Datasheet

NCP1010, NCP1011, NCP1012, NCP1013, NCP1014
http://onsemi.com
10
Figure 16. NCP101X Facing a Fault Condition (Vin = 150 Vdc)
Tsta r t
Tsw
TLatch
1 V Ripple
Latch--off
Level
The rising slope from the latch--off level up to 8.5 V
is expressed by:
Tstart =
ΔV1 · C
IC1
. The time during which
the IC actually pulses is given by
tsw =
ΔV2 · C
ICC1
.
Finally, the latch-- off time can be derived
using the same f ormula topology:
TLatch =
ΔV3 · C
ICC2
.
From these thr ee definitions, the burst duty -- cycle
can be computed:
dc =
Tsw
Tstart + Tsw + TLatch
(eq. 2)
.
dc =
ΔV2
ICC1 ·

ΔV2
ICC1
+
ΔV1
IC1
+
ΔV3
ICC2

(eq. 3)
. Feeding the
equation with values extracted from the parameter section
gives a typical duty--cycle of 13%, precluding any lethal
thermal runaway while in a fault condition.
DSS Internal Dissipation
The Dynamic Self--Supplied pulls energy out from the
drain pin. In Flyback--based converters, this drain level can
easily go above 600 V peak and thus increase the stress on the
DSS startup source. However, the drain voltage evolves with
time and its period is small compared to that of the DSS. As
a result, the averaged dissipation, excluding capacitive losses,
can be derived by:
P
DSS
= ICC1 · < Vds(t) > .
(eq. 4)
.
Figure 17 portrays a typical drain--ground waveshape where
leakage effects have been removed.
Figure 17. A typical drain--ground waveshape
where leakage effects are not accounted for.
Vds(t)
Vin
Vr
toff
dt
ton
t
Tsw
By looking at Figure 17, the average result can easily be
derived by additive square area calculation:
< Vds(t) >= Vin · (1 − d) + Vr ·
toff
Tsw
(eq. 5)
By developing Equation 5, we obtain:
< Vds(t) >= Vin − Vin ·
ton
Tsw
+ Vr ·
toff
Tsw
(eq. 6)
toff can be expressed by:
toff = Ip ·
Lp
Vr
(eq. 7)
where ton
can be evaluated by:
ton = Ip ·
Lp
Vin
(eq. 8)
.