CS1600 Datasheet, PDF (8/18 Pages) Cirrus Logic – Low-cost PFC Controller for Electronic Ballasts

CS1600
4.1 PFC Implementation
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.
f_SW
[kHz]
100
Burst Mode
Max f_SW
70
35
Min f_SW
5%
50%
100% _{PO [W]}
Figure 8. Switching Frequency vs. Output Power
V_in < 165 VAC
f_SW
[kHz]
60
48
Burst Mode
Max f_SW
24
Min f_SW
5%
50%
100% _{PO [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-
CRM mode near the peaks of the input line, in order to enable
maximum power delivery, as illustrated in Figure 10 below.
Quasi
DCM CRM
DCM
Quasi
CRM
DCM
I_LB
Figure 10. DCM and quasi-CRM Operation with CS1600
4.1.1 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:
Once V_link reaches its nominal value, the chip operates in the
normal mode. Here, the frequency follows the profile shown in
Figure 7, and the ON time is varied to achieve PFC. Any drop
in V_link to below its undervoltage threshold, as defined in
Section 2.2. Electrical Characteristics re-triggers the start-up
mode of operation. A simplified illustration of operation in
these two modes is shown below in Figure 11.
100%
90%
Normal Mode
Normal Mode
t [ms]
Figure 11. Start-up and Normal Modes
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DS904A5