DocID029495 Rev 1 13/20
SRK2001A Operation description
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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
target value T
diode
, the comparator threshold is increased (by an amount proportional to 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
sensed signal: this is worse with lower R
DS_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.
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.
The high-level voltage provided by the driver is clamped at V
GDclamp
in order to avoid
excessive voltage levels on the gate in case the device is supplied with a high V
CC
, thus
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
spuriously turned on even at low V
CC
: in fact, the drivers have a 1 V (typ.) saturation level at
V
CC
below the turn-on threshold.
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
(equal to V
F
); therefore there is no Miller effect nor switching losses at the MOSFET turn-on,
in which case the drive doesn't need to provide a fast turn-on.
