NCP1379
http://onsemi.com
10
9.2
9.4
9.6
9.8
10.0
10.2
10.4
−40 −20 0 20 40 60 80 100 120
Figure 21. I
BO
vs. Junction Temperature
I
BO
, (mA)
T
J
, JUNCTION TEMPERATURE (°C)
APPLICATION INFORMATION
NCP1379 implements a standard current−mode
architecture operating in quasi−resonant mode. Thanks to a
proprietary circuitry, the controller prevents
valley−jumping instability and steadily locks out in selected
valley as the power demand goes down. Once the fourth
valley is reached, the controller continues to reduce the
frequency further down, offering excellent efficiency over
a wide operating range. Due to a fault timer combined to an
OPP circuitry, the controller is able to efficiently limit the
output power at high−line.
• Quasi−Resonance Current−mode operation:
implementing quasi−resonance operation in peak
current−mode control, the NCP1379 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 output loading significantly changes.
This behavior is obtained by monitoring the feedback
voltage. When the load becomes lighter, the feedback
setpoint changes and the controller jumps into the next
valley. It can go down to the 4
th
valley if necessary.
Beyond this point, the controller reduces its switching
frequency by freezing the peak current setpoint. During
quasi−resonance operation, in case of very damped
valleys, a 5.9 ms timer adds the missing valleys.
• Frequency reduction in light−load conditions: when the
4
th
valley is left, the controller reduces the switching
frequency which naturally improves the standby power
by a reduction of all switching losses.
• Overpower protection (OPP): When the voltage on
ZCD pin swings in flyback polarity, a direct image of
the input voltage is applied on ZCD pin. We can thus
reduce the peak current depending of the ZCD pin
voltage level during the on−time.
• Internal soft−start: a soft−start precludes the main
power switch from being stressed upon start−up. Its
duration is fixed and equal to 3.8 ms.
• Fault input: the NCP1379 and D versions include a
brown−out circuit which safely stops the controller in
case the input voltage is too low. Restart occurs via a
complete startup sequence (latch reset and soft−start).
During normal operation, the voltage on this pin is
clamped to 1.2 V to give enough room for OVP
detection. If the voltage on this pin increases above
2.5 V, the part latches−off.
• Short−circuit protection: short−circuit and especially
over−load protections are difficult to implement when a
strong leakage inductance between auxiliary and power
windings affects the transformer (where the auxiliary
winding level does not properly collapse in presence of
an output short). Here, when the internal 0.8 V
maximum peak current limit is activated, the timer
starts counting up. If the fault disappears, the timer
counts down. If the timer reaches completion while the
error flag is still present, the controller stops the pulses
and goes into auto−recovery mode.
