Datasheet

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SLUS577C − SEPTEMBER, 2003 − REVISED MARCH 2009
12
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APPLICATION INFORMATION
Because the VFF voltage is generated from line voltage it needs to be adequately filtered to reduce total
harmonic distortion caused by the 120 Hz rectified line voltage. Refer to Unitrode Power Supply Design
Seminar, SEM−700 Topic 7, [Optimizing the Design of a High Power Factor Preregulator.] A single pole filter
was adequate for this design. Assuming that an allocation of 1.5% total harmonic distortion from this input is
allowed, and that the second harmonic ripple is 66% of the input ac line voltage, the amount of attenuation
required by this filter is:
1.5 %
66 %
+ 0.022
With a ripple frequency (f
R
) of 120 Hz and an attenuation of 0.022 requires that the pole of the filter (f
P
) be placed
at:
f
P
+ 120 Hz 0.022 [ 2.6 Hz
The following equation can be used to select the filter capacitor (C
VFF
) required to produce the desired low pass
filter.
C
VFF
+
1
2 p R
VFF
f
P
[ 2.2 mF
The R
MOUT
resistor is sized to match the maximum current through the sense resistor to the maximum multiplier
current. The maximum multiplier current, or I
MOUT(max)
, can be determined by the equation:
I
MOUT(max)
+
I
IAC
@V
IN(min)
ǒ
V
VAOUT(max)
* 1V
Ǔ
K V
VFF
2
(min)
I
MOUT(max)
for this design is approximately 315 µA. The R
MOUT
resistor can then be determined by:
R
MOUT
+
V
RSENSE
I
MOUT(max)
In this example V
RSENSE
was selected to give a dynamic operating range of 1.25 V, which gives an R
MOUT
of
roughly 3.91 kΩ.
(11)
(12)
(13)
(14)
(15)