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
Latching Off the NCP1207
In certain cases, it can be very convenient to externally
shut down permanently the NCP1207 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 25) or a simple
NPN transistor (Figure 26) can do the work.
CTN
NCP1207
Aux
1
8
2
7
3
6
4
5
Figure 25. A simple CTN triggers the latchoff as
soon as the temperature exceeds a given setpoint
ON/OFF
NCP1207
Aux
1
8
2
7
3
6
4
5
Figure 26. A simple transistor arrangement allows
to trigger the latchoff by an external signal
Shutting Off the NCP1207
Shutdown can easily be implemented through a simple
NPN bipolar transistor as depicted by Figure 27. 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 27).
NCP1207
1
8
10 k
ON/OFF
2
Q1
1
3
7
6
3
2
4
5
10 nF
Figure 27. A simple bipolar transistor totally
disables the IC
Power Dissipation
The NCP1207 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
NCP1207 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 NCP1207 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 NCP1207 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
12
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