NCP1600 Datasheet, PDF (21/23 Pages) ON Semiconductor – High Voltage PFC Controller

NCP1600
Consequently, for a given peak inductor current, the
longer the off−time, the smaller power switch duty cycle
and then its conduction dissipation. This is the first benefit
of this technique: the MOSFET on–time losses are reduced.
The increase of the off−time duration also results in a
switching frequency reduction (for a given inductor value).
Given that, in practice, the boost inductor is selected to be
big enough to limit the switching frequency down to an
acceptable level, one can immediately see the second
benefit of the follower boost: it allows the use of smaller,
lighter and cheaper inductors compared to traditional
systems. Finally, this technique utilization brings a drastic
system cost reduction by lowering the cost of both the
inductor and the power switch.
I_L
I_pk
Traditional Preconverter
Follower Boost Preconverter
time
V_in
I_L
V_in
The Power
Switch is On
V_in
I_L V_out
V_in
The Power
Switch is Off
Figure 47. Off−Time Duration Increase
Follower Boost Implementation
In the NCP1600, the on–time is controlled differently
according to the feedback current level. Two areas can be
defined:
• When the feedback current is higher than I_reg−L (refer
to regulation section), the regulation block output
(V_control) is modulated to force the output voltage to
a desired value.
• On the other hand, when the feedback current is lower
than I_reg−L, the regulation block output and therefore,
the on–time, are maximum. As explained in the PWM
Latch Section, the on–time is then inversely
proportional to the output voltage square. The
Follower Boost is active in these conditions in which
the on–time is simply limited by the output voltage
level. Note: In this equation, the feedback pin voltage
(V_Pin1) is neglected compared to the output voltage
(refer to the PWM Latch Section).
tON
+
(tON)
max
+
CPin12
KOSC
RO2
VO2
where:
C_Pin12 is the total oscillator capacitor (sum of the
internal and external capacitors – C_int + C_T),
K_OSC is the ratio (oscillator swing over oscillator gain),
V_O is the output voltage,
R_O is the feedback resistor.
On the other hand, the boost topology has its own rule
that dictates the on–time necessary to deliver the required
power:
4
tON +
Lp PIN
Vpk2
where:
V_pk is the peak AC line voltage,
L_p is the inductor value,
P_IN is the input power.
Combining the two equations, one can obtain the
Follower Boost equation:
ǒ Ǔ ^Ǹ VO +
RO
2
CPin12
KOSC Lp PIN
Vpk
Consequently, a linear dependency links the output
voltage to the AC line amplitude at a given input power.
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