1207 データシートの表示（PDF） - ON Semiconductor
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1207 Datasheet PDF : 18 Pages
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NCP1207
Calculating the VCC Capacitor
As the above section describes, the fall down sequence
depends upon the VCC level: how long does it take for the
VCC line to go from 12 V to 10 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 12 V to 10 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 6.0 ms. Therefore a
VCC 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 1.8 mA (e.g. with an 11 nC MOSFET),
we can calculate the required capacitor using the following
formula:
Dt
+
DV @
i
C,
with
DV
=
2.0
V.
Then
for
a
wanted
Dt of 10 ms, C equals 9.0 mF or 22 mF for a standard value.
When an overload condition occurs, the IC blocks its
internal circuitry and its consumption drops to 330 mA
typical. This happens at VCC = 10 V and it remains stuck
until VCC reaches 5.3 V: we are in latchoff phase. Again,
using the calculated 22 mF and 330 mA current consumption,
this latchoff phase lasts: 313 ms.
HV Pin Recommended Protection
When the user unplugs a power supply built with a QR
controller such as the NCP1207, two phenomena can
appear:
1. A negative ringing can take place on pin8 due to a
resonance between the primary inductance and
the bulk capacitor. As any CMOS device, the
NCP1207 is sensitive to negative voltages that
could appear on it’s pins and could create an
internal latchup condition.
2. When the bulk capacitor discharges, the internal
latch is reset by the voltage developed over the
sense resistor and the ON time expands as less
voltage is available. When the high−voltage rail
becomes too low, the gate drives permanently
stays high since no reset occurs. This situation is
not desirable in many applications.
For the above reasons, we strongly recommend to add a
high−voltage diode like a 1N4007 between the bulk
capacitor and the VCC pin. When the bulk level collapses, it
naturally shuts the controller down and eradicates the two
above problems.
+
Cbulk
HV
NCP1207
1
8
2
7
3
6
4
5
D1
1N4007
+
Figure 29.
Operation Shots
Below are some oscilloscope shots captured at
Vin = 120 VDC with a transformer featuring a 800 mH
primary inductance.
Figure 30.
This plot gathers waveforms captured at three different
operating points:
1st upper plot: free run, valley switching operation,
Pout = 26 W
2nd middle plot: min Toff clamps the switching frequency
and selects the second valley
3rd lowest plot: the skip slices the second valley pattern
and will further expand the burst as Pout goes low
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