ncp1608 ON Semiconductor, ncp1608 Datasheet - Page 14

• Manufacturer Part Number: ncp1608
• Description: Critical Conduction Mode Pfc Controller Utilizing A Transconductance Error Amplifier
• Manufacturer: ON Semiconductor

load, which naturally satisfies Equation 1. The on time is
constant during the ac line cycle if the values of
compensation components are sufficient to filter out the
V
when V
to ensure that the required on time is reached at maximum
output power and the minimum input voltage condition.
The on time is calculated using Equation 7:
Equation 8:
To calculate the minimum Ct value:
minimum rms input voltage, and L
inductor value.
Off Time Sequence
line cycle and the off time varies with the instantaneous
input voltage. When the inductor current reaches zero, the
drain voltage (V
Measuring V
current reaches zero. To measure the high voltage V
directly is generally not economical or practical. Instead,
a winding is added to the boost inductor. This winding,
called the Zero Current Detection (ZCD) winding,
provides a scaled representation of the inductor voltage
that is sensed by the controller. Figure 32 shows waveforms
of ideal CrM operation using a ZCD winding.
out
V
Combining Equation 7 with Equation 1, results in
V
I
In CrM operation, the on time is constant during the ac
charge
Control
Ct(MAX)
ripple. The maximum on time of the controller occurs
Control
Control
= 297 mA (maximum value), Vac
Ct
varies with the rms input voltage and output
= 4.775 V (minimum value),
Figure 31. On Time Generation
drain
is at the maximum. The Ct capacitor is sized
Ct w
V
drain
DD
t
on
is a way to determine when the inductor
2 @ P
V
+
I
charge
h @ Vac
in Figure 27) resonates towards V
Ct(off)
DRV
Ct @ V
out
DRV
I
charge
@ L
LL
Ct(MAX)
V
V
Control
Ct
V
2
MAX
Control
@ V
Ct
t
on
(offset)
+
@ I
Ct(MAX)
MAX
− Ct
charge
(offset)
is the maximum
PWM
+
−
LL
http://onsemi.com
(eq. 7)
(eq. 8)
is the
t
on
drain
in
.
14
on time (V
is the turns ratio of the boost winding to the ZCD winding.
switch off time (V
Equation 10:
voltage (V
(V
volts. The NCP1608 detects the falling edge of V
turns the driver on. To ensure that a ZCD event is not
inadvertently detected, the NCP1608 logic verifies that
V
decreases to less than V
V
V
ZCD
ZCD(WIND),off
ZCD(WIND),on
The voltage induced on the ZCD winding during the switch
Where V
The voltage induced on the ZCD winding during the
When the inductor current reaches zero, the ZCD pin
ZCD(WIND)
V
V
V
ZCD(WIND)
ZCD(TRIG)
V
V
ZCD(ARM)
Figure 32. Ideal CrM Waveforms Using a ZCD
CL(NEG)
CL(POS)
I
exceeds V
L(peak)
V
V
DRV
drain
V
ZCD
out
in
ZCD(WIND),on
I
ZCD
L
is the instantaneous input voltage and N
) and begins to decrease and ring towards zero
V
V
MOSFET Conduction
) follows the ZCD winding voltage
ZCD(WIND),off
ZCD(WIND),on
t
ZCD(ARM)
on
T
ZCD(WIND),off
SW
) is calculated using Equation 9:
ZCD(TRIG)
Diode Conduction
Winding
t
diode
t
off
and then senses that V
+
+
N
V
N
out
B
B
(Figure 33).
) is calculated using
−V
: N
: N
* V
in
ZCD
ZCD
in
ZCD
(eq. 10)
B
(eq. 9)
:N
0 A
0 V
0 V
0 V
0 V
ZCD
ZCD
and