AN-9719 データシートの表示（PDF） - Fairchild Semiconductor
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AN-9719 Datasheet PDF : 12 Pages
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AN-9719
(Design Example) EE-16 core is selected, whose
effective cross-sectional area is 19.2mm2. Choosing the
saturation flux density as 0.3T, the minimum number of
turns for the primary-side is obtained as:
·
10
·
540 10 · 0.8
10 75
0.3 · 19.2
[STEP-7] Determine the Number of Turns for each
Winding
Figure 12 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
(VRO) determined in Step-3 as:
(13)
where NP and NS are the number of turns for primary-side
and secondary-side, respectively; VO is the output voltage;
and VF is the output diode (DO) forward-voltage drop.
Next, determine the proper integer for NS such that the
resulting NP is larger than NPMIN obtained from Equation 12.
The number of turns for the auxiliary winding for VDD
supply is determined as:
·
(14)
where VDD is the supply voltage nominal value and VFA is
the forward-voltage drop of DDD as defined in Figure 12.
Since VDD increases as the output load increases, set VDD at
5~8V higher than VDD UVLO level (8V) to avoid the over-
voltage protection condition during the peak-load operation.
APPLICATION NOTE
(Design Example) Assuming the diode forward-
voltage drop is 0.85V, the turn ratio is obtained as:
74
5.8
12 0.85
Then determine the proper integer for NS such that the
resulting NP is larger than NPMIN as:
13,
· 75
Setting
as 12V, the number of turns for the
auxiliary winding is obtained as:
12 0.85
·
12 0.5 · 13 13
[STEP-8] Determine the Wire Diameter for Each Winding
Based on the RMS Current of Winding
The maximum RMS current of the secondary winding is
obtained as:
1
·
(15)
The current density is typically 3~5A/mm2 when the wire is
long (>1m). When the wire is short with a small number of
turns, a current density of 5~10A/mm2 is acceptable. Avoid
using wire with a diameter larger than 1mm to avoid severe
eddy current or ac losses. For high-current output, it is better
to use parallel windings with multiple strands of thinner
wire to minimize skin effect.
(Design Example) The RMS current of primary-side
winding is obtained from Step-4 as 0.31A. The RMS
current of secondary-side winding is calculated as:
1
·
Figure 12. Simplified Transformer Diagram
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/2/10
1 0.48
5.8 · 0.31 0.48 1.87
0.3mm (5A/mm2) and 0.4mm (8A/mm2) diameter wires
are selected for primary and secondary windings,
respectively.
[STEP-9] Choose the Rectifier Diode in the Secondary-
Side Based on the Voltage and Current Ratings
The maximum reverse voltage and the RMS current of the
rectifier diode are obtained as:
(16)
1
·
7
(17)
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