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NCL30001
converter. There is no error in the output signal due to the
series rectifier as shown in Figure 58.
The scaled version of the full wave rectified input ac wave
is applied to the AC_IN pin by means of a resistive voltage
divider. The multiplier ramp is generated by comparing the
scaled line voltage to the oscillator ramp with the AC_IN
Comparator. The current signal from the V-I converter is
FB
Multiplier
V ? to ? I
factored by the AC_IN comparator output. The resulting
signal is filtered by the low pass R-C filter on the CM pin.
The low pass filter removes the high frequency content. The
gain of the multiplier is determined by the V-I converter, the
resistor on the CM pin, and the peak and valley voltages of
the oscillator sawtooth ramp.
FB
AC_IN
AC IN
+
?
Oscillator
V
AC_REF
+ k @
V
@ V
FF
V 2
CM
Divide
AC_REF
V FF
Square
V FF2
Figure 58. Reference Generator
V AC_REF + @ k (eq. 8)
V FF
V FF +
Vac 2 a
The third input to the reference generator is the V FF signal.
The V FF signal is a dc voltage proportional to the ac line
voltage. A resistive voltage divider attenuates the full wave
rectified line voltage between 0.7 and 5.0 V. The full wave
rectified line is then averaged with a capacitor. The ac
average voltage must be constant over each half cycle of the
line. Line voltage ripple (120 Hz or 100 Hz) ripple on the
V FF signal adds ripple to the output of the multiplier. This
will distort the ac reference signal and reduce the power
factor and increase the line current distortion. Excessive
filtering delays the feedforward signal reducing the line
transient response. A good starting point is to set the filter
time constant to one cycle of the line voltage. The user can
then optimize the filter for line transient response versus
power factor. The average voltage on the V FF pin is:
2
p (eq. 7)
Where, a is the voltage divider ratio, normally 0.01.
V FB @ V AC_IN
2
The multiplier transfer function is given by Equation 8.
The output of the multiplier is the AC_REF. It connects to
the AC Error Amplifier.
where, k is the reference generator gain, typically 0.55. The
output of the reference generator is clamped at 4.5 V to limit
the maximum output power.
Feedforward maintains a constant input power
independent of the line voltage. That is, for a given FB
voltage, if the line voltage doubles (AC_IN), the
feedforward term quadruples and reduces the output of the
error amplifier in half to maintain the same input power.
AC Error Amplifier Compensation
A pole-zero pair is created by placing a series combination
of R COMP and C COMP at the output of the AC error amplifier
(EA). The value of the compensation components is
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