NCP1651D Datasheet, PDF (28/32 Pages) ON Semiconductor – Single Stage Power Factor Controller

NCP1651
Current Scaling Resistor & Filter Capacitor
R₇ sets the gain of the averaged current signal out of the
current sense amplifier which is fed into the AC error
amplifier. R₇ is used to scale the current to the appropriate
level for protection purposes in the AC error amplifier
circuit.
R₇ should be calculated to limit the maximum current
signal at the input to the AC error amplifier to less than
4.5 volts at low line and full load. 4.5 volts is the clamp
voltage at the output of the reference amplifier and limits the
maximum averaged current that the unit can process. The
equation for R₇ is:
R7
=
VinLL
_4.5
−
212 k
(0.75
⋅
⋅
RS ⋅ Pin
ACratio ⋅
VinLL
⋅
₂
)^
Where: P_in = rated input power (W)
Where: R_S = Shunt resistance (W)
Where: V_inLL = min. operating rms input voltage (W)
Where: AC_ratio = AC attenuation factor at pin 9
This equation does not allow for tolerances, and it would
be advisable to increase the input power to assure operation
at maximum power over production tolerance variations.
The current sense filter capacitor should be selected to set
its pole about a factor of 10 below the switching frequency.
C6
=
5.3
f
Where: C₆ = Pin 6 capacitance (nF)
Where: f = pole frequency (kHz)
so, for a 100 kHz switching frequency, a 10 kHz pole is
desirable, and C₆ would be 0.5 nF.
Reference Multiplier
The output of the reference multiplier is a pulse width
modulated representation of the analog input. The multiplier
is internally loaded with a resistor to ground which will set
the DC gain. An external capacitor is required to filter the
signal back into one that resembles the input fullwave
rectified sinewave. The pole for this circuit should be greater
than the line frequency and lower than the switching
frequency.
1/15th of the switching frequency is a recommended
starting value for a 60 Hz line frequency. The filter capacitor
for pin 10 can be determined by the following equation:
C10 = ₂
3.14
1
25 k
fpole
=
6.366E--6
fpole
Where: C₁₀ = Pin 10 capacitance (F)
Where: f_pole = Ref gain pole freq (Hz)
AC Error Amplifier
The AC error amplifier is a transconductance amplifier
that is terminated with a series R_C impedance. This creates
a pole--zero pair.
To determine the values of R₃ and C₃, it is necessary to
look at the two signals that reach the PWM inputs. The
non--inverting input is a slow loop using the averaged
current signal. It’s gain is:
AIf
=
30 k
3k
⋅
15 k
R7
⋅
(gm
⋅
R11)
⋅
2.3
Where the first two terms are the gains in the current sense
amplifier averaging circuit. The next term is the gain of the
transconductance amplifier and the constant is the gain of
the AC Reference Buffer.
The high frequency path is that of the instantaneous
current signal to the PWM non--inverting input. This gain is
16 k/3 k = 5.33, since the input signal is converted to a
current through a 3 k resistor in the current sense amplifier,
and then terminated by the 16 k resistor at the PWM input.
For stability, the gain of the low frequency path must be
less than the gain of the high frequency path. This can be
written as:
345,
000
⋅ ^gm
R7
⋅
R11
<
5.3
The suggested resistor and capacitor values are:
R11
=
R7
130,000
gm
and for a zero at 1/10th of the switching frequency
C11
=
1.59
fSW R11
Where: R₇ & R₁₁ are in units of Ohms
Where: g_m is in units of mhos
Where: C₁₁ is in Farads
Where: f_sw is in Hz
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