NCP1072 ONSEMI [ON Semiconductor], NCP1072 Datasheet - Page 13

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

Fault Condition – Output Short−Circuit
internally enabled. If everything is correct, the auxiliary
winding increases the voltage on the V
voltage rises. During the start−sequence, the controller
smoothly ramps up the peak drain current to maximum
setting, i.e. I
ms. When the output voltage is not regulated, the current
coming through FB pin is below I
typically), which is not only during the startup period but
also anytime an overload occurs, an internal error flag is
Auto−Recovery Over Voltage Protection
way to prevent output voltage runaway when the
optocoupler fails. As Figure 24 shows, an active zener diode
monitors and protects the V
breakdown voltage, that is to say 8.4 V typical, no current
flows in it. If the auxiliary V
inside the zener, then the controller considers an OVP
situation and stops the internal drivers. When an OVP
occurs, all switching pulses are permanently disabled. After
t
symptom still exists, e.g. feedback opto−coupler fails, the
device keeps the auto−recovery OVP mode.
between the auxiliary dc level and the V
a) not to damage the internal 8.4 V zener diode during an
overshoot for instance (absolute maximum current is
15 mA) b) to implement the fail−safe optocoupler protection
(OVP) as offered by the active clamp. Please note that there
cannot be bad interaction between the clamping voltage of
the internal zener and V
actually built on top of V
(200 mV typical). R
recovery
As soon as V
The particular NCP107X arrangement offers a simple
Figure 24 shows that the insertion of a resistor (R
Figure 23. In Case of Short−Circuit or Overload, the NCP107X Protects Itself and the Power Supply Via a Low
Frequency Burst Mode. The V
delay, it resumes the internal drivers. If the failure
IPK
, which is reached after a typical period of 1
CC
limit
reaches V
CC(on)
should be carefully selected to avoid
CC(on)
CC
since this clamping voltage is
CC
with a fixed amount of offset
pin. Below its equivalent
CC(on)
pushes too much current
FBfault
CC
CC
, drive pulses are
CC
is Maintained by the Current Source and Self−supplies the Controller.
pin as the output
pin is mandatory
level (35 mA
http://onsemi.com
limit
)
13
asserted, Ipflag, indicating that the system has reached its
maximum current limit set point. The assertion of this flag
triggers a fault counter t
completion, Ipflag remains asserted, all driving pulses are
stopped and the part stays off in t
420 ms). A new attempt to re−start occurs and will last 53 ms
providing the fault is still present. If the fault still affects the
output, a safe burst mode is entered, affected by a low
duty−cycle operation (11%). When the fault disappears, the
power supply quickly resumes operation. Figure 23 depicts
this particular mode:
triggering the OVP as we discussed, but also to avoid
disturbing the V
lines detail how to evaluate the R
applications often puzzles the designer. Actually, if a SMPS
operated at nominal load can deliver an auxiliary voltage of
an arbitrary 16 V (V
(V
recurrence of the switching pulses expands so much that the
low frequency re−fueling rate of the V
enough to keep a proper auxiliary voltage. Figure 25
portrays a typical scope shot of a SMPS entering deep
standby (output un−loaded). Thus, care must be taken when
calculating R
(by injecting 6 mA into the active clamp – always use the
minimum value for worse case design) in normal operation
but 2) not to drop too much voltage over R
standby. Otherwise, the converter will enter dynamic self
supply mode (DSS mode), which increases the power
dissipation. Based on these recommendations, we are able to
bound R
Self−supplying controllers in extremely low standby
stby
) when entering standby. This is because the
limit
between two equations:
limit
CC
1) to not trigger the V
in low / light load conditions. The below
nom
), this voltage can drop below 10 V
SCP
(53 ms typically). If at counter
limit
recovery
value...
CC
CC
limit
over current latch
duration (about
capacitor is not
when entering