CS1500_1 Datasheet, PDF (8/22 Pages) Cirrus Logic – Digital Power Factor Correction IC

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CONFIDENTIAL
CS1500
3.1 PFC Operating Frequency
One key feature of the CS1500 is its operating frequency
profile. Figure 10 illustrates how the frequency varies over half
cycle of the line voltage in steady-state operation. When
power is first applied to the CS1500, it first examines the line
voltage and adapts its operating frequency to the exposed line
voltage as shown in Figure 11. The operating frequency is
varied in about a 2-to-1 ratio from the peak to the trough.
During start-up the control algorithm limits the maximum on-
time, provides nearly square-wave envelop current within
every half line cycle by adjusting the operating frequency for
fast startup behavior.
120
Switching Freq. (% of Max.)
100
80
60
40
Line Voltage (% of Max.)
20
0
0
45
90
135
180
Rectified Line Voltage Phase (Deg.)
Figure 10. Switching Frequency vs. Phase Angle
70
Vin < 150 VAC
60
56
50
46
Vin > 150 VAC
40
The CS1500 is designed to function as a DCM (discontinuous
conduction mode) controller, however it may operate in a
quasi-CRM operation mode near the peak periods. For
90~265VAC main input applications, PFC can be also
designed in quasi-CRM at a peak of 90VAC and full load as
shown in Figure 12.
DCM Quasi CRM
DCM
I_LB
Quasi CRM DCM
I_AC
t [ms]
Figure 12. DCM and quasi-CRM Operation with CS1500
3.2 Start-up vs. Normal Operation Mode
CS1500 has two discrete operation modes: Start-up and
Normal. Start-up mode will be activated when V_link is less than
90% of nominal value and remains active until V_link reaches
100% of nominal value, as shown in Figure 13. Startup mode
is activated during initial system power-up. Any V_link drop to
less than 90% of nominal value, such as load change, can
cause the system to enter Start-up Mode until V_link is brought
back into regulation.
V_link
[V]
100%
90%
Normal
Mode
Normal
Mode
20
0
5
20
40
60
80
100
% P_{O max}
Figure 11. Switching Frequency vs. Output Power
Figure 11 illustrates how the operating frequency (as a
percentage of maximum frequency) changes with output
power and the peak of the line voltage. Burst mode (when P_o
below 5%) will be discussed in a later section.
t [ms]
Figure 13. Start-up and Normal Modes
3.3 Burst Mode
Burst mode is utilized to improve system efficiency when the
system output power (P_o) is < 5% of nominal. Burst mode is
implemented by intermittently disabling the PFC over a full
half-line period cycle under light load conditions, as shown in
Figure 14.
8
DS849A7