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FAN6747 Fiches technique(PDF) 5 Page - Fairchild Semiconductor

No de pièce FAN6747
Description  Control a Flyback Power Supply with Peak Current Output
Download  12 Pages
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Fabricant  FAIRCHILD [Fairchild Semiconductor]
Site Internet  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN6747 Fiches technique(HTML) 5 Page - Fairchild Semiconductor

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AN-6747
APPLICATION NOTE
© 2010 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.1 • 9/16/10
5
[STEP-6] Determine the Minimum Primary Turns
With a given core, the minimum number of turns for the
transformer primary side to avoid the core saturation is
given by:
6
e
SAT
CS
M
6
e
SAT
LIM
M
MIN
P
10
A
B
R
/
825
.
0
L
10
A
B
I
L
N
×
=
×
=
(19)
where A
e is the cross-sectional area of the core in mm
2, ILIM
is the pulse-by-pulse current limit level determined by
0.825V threshold, RCS is current sensing resistor, and B
SAT
is the saturation flux density in Tesla.
The pulse-by-pulse current limit level is included in
Equation (19) because the inductor current reaches the
pulse-by-pulse current limit level during the load transient
or overload condition. Figure 6 shows the typical
characteristics of ferrite core from TDK (PC40). Since the
saturation flux density (B
SAT) decreases as the temperature
rises, the high-temperature characteristics should be
considered. If there is no reference data, use B
MAX =0.3T.
Figure 6. Typical B-H Characteristics of Ferrite Core
(TDK/PC40)
(Design Example)
An EF25/13/11 core is selected
with effective cross-sectional area of 78mm2. Choosing
the saturation flux density as 0.27T, the minimum
number of turns for the primary side is obtained as:
60
10
78
27
.
0
33
.
0
/
825
.
0
10
508
10
A
B
R
/
825
.
0
L
N
6
6
6
e
SAT
CS
M
MIN
P
=
×
×
=
×
=
[STEP-7] Determine the Number of Turns for Each
Winding
Figure 7 shows a simplified diagram of the transformer.
First, calculate the turn ratio (n) between the primary side
and the secondary side from the reflected output voltage
determined in step 3 as:
F
O
RO
S
P
V
V
V
N
N
n
+
=
=
(20)
where N
P and NS are the number of turns for primary side
and secondary side, respectively, V
O is the output voltage;
and V
F is the diode (DO) forward-voltage drop.
Determine the proper integer for N
S such that the resulting
NP is larger than NP
min obtained from Equation (19).
The number of turns for the auxiliary winding for VDD
supply is determined as:
S
F
O
FA
DD
A
N
V
V
V
*
V
N
+
+
=
(21)
where V
DD is the nominal value of the supply voltage and
VFA is the forward-voltage drop of DDD as defined in Figure
7. Since VDD increases as the output load increases, it is
proper to set V
DD at 3~5V higher than VDD UVLO level (9V)
to avoid the over-voltage protection condition during the
peak load operation.
Figure 7.
Simplified Transformer Diagram
(Design
Example)
Assuming the diode forward-
voltage drop is 1V, the turn ratio is obtained as:
03
.
3
1
32
100
V
V
V
N
N
n
F
O
RO
S
P
=
+
=
+
=
=
Then, determine the proper integer for N
S such that the
resulting N
P is larger than NP
min as:
MIN
P
S
P
S
N
61
N
n
N
,
20
N
>
=
=
=
Setting VDD* as 13V, the number of turns for the
auxiliary winding is obtained as:
9
20
1
32
1
13
N
V
V
V
*
V
N
S
F
O
FA
DD
A
=
+
+
=
+
+
=


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