NCP1236
VFB
VFB(fault)
Time
Time
Time
VFB takes
over soft-start
Soft-start ramp
VILIM
tSSTART
CS Setpoint
VILIMI
Figure 37. Soft−Start
Overpower compensation
Under some conditions, like a winding short−circuit for
instance, not all the energy stored during the on time is
transferred to the output during the off time, even if the on
time duration is at its minimum (imposed by the propagation
delay of the detector added to the LEB duration). As a result,
The power delivered by a flyback power supply is
proportional to the square of the peak current in the
discontinuous conduction mode:
1
2
2
(eq. 1)
POUT
+
@ h @ Lp @ FSW @ Ip
the current sense voltage keeps on increasing above V
,
ILIM
because the controller is blind during the LEB blanking
time. Dangerously high current can grow in the system if
nothing is done to stop the controller. That’s what the
additional comparator, that senses when the current sense
Unfortunately, due to the inherent propagation delay of
the logic, the actual peak current is higher at high input
voltage than at low input voltage, leading to a significant
difference in the maximum output power delivered by the
power supply.
voltage on CS pin reaches V
(= 1.5 x V ), does:
ILIM
CS(stop)
as soon as this comparator toggles, the controller
immediately enters the protection mode (latched or
autorecovery according to the chosen option).
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