations.

5.4

ZCD comparator

The IC is equipped with a ZCD comparator that is always ready to quickly turn-off the SR

MOSFETs, avoiding in this way current inversion, that would cause SR MOSFETs failure

and even half bridge destruction, in case of the primary controller not equipped with proper

protections.

The ZCD (zero current detection) comparator acts during fast load transitions or the short-

circuit operation and when the above resonance operation occurs. It senses that the current

has reached the zero level and triggers the gate drive circuit for a very fast MOSFET turn-off

The ZCD comparator threshold is not fixed but self-adaptive.

In the steady-state load operation and in case of slow load transitions, the turn-off is

prevalently managed by the adaptive mechanism (characterized by the two slope turn-off

driving). Instead, during fast transitions or during above resonance operation, the ZCD_OFF

comparator will take over, causing a fast MOSFET switch-off that prevents undesired

current inversions.

The ZCD_OFF comparator is blanked for 450 ns after the turn-on.

Depending on SR MOSFET choice, some premature turn-off triggered by the ZCD_OFF

comparator may be found due to the noise present on the drain-source sensed signal: this is

worse with lower RDS_ON (due to worse signal to noise ratio) and lower stray inductance of

the MOSFET package. Normally the load level where this may happen is such that the

circuit has already entered a low consumption state (for example in burst mode from primary

controller); if this is not the case, some noise reduction may be helpful, for example by using

RC snubbers across the SR MOSFETs drain-source.