FAN501A Datasheet, PDF (10/16 Pages) Fairchild Semiconductor – Offline DCM / CCM Flyback PWM Controller for Charger Applications

The unified output current equation both for DCM and
CCM operation is obtained as:
𝐼_𝑂
=
𝑁_𝑃
𝑁_𝑆
∙ 𝐼_{𝐷𝑆_𝑀𝑖𝑑}
∙
𝑡_{𝐷𝐼𝑆}
𝑡_𝑆
=
𝑁_𝑃
𝑁_𝑆
∙
𝑉_{𝐶𝑆_𝑀𝑖𝑑}
𝑅_{𝐶𝑆}
∙
𝑡_{𝐷𝐼𝑆}
𝑡_𝑆
(4)
VCS_Mid is obtained by sampling the current-sense
voltage at the middle of the MOSFET conduction time.
The diode current discharge time is obtained by
detecting the diode current zero-crossing instant. Since
the diode current cannot be sensed directly in the
primary side, Zero-Crossing Detection (ZCD) is
accomplished indirectly by monitoring the auxiliary
winding voltage in the primary side. When the diode
current reaches zero, the transformer winding voltage
begins to drop sharply. To detect the corner voltage, the
VS is sampled, called VSH, at 85% of diode current
discharge time (tDIS) of the previous switching cycle and
compared with the instantaneous VS voltage. When
instantaneous voltage of the VS pin drops below VSH by
more than 200 mV, the ZCD of diode current is
obtained, as shown in Figure 23.
The output current can be programmable by setting
current sensing resistor as:
R_CS

1
I_O
 ^NP
N_S
 ^VCCR
K_CC
(5)
where VCCR is the internal voltage for CC control and
KCC is the IC design parameter, 12 for the FAN501A.
DCM Waveform
IDS-Mid
IDS
½ tON
ID
ID-Mid = NPS∙IDS-Mid
½ tDIS
IO = <ID>Ts
VS
t_DIS(n)
0.85∙t_DIS(n-1)
V_S
V_SH
ZCD
200mV
Figure 23. Operation Waveform for ZCD Function
Line Voltage Detection and its Utilization
The FAN501A indirectly senses line voltage using the
current flowing out of the VS pin while the MOSFET is
turned on, as illustrated in Figure 25 and Figure 26.
During the MOSFET turn-on period, auxiliary winding
voltage, VAux, reflects input bulk capacitor voltage, VBLK,
by the transformer coupling between primary and
auxiliary. During MOSFET conduction time, the line
voltage detector clamps the VS pin voltage ~0.5 V and
the current, IVS, flowing from the VS pin is expressed as:
I_VS

N_A
/ N_P V_BLK
R_VS1
+
0.5
R_VS1 / / R_{VS 2}
(6)
Typically, the second term in Equation (6) can be
ignored because it is much smaller than the first term.
The current, IVS, is approximately proportional to the line
voltage, calculated as:
I_VS

N_A / N_P
R_VS1
 V_BLK
(7)
The IVS current, reflecting the line voltage information, is
used for dual switching frequency operation, CC control
correction weighting, and brownout protection; as
illustrated in Figure 25.
tON
Figure 21.
IDS-Mid
tDIS
tS
Waveforms of DCM Flyback Converter
CCM Waveform
IDS
½ tON
ID
ID-Mid = NPS∙IDS-Mid
IO = <ID>Ts
½ tDIS
VS
tON
Figure 22.
tDIS
tS
Waveforms of CCM Flyback Converter
© 2014 Fairchild Semiconductor Corporation
FAN501A • Rev. 1.0.0
Dual Switching Frequency
The FAN501A changes the switching frequency
between 85 kHz and 140 kHz according to the line
voltage. It is typical to design the flyback converter to
operate in CCM for low line and DCM in high line.
Therefore, the peak transformer current decreases as
the operation mode changes from CCM to DCM, as
shown in Figure 24(a), for single-frequency operation.
The transformer is not fully utilized at high line when a
single switching frequency is used. The peak
transformer current can be maintained almost constant
when the flyback converter operates at lower frequency
at high line, as illustrated in Figure 24(b). This allows full
transformer utilization and improves the efficiency by
decreasing the switching losses at high line.
When IVS is larger than IVS-H (750 µA), the switching
frequency is set at fOSC-L (85 kHz) in CV Mode. When IVS
is less than IVS-L (680 µA), the switching frequency is set
at fOSC-H (140 kHz) in CV Mode. For the universal line
range, the frequency change should occur between 132
~ 180 VAC to avoid the transition within the actual
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