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NCP1237

Fixed Frequency Current Mode Controller for Flyback Converters

ON Semiconductor 

NCP1237

• Normal operation: As long as the feedback voltage is

within the regulation range, the NCP1237 runs at a

fixed frequency (with jittering) in current−mode

control, where the peak current (sensed on the CS pin)

is set by the voltage on the FB pin. A fixed ramp

compensation is applied internally to prevent

sub−harmonic oscillations from occurring. The VCC

must be supplied by an external source (such as an

auxiliary winding), as the startup current source cannot

permanently supply the controller without overheating.

• Light load operation: When the FB voltage decreases

below VFB(fold), typically corresponding to a load of

20% (DCM design only) to 30% (CCM / DCM design)

of the maximum load, the switching frequency starts to

decrease down to fOSC(min). By lowering the switching

losses, this feature helps to improve the efficiency in

light load conditions. The frequency jittering is disabled

in light load operation.

• No load operation: When the FB voltage decreases

below Vskip(in), typically corresponding to a load of 1%

of the maximum load, the controller enters Skip mode.

By completely stopping the switching while the

feedback voltage is below Vskip(out), the losses are

further reduced, allowing to minimize the power

dissipation under extremely low load conditions. In

order to avoid audible noise, the peak current is

gradually increased during the tSSKIP duration while

exiting the skip mode (Soft−Skip function). In case of

abrupt load increase during Soft−Skip mode, the

soft−skip portion is bypassed and the peak current

needed for regulation is directly applied. VCC can be

maintained between VCC(on) and VCC(min) by the DSS.

• Overload: The NCP1237 features a timer−based

dual−level overload detection, solely dependent on the

feedback information: as soon as the internal peak

current setpoint goes above the VCS(tran) threshold, a

first internal timer starts to count, but the controller is

still able to regulate up to VILIM. Once it reaches the

VILIM clamp, the internal overload timer starts to count.

When either timer times out, the controller stops and

enters the protection mode, autorecovery for the B

version (the controller initiates a new start−up after

tautorec elapses), or latched for the A version (the latch

is released if a brown−out event occurs or VCC is reset).

• Brown−out: The NCP1237 features on its HV pin a

true AC line monitoring circuitry which includes a

minimum startup threshold, brown−out protection, and

overvoltage protection. All of these circuits are

autorecovery and operate independently of any ripple

on the input voltage. They can even work with an

unfiltered, rectified AC input. All 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 overvoltage condition), or pulled low (typically

by an overtemperature condition, using the provided

current source with an NTC), the controller latches off.

The latch is released when a brown−out condition

occurs, or when VCC decreases below VCC(reset).

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