NCP1608BOOSTGEVB
http://onsemi.com
11
Figure 15. On Time and Switching Frequency With and Without R
CTUP
time
Vac(t)
t
on
f
SW
with R
CTUP
with R
CTUP
no R
CTUP
no R
CTUP
The dependency of THD on R
CTUP
is illustrated in
Figure 16.
R
CTUP
= open
Ct = 1 nF
R
CTUP
= 1.5 M
W
Ct = 1.22 nF
Figure 16. Dependency of THD on R
CTUP
(I
out
= 250 mA)
85
115
145
175
205
235
265
0
2
4
6
8
10
12
14
THD (%)
Vin (Vac)
2. Improve the THD/PF at Maximum Input Voltage or
Low Output Current:
If the required on time at maximum input voltage or low
output current is less than the minimum on time (t
PWM
), then
DRV pulses must be skipped to prevent excessive power
delivery to the output. This results in the following
sequence:
1. The excessive on time causes V
Control
to decrease
to Ct
(offset)
.
2. When V
Control
< Ct
(offset)
, the drive is disabled.
3. The drive is disabled and V
out
decreases.
4. As V
out
decreases, V
Control
increases.
5. The sequence repeats. Figure 17 depicts the
sequence:
Figure 17. Required On Time Less Than the
Minimum On Time
DRV
Ct
(offset)
V
Control
V
REF
V
FB
V
out
V
out
This sequence increases the input current distortion.
There are two solutions to improve THD/PF at maximum
input voltage or low output current:
1. Properly size the Ct capacitor. As previously
mentioned, the Ct capacitor is sized to set the
maximum on time for minimum line input voltage
and maximum output power. Sizing Ct to an
excessively large value reduces the control range
at Vac
HL
or low output power.
2. Compensate for propagation delays. If optimizing
the Ct capacitor does not achieve the desired
performance, then it may be necessary to
compensate for the PWM propagation delay by
connecting a resistor (R
CT
) in series with Ct.
When the Ct voltage reaches the V
Control
setpoint,
the PWM comparator sends a signal to end the on
time of the driver as shown in Figure 18.
