NCP1650_15 Datasheet, PDF (25/31 Pages) ON Semiconductor – Power Factor Controller

NCP1650
Current Sense Resistor/Ramp Compensation
The combination of the voltage developed across the
current sense resistor and ramp compensation signal, will
determine the peak instantaneous current that the power
switch will be allowed to conduct before it is turned off.
The vector sum of the three signals that combine to create
the signal at the non−inverting input to the PWM comparator
must add up to 4.0 volts in order to terminate the switch
cycle. These signals are the error signal from the AC error
amp, the ramp compensation signal, and the instantaneous
current. For a worst case condition, the output of the AC
error amp could be zero (current), which would require that
the sum of the ramp compensation signal and current signal
be 4.0 volts. This must be evaluated under full load and low
line conditions.
Equation 1)
1.6 * Voscpk * 16 k
VRCOMP +
RRC
Where: V_oscpk = 4.0 V
VRCOMP + ^1.6
*4*
RRC
16
k
+
102, 400
RRC
For proper ramp compensation, the ramp signal should
match the falling di/dt (which has been converted to a dv/dt)
of the inductor at 50% duty cycle. 50% duty cycle will occur
when the input voltage is 50% of the output voltage. Thus the
following equations must be satisfied:
Equation 2)
di
dt
*
T
*
RS
*
High Frequency Current Gain
Vo
*
T * RS
L*2
*
16
+
102, 400
RRC
RS
+
12800
Vo * T *
*L
RRC
R_S = Shunt resistance (W)
P_O = Output power (W)
L = Inductance (H)
ǒ Ǔ ton + T
1
*
Ǹ₂
· VinLL
Vout
Where: t_on = Switch on time (s)
T = Period (s)
Vin_LL = Low line input voltage (Vrms)
Vout = DC output voltage (V)
ipk
+
Ǹ_{2 @ Pin}
VinLL
)
VinLL @ ton
Ǹ_{2 @ L}
Ramp Compensation:
Equation 3)
Vrefpwm + Vinst ) VRCOMP
Where: V_refpwm = 3.8 V
3.8 + (ipk
*
RS
*
16 ) ^{10R2R, 4C00)}
*
ton
T
RRC
+
(3.8
*
102,
(16 *
400
ipk
*
RS))
*
ton
T
Where:
R_S = Shunt resistance (W)
L = Inductance (H)
V_out = Output voltage (V)
R_RC = Ramp comp resistor (kW)
Current Shunt:
Equation 4)
Combining equations 2 and 3:
RS
+
12800
Vo * T *
*L
RRC
*
T
ton
*
(3.8 * 16 * ipk
102, 400
*
RS)
ǒ Ǔ ^{RS +}
3.8
8 * Vo * ton
L
) ^ǒ16 * ipk^Ǔ
Solve for R_S and then R_RC, using the above equations. It
should be understood that these equations do not take into
account tolerances of the inductor, switching frequency, etc…
The shunt should be a non−wirewound (low inductance)
type of resistor. There are several types of metal film
resistors available for shunt applications.
Current Scaling Resistor and Filter Capacitor
R₁₀ sets the gain of the averaged current signal out of the
current sense amplifier. This signal is fed into the AC error
amplifier and is also used in the power multiplier. R₁₀ is used
to scale the current to the appropriate level for protection
purposes in the AC error amplifier circuit. The power
multiplier has an external resistor, R₉ that will adjust the gain
of that circuit.
R₁₀ 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 R₁₀ is:
R10
+
318, 200 ·
4.5 * (1.06
Pin · RSńVinLL
· VinLL · ACratio)
Where:
Pin = rated input power (w)
R_S = Shunt resistance (W)
Vin_LL = min. operating rms input voltage (v)
AC_ratio = AC attenuation factor at pin 5
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