NCP1608BOOSTGEVB Datasheet, PDF (12/24 Pages) ON Semiconductor – Critical Conduction Mode PFC Controller Utilizing a Transconductance Error Amplifier

NCP1608
When the switch is closed, the inductor current increases
linearly to the peak value. When the switch opens, the
inductor current linearly decreases to zero. When the
inductor current decreases to zero, the drain voltage of the
switch (V_drain) is floating and begins to decrease. If the next
switching cycle does not begin, then V_drain rings towards
V_in. A derivation of equations found in AND8123 leads to
the result that high power factor in CrM operation is
achieved when the on time (t_on) of the switch is constant
during an ac cycle and is calculated using Equation 1.
ton
+
2 @ P_out @ L
h @ Vac²
(eq. 1)
Where P_out is the output power, L is the inductor value, h
is the efficiency, and Vac is the rms input voltage.
A description of the switching over an ac line cycle is
illustrated in Figure 28. The on time is constant, but the off
time varies and is dependent on the instantaneous line
voltage. The constant on time causes the peak inductor
current (I_L(peak)) to scale with the ac line voltage. The
NCP1608 represents an ideal method to implement a
constant on time CrM control in a cost−effective and robust
solution by incorporating an accurate regulation circuit, a
low current consumption startup circuit, and advanced
protection features.
V_in(peak)
I_L(peak)
I_in(peak)
V_in(t)
I_L(t)
I_in(t)
ON
MOSFET
OFF
Figure 28. Inductor Waveform During CrM Operation
Error Amplifier Regulation
The NCP1608 regulates the boost output voltage using
an internal error amplifier (EA). The negative terminal of
the EA is pinned out to FB, the positive terminal is
connected to a 2.5 V ± 1.6% reference (V_REF), and the EA
output is pinned out to Control (Figure 29).
A feature of using a transconductance error amplifier is
that the FB pin voltage is only determined by the resistor
divider network connected to the output voltage, not the
operation of the amplifier. This enables the FB pin to be
used for sensing overvoltage or undervoltage conditions
independently of the error amplifier.
V_out
R_out1
FB
R_out2
R_FB
Control
C_COMP
+ ^OVP
−
V_OVP
− ^UVP
+
V_UVP
EA
−
+
gm
V_REF
OVP Fault
POK
UVP Fault
(Enable EA)
t_on(MAX)
V_Control
t_on
PWM BLOCK
Slope
+
Ct
I_charge
t_PWM
Ct_(offset)
V_Control
V_EAH
Figure 29. Error Amplifier and On Time Regulation Circuits
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