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參數資料
| 型號： | NCP1207DR2 |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩壓器 |
| 英文描述： | PWM Current-Mode Controller for Free Running Quasi-Resonant Operation |
| 中文描述： | 0.5 A SWITCHING CONTROLLER, PDSO8 |
| 封裝： | SOIC-8 |
| 文件頁數： | 12/18頁 |
| 文件大小： | 202K |
| 代理商： | NCP1207DR2 |
NCP1207
http://onsemi.com
12
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.
Figure 25. A simple CTN triggers the latchoff as
soon as the temperature exceeds a given setpoint
1
2
3
4
8
7
6
5
CTN
Aux
NCP1207
ON/OFF
Figure 26. A simple transistor arrangement allows
to trigger the latchoff by an external signal
1
2
3
4
8
7
6
5
NCP1207
Aux
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 (V
CE(sat)
≈
200 mV) and
permanently disables the IC. A small time constant on the
transistor base will avoid false triggering (Figure 27).
Figure 27. A simple bipolar transistor totally
disables the IC
1
2
3
4
8
7
6
5
NCP1207
10 nF
Q1
10 k
ON/OFF
1
2
3
Power Dissipation
The NCP1207 is directly supplied from the DC rail
through the internal DSS circuitry. The DSS being an
autoadaptive circuit (e.g. the ON/OFF dutycycle 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
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 dutycycle 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 junctiontoambient thermal
resistance R
JA
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):
TJmax
TAmax
RJA
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 mm
2
of 35 m copper (1 oz.) R
JA
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 turnoff (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
Vbulkmin
11 V)
ICC2
. If we
Pmax
550 mW
. As we can see, we
our case):
Rdrop
power dissipated by the resistor is thus:
50 V
7.0 mA
7.1 k
. The
Pdrop
VdropRMS
2
Rdrop
IDSS
Rdrop
DSSduty
cycle
2
Rdrop
7.0 mA
7.1 k
7.1 k
0.286
2
99.5 mW
Please refer to the application note AND8069 available
from www.onsemi.com/pub/ncp1200.
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