參數(shù)資料
型號： 88EM8081XX-SAG2C000-T
廠商： MARVELL SEMICONDUCTOR INC
元件分類： 穩(wěn)壓器
英文描述： 2 A POWER FACTOR CONTROLLER, 120 kHz SWITCHING FREQ-MAX, PDSO8
封裝： GREEN, SOIC-8
文件頁數(shù)： 42/64頁
文件大?。?/td> 667K
代理商： 88EM8081XX-SAG2C000-T
Design and Applications Information
Current Sensing and Over Current Protection
Copyright 2010 Marvell
Doc. No. MV-S106340-01 Rev. B
August 6, 2010, Preliminary Information
Document Classification: Proprietary
Page 47
The resistor (Rsen) should be designed as the example in Table 7 where Rsen is designed for a 60W
power supply.
Table 8 shows the reference value of the current sensing resistor for different input power levels. In
the practical design, the current sensing resistor value could be fine tuned around the value shown
in the table based on the specification and the primary inductance of the flyback transformer.
5.4.2
Average Current Signal and Over Power Limitation
The peak current is sensed as the voltage across the sense resistor. To convert flyback peak current
into an average current signal, an RC filter is required as described in the previous section.
Figure 25 shows how the addition of the filter will result in an average current signal. This average
current signal, Vcs is fed back onto the ISNS pin and used to achieve a sinusoidal current waveform
by an internal current control loop. It is also used to achieve power limitation. The corner frequency
of the RC filter is recommended approximately 1/10~1/6 of the switching frequency. The
recommended value for Rcs1 is 187, Rcs2 is 200. Rcs1 and Rcs2 are used for the purpose of
blocking excessive negative and surge voltages. A single stage PFC operates at 120kHz (typical)
using the 88EM8081 device. Ccs is designed as 47nF which results in a corner frequency of 18Hz.
The corner frequency of the low pass filter is defined by the following equation;
Equation (30)
U3 is designed to perform over power limitation according to different over current thresholds as
shown in Table 4, Electrical Characteristics, on page 16. The adaptive over current and hence
adaptive over power protection feature is described in Section 3.7.3
Table 7:
Current Sensing Circuit
Input Power
Pin
60W
Minimum input voltage
Vin_min
85V
Maximum average input current
1A
Over current threshold Zone 1
VIOVER_TH1
0.39V
Over current margin
IMargin
30%
Current sensing resistor calculation
0.30
Current sensing resistor selection
Rsns
0.30
I
in_max
2
P
in
V
in_min
-----------------
×
=
R
sns
V
IOVER_TH1
i
in_max
1
I
min
arg
+
()
×
-----------------------------------------------------
=
Table 8:
Current Sensing Resistor Selection Reference
Input Power (W)
10
30
60
120
Current Sensing Resistor (
)
1.0 - 2.0
0.50 - 0.70
0.25 - 0.35
0.12 - 0.15
f
corner
1
2
πR
cs1Ccs
--------------------------
=
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