AN-9719 데이터 시트보기 (PDF) - Fairchild Semiconductor
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AN-9719 Datasheet PDF : 12 Pages
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AN-9719
Once LM is calculated from equation (7) after choosing KRF,
the peak and RMS current of the MOSFET for the minimum
input voltage and nominal load condition are obtained as:
∆
2
(8)
∆
3
23
(9)
where:
·
(10)
and
·
∆
·
(11)
K RF
=
ΔI
2 I EDC
ΔI
I PK
DS
Figure 10. MOSFET Current and Ripple Factor (KRF)
(Design Example) Choosing the ripple factor as 0.88,
·
2· · ·
79 · 0.48
540
2 · 15 · 100 10 · 0.88
15
0.4
·
79 · 0.48
APPLICATION NOTE
[STEP-5] Choose the Proper FPS Considering Input
Power and Peak Drain Current
With the resulting maximum peak drain current of the
MOSFET (IDSPK) from Equation 8, choose the proper FPS
for which the pulse-by-pulse current limit level (ILIM) is
higher than IDSPK. Since FPS has ±10% tolerance of ILIM,
there should be some margin when choosing the proper FPS
device. The FSL-series lineup with proper power rating is
summarized in Table 3.
Table 3. Lineup of FSL1x7-Series with Power Rating
Product
ILIM at VIN=1.2V
Min. Typ. Max.
85-265VAC
Open Frame
FSL127H
FSL137H
0.51A 0.61A 0.71A
0.74A 0.84A 0.94A
16W
19W
(Design Example)
0.75 0.84
.
FSL137H is selected.
[STEP-6] Determine the Minimum Primary Turns
With a given magnetic core, the minimum number of turns
for the transformer primary side to avoid the core saturation
is given by:
·
10
(12)
·
where Ae is the cross-sectional area of the core in mm2, ILIM
is the pulse-by-pulse current limit level, and BSAT is the
saturation flux density in Tesla.
The pulse-by-pulse current limit level is included in
Equation 12 because the inductor current reaches the pulse-
by-pulse current limit level during the load transient or
overload condition. Figure 11 shows the typical
characteristics of ferrite core from TDK (PC40). Since the
saturation flux density (BSAT) decreases as temperature
increases, the high-temperature characteristics should be
considered. If there is no reference data, use BSAT =0.3 T.
·
79 · 0.48
∆
0.7
·
540 10 · 100 10
∆
0.4 0.35 0.75
2
∆
3
23
3 0.4
0.48
0.35
0.31
3
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/2/10
Figure 11.Typical B-H Characteristics of Ferrite Core
(TDK/PC40)
www.fairchildsemi.com
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