NCP1238
http://onsemi.com
15
APPLICATION INFORMATION
Introduction
The NCP1238 includes all of the 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 remains first order if in DCM, which eases the
design of the feedback loop. The controller can also be
used in CCM with a wide input voltage range due to its
fixed ramp compensation that prevents the appearance
of sub−harmonic oscillations in most of the
applications.
• Fixed−Frequency Oscillator with Jittering: The
NCP1238 is available in various frequency options to
fit any application. The internal oscillator features a
low−frequency jittering that helps to pass the EMI
requirements by spreading out the energy content of
frequency peaks in quasi−peak and average mode.
• Latched / Autorecovery Timer−Based Overcurrent
Protection: The overcurrent protection depends only
on the FB signal, enabling it to work with any
transformer, even with very poor coupling or high
leakage inductance. The protection is fully latched on
the A version (the power supply has to be unplugged
then restarted in order to resume operation, even if the
overload condition disappears), and autorecovery on the
B version. The timer’s duration is fixed. The controller
also enters the same protection mode if the voltage on
the CS pin reaches 1.5 times the maximum internal
setpoint, which enables to detect winding short circuits.
• High Voltage Startup Current Source with
Brown−Out and Line Overvoltage Detections: Due
to On Semiconductor’s Very High Voltage technology,
the NCP1238 can directly be connected to the high
input voltage. The startup current source ensures a
clean startup while ensuring low losses when it is off,
and the Dynamic Self−Supply (DSS) restarts the startup
current source to supply the controller if the V
CC
supply 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) or too high (line overvoltage).
This protection works either with a DC input voltage or
a rectified AC input voltage, and is independent of the
high voltage ripple. It uses a peak detector
synchronized with line frequency, or with the internal
watchdog timer if the HV pin is tied to a dc voltage.
• Adjustable Overpower Compensation: The high
voltage sensed on the HV pin is converted into a
current to add to 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 Soft−Skip mode for light
load operation: In order to ensure a high efficiency in
all load conditions, the NCP1238 implements a
frequency foldback (the switching frequency is lowered
to reduce switching losses) for light load condition; and
a Soft−Skip (disabled in case of fast load transients) for
extremely low load condition.
• Extended V
CC
range: The NCP1238 accepts a supply
voltage as high as 28 V, making the design of the power
supply easier.
• Clamped Driver Stage: Despite the high 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 NCP1238 can be latched
off by an increasing voltage applied to its Latch pin
(typically an overvoltage) or by a decreasing one, and
an NTC can be directly connected to the latch pin
thanks to the precise internal current source.
• Soft−Start: At every startup the peak current is
gradually increased during 4 ms to minimize the stress
on power components.
• Temperature Shutdown: The NCP1238 is internally
protected against self−heating: if the die temperature is
too high, the controller shuts all circuitries down
(including the HV startup current source), allowing the
silicon to cool down before attempting to restart. This
ensures a safe behavior in case of failure.
Typical Operation
• Startup: The HV startup current source ensures the
charging of the V
CC
capacitor up to the startup
threshold V
CC(on)
, until the input voltage is high
enough (above V
HV(start)
) to enable the switching. 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 decoupling capacitor on the Latch
pin.
• Normal operation: As long as the feedback voltage is
within the regulation range, the NCP1238 runs at a
fixed frequency (with jittering) in current−mode
control, where the peak current (sensed on the CS pin)
