NCP1028 データシートの表示（PDF） - ON Semiconductor

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NCP1028

High−Voltage Switcher for Medium Power Offline SMPS Featuring Low Standby Power

ON Semiconductor 

NCP1028

APPLICATION INFORMATION

Introduction

The NCP1028 offers a complete current−mode control

solution and enhances the NCP101X series. The

component integrates everything needed to build a rugged

and low−cost Switch−Mode Power Supply (SMPS)

featuring low standby power.

• Current−Mode Operation: The controller uses a

current−mode control architecture, which, together

with an adjustable ramp compensation circuitry,

ensures efficient and stable continuous or

discontinuous conduction designs.

• 700 V–5.8 W Power Switch Circuit: Due to

ON Semiconductor Very High Voltage Integrated

Circuit technology, the circuit hosts a high−voltage

power switch circuit featuring a 5.8 W RDS(on) – TJ =

25°C. This value lets the designer build a 15 W

power supply operated on universal mains as

long as sufficient copper area exists to lower the

junction−to−ambient thermal resistance. An internal

current source delivers the startup current, necessary

to crank the power supply.

• Short−Circuit Protection: By permanently

monitoring the feedback line activity, the circuit is

able to detect the presence of a short−circuit,

immediately reducing the output power for a total

system protection. A 55 ms timer is started as soon as

the feedback pin asks for the maximum peak current.

At the end of this timer, if the fault is still present,

then the device enters a safe, auto−recovery burst

mode, affected by a fixed 440 ms recurrence. Once the

short has disappeared, the controller resumes and goes

back to normal operation. The timer duration is fully

independent from the VCC capacitor value.

• Over Power Protection: A possibility exists to reduce

the maximum output power capability in high line

conditions. A simple two resistor network wired to the

bulk capacitor will program the maximum current

reduction for a given input voltage (down to 20% of

the maximum peak current).

• Brown−Out Input: A fraction of the input voltage

appears on pin 3, due to a resistive divider. If the

mains drops below a level adjusted by this resistive

divider, the circuit does not switch. As soon as the

mains goes back within its normal range, the device

resumes operation and operates normally. By adjusting

the bridge resistors, it becomes possible to set the

brown−out levels (on and off) independently.

• Latchoff: Pin 3 also welcomes a comparator who

offers a way to fully latch the controller. If an external

event (e.g. an overtemperature) brings the brown−out

pin above 3.5 V, the circuit stays permanently off

until the user cycles its VCC down, for instance by

unplugging the converter from the mains outlet.

• Frequency Jittering: The internal clock receives a

low frequency modulation which helps smoothing the

power supply EMI signature.

• Soft−Start: A 1.0 ms soft−start ensures a smooth

startup sequence, reducing output overshoots.

• Skip Cycle: If SMPS naturally exhibit a good

efficiency at nominal load, they begin to be less

efficient when the output power demand diminishes.

By skipping unneeded switching cycles, the NCP1028

drastically reduces the power wasted during light load

conditions. Experiments carried over the 5.0 V/2.0 A

demonstration board reveal a standby power at

no−load and 265 Vac of 85 mW and an efficiency for

500 mW output power of 64% at 230 Vac.

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