NCP1075A/B, NCP1076A/B, NCP1077A/B, NCP1079A/B
www.onsemi.com
14
APPLICATION INFORMATION
Introduction
Thanks to ON Semiconductor Very High Voltage
Integrated Circuit technology, the circuit hosts a
high−voltage power MOSFET featuring a 13.5/4.8/2.9 W
R
DS(ON)
– T
J
= 25°C. An internal current source delivers the
start−up current, necessary to crank the power supply.
• Current−mode operation: The controller uses
current−mode control architecture.
• 700 V Power MOSFET: Thanks to ON Semiconductor
Very High Voltage Integrated Circuit technology, the
circuit hosts a high−voltage power MOSFET featuring
a 4.8 and 2.9 W R
DS(ON)
– T
J
= 25°C. This value lets
the designer build a power supply up to 28 W operated
on universal mains. An internal current source delivers
the start−up current, necessary to crank the power
supply.
• Dynamic Self−Supply: This device could be used in an
application without an auxiliary winding to provide
supply voltage via an internal high−voltage current
source.
• Short−circuit protection: By permanently monitoring
the feedback line activity, the IC is able to detect the
presence of a short−circuit, immediately reducing the
output power for a total system protection. A t
SCP
timer
is started as soon as the feedback current is below
threshold, I
FB(fault)
, which indicates a maximum peak
current condition. If at the end of this timer the fault is
still present, then the device enters a safe,
auto−recovery burst mode, affected by a fixed timer
recurrence, t
recovery
. Once the short has disappeared, the
controller resumes and goes back to normal operation.
• Built−in VCC Over−Voltage Protection: When the
auxiliary winding is used to bias the VCC pin (no
DSS), an internal comparator is connected to VCC pin.
In case the voltage on the pin exceeds the V
OVP
level
(18 V typically), the controller immediately stops
switching and awaits a full timer period (t
recovery
)
before attempting to re−start. If the fault is gone, the
controller resumes operation. If the fault is still there,
e.g. in the case of a broken opto−coupler, the controller
protects the load through a safe burst mode.
• Line detection: An internal comparator monitors the
drain voltage. If the drain voltage is lower than the
internal threshold (V
HV(EN)
), the internal power switch
is inhibited. This avoids operating at too low an ac
input. Line detection is active, when BO/AC_OVP pin
is grounded.
• Brown−out detection and AC line Over−Voltage
Protection: The BO/AC_OVP input monitors bulk
voltage level via resistive divider and thus assures that
the application is working only for designed bulk
voltage. When BO/AC_OVP pin is connected to
ground, Line detection is inhibited.
• Internal OPP: An internal function using the bulk
voltage to program the maximum current reduction for
a given input voltage. Internal OPP is active when
BO/AC_OVP pin is connected via resistive divider to
the bulk voltage.
• 2
nd
LEB: Second level of current protection. If peak
current is 150% max peak current limit, then the
controller stops switching after three pulses and waits
for an auto−recovery period (t
recovery
) before
attempting to re−start.
• Frequency jittering: An internal low−frequency
modulation signal varies the pace at which the
oscillator frequency is modulated. This helps spreading
out energy in conducted noise analysis. To improve the
EMI signature at low power levels, the jittering remains
active in frequency foldback mode.
• Soft−Start: A 10 ms soft−start ensures a smooth
start−up sequence, reducing output overshoots.
• Frequency foldback capability: A continuous flow of
pulses is not compatible with no−load/light−load
standby power requirements. To excel in this domain,
the controller observes the feedback current
information and when it reaches a level of I
FBfold
, the
oscillator then starts to reduce its switching frequency
as the feedback current continues to increase (the power
demand continues to reduce). It can go down to 27 kHz
(typical) reached for a feedback level of I
FBfold(END)
(100 mA roughly). At this point, if the power continues
to drop, the controller enters classical skip−cycle mode.
• Skip: If SMPS naturally exhibits a good efficiency at
nominal load, they begin to be less efficient when the
output power demand diminishes. By skipping
un−needed switching cycles, the NCP107xu drastically
reduces the power wasted during light load conditions.
