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

Figure 25. A typical startup sequence showing the VCC capacitor

voltage evolution versus time.

Suppose our power supply takes 10 ms (tstartup) to bring

the output voltage to its target value. We know that the

switcher consumption is around 2.0 mA (ICC1). Therefore,

we can calculate the amount of capacitance we need, to

hold VCC above 7.5 V at least for 10 ms while delivering

2.0 mA:

C

w

ICC1tstartup

DVCC

or,

by

replacing

with

the

above

values,

C

w

2m

· 10

1

m

w

20

mF

then

select

a

33

mF

for

the

VCC

capacitor.

Fault Condition – Short−Circuit on VCC

In some fault situations, a short−circuit can purposely

occur between VCC and GND. In high line conditions

(VHV = 370 VDC) the current delivered by the startup

device will seriously increase the junction temperature. For

instance, since IC1 equals 3.0 mA (the min corresponds to

the highest TJ), the device would dissipate 370 3 m =

1.1 W. To avoid this situation, the controller includes a

novel circuitry made of two startup levels, IC1 and IC2. At

powerup, as long as VCC is below a 1.3 V level, the source

delivers IC1 (around 650 mA typical), then, when VCC

reaches 1.3 V, the source smoothly transitions to IC2 and

delivers its nominal value. As a result, in case of

short−circuit between VCC and GND, the power dissipation

will drop to 370 650 m = 240 mW. Figure 26 portrays

this particular behavior.

Figure 26. The startup source now features a

dual−level startup current.

The first startup period is calculated by the formula

C V = I t, which implies a 33 m 1.3/650 m = 66 ms

startup time for the first sequence (t1). The second

sequence (t2) is obtained by toggling the source to 4.0 mA

with a delta V of VCCON – VCCth = 8.5 – 1.5 = 7.0 V,

which finally leads to a second startup time of

7 33 m/6.0 m = 39 ms. The total startup time becomes

66 m + 39 m = 105 ms as a typical value. Please note that

this calculation is approximated by the presence of the knee

in the vicinity of the transition.

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