NCP1361, NCP1366
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16
APPLICATION INFORMATION
The NCP1366/61 is a flyback power supply controller
providing a means to implement primary side
constant−current regulation. This technique does not need a
secondary side feedback circuitry, associated bias current
and an opto−coupler. NCP1366/61 implements a
current−mode architecture operating in quasi−resonant
mode. The controller prevents valley−jumping instability
and steadily locks out in a selected valley as the power
demand goes down. As long as the controller is able to detect
a valley, the new cycle or the following drive remains in a
valley. Due to a dedicated valley detection circuitry
operating at any line and load conditions, the power supply
efficiency will always be optimized. In order to prevent any
high switching frequency two frequency clamp options are
available.
• Quasi−Resonance Current−mode operation:
implementing quasi−resonance operation in peak
current−mode control optimizes the efficiency by
switching in the valley of the MOSFET drain−source
voltage. Due to a proprietary circuitry, the controller
locks−out in a selected valley and remains locked until
the input voltage significantly changes. Only the four
first valleys could be locked out. When the load current
diminishes, valley switching mode of operation is kept
but without valley lock−out. Valley−switching
operation across the entire input/output conditions
brings efficiency improvement and lets the designer
build higher−density converters.
• Frequency Clamp: As the frequency is not fixed and
dependent on the line, load and transformer
specifications, it is important to prevent switching
frequency runaway for applications requiring maximum
switching frequencies up to 90 kHz or 130 kHz. Two
frequency clamp options at 80 kHz or 110 kHz are
available for this purpose. In case frequency clamp is
not needed, a specific version of the 1361/66 exists in
which the clamp is deactivated.
• Primary Side Constant Current Regulation: Battery
charging applications request constant current
regulation. NCP1361/66 controls and regulates the
output current at a constant level regardless of the input
and output voltage conditions. This function offers tight
over power protection by estimating and limiting the
maximum output current from the primary side, without
any particular sensor.
• Optocoupler−based feedback: the voltage feedback
loop is classically implemented with an optocoupler
and a NCP431 voltage reference in the secondary side.
By pulling the feedback pin low, the controller adjusts
the peak current setpoint and regulates V
out
.
V
out
I
out
Optocoupler−based feedback
CC mode
0
V
nom
I
nom
Figure 39. Constant−Voltage & Constant−Current
Mode
Primary−side
• Soft−Start: 4 ms internal fixed soft start guarantees a
peak current starting from zero to its nominal value
with smooth transition in order to prevent any
overstress on the power components at each startup.
• Cycle−by−Cycle peak current limit: If the max peak
current reaches the V
ILIM
level, the over current
protection timer is enabled and starts counting. If the
overload lasts T
OCP
delay, then the fault is latched and
the controller stops immediately driving the power
MOSFET. The controller enters in a double hiccup
mode before autorecovering with a new startup cycle.
• V
CC
Over Voltage Protection: If the V
CC
voltage
reaches the V
CC(OVP)
threshold the controller enters in
latch mode. Thus it stops driving pulse on DRV pin:
♦ A & C version − (Latched V
CC(OVP)
): V
CC
capacitor is internally discharged to the V
CC(Clamp)
level with a very low power consumption: the
controller is completely disabled. Resuming
operation is possible by unplugging the line in order
to releasing the internal V
CC
thyristor with a V
CC
current lower than the I
CC(Clamp)
.
♦ B version − (Autorecovery): it enters in double
hiccup mode before resuming operation.
• Winding Short−Circuit Protection: An additional
comparator senses the CS signal and stops the
controller if V
CS
reaches V
ILIM
+50% (after a reduced
LEB: t
LEB2
). Short circuit protection is enabled only if
4 consecutive pulses reach SCP level. This small
counter prevents any false triggering of short circuit
protection during surge test for instance. This fault is
latched and operations will be resumed like in a case of
V
CC
Over Voltage Protection.
