AN-6920
APPLICATION NOTE
Non-Conduction Time of the MOSFET
(t
OFF
)
FAN6920 has a minimum non-conduction time of MOSFET
(5 s), during which turning on the MOSFET is prohibited.
To maximize the efficiency, it is necessary to turn on the
MOSFET at the first valley of MOSFET drain-to-source
voltage at heavy-load condition. Therefore, the MOSFET
non-conduction time at heavy load condition should be
larger than 5 s.
After determining f
S.QRmin
and t
F
, the maximum duty cycle is
calculated as:
When designing the transformer, the maximum flux density
swing in normal operation (B) as well as the maximum flux
density in transient (B
max
) should be considered. The
maximum flux density swing in normal operation is related
to the hysteresis loss in the core, while the maximum flux
density in transient is related to the core saturation.
The minimum number of turns for the transformer primary
side to avoid over temperature in the core is given by:
N
P
min
D
max
=
V
RO
⋅
(1
−
f
S
.
QR
min
⋅
t
F
)
V
RO
+
V
O
.
PFC
.
L
2
L
m
I
DS PK
=
A
e
∆
B
(
26
)
(
20
)
where
B
is the maximum flux density swing in Tesla.
If there is no reference data, use
B
=0.25~0.30T.
Once the minimum number of turns for the primary side is
determined, calculate the proper integer for
N
S
so that the
resulting
N
P
is larger than
N
pmin
as:
Then, the primary-side inductance is obtained as:
L
m
=
η
QR
⋅
(
V
O
.
PFC
.
L
⋅
D
max
)
2
⋅
f
S
.
QR
min
P
OUT
(
21
)
Once
L
m
is determined, the maximum peak current and
RMS current of the MOSFET in normal operation are
obtained as:
N
P
=
n
⋅
N
S
>
N
P
min
(
27
)
I
DS PK
=
V
O
.
PFC
.
L
⋅
D
max
L
m
f
S
.
QR
min
D
max
3
The number of turns of the auxiliary winding for V
DD
is
given as:
(
22
)
N
AUX
(
23
)
V
+
V
FA
=
DD
⋅
N
S
(
V
O
+
V
F
)
nom
(
28
)
I
DS RMS
=
I
DS PK
The MOSFET non-conduction time at heavy load and low
line is obtained as:
where V
DDnom
is the nominal V
DD
voltage, the range about
12~20V, and the V
FA
is forward-voltage drop of V
DD
diode,
about 1V.
Once the number of turns of the primary winding is
determined, the maximum flux density when the drain
current reaches its pulse-by-pulse current limit level should
be checked to guarantee the transformer is not saturated
during transient or fault condition.
The maximum flux density (B
max
) when drain current
reaches I
LIM
is given as:
t
OFF
.
L
=
(1
−
D
max
)
f
S
.
QR min
(
24
)
The MOSFET non-conduction time at heavy load and
higher voltage of PFC output (V
O.PFC.H
) is obtained as:
t
OFF
.
H
=
t
OFF
.
L
⋅
V
O
.
PFC
.
L
V
O
.
PFC
.
H
+
V
RO
⋅
V
O
.
PFC
.
H
V
O
.
PFC
.
L
+
V
RO
(
25
)
B
max
=
L
m
I
LIM
<
B
sat
A
e
N
P
(
29
)
To guarantee the first valley switching at high line and
heavy-load condition, t
OFF.H
should be larger than 5 s.
B
max
should be smaller than the saturation flux density.
If there is no reference data, use
B
sat
=0.35~0.40T.
(Design Example)
Setting the minimum frequency is
65kHz and the falling time is 1 s, and assuming
V
O.PFC.L
=300V:
D
max
=
=
V
RO
V
RO
min
⋅
(1
−
f
S
.
QR
⋅
t
F
)
+
V
O
.
PFC
.
L
240
⋅
(1
−
70
×
10
3
⋅
1
×
10
−
6
)
=
0.413
240
+
300
Figure 14. Switching Timing of QR Flyback Converter
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • March 10, 2011
www.fairchildsemi.com
9
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