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NCP1631 Datasheet, PDF (9/23 Pages) ON Semiconductor – Interleaved, 2-Phase Power Factor Controller
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NCP1631
Figure 3. DCM and CRM Operation Within a Sinusoid Cycle for One Branch
NCP1631 On−time Modulation
Let’s study the ac line current absorbed by one phase of
the interleaved PFC converter.
The current waveform of the inductor (L) during one
switching period (Tsw) is portrayed by Figure 5.
The ac line current is the averaged value of the coil
current as the result of the EMI filter “polishing” action.
Hence, the line current produced by one of the phase is:
ǒ Ǔǒ Ǔ Iin
+
1
2
t1
L
t1 ) t2
Tsw
Vin
(eq. 1)
Where (Tsw = t1 + t2 + t3) is the switching period and Vin
is the ac line rectified voltage.
Equation 1 shows that Iin is proportional to Vin if
ǒ Ǔ t1(t1 ) t2)
Tsw
is a constant.
ǒ Ǔ Forcing
t1(t1 ) t2)
Tsw
constant is what the NCP1631 does to perform FCCrM
operation that is, to operate in discontinuous or critical
conduction mode according to the conditions, without
degradation of the power factor.
Figure 4. Boost Converter
Figure 5. Inductor Current in DCM
The NCP1631 operates in voltage mode. As portrayed by
Figure 6, the MOSFET on time t1 is controlled by the signal
Vton generated by the regulation block as follows:
t1
+
CtVTON
It
(eq. 2)
Where:
• Ct is the internal timing capacitor
• It is the internal current source for the timing capacitor.
The It charge current is constant for a given resistor
placed on the Rt pin. Ct is also a constant. Hence, the
condition
ǒ Ǔ t1(t1 ) t2)
Tsw
to be a constant for proper power factor correction can be
changed into:
ǒ Ǔ VTON(t1 ) t2)
Tsw
is constant.
The output of the regulation block (VCONTROL) is
linearly changed into a signal (VREGUL) varying between
0 and 1.66 V. (VREGUL) is the voltage that is injected into
the PWM section to modulate the MOSFET duty−cycle.
However, the NCP1631 inserts some circuitry that
processes (VREGUL) to form the signal (VTON) that is used
in the PWM section instead of (VREGUL) (see Figure 7).
(VTON) is modulated in response to the dead−time sensed
during the precedent current cycles, that is, for a proper
shaping of the ac line current. This modulation leads to:
VTON
+
TswVREGUL
t1 ) t2
or:
VTON
t1 ) t2
Tsw
+
VREGUL
(eq. 3)
Substitution of Equation 3 into Equation 2 leads to the
following on−time expression:
ǒ Ǔ Ct
TswVREGUL
t1)t2
t1 +
It
(eq. 4)
Replacing “t1” by its expression of Equation 4,
Equation 1 simplifies as follows:
Iin(phase1)
+
Iin(phase2)
+
Vin
2L
CtVREGUL
It
(eq. 5)
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