ON Semiconductor NCP1608BOOSTGEVB, User Manual

Page: 9 / 25

NCP1608BOOSTGEVB
http://onsemi.com
9
The type 2 network is composed of C
COMP
, C
COMP1
, and
R
COMP1
. C
COMP1
sets the crossover frequency (f
CROSS
) and
is calculated using Equation 23:
C
COMP1
+
gm
2 @ p @ f
CROSS
(eq. 23)
For this design, f
CROSS
is set to 5 Hz at the average input
voltage (175 Vac) to decrease THD and gm is specified in the
NCP1608 datasheet:
C
COMP1
+
110 m
2 @ p @ 5
+ 3.5 m F
A normalized value of 3.3 m F is selected, which sets
f
CROSS
to 5.3 Hz.
The addition of R
COMP1
causes a zero in the loop
response. The zero frequency (f
zero
) is typically set to half
the crossover frequency, which is 2.5 Hz for this case.
R
COMP1
is calculated using Equation 24:
R
COMP1
+
1
2 @ p @ f
zero
@ C
COMP
(eq. 24)
R
COMP1
+
1
2 @ p @ 2.5 @ 3.3 m
+ 19.3 k W
R
COMP1
is selected as 20 k W .
C
COMP
is used to filter high frequency noise and is set to
between 1/10 and 1/5 of C
COMP1
. For this design, C
COMP
is
selected to be 1/5 of C
COMP1
.
C
COMP
+
ǒ
1
5
Ǔ
@ 3.3 m + 0.66 m F
C
COMP
is selected as 0.68 m F.
The phase margin and crossover frequency change with
the ac line voltage. It is critical that the gain and phase are
measured for all operating conditions. The measurement
setup using a network analyzer is shown in Figure 12.
Ch A
High − Voltage
(> 450 V)
Isolation Probe
Ch B
High − Voltage
(> 450 V)
Isolation Probe
Figure 12. Gain-Phase Measurement Setup for a Boost PFC Pre − Converter
+
AC Line
EMI
Filter
GND
ZCD
+
Load
Vout
Ct
Isolator
Network Analyzer
L
NCP1608
D
M
C
bulk
R
sense
1 k W
V
CC
C
in
R
ZCD
R
out1
R
out2
C
COMP
V
CC
DRV
4
3
2
1
CS
Ct
Control
FB
5
6
7
8
There is a tradeoff of transient response for PF and THD.
The low bandwidth of the feedback loop reduces the Control
pin ripple voltage. The reduction of the Control pin ripple
voltage increases PF and reduces THD, but increases the
magnitude of overshoots and undershoots.
Additional THD Reduction
The constant on time architecture of the NCP1608
provides flexibility in optimizing each design.
The following design guidelines provide methods to
further improve PF and THD.
1. Improve the THD/PF at Maximum Output Power by
Increasing the On Time at the Zero Crossing:
One disadvantage of constant on time CrM control is that
at the zero crossing of the ac line, the instantaneous input
voltage is not large enough to store sufficient energy in the
inductor during the constant on time. Minimal energy is
processed and “zero crossing distortion” is produced as
shown in Figure 13.