Datasheet
R
MOUT
+
V
RSENSE
I
MOUT(max)
Voltage Loop
R
IN
R
D
+
−
R
f
C
f
V
REF
V
OUT
C
Z
V
OPK
+
P
IN
2 p f
R
C
OUT
V
OUT
G
VA
+
ǒ
DV
VAOUT
Ǔ
0.015
2 V
OPK
C
f
+
1
2 p f
R
G
VA
R
IN
UCC2817-EP
UCC2818-EP
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........................................................................................................................................................................................... SLUS716 – DECEMBER 2008
I
MOUT(max)
for this design is approximately 315 µ A. The R
MOUT
resistor can then be determined by:
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 k Ω .
The second major source of harmonic distortion is the ripple on the output capacitor at the second harmonic of
the line frequency. This ripple is fed back through the error amplifier and appears as a third harmonic ripple at
the input to the multiplier. The voltage loop must be compensated not just for stability but also to attenuate the
contribution of this ripple to the total harmonic distortion of the system (see Figure 2 ).
Figure 2. Voltage Amplifier Configuration
The gain of the voltage amplifier, G
VA
, can be determined by first calculating the amount of ripple present on the
output capacitor. The peak value of the second harmonic voltage is given by the equation:
In this example, V
OPK
= 3.91 V. Assuming an allowable contribution of 0.75% (1.5% peak to peak) from the
voltage loop to the THD budget, set the gain equal to:
Where:
Δ V
VAOUT
= Effective output voltage range of the error amplifier (5 V for the UCC2817).
The network needed to realize this filter is comprised of an input resistor, R
IN
, and feedback components C
f
, C
Z
,
and R
f
. The value of R
IN
is already determined because of its function as one-half of a resistor divider from V
OUT
feeding back to the voltage amplifier for output voltage regulation. In this case, the value was chosen to be
1 M Ω . This high value was chosen to reduce power dissipation in the resistor. In practice, the resistor value
would be realized by the use of two 500-k Ω resistors in series because of the voltage rating constraints of most
standard 1/4-W resistors. The value of C
f
is determined by the equation:
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