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FAN6204MY Scheda tecnica(PDF) 10 Page - Fairchild Semiconductor

Il numero della parte FAN6204MY
Spiegazioni elettronici  Synchronous Rectification Controller
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Produttore elettronici  FAIRCHILD [Fairchild Semiconductor]
Homepage  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN6204MY Scheda tecnica(HTML) 10 Page - Fairchild Semiconductor

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FAN6204 • Rev. 1.0.0
10
Figure 19. Simplified Linear-Predict Block
The voltage-second balance equation for the primary-
side inductance of the flyback converter is given in
Equation 1. Inductor current discharge time is given as:
.
.
IN
PM ON
LDIS
OUT
Vt
t
nV
=
(2)
The voltage scale-down ratio between RES and LPC is
defined as K below:
()
()
43
4
21
2
/
/
R
RR
K
R
RR
+
=
+
(3)
During tPM.ON, the charge current of CT is iCHR-iDICHR,
while during tL.DIS, the discharge current is iDICHR. As a
result, the current-second balance equation for internal
timing capacitor (CT) can be derived from:
..
5
((
)
)
IN
OUT
OUT
PM ON
OUT
CT DIS
V
VV
t
V
t
Kn
⋅+
=
(4)
Therefore, the discharge time of CT is given as:
.
.
5
((
)
)
IN
OUT
OUT
PM ON
CT DIS
OUT
V
VV
t
Kn
t
V
⋅+
=
(5)
When the voltage scale-down ratio between RES and
LPC (K) is five (5), the discharge time of CT (tCT.DIS) is
the same as inductor current discharge time (tL.DIS).
However, considering the tolerance of voltage divider
resistors and internal circuit, the scale-down ratio (K)
should be larger than five (5) to guarantee that tCT.DIS is
shorter than tL.DIS. It is typical to set K around 5~5.5.
Referring to Figure 18; when LPC voltage is higher than
VLPC-EN over a blanking time (tLPC-EN) and lower than
VLPC-TH-HIGH (0.05VOUT), then SR MOSFET can be
triggered. Therefore, VLPC-EN must be lager than VLPC-TH-
HIGH or the SR MOSFET cannot be turned on. When
designing the voltage divider of LPC, R1 and R2 should
be considered as:
.
2
12
0.83
(
)
0.05
0.3
IN MIN
OUT
OUT
V
R
VV
RR
n
⋅⋅
+
>
+
+
(6)
On the other hand, the linear operation range of LPC
and RES (1~4V) should be considered as:
.
2
12
()
4
IN MAX
OUT
V
R
V
RR
n
⋅+
<
+
(7)
4
4
3
4
<
+
OUT
V
R
R
R
(8)
CCM Operation
The typical waveforms of CCM operation in steady state
are shown as Figure 18. When the primary-side
MOSFET is turned on, the energy is stored in Lm.
During the on-time of the primary-side MOSFET
(tPM.ON), the magnetizing current (IM) increases linearly
from IM,min to IM,max. Meanwhile, internal timing capacitor
(CT) is charged by current source (iCHR-iDICHR)
proportional to VIN, so VCT also increases linearly.
When the primary-side MOSFET is turned off, the
energy stored in Lm is released to the output. During the
inductor discharge time (tL.DIS), the magnetizing current
(IM) decreases linearly from IM,max to IM,min. At the same
time, the internal timing capacitor (CT) is discharged by
current source (iDISCHR) proportional to VOUT, so VCT also
decreases linearly. To guarantee the proper operation
of SR, it is important to turn off SR MOSFET just before
SR current reaches IM,min so that the body diode of SR
MOSFET is naturally turned off.
DCM / QR Operation
In DCM / QR operation, when primary-side MOSFET is
turned off, the energy stored in Lm is fully released to
the output at the turn-off timing of primary-side
MOSFET. Therefore, the DET voltage continues
resonating until the primary-side MOSFET is turned on,
as depicted in Figure 18. While DET voltage is
resonating, DET voltage and LPC voltage drop to zero
by resonance, which can trigger the turn-on of the SR
MOSFET. To prevent fault triggering of the SR
MOSFET in DCM operation, blanking time is introduced
to LPC voltage. The SR MOSFET is not turned on even
when LPC voltage drops below 0.05VOUT unless LPC
voltage stays above 0.83VLPC-HIGH longer than the
blanking time (tLPC-EN). The turn-on timing of the SR
MOFET is inhibited by gate inhibit time (tINHIBIT), once
the SR MOSFET turns off, to prevent fault triggering.
Green-Mode Operation
To minimize the power consumption at light-load
condition, the SR circuit is disabled when the load
decreases. As illustrated in Figure 20, the discharge
times of inductor and internal timing capacitor decrease
as load decreases. If the discharge time of the internal
timing capacitor is shorter than tGREEN-ON (around 4.8µs)
for more than three cycles, the SR circuit enters Green
Mode. Once FAN6204 enters Green Mode, the SR
MOSFET stops switching and the major internal block is
shut down to further reduce operating current of the SR
controller. In Green Mode, the operating current
reduces to 800µA. This allows power supplies to meet
the most stringent power conservation requirements.
When the discharge time of the internal capacitor is
longer than tGREEN-OFF (around 5.35µs) for more than
seven cycles, the SR circuit is enabled and resumes the
normal operation, as shown in Figure 21.


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