NCP1072 ONSEMI [ON Semiconductor], NCP1072 Datasheet - Page 12

• Manufacturer Part Number: NCP1072
• Description: High-Voltage Switcher for Low Power Offline SMPS
• Manufacturer: ONSEMI [ON Semiconductor]

provide energy for V
biased by DSS during start−up time or some fault mode
when the voltage on auxiliary winding is not ready yet. The
V
V
V
charge V
close to zero. Hence the V
using
Take the 65 kHz device as an example. C
above
A margin that covers the temperature drift and the voltage
drop due to switching inside FET should be considered, and
thus a capacitor above 0.1 mF is appropriate.
does not impact other parameters such as fault duration or
the frequency sweep period for instance. As one can see on
Figure 21, an internal active zener diode, protects the
switcher against lethal V
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 V
CC
CC(off)
CC(min)
As one can see, even if there is auxiliary winding to
The V
capacitor shall be dimensioned to avoid V
CC
CC
level, which stops operation. The DV between
and V
capacitor when driver is on, i.e. drain voltage is
capacitor has only a supply role and its value
CC(off)
CC
C
rapid growth triggers the over voltage
CC
VCC
is 0.4 V. There is no current source to
59 kHz @ 0.4
0.8m @ 72%
, it happens that the device is still
CC
w
Figure 22. The Charge/Discharge Cycle Over a 1 mF V
CC
f
I
runaways. This situation can
OSC
CC1
capacitor can be calculated
D
@ DV
max
VCC
CC
should be
crosses
(eq. 1)
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12
protection (OVP) circuit and immediately stops the output
pulses for t
start−up attempt takes place to check whether the fault has
disappeared or not. The OVP paragraph gives more design
details on this particular section.
Fault Condition – Short−Circuit on V
occur between V
= 370 V
seriously increase the junction temperature. For instance,
since I
T
this situation, the controller includes a novel circuitry made
of two startup levels, I
as V
(around 500 mA typical), then, when V
source smoothly transitions to I
value. As a result, in case of short−circuit between V
GND, the power dissipation will drop to 370 x 500u =
185 mW. Figure 22 portrays this particular behavior.
= I x t, which implies a 1m x 2.4 / 500u = 4.8 ms startup time
for the first sequence. The second sequence is obtained by
toggling the source to 8 mA with a delta V of V
V
startup time of 1m x 5.8 / 8m = 0.725 ms. The total startup
time becomes 4.8m + 0.725m = 5.525 ms. Please note that
this calculation is approximated by the presence of the knee
in the vicinity of the transition.
j
CCTH
), the device would dissipate 370 x 5 m = 1.85 W. To avoid
In some fault situations, a short−circuit can purposely
The first startup period is calculated by the formula C x V
CC
start1
= 8.2 – 2.4 = 5.8 V, which finally leads to a second
DC
is below a 2.4 V level, the source delivers I
recovery
) the current delivered by the startup device will
equals 5 mA (the min corresponds to the highest
CC
CC
duration (420 ms typically). Then a new
and GND. In high line conditions (V
Capacitor
start1
and I
start1
start2
and delivers its nominal
. At power−up, as long
CC
CC
reaches 2.4 V, the
CC(on)
CC
start2
and
HV
–