FAN302HLMY Datasheet, PDF (11/18 Pages) Fairchild Semiconductor – PWM Controller for Low Standby Power Battery- Charger Applications — mWSaver™ Technology

I _PK
I _PK
⋅
N_P
N_S
< I_D >_AVG = I_O
Figure 26. Key Waveforms of DCM Flyback
Converter
Figure 28. t_DIS 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 (t_DIS) 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 (V_SH) of V_S,
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 V_SH
Δf
ΔV
= S_G−CC
Figure 30. Frequency Reduction Curve in
CC Regulation
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