NCP1631_16 Datasheet, PDF (18/24 Pages) ON Semiconductor – Interleaved, 2-Phase Power Factor Controller

NCP1631
Figure 17. Typical Waveforms (T_delay not shown here for the sake of simplicity)
Frequency Foldback
In addition, the circuit features the frequency fold−back
function to improve the light load efficiency. Practically,
the oscillator charge and discharge currents (I_OSC(CH) and
I_OSC(DISCH) of Figure 16) are not constant but dependent on
the power level. More specifically, I_OSC(CH) and
I_OSC(DISCH) linearly vary as a function of V_control output of
the regulation block that thanks to the feed−forward
“V_REGUL” is the signal derived from V_control that is
effectively used to modulate the MOSFET on−time.
V_REGUL is buffered and applied to pin 6 (“Frequency
fold−back” pin). A resistor R_FF is to be connected to pin 6
to sink a current proportional to V_REGUL
ǒ Ǔ I_pin6
+
I_FF
+
V_REGUL
R_FF
.
featured by the NCP1631, is representative of the load.
The practical implementation is portrayed by Figure 16.
This current is clamped not to exceed 105 mA and copied
by a current mirror to form I_OSC(CH) and I_OSC(DISCH).
As a matter of fact, the oscillator charge current is:
I_OSC(CH)
+
I_OSC(clamp)
)
V_REGUL
R_FF
ǒ Ǔ if
V_REGUL
R_FF
v
105
mA
(eq. 18)
I_OSC(CH) + I_OSC(clamp) ) I_OSC(CH1) + I_OSC(CHT1) + 140 mA
otherwise
The oscillator charge current is then an increasing function of V_REGUL and is clamped to 140 mA.
The oscillator discharge current is:
I_OSC(DISCH)
+
V_REGUL
R_FF
ǒ Ǔ if
V_REGUL
R_FF
v
105
mA
(eq. 19)
I_OSC(DISCH) + I_OSC(DISCH1) + 105 mA
otherwise
The oscillator discharge current is also an increasing
function of V_REGUL and is clamped to105 mA.
As a consequence, the clamp frequency is also an
increasing function of V_REGUL until it reaches a maximum
value for (I_FF = 105 mA). If we consider the clamp
frequency f_OSC computed by Equation 17 as the nominal
value obtained at full load and if we name it “f_OSC(nom)”:
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