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NCP1650
Current Sense Resistor/Ramp Compensation
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
Equation 3)
Vrefpwm = Vinst + VRCOMP
) * on
3.8 = (ipk * RS * 16 +
* on
RRC =
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)
Where: V refpwm = 3.8 V
Where:
R S = Shunt resistance ( Ω )
L = Inductance (H)
V out = Output voltage (V)
R RC = Ramp comp resistor (k Ω )
102, 400 t
102, 400 t
(3.8 ? (16 * ipk * RS)) T
RRC T
VRCOMP =
Where: V oscpk = 4.0 V
1.6 * Voscpk * 16 k
RRC
Current Shunt:
Equation 4)
Combining equations 2 and 3:
VRCOMP =
RRC
RRC
di * T * R S * High Frequency Current Gain
* T *
RS =
RS =
? ? + ? 16 * ipk ?
1.6 * 4 * 16 k   102, 400
=
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)
dt
12800 * L (3.8 ? 16 * ipk * RS)
Vo * T * RRC ton 102, 400
3.8
8 * Vo * ton
L
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
=
RS =
Vo * T * RS * 16
L * 2
12800 * L
Vo * T * RRC
102, 400
RRC
resistors available for shunt applications.
Current Scaling Resistor and Filter Capacitor
R 10 sets the gain of the averaged current signal out of the
current sense amplifier. This signal is fed into the AC error
? ?
ton = T 1 ?
? 2 ? P in VinLL ? ton
ipk = +
4.5 ? (1.06 · VinLL · ACratio)
R S = Shunt resistance ( Ω )
P O = Output power (W)
L = Inductance (H)
? 2 · Vin LL
Vout
Where: t on = Switch on time (s)
T = Period (s)
Vin LL = Low line input voltage (Vrms)
Vout = DC output voltage (V)
VinLL ? 2 ? L
amplifier and is also used in the power multiplier. R 10 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, R 9 that will adjust the gain
of that circuit.
R 10 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 R 10 is:
318, 200 · Pin · RS ∕ VinLL
R10 =
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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