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NCP1234BD65R2G

Fixed Frequency Current Mode Controller for Flyback Converters

ON Semiconductor 

NCP1234

APPLICATION INFORMATION

Introduction

The NCP1234 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 in most

applications.

 Fixed−Frequency Oscillator with Jittering: The

NCP1234 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 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. The

protection is fully latched on the A version (the power

supply has to be stopped then restarted in order to

resume operation, even if the overload condition

disapears), and autorecovery on the B version. The

timer 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

(allows to detect winding short-circuits).

 High Voltage Start−Up Current Source: Thanks to

ON Semiconductor’s Very High Voltage technology,

the NCP1234 can directly be connected to the high

input voltage. The start-up current source ensures a

clean start-up while ensuring low losses when it is off,

and the Dynamic Self-Supply (DSS) restarts the

start-up current source to supply the controller if the

VCC supply transiently drops.

 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 NCP1234 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 NCP1234 accepts a supply

voltage as high as 28 V, with an overvoltage threshold

VCC(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 NCP1234 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 NCP1234 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.

Typical Operation

 Start−up: The HV start−up current source ensures the

charging of the VCC capacitor up to the start−up

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

enough (above VHV(start)) to allow the switching to

start. 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 Latch pin decoupling capacitor.

 Normal operation: As long as the feedback voltage is

within the regulation range and VCC is maintained

above VCC(min), the NCP1234 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 VFB(foldS), typically corresponding to a load of

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