NCP1207A(2004) 查看數據表（PDF） - ON Semiconductor
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NCP1207A
(Rev.:2004)
(Rev.:2004)
ON Semiconductor
NCP1207A Datasheet PDF : 16 Pages
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NCP1207A
Latching Off the NCP1207A
In certain cases, it can be very convenient to externally
shut down permanently the NCP1207A via a dedicated
signal, e.g. coming from a temperature sensor. The reset
occurs when the user unplugs the power supply from the
mains outlet. To trigger the latchoff, a CTN (Figure 21) or
a simple NPN transistor (Figure 22) can do the work.
CTN
NCP1207A
Aux
1
8
2
7
3
6
4
5
Figure 21. A simple CTN triggers the latchoff as
soon as the temperature exceeds a given setpoint
ON/OFF
NCP1207A
Aux
1
8
2
7
3
6
4
5
Figure 22. A simple transistor arrangement allows
to trigger the latchoff by an external signal
Shutting Off the NCP1207A
Shutdown can easily be implemented through a simple
NPN bipolar transistor as depicted by Figure 23. When OFF,
Q1 is transparent to the operation. When forward biased, the
transistor pulls the FB pin to ground (VCE(sat) ≈ 200 mV) and
permanently disables the IC. A small time constant on the
transistor base will avoid false triggering (Figure 23).
NCP1207A
1
8
10 k
ON/OFF
2
Q1
1
3
7
6
3
2
4
5
10 nF
Figure 23. A simple bipolar transistor totally
disables the IC
Power Dissipation
The NCP1207A is directly supplied from the DC rail
through the internal DSS circuitry. The DSS being an
auto−adaptive circuit (e.g. the ON/OFF duty−cycle adjusts
itself depending on the current demand), the current flowing
through the DSS is therefore the direct image of the
NCP1207A current consumption. The total power
dissipation
can
be
evaluated
using:
(VHVDC * 11 V) @ ICC2. If we operate the device on a 250
Vac rail, the maximum rectified voltage can go up to 350
Vdc. As a result, the worse case dissipation occurs at the
maximum switching frequency and the highest line. The
dissipation is actually given by the internal consumption of
the NCP1207A when driving the selected MOSFET. The
best method to evaluate this total consumption is probably
to run the final circuit from a 50 Vdc source applied to pin 8
and measure the average current flowing into this pin.
Suppose that we find 2.0 mA, meaning that the DSS
duty−cycle will be 2.0/7.0 = 28.6%.
From the 350 Vdc rail, the part will dissipate:
350 V @ 2.0 mA + 700 mW (however this 2.0 mA number
will drop at higher operating junction temperatures).
A DIP8 package offers a junction−to−ambient thermal
resistance RqJA of 100°C/W. The maximum power
dissipation can thus be computed knowing the maximum
operating ambient temperature (e.g. 70°C) together with
the maximum allowable junction temperature (125°C):
P
max
+
Tjmax * TAmax
RqJA
t
550
mW.
As
we
can
see,
we
do not reach the worse consumption budget imposed by the
operating conditions. Several solutions exist to cure this
trouble:
• The first one consists in adding some copper area around
the NCP1207A DIP8 footprint. By adding a min pad area
of 80 mm2 of 35 mm copper (1 oz.), RqJA drops to about
75°C/W. Maximum power then grows up to 730 mW.
• A resistor Rdrop needs to be inserted with pin 8 to a) avoid
negative spikes at turn−off (see below)
b) split the power budget between this resistor and the
package. The resistor is calculated by leaving at least 50 V
on pin 8 at minimum input voltage (suppose 100 Vdc in
our
case):
Rdrop
v
Vbulkmin *
7.0 mA
50
V
t
7.1
kW.
The
power dissipated by the resistor is thus:
Pdrop + VdropRMS2ńRdrop
ǒ Ǔ IDSS @ Rdrop @ ǸDSSduty * cycle 2
+
Rdrop
ǒ7.0 mA @ 7.1 kW @ Ǹ0.286Ǔ2
+
7.1 kW
+ 99.5 mW
Please refer to the application note AND8069 available
from www.onsemi.com/pub/ncp1200.
http://onsemi.com
11
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