Datasheet

NCP1028
http://onsemi.com
11
Startup Sequence
The NCP1028 includes a high-voltage startup circuitry,
directly deriving current from the bulk line to charge the
V
CC
capacitor. Figure 23 details the simplified internal
arrangement.
Vbulk
I1
RV
CC
1
I2
+
CV
CC
I
clamp
Vz = 8.7 V
ICC1
+
-
V
CCon
V
CCoff
+
5
8
+
IC1
Figure 23. 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 V
CC
capacitor. When the voltage on this V
CC
capacitor
reaches the VCC
ON
level (typically 8.5 V), the current
source turns off, reducing the amount of power being
dissipated. At this time, the V
CC
capacitor only supplies the
controller, and the auxiliary supply should take over before
V
CC
collapses below VCC
(min)
. This V
CC
capacitor, CV
CC
,
must therefore be calculated to hold enough energy so that
V
CC
stays above VCC
(min)
(7.3 V typical) until the
auxiliary voltage fully takes over.
An auxiliary winding is needed to maintain the V
CC
in
order to self-supply the switcher. The V
CC
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 23, an internal
active Zener diode, protects the switcher against lethal V
CC
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.
The V
CC
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 24 shows how the voltage evolves
on the V
CC
capacitor upon startup.