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NCP1650
Loop Compensation
R dc1
R ac2
V ac
R ac1
V line
V o
V ref
V ?
FB/SD
6
R dc2
4V
+
--
V e/a
ERROR
AMP
ORing NET
--0.32 mA/V
REFERENCE
MULTIPLIER
25 k
AC
ERROR
AMP
--
+
4V
--
+
PWM
LOGIC
OUT
16
Q1
R S
C
R L
C.S. Amp
12
I S--
LOOP COMP
7
I avg
10
RECTIFIER
R 7
C 7
R 10
DIVIDER
ERROR AMP
REFERENCE SIGNAL
MODULATOR AND OUTPUT STAGE
V ′ =
Vo
Rdc2
Rdc1 + Rdc2
funity =
Gm
2 π C7
Vref
Ve ∕ a
= --2 Vac
Δ Vo
Δ Vref
=
RL R10
225k RS
fz =
1
2 π C7 R7
Vac =
R
Vline Rac2
ac1 + Rac2
fp =
1
2 π RC
Voltage Loop
Av = Gm R7
(High Frequency Gain, Past Zero)
Figure 41. Voltage Loop Model
Block Diagram
The block diagram for the voltage loop has been broken down
into four sections. These are the voltage divider, voltage error
20
amplifier, reference signal and modulator and output stage.
The modulator and output stage circuitry is greatly
simplified based on the assumption that that poles and zeros
in the current feedback loop are considerably greater than
0
UNITY GAIN
Av
the bandwidth of the overall loop. This should be a good
assumption, because a bandwidth in the kilohertz is
necessary for a good current waveform, and the voltage error
amplifier needs to have a bandwidth of less than the lowest
line frequency that will be used.
There are two poles in this circuit. The output filter has a
pole that varies with the load. The pole on the voltage error
amplifier will be determined by this analysis.
Voltage Divider
The voltage divider is a simple resistive divider that
reduces the output voltage to the 4.0 volt level required by
the internal reference on the voltage error amplifier.
Voltage Error Amplifier
The voltage error amplifier is constrained by the three
equations. When this amplifier is compensated with a
pole--zero pair, there will be a unity gain pole which will be
cancelled by the zero at frequency f Z . The corresponding
bode plot would be:
--20
FREQUENCY
Figure 42. Pole--Zero Bode Plot
Reference Signal
The output of the error amplifier is modified by the ORing
network, which has a negative gain, and is then used as an
input to the reference multiplier. The gain of this block is
dependent on the AC input voltage, because of the multiplier
which requires two inputs for one output.
Modulator and Output Stage
The AC error amplifier receives an input from the
reference multiplier and forces the current to follow the
shape and amplitude of the reference signal. The current
shaping circuit is an internal loop within this section due to
the current sense amplifier. Based on the assumptions listed
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