NCL30000
http://onsemi.com
14
ZCD Detection Block
A dedicated circuit block is necessary to implement the
zero current detection. The NCL30000 provides a separate
input pin to signal the controller to turn the power switch
back on just after the flyback transformer discharges all the
stored energy to the secondary winding. When the output
winding current reaches zero the winding voltage will
reverse. Since all windings of the transformer reflect the
same voltage characteristic this voltage reversal appears on
the primary bias winding. Coupling the winding voltage to
the ZCD input of the NCL30000 allows the controller to start
the next switching cycle at the precise time. To avoid
inadvertent false triggering, the ZCD input has a dual
comparator input structure to arm the latch when the ZCD
detect voltage rises above 1.4 V (nominal) thus setting the
latch. When the voltage on ZCD falls below 0.7 V (nominal)
a zero current event is detected and a signal is asserted which
initiates the next switching cycle. This is illustrated in
Figure 27. The input of the ZCD has an internal circuit
which clamps the positive and negative voltage excursions
on this pin. The current into or out of the ZCD pin must be
limited to 10 mA with an external resistor.
Figure 27. ZCD Operation
ZCD
+
−
+
Demag
+
−
+
Reset
Dominant
Latch
R
QS
DRIVE
ZCD Clamp
R
ZCD
N
ZCD
V
ZCD(ARM)
V
ZCD(TRIG)
Q
Bias Winding Voltage
V
trig
V
arm
At startup, there is no energy in the ZCD winding and no
voltage signal to activate the ZCD comparators. To enable
the controller to start under these conditions, an internal
watchdog timer is provided which initiates a switching cycle
in the event that the output drive has been off for more than
180 ms (nominal).
The timer is deactivated only under an OVP or UVP fault
condition which will be discussed in the next section.
Overcurrent Protection (OCP)
The dedicated CS pin of the NCL30000 senses the current
through the MOSFET switch and the primary side of the
transformer. This provides an additional level of protection
in the event of a fault. If the voltage of the CS pin exceeds
V
ILIM
, the internal comparator will detect the event and turn
off the MOSFET. The peak switch current is calculated
using Equation 1:
I
SW(peak)
+
V
ILIM
R
sense
(eq. 1)
To avoid the probability of false switching, the
NCL30000 incorporated a built in leading edge blanking
circuit (LEB) which masks the CS signal for a nominal time
of 190 ns. If required, an optional RC filter can be added
between R
sense
and CS to provide additional filtering. This
is illustrated below.
Figure 28. OCP Circuitry with Optional
External RC Filter
CS
+
−
+
OCP
LEB
DRV
optional
R
sense
V
ILIM
MFP Input
The multi-function pin is connected to the input of the
transconductance amplifier, the undervoltage and
overvoltage protection comparators. This allows this pin to
perform several functions. To place the device in standby,
the MFP pin should be pulled below the V
uvp
threshold. This
is illustrated in Figure 29. Additionally, raising the MFP pin
above V
ovp
will also suspend switching activity but not place
the controller in the standby mode. This can be used
implement overvoltage monitoring on the bias winding and
add an additional layer of fault protection.
