FAN302HLMY 查看數據表（PDF） - Fairchild Semiconductor

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FAN302HLMY

PWM Controller for Low Standby Power Battery Charger Applications — mWSaver™ Technology

Fairchild Semiconductor 

I PK

I PK

⋅

NP

NS

< ID >AVG = IO

Figure 26. Key Waveforms of DCM Flyback

Converter

Figure 28. tDIS Variation in CC Mode

Figure 27. Detailed Waveform for ZCD

Frequency Reduction in CC Mode

An important design consideration is that the

transformer guarantee DCM operation across the whole

range since the output current is properly estimated only

in DCM operation. As can be seen in Figure 28, the

discharge time (tDIS) of the diode current increases as

the output voltage decreases in CC Mode. The

converter tends to go into CCM as output voltage drops

in CC Mode when operating at the fixed switching

frequency. To prevent this CCM operation while

maintaining good output current estimation in DCM,

FAN302HLMY_F117 decreases switching frequency as

output voltage drops, as shown in Figure 28 and Figure

29. FAN302HLMY_F117 indirectly monitors the output

voltage by the sample-and-hold voltage (VSH) of VS,

which is taken at 85% of diode current discharge time of

the previous switching cycle, as shown in Figure 27.

Figure 30 shows how the frequency reduces as the

sample-and-hold voltage of VS decreases.

© 2012 Fairchild Semiconductor Corporation

FAN302HLMY_F117 • Rev. 1.0.1

11

Figure 29. Frequency Reduction with VSH

Δf

ΔV

= SG−CC

Figure 30. Frequency Reduction Curve in

CC Regulation

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