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NCP1236BD100R2G

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NCP1236
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13
APPLICATION INFORMATION
Introduction
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.
• 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
measurement.
• 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.
• 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
current source to supply the controller if the V
CC
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.
• 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.
• 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
off.
• 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.
• 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.
NCP1236
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14
Typical Operation
• Start−up: The HV start−up current source ensures the
charging of the V
CC
capacitor up to the start−up
threshold V
CC(on)
, until the input voltage is high
enough (above V
HV(start)
) to allow the switching to
start. The controller then delivers pulses, starting with a
soft−start period t
SSTART
during which the peak current
linearly increases before the current−mode control takes
over. During the soft−start period, the low level latch is
ignored, and the latch current is double, to ensure a fast
pre−charge of the Latch pin decoupling capacitor.
• Normal operation: As long as the feedback voltage is
within the regulation range and V
CC
is maintained
above V
CC(min)
, the NCP1236 runs at a fixed frequency
(with jittering) in current−mode control. The peak
current (sensed on the CS pin) is set by the voltage on
the FB pin. Fixed ramp compensation is applied
internally to prevent sub−harmonic oscillations from
occurring.
• Light load operation: When the FB voltage decreases
below V
FB(foldS)
, typically corresponding to a load of
33 % of the maximum load (for a DCM design), the
switching frequency starts to decrease down to
f
OSC(min)
. By lowering the switching losses, this feature
helps to improve the efficiency in light load conditions.
The frequency jittering is enabled in light load
operation as well.
• No load operation: When the FB voltage decreases
below V
skip(in)
, typically corresponding to a load of 2
% of the maximum load, the controller enters skip
mode. By completely stopping the switching while the
feedback voltage is below V
skip(out)
, the losses are
further reduced. This allows minimizing the power
dissipation under extremely low load conditions. As the
skip mode is entered at very light loads, for which the
peak current is very small, there is no risk of audible
noise. V
CC
can be maintained between V
CC(on)
and
V
CC(min)
by the DSS, if the auxiliary winding does not
provide sufficient level of V
CC
voltage under this
condition.
• Overload: The NCP1236 features timer−based
overload detection, solely dependent on the feedback
information: as soon as the internal peak current
setpoint hits the V
ILIM
clamp, an internal timer starts to
count. When the timer elapses, the controller stops and
enter the protection mode, autorecovery for the B
version (the controller initiates a new start−up after
t
autorec
elapses), or latched for the A version (the latch
is released if a brown−out event occurs or V
CC
is reset).
• Brown−out: The NCP1236 features a true AC line
monitoring circuitry. It includes a minimum start−up
threshold and an autorecovery brown−out protection;
both of them independent of the ripple on the input
voltage. It can even work with an unfiltered, rectified
AC input. The thresholds are fixed, but they are
designed to fit most of the standard AC−DC conversion
applications.
• Latch−off: When the Latch input is pulled up (typically
by an over−voltage condition), or pulled down
(typically by an over−temperature condition, using the
provided current source with an NTC), the controller
latches off. A voltage higher than V
CC(ovp)
on the VCC
pin has the same effect. The latch is released when a
brown−out condition occurs, or when the V
CC
is reset.
NCP1236
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15
DETAILED DESCRIPTION
High−Voltage Current Source with Built−in Brown−out
Detection
The NCP1236 HV pin can be connected either to the
rectified bulk voltage, or to the ac line through a rectifier.
Start−up
−
+
−
+
+
+
R
S
Q
TSD
HV
VCC
Istart
VCC(on)
VCC(off )
tUVLO(blank)
blanking
Control
UVLO
−
+
+
VCC(reset)
Reset
IC Start
−
+
+
VCC(min)
Figure 30. HV Start−up Current Source Functional Schematic
At start−up, the current source turns on when the voltage
on the HV pin is higher than V
HV(min)
, and turns off when
V
CC
reaches V
CC(on)
, then turns on again when V
CC
reaches
V
CC(min)
, until the input voltage is high enough to ensure a
proper start−up, i.e. when V
HV
reaches V
HV(start)
. The
controller actually starts the next time V
CC
reaches V
CC(on)
.
Even though the DSS is able to maintain the V
CC
voltage
between V
CC(on)
and V
CC(min)
by turning the HV start−up
current source on and off, it can only be used in light load
condition, otherwise the power dissipation on the die would
be too much. As a result, an auxiliary voltage source is
needed to supply V
CC
during normal operation.
The DSS is useful to keep the controller alive when no
switching pulses are delivered, e.g. in brown−out condition,
or to prevent the controller from stopping during load
transients when the V
CC
might drop.
If the voltage increases above the overvoltage protection
threshold V
CC(ovp)
, the controller is latched off.
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NCP1236BD100R2G

Mfr. #:
Buy NCP1236BD100R2G
Manufacturer:
ON Semiconductor
Description:
Switching Controllers BRWN AREC OCP OVP 100KHZ
Lifecycle:
New from this manufacturer.
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