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| 型号： | NCP1351BPG |
| 厂商： | ON Semiconductor |
| 文件页数： | 13/27页 |
| 文件大小： | 0K |
| 描述： | IC CTRLR PWM PROG CM OTP 8DIP |
| 标准包装： | 50 |
| 输出隔离： | 隔离 |
| 频率范围： | 调节 |
| 输入电压： | 9.5 V ~ 28 V |
| 工作温度： | -25°C ~ 125°C |
| 封装/外壳： | 8-DIP（0.300"，7.62mm） |
| 供应商设备封装： | 8-PDIP |
| 包装： | 管件 |
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NCP1351
Over power protection can be done without power
dissipation penalty by arranging components around the
auxiliary as suggested by Figure 14. On this schematic, the
diode anode swings negative during the on time. This
negative level directly depends on the input voltage and
offsets the current sense pin via the R OPP resistor. A small
integration is necessary to reduce the O PP action in light load
the standby power can be affected. Again, the resistor R OPP
should be placed as close as possible to the CS pin. The
22 pF can help to circumvent any picked-up noise and D 2
prevents the positive loading of the 270 pF capacitor during
the flyback swing. We have put a typical 100 k W O PP
resistor but a tweak is required depending on your
application.
conditions. However, depending on the compensation level,
L P
DRV
DRV
+
CS
V CC
D aux
C Bulk
C4
22p
+
CV CC
L aux
R1
I Lp
150k
R offset
R sense
D2
1N4148
C3
270p
R OPP
100k
Figure 14. The OPP is Relatively Easy to Implement and It Does not Waste Power
Suppose you would need to reduce the peak current by
15% in high-line conditions. The turn-ratio between the
auxiliary winding and the primary winding is N aux . Assume
its value is 0.15. Thus, the voltage on D aux cathode swings
Typically, we measured around –4 V on our 50 W prototype.
By calculation, we want to decrease the peak current by
15%. Compared to the internal 270 m A source, we need to
derive:
negative during the on time to a level of:
Vaux_peak + -Vin_max Naux + -375
0.15 + -56 V
Ioffset + -0.15
270 m + -40.5 m A
(eq. 11)
(eq. 8)
Thus, from the –4 V excursion, the R OPP resistor is
If we selected a 3.7 k W resistor for R offset , then the
maximum sense voltage being developed is:
Vsense + 3.7 k 270 m + 1 V (eq. 9)
derived by:
ROPP +
4
40.5 m
+ 98 k W
(eq. 12)
The small RC network made of R 1 and C 3 , purposely limits
the voltage excursion on D 2 anode. Assume the primary
inductance value gives an on time of 3 m s at high-line. The
voltage across C 3 thus swings down to:
After experimental measurements, the resistor was
normalized down to 100 k W .
Feedback
Unlike other controllers, the feedback in the NCP1351
VC3 +
tonVaux_peak
R1C3
+ -
3 m
150 k
56
270 p
+ -4.2 V
(eq.
10)
works in current rather than voltage. Figure 15 details the
internal circuitry of this particular section. The optocoupler
injects a current into the FB pin in relationship with the
input/output conditions.
http://onsemi.com
13
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