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AN4129 Datasheet(PDF) 7 Page - Fairchild Semiconductor

Part # AN4129
Description  Green Current Mode PWM Controller FAN7601
Download  12 Pages
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Manufacturer  FAIRCHILD [Fairchild Semiconductor]
Direct Link  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

AN4129 Datasheet(HTML) 7 Page - Fairchild Semiconductor

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APPLICATION NOTE
AN4129
7
©2003 Fairchild Semiconductor Corporation
6. Output Voltage Sensing Resistor and
Feedback Loop Design
The output voltage sensing circuits cause power loss if the
impedance is too low. The values of the output voltage
sensing resistors are selected so that that power loss is under
10mW. The designed values are 27k
Ω for the upper resistor
and 7k
Ω for the bottom resistor.
To control output voltage, a KA431 and an optocoupler are
used as shown in Fig. 15. Equation (15) is the compensator
transfer function. In this equation, k is the current transfer
ratio of the optocoupler and this value is nonlinear.
Figure 15. Output Voltage Compensation Circuit
The FAN7601 does not contain an LEB(Leading Edge
Blanking) circuit, but the parasitic capacitance and
resistance of the internal circuit work as an RC filter which
filters switching noise. The parasitic capacitance is about
10pF and the parasitic resistance is about 20k
Ω. These
values are sufficient to filter switching noise; therefore a
small capacitor can be used for Cf . The value of Rf should
be 1000~2000 times higher than that of Rs. The filter resistor
Rf causes some sensing delay so that the peak value of the
filtered information is less than that of the real current
information. The higher resistance causes a greater
difference as shown in Fig. 16. The black line is the CS/FB
voltage and the red and blue lines are the real current
waveforms before filtering . The red line is the current
waveform when the resistance is low and the blue line is the
current waveform when the resistance is high. Because the
current peak of the blue line is higher than that of the red
line, more energy is transferred to the secondary side.
Therefore standby power is lower with the higher resistance.
But if the resistance is too high, the system can become
unstable. The selected values are 10pF for Cf and 1kΩ for Rf
.
Figure 16. Current Sense Waveforms
For the stability of the system, capacitor C1 must be high
enough. C1 can be selected as in (16). The selected value is
1nF.
R3 determines the control loop gain. If the value is too low,
the system can become unstable. And if the value is too high,
the output voltage regulation characteristics may be poor.
The selected value is 1.5k
Ω. If the value of Rfb is too high
then the output voltage is not regulated at no load because
the offset voltage of the CS/FB pin is lower than necessary.
If the value is too low the audible noise increases. Because
the current transfer ratio of the photocoupler is nonlinear, the
value of Rfb has to be selected by experiment at no load.
The selected value is 3.9k
Ω.
7. Transformer Audible Noise
Because the FAN7601 operates in burst mode at light load
and no load, it has a switching period and a non-switching
period. Figure 17 shows the gate and output voltage at no
load. Burst operation frequency is about 114Hz. The burst
operation frequency varies according to load condition and
the frequency is in the range of the audible frequency.
Therefore the transformer may generate audible noise.
The audible noise level depends on the control loop
characteristic. If the loop speed is fast, audible noise
increases but power loss decreases. If the loop speed is slow,
audible noise decreases but power loss increases. There has
to be a compromise between power loss and audible noise.
Varnishing the transformer helps in reducing audible noise.
Vfb
VO
--------
k
=
Rf
R3
--------
1
Rf Cf s
⋅⋅
1
+
---------------------------------
1
1
R1 C1 s
⋅⋅
----------------------------
+


⋅⋅
(15
CS/FB
Vcc
R
fb
R
f
R
S
C
f
Vo
C1
R1
R2
R3
1V
CS/FB
Low resistance
High resistance
Filtered informations
C1
10
fsw
---------
>
1
2
π R1
-------------------
(16)


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