DocID029495 Rev 1

13/20

SRK2001A

Operation description

20

7.4

ZCD_OFF comparator turn-off

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

SR MOSFETs, avoiding in this way current inversions, that would cause SR MOSFETs

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

proper protections.

The ZCD_OFF comparator acts during fast transient conditions, where a sudden slope

variation of the rectified current presents or when (after a quick frequency change) 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_OFF

comparator threshold is not fixed but self-adaptive. In fact, after a turn-off by the ZCD_OFF

comparator, the circuit senses the body diode residual conduction: if it is longer than the

difference between the residual diode conduction and the target value), so that in the next

cycle the residual body diode conduction is decreased. At the end, the comparator threshold

sets to such a level that the turn-off is accomplished by the adaptive turn-off mechanism,

while the ZCD_OFF comparator is ready to protect in case of a transient condition.

Therefore, 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 sudden above resonance operation, the

ZCD_OFF comparator will take over, driving a fast MOSFET switch-off that prevents

undesired current inversions.

The ZCD_OFF comparator is blanked for 300 ns after the turn-on time instant in order to

avoid a premature turn-off: in fact, soon after the turn-on, the sensed drain-source voltage

may cross the turn-off threshold due to switching noise on parasitic stray inductance, which

may trigger the ZCD_OFF comparator.

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

comparator may be found at the low-load, due to the noise present on the drain-source

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.

7.5

Gate drive

The IC is provided with two high current gate-drive outputs, each capable of driving one or

more N-channel power MOSFETs in parallel.

minimizing the gate charge provided in each switching cycle.

The two gate drivers have a pull-down capability that ensures the SR MOSFETs cannot be

As described in the previous paragraphs, either the SR MOSFET is switched on after the

current starts flowing through the body diode, when the drain-source voltage is already low

in which case the drive doesn't need to provide a fast turn-on.