CS1600
4.1
PFC Implementation
CRM mode near the peaks of the input line, in order to enable
maximum power delivery, as illustrated in Figure 10 below.
The PFC switching frequency profile over the line period has
been discussed in detail in Section 4. In addition, the digital
control algorithm tracks changes the AC input and operates in
different frequency bands at different line voltages as
illustrated in Figure 8 and Figure 9 below.
DCM
Quasi
CRM
DCM
Quasi
CRM
DCM
I
LB
f
SW
[kHz]
100
Burst Mode
70
Max f
SW
Figure 10. DCM and quasi-CRM Operation with CS1600
4.1.1
35
Min f
SW
Start-up Mode vs. Normal Mode
CS1600 operates in two discrete states:
Start-up mode:
When the output voltage of the PFC stage, V
link
, is <90% of its
nominal value, the device operates in the start-up mode. It
continues operating in this mode till the nominal V
link
voltage
is reached. The start-up algorithm provides an ON time which
is varied in proportion to the sensed rectified voltage, while
changing the switching frequency to provide maximum power.
During this start-up phase of operation, the switching
frequency could be significantly lower than the normal
operating frequency, and the input current waveform is forced
into following a trapezoidal envelope in phase with the line
voltage, to maximize energy transfer. The ON time and the
switching frequency of the IC ensure that peak currents are
kept controlled to prevent saturation of the boost inductor
during this period.
Normal mode:
Min f
SW
5%
50%
100%
P
O
[W]
Figure 8. Switching Frequency vs. Output Power
V
in
< 165 VAC
f
SW
[kHz]
60
48
Burst Mode
Max f
SW
24
5%
50%
100%
P
O
[W]
Figure 9. Switching Frequency vs. Output Power
V
in
> 165 VAC
The CS1600 primarily operates in the DCM mode with a
properly sized inductor. However, it will move into a quasi-
Once V
link
reaches its nominal value, the chip operates in the
normal mode. Here, the frequency follows the profile shown in
in V
link
to below its undervoltage threshold, as defined in
mode of operation. A simplified illustration of operation in
these two modes is shown below in Figure 11.
100%
Startup Mode
Startup Mode
90%
Normal Mode
Normal Mode
t [ms]
Figure 11. Start-up and Normal Modes
8
DS904A5
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