1207A ON Semiconductor, 1207A Datasheet - Page 11

• Manufacturer Part Number: 1207A
• Description: Search -----> NCP1207A
• Manufacturer: ON Semiconductor

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Latching Off the NCP1207A
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.
Shutting Off the NCP1207A
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 (V
permanently disables the IC. A small time constant on the
transistor base will avoid false triggering (Figure 23).
ON/OFF
ON/OFF
In certain cases, it can be very convenient to externally
Shutdown can easily be implemented through a simple
soon as the temperature exceeds a given setpoint
Figure 22. A simple transistor arrangement allows
Figure 21. A simple CTN triggers the latchoff as
Figure 23. A simple bipolar transistor totally
to trigger the latchoff by an external signal
3
10 k
1
2
3
4
NCP1207A
disables the IC
10 nF
CTN
2
Q1
1
2
3
4
8
7
6
5
NCP1207A
1
CE(sat)
Aux
1
2
3
4
NCP1207A
8
7
6
5
200 mV) and
Aux
http://onsemi.com
8
7
6
5
NCP1207A
11
Power Dissipation
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
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
From the 350 Vdc rail, the part will dissipate:
will drop at higher operating junction temperatures).
A DIP8 package offers a junction−to−ambient thermal
resistance R
dissipation can thus be computed knowing the maximum
operating ambient temperature (e.g. 70 C) together with
the maximum allowable junction temperature (125 C):
do not reach the worse consumption budget imposed by the
operating conditions. Several solutions exist to cure this
trouble:
(V HVDC * 11 V) @ I CC2
350 V @ 2.0 mA + 700 mW
P max +
The NCP1207A is directly supplied from the DC rail
The first one consists in adding some copper area around
the NCP1207A DIP8 footprint. By adding a min pad area
of 80 mm
75 C/W. Maximum power then grows up to 730 mW.
A resistor R
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):
power dissipated by the resistor is thus:
P drop + V dropRMS
Please refer to the application note AND8069 available
from www.onsemi.com/pub/ncp1200.
+
+
T jmax * T Amax
2
I DSS @ R drop @ DSS duty * cycle
7.0 mA @ 7.1 kW @ 0.286
qJA
of 35 mm copper (1 oz.), R
drop
R qJA
R drop v
will
can
needs to be inserted with pin 8 to a) avoid
of 100 C/W. The maximum power
7.1 kW
2
. If we operate the device on a 250
V bulkmin * 50 V
R drop
be
t 550 mW
be
R drop
(however this 2.0 mA number
7.0 mA
2.0/7.0
evaluated
. As we can see, we
2
+ 99.5 mW
qJA
t 7.1 kW
drops to about
=
2
28.6%.
. The
using:
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