Design and Applications Information
Isolated Voltage Loop and Output Voltage Feedback on FB Pin
5.2.3
RS2 and Rf1 Design
The RS2 and Rf1 design is mainly based on the opto-couplers current transfer ratio(CTR) Which
should be around 100% to 200%. RS2 is designed to produce around 1mA current at the LED side
of the opto-coupler. Rf1 is designed to produce 2.5V feedback voltage to close the loop under a
stead state. If Rf1 is designed at 1.24kΩ, the current at transistor side of the opto-coupler should be
designed at 2mA (typical). This should have enough signal to noise ratio in the practical design. The
feedback resistor (Rf1) should be kept close to the opto-coupler to avoid noise in the layout.
The output of PFC Flyback has double line frequency ripple voltage. At the steady state operation
condition, the FB pin voltage transferred from secondary side also has this double line frequency
ripple voltage. The ripple voltage amplitude on the FB pin is determined by the output voltage ripple
amplitude and the gain from the output voltage to the FB pin (referred in the previous section
Section 5.2.2, Compensation Network Design, on page 31). It is noticed that there is an attenuated
ripple appearing at the output of the amplifier with the minus phase shift from the output voltage
ripple. Therefore, the ratio of ripple voltage amplitude over the DC voltage value of FB pin is bigger
than the ratio of output ripple over DC output voltage. If the output ripple voltage is too big in certain
applications, the FB pin voltage peak value might trigger the internal FB OVP threshold, which is
about 7% on the top of the reference value. This will heavily distort the input current waveform and
disturb the stability of the system. In order to solve this issue, it is recommended to use a constant
offset voltage circuit, as show in Figure 22. This circuit consists of a diode (ZD1), and two resistors
(Rf2 and Rf3). This will provide a bias current from the bias winding so as to produce a bias voltage
on the FB pin. Therefore, the ripple voltage amplitude of the FB pin is decreased below the FB OVP
threshold. In the 90W reference design, the winding bias voltage provides 1mA (typical) offset
current to the FB pin and the ripple voltage amplitude on the FB pin can decrease to be around half
of that with this bias circuit. Rf2 can be calculated by equation (10), in which VFB is 2.5V reference
voltage.
If the cathode voltage VZD1 is 9.1V, Rf2 is calculated as 6.8kΩ⋅
VZD1 – VFB
---------------------------
= 1mA
Equation (10)
Rf2
Figure 22: Bias Current for Offset Voltage on FB Pin
OCP SW
SGND
VOut
ISNS
VIN
PGND
88EM8040
/8041
RS4
CS2
VDD
FB
CS1 RS5
RS2
Opto- Coupler
VZD1
R
f3
Rf2
Cvout
CVCC
CVDD
R
f1
+
Vref
RS3
–
ZD1
Copyright © 2009 Marvell
Doc. No. MV-S104983-01 Rev. A
Page 33
October 5, 2009, Preliminary
Document Classification: Proprietary
MARVELL [ MARVELL TECHNOLOGY GROUP LTD. ]
