CS1501

DS927PP6

9

5. GENERAL DESCRIPTION

The CS1501 offers numerous features, options, and

functional capabilities to the designer of switching power

converters. This digital PFC control IC is designed to replace

legacy analog PFC controllers with minimal design effort.

5.1 PFC Operation

One key feature of the CS1501 is its operating frequency

profile. Figure 11 illustrates how the frequency varies over one

half cycle of the line voltage in steady-state operation. When

power is first applied to the CS1501, it examines the line

voltage and adapts its operating frequency to the line voltage

as shown in Figure 11. The operating frequency is varied from

the peak to the trough of the AC input. During startup, the

control algorithm generates maximum power while operating

in critical conduction mode (CRM), providing an approximate

square-wave current envelop within every half-line cycle.

Figure 11. Switching Frequency vs. Phase Angle

Figure 12 illustrates how the operating frequency (as a

percentage of maximum frequency) changes with output

power and the peak of the line voltage.

Figure 12. Max. Switching Frequency vs. Output Power

(Refer to Burst Mode section for more information.)

The CS1501 is designed to function as a DCM controller.

However, during peak periods, the controller may interchange

control methods and operate in a quasi-critical-conduction

mode (quasi-CRM) at low line. For example, at 90VAC main

input under full load, the PFC controller will function as a

quasi-CRM controller at the peak of the AC line cycle, as

shown in Figure 13.

Figure 13. DCM and quasi-CRM Operation with CS1501

The zero-current detection (ZCD) of the boost inductor is

achieved using an auxiliary winding. When the stored energy

of the inductor is fully released to the output, the voltage on the

ZCD pin decreases, triggering a new switching cycle. This

quasi-resonant switching allows the active switch to be turned

on with near-zero inductor current, resulting in a nearly

lossless switch event. This minimizes turn-on losses and EMI

noise created by the switching cycle. Power factor correction

control is achieved during light load by using on-time

modulation.

5.2 Startup vs. Normal Operation Mode

The CS1501 has two discrete operation modes: startup and

reaches 100% of nominal value, as shown in Figure 14.

Startup mode is activated during initial system power-up. Any

back into regulation.

Figure 14. Startup and Normal Modes

Startup mode is defined as a surge of current delivering

maximum power to the output regardless of the load. During

every active switch cycle, the 'ON' time is calculated to drive a

constant peak current over the entire line cycle. However, the

'OFF' time is calculated based on the DCM/CCM boundary

equation.

0

20

40

60

80

100

120

045

90

135

180

Rectified Line Voltage Phase (Deg.)

Switching Freq. (% of Max.)

Line Voltage (% of Max.)

Vin < 181 VAC

20

70

50

60

40

40

5

20

0

60

80

100

Vin > 147 VAC

46

56

DCM

Quasi CRM

DCM

Quasi CRM

DCM

t [ms]

t[ms]

[V]

100%

90%

Normal

Mode

Normal

Mode