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NCP1238

Fixed Frequency Current Mode Controller for Flyback Converters

ON Semiconductor 

NCP1238

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 VCC

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 VCC 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 VCC capacitor up to the startup

threshold VCC(on), until the input voltage is high

enough (above VHV(start)) to enable the switching. The

controller then delivers pulses, starting with a soft−start

period tSSTART 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)

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