NCP1200_15 Datasheet, PDF (10/16 Pages) ON Semiconductor – PWM Current-Mode Controller for Low-Power Universal Off-Line Supplies

V_CC
11.4 V
9.8 V
6.3 V
NCP1200
Regulation
Occurs Here
Latchoff
Phase
Drv
Time
Internal
Fault
Flag
Driver
Pulses
Driver
Pulses
Time
Startup Phase
Fault is
Relaxed
Fault Occurs Here
Time
Figure 20. If the fault is relaxed during the V_CC natural fall down sequence, the IC automatically resumes.
If the fault persists when V_CC reached UVLO_L, then the controller cuts everything off until recovery.
Calculating the V_CC Capacitor
As the above section describes, the fall down sequence
depends upon the V_CC level: how long does it take for the
V_CC line to go from 11.4 V to 9.8 V? The required time
depends on the startup sequence of your system, i.e. when
you first apply the power to the IC. The corresponding
transient fault duration due to the output capacitor charging
must be less than the time needed to discharge from 11.4 V
to 9.8 V, otherwise the supply will not properly start. The test
consists in either simulating or measuring in the lab how
much time the system takes to reach the regulation at full
load. Let’s suppose that this time corresponds to 6ms.
Therefore a V_CC fall time of 10 ms could be well
appropriated in order to not trigger the overload detection
circuitry. If the corresponding IC consumption, including
the MOSFET drive, establishes at 1.5 mA, we can calculate
the required capacitor using the following formula:
Dt
+
DV
@
i
C_,
with
DV
=
2V.
Then
for
a
wanted
Dt
of
10
ms,
C equals 8 mF or 10 mF for a standard value. When an
overload condition occurs, the IC blocks its internal
circuitry and its consumption drops to 350 mA typical. This
appends at V_CC = 9.8 V and it remains stuck until V_CC
reaches 6.5 V: we are in latchoff phase. Again, using the
calculated 10 mF and 350 mA current consumption, this
latchoff phase lasts: 109 ms.
Protecting the Controller Against Negative Spikes
As with any controller built upon a CMOS technology, it
is the designer’s duty to avoid the presence of negative
spikes on sensitive pins. Negative signals have the bad habit
to forward bias the controller substrate and induce erratic
behaviors. Sometimes, the injection can be so strong that
internal parasitic SCRs are triggered, engendering
irremediable damages to the IC if they are a low impedance
path is offered between V_CC and GND. If the current sense
pin is often the seat of such spurious signals, the
high−voltage pin can also be the source of problems in
certain circumstances. During the turn−off sequence, e.g.
when the user unplugs the power supply, the controller is still
fed by its V_CC capacitor and keeps activating the MOSFET
ON and OFF with a peak current limited by Rsense.
Unfortunately, if the quality coefficient Q of the resonating
network formed by Lp and Cbulk is low (e.g. the MOSFET
Rdson + Rsense are small), conditions are met to make the
circuit resonate and thus negatively bias the controller. Since
we are talking about ms pulses, the amount of injected
charge (Q = I x t) immediately latches the controller which
brutally discharges its V_CC capacitor. If this V_CC capacitor
is of sufficient value, its stored energy damages the
controller. Figure 21 depicts a typical negative shot
occurring on the HV pin where the brutal V_CC discharge
testifies for latchup.
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