NCP1028 ON Semiconductor, NCP1028 Datasheet - Page 12

• Manufacturer Part Number: NCP1028
• Description: High?voltage Switcher For Medium Power Offline Smps
• Manufacturer: ON Semiconductor

the output voltage to its target value. We know that the
switcher consumption is around 2.0 mA (I
we can calculate the amount of capacitance we need, to
hold V
2.0 mA:
capacitor.
Fault Condition – Short-Circuit on V
occur between V
(V
device will seriously increase the junction temperature. For
instance, since IC1 equals 3.0 mA (the min corresponds to
the highest T
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 V
delivers IC1 (around 650 mA typical), then, when V
reaches 1.3 V, the source smoothly transitions to IC2 and
delivers its nominal value. As a result, in case of
short-circuit between V
will drop to 370
this particular behavior.
C w 2m · 10 m
Suppose our power supply takes 10 ms (t
In some fault situations, a short-circuit can purposely
C w
HV
= 370 V
CC
I CC1 t startup
1
above 7.5 V at least for 10 ms while delivering
DV CC
J
), the device would dissipate 370
DC
w 20 mF
CC
) the current delivered by the startup
or, by replacing with the above values,
650 m = 240 mW. Figure 25 portrays
and GND. In high line conditions
CC
CC
then select a 33 mF for the V
is below a 1.3 V level, the source
and GND, the power dissipation
Figure 24. A typical startup sequence showing the V
CC1
startup
CC
). Therefore,
) to bring
voltage evolution versus time.
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3 m =
CC
CC
NCP1028
12
C
startup time for the first sequence (t
sequence (t
with a delta V of VCC
which finally leads to a second startup time of
7
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.
The first startup period is calculated by the formula
33 m/6.0 m = 39 ms. The total startup time becomes
Figure 25. The startup source now features a
V = I
2
) is obtained by toggling the source to 4.0 mA
t, which implies a 33 m
dual-level startup current.
CC
ON
capacitor
– VCCth = 8.5 – 1.5 = 7.0 V,
1.3/650 m = 66 ms
1
). The second