NCP1027 データシートの表示（PDF） - ON Semiconductor

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NCP1027

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

ON Semiconductor 

NCP1027

Startup Sequence

The NCP1027 includes a high−voltage startup circuitry,

directly deriving current from the bulk line to charge the

Vbulk

VCC capacitor. Figure 24 details the simplified internal

arrangement.

+

I1

RVCC

IC1

5

ICC1

←

1

I2

Iclamp

−+

+

CVCC

Vz = 8.7 V

VCCon +

VCCoff

Iclamp > 6 mA

→ OVP fault

8

Figure 24. Internal Arrangement of the Startup Circuitry

When the power supply is first connected to the mains

outlet, the internal current source is biased and charges up

the VCC capacitor. When the voltage on this VCC capacitor

reaches the VCCON level (typically 8.5 V), the current

source turns off, reducing the amount of power being

dissipated. At this time, the VCC capacitor only supplies the

controller, and the auxiliary supply should take over before

VCC collapses below VCC(min). This VCC capacitor, CVCC,

must therefore be calculated to hold enough energy so that

VCC stays above VCC(min) (7.3 V typical) until the

auxiliary voltage fully takes over.

An auxiliary winding is needed to maintain the VCC in

order to self−supply the switcher. The VCC capacitor has

only a supply role and its value does not impact other

parameters such as fault duration or the frequency sweep

period for instance. As one can see in Figure 24, an internal

active Zener diode, protects the switcher against lethal VCC

runaways. This situation can occur if the feedback loop

optocoupler fails, for instance, and you would like to

protect the converter against an over voltage event. In that

case, the internal current increase incurred by the VCC

rapid growth triggers the over voltage protection (OVP)

circuit and immediately stops the output pulses for 440 ms.

Then a new startup attempt takes place to check whether

the fault has disappeared or not. The OVP paragraph gives

more design details on this particular section.

The VCC capacitor can be calculated knowing a) the

amount of energy that needs to be stored; b) the time it

takes for the auxiliary voltage to appear, and; c) the current

consumed by the controller at that time. For a better

understanding, Figure 25 shows how the voltage evolves

on the VCC capacitor upon startup.

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