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NCP1378P

PWM Current−Mode Controller for Free−Running Quasi−Resonant Operation

ON Semiconductor 

NCP1378

APPLICATION INFORMATION

INTRODUCTION

The NCP1378 implements a standard current mode

architecture where the switch−off time is dictated by the

peak current setpoint, whereas the core reset detection

triggers the turn−on event . This component represents the

ideal candidate where low part−count is the key parameter

in applications supplied by a battery. Due to its high−

performance High−Voltage technology, the NCP1378

incorporates all the necessary components/features needed

to build a rugged and reliable Switchmode Power Supply

(SMPS):

• Transformer Core Reset Detection: Borderline/critical

operation is ensured whatever the operating conditions

are. As a result, there are virtually no primary switch

turn−on losses and no secondary diode recovery

losses. The converter also stays a first−order system

and accordingly eases the feedback loop design.

• Quasi−Resonant Operation: By delaying the turn−on

event, it is possible to restart the MOSFET in the

minimum of the drain−source wave, ensuring reduced

EMI/video noise perturbations. In nominal power

conditions, the NCP1378 operates in Borderline

Conduction Mode (BCM) also called Critical

Conduction Mode.

• Undervoltage Lockout (UVLO): When VCC falls

below UVLO, all pulses are stopped and the IC

consumption drops down to a few hundreds of mA

(ICC3 data). When VCC reaches the latchoff level

(5.5 V typical), the startup current source is activated

and brings VCC back to VCCON where the IC attempts

to startup.

• Overvoltage Protection (OVP): By sampling the

plateau voltage on the demagnetization winding, the

NCP1378 goes into latched fault condition whenever

an over−voltage condition is detected. The controller

stays fully latched in this position until the VCC is

cycled down to 4.0 V, e.g. when the user unplugs the

power supply from the mains outlet and replugs it.

• External LatchTrip Point: By externally forcing a

level on the OVP greater than the internal setpoint, it

is possible to latchoff the IC, e.g. with a signal coming

from a temperature sensor.

• Adjustable Skip Cycle Level: By offering the ability

to tailor the level at which the skip cycle takes place,

the designer can make sure that the skip operation

only occurs at low peak current. This point guarantees

a noise−free operation with cheap transformer. This

option also offers the ability to fix the maximum

switching frequency when entering light load conditions.

• Overcurrent Protection (OCP): NCP1378 enters burst

mode as soon as the power supply undergoes an

overload, which is detected through the sense of the

auxiliary voltage. As detailed above, as soon as VCC

crosses the UVLO level (called VCCOFF in the

electrical table), all pulses are stopped and the device

enters a safe low power operation that prevents from

any lethal thermal runaway. By monitoring the VCC

level, the startup current source is activated ON and

OFF to create a kind of burst mode where the SMPS

tries to restart. If the fault has gone, the SMPS

resumes operation. On the other hand, if the fault is

still there, the burst sequence starts again.

Startup Sequence

When the power supply is first powered from the mains

outlet, the internal current source (typically 4.0 mA) is

biased and charges up the VCC capacitor. When the voltage

on this VCC capacitor reaches the VCCON level (typically

8.4 V), the current source turns off and no longer wastes

any power. At this time, the VCC capacitor only supplies the

controller and the auxiliary supply is supposed to take over

before VCC collapses below VCCOFF. Figure 15 shows the

internal arrangement of this structure.

VCCON/VCCOFF

+

−

8

HV

IC1 or 0

6

SVCC Aux

4

Figure 15. The Current Source Brings Vcc Above

VccON and Then Turns Off

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