NCP1070_16 Datasheet, PDF (13/30 Pages) ON Semiconductor – High-Voltage Switcher for Low Power Offline SMPS

NCP1070, NCP1071, NCP1072, NCP1075, NCP1076, NCP1077
APPLICATION INFORMATION
Introduction
The NCP107x offers a complete current−mode control
solution. The component integrates everything needed to
build a rugged and low−cost Switch−Mode Power Supply
(SMPS) featuring low standby power. The Quick Selection
Table on page 2 details the differences between references,
mainly peak current setpoints and operating frequency.
• Current−mode operation: the controller uses
current−mode control architecture.
• 700 V Power MOSFET: Due to ON Semiconductor
Very High Voltage Integrated Circuit technology, the
circuit hosts a high*voltage power MOSFET featuring
a 22/11/4.7 W R_DS(on) – T_J = 25°C. This value lets the
designer build a power supply up to respectively
7.75 W, 10 W and 15 W operated on universal mains.
An internal current source delivers the startup current,
necessary to crank the power supply.
• Dynamic Self−Supply: Due to the internal high voltage
current source, this device could be used in the
application without the auxiliary winding to provide
supply voltage.
• 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 the maximum peak
current. 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 V_CC Over Voltage Protection: when the
auxiliary winding is used to bias the V_CC pin (no DSS),
an internal active clamp connected between V_CC and
ground limits the supply dynamics to V_CC(clamp). In
case the current injected in this clamp exceeds a level
of 6.0 mA (minimum), the controller immediately stops
switching and waits a full timer period (t_recovery) before
attempting to restart. If the fault is gone, the controller
resumes operation. If the fault is still there, e.g. a
broken opto−coupler, the controller protects the load
through a safe burst mode.
• Line detection: An internal comparator monitors the
drain voltage as recovering from one of the following
situations:
♦ Short Circuit Protection,
♦ V_CC OVP is confirmed,
♦ UVLO
♦ TSD
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 ac input. This is also called
brown−in function in some fields.
• 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 1 ms soft−start ensures a smooth startup
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 25 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 NCP107x drastically
reduces the power wasted during light load conditions.
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