NCP1236
APPLICATION INFORMATION
Introduction
voltage transiently drops. The high voltage pin also
features a high−voltage sensing circuitry, which is able
to turn the controller off if the input voltage is too low
(brown−out condition). This protection works either
with a DC input voltage or a rectified AC input voltage,
and is independent of the high voltage ripple.
The NCP1236 includes all necessary features to build a safe
and efficient power supply based on a fixed−frequency
flyback converter. It is particularly well suited for
applications where low part count is a key parameter,
without sacrificing safety.
Adjustable Overpower Compensation: The high
input voltage sensed on the HV pin is converted into a
current to build on the current sense voltage an offset
proportional to the input voltage. By choosing the value
of the resistor in series with the CS pin, the amount of
compensation can be adjusted to the application.
Frequency foldback then skip mode for light load
operation: In order to ensure a high efficiency under all
load conditions, the NCP1236 implements a frequency
foldback for light load condition and a skip mode for
extremely low load condition. The switching frequency
is decreased down to 27 kHz to reduce switching
losses.
Current−Mode Operation with slope compensation:
The primary peak current is permanently controlled by
the FB voltage, ensuring maximum safety: the DRV
turn−off event is dictated by the peak current setpoint.
It also ensures that the frequency response of the
system stays a first order if in DCM, which eases the
design of the FB loop. The controller can be also used
in CCM applications with a wide input voltage range
thanks to its fixed ramp compensation that prevents the
appearance of sub−harmonic oscillations.
Fixed−Frequency Oscillator with Jittering: The
NCP1236 is available in different frequency options to
fit any application. The internal oscillator features a
low−frequency jittering that helps passing the EMI
limits by spreading out the energy content of frequency
peaks in quasi−peak and average mode of
Extended VCC range: The NCP1236 accepts a supply
voltage as high as 28 V, with an overvoltage threshold
V
CC(ovp)
(typically 26.5 V) that latches the controller
measurement.
off.
Latched / Autorecovery Timer−Based Overload
Protection: The overload protection depends only on
the FB signal, making it able to work with any
transformer, even with very poor coupling or high
leakage inductance. When the fault timer elapses the
device can be permanently latched in version A or the
latch can be reset by an autorecovery restart of the
device in version B. The power supply has to be
stopped then restarted in order to resume operation,
even if the overload condition disapears, in case of
usage the A version of the NCP1236. The fault timer
duration is internally fixed. The controller also latches
off if the voltage on the CS pin reaches 1.5 times the
maximum internal setpoint (allowing to detect winding
short−circuits), with the same modes of releasing the
latch in A or B version.
Clamped Driver Stage: Despite the high maximum
supply voltage, the voltage on DRV pin is safely
clamped below 16 V, allowing the use of any standard
MOSFET, and reducing the current consumption of the
controller.
Dual Latch−off Input: The NCP1236 can be latched
off by 2 ways: The voltage increase applied to its Latch
pin (typically an overvoltage) or by a decrease this
voltage. Thanks to the internal precise pull−up current
source a NTC can be directly connected to the latch pin.
This NTC will provide an overtemperature protection
by decreasing its resistance and consequently the
voltage at Latch pin,
Soft−Start: At every start−up the peak current is
gradually increased during 4.0 ms to minimize the
stress on power components.
High Voltage Start−Up Current Source with
Brown−Out Detection: Due to ON Semiconductor’s
Very High Voltage technology, the NCP1236 can be
directly connected to the high input voltage. The
start−up current source ensures a clean start−up and the
Dynamic Self−Supply (DSS) restarting the start−up
Temperature Shutdown: The NCP1236 is internally
protected against self−overheating: if the die
temperature is too high, the controller shuts all
circuitries down (including the HV start−up current
source), allowing the silicon to cool down before
attempting to restart. This ensures a safe behavior in
case of failure.
current source to supply the controller if the V
CC
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