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AN701 Datasheet(PDF) 16 Page - Vishay Siliconix

Part No. AN701
Description  reduce the size of energy storage components
Download  19 Pages
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Maker  VISHAY [Vishay Siliconix]
Homepage  http://www.vishay.com
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AN701 Datasheet(HTML) 16 Page - Vishay Siliconix

 
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AN701
Vishay Siliconix
www.vishay.com
16
Document Number: 70575
16-Jan-01
The volume of the EF12.6 core is specified as 384 mm3 or
0.384 cm3.
Therefore the core loss will be:
P = 0.742
0.384
P = 0.285 W
[ 1/4 W
which represents less than 2% of the total converter losses.
The converter transformer can be calculated by using the
following adapted version of Faraday’s equation:
N
+
V
in min xton max
Bmax xAeff
Where:
Vinmax = minimum input voltage (V)
N = number of turns (Integer)
Bmax = maximum peak flux (Tesla)
Aef = effective area of core (m2)
In the case of the EF12.6, the minimum number of turns will be:
N
+
36 V x 1
ms
85 mT x 12.2 mm2
N = 36.64 Turns [ 37 Turns
This is the calculated minimum number of turns that should be
applied. To determine the turns for a given output voltage, first
determine the output voltage (5 V). Then determine all the other
losses that will exist in series with the output: including the
choke, transformer dc losses, and rectifier forward drop.
Assuming these add up to 0.5 V, with a duty cycle of 50%, the
transformer will be required to supply a peak voltage of
Vsec = (Vout+Vloss)/Dmax
Vsec = (5 V + 0.5 V)/0.5 = 11 V
The transformer turns ratio is thus determined: for 36-V input,
11-V output is required. The turns ratio is therefore TR=36/11
= 3.27.
For the 5-V output, the number of turns required is:
NS=Np/TR = 11.01
Therefore, the number of turns selected is 11. The Si9114A will
require a few miliamps of current to power itself and drive the
power MOSFET. This power can be taken from a winding
which peak charges a capacitor through a diode and does not
require an inductor. In this case, 11 turns can also be taken (as
this was calculated to be sufficient for 11 V).
SKIN DEPTH
Conductors carrying high-frequency ac current are subject to
a “skin effect” in which the current has a tendency to flow
predominantly on the surface of the conductor instead through
the whole cross section. The value at which the current falls to
1/e (37%) is called the “skin depth.” Below this depth, very little
usage of copper is made, and multiple strands are required for
applications where higher current is required. In some
instances, it may be necessary to use larger wire than
indicated, due to mechanical assembly constraints and ease
of manufacture. The skin depth can be calculated from:
X
D +
66
f
mm
Operation at 500 kHz means that the skin depth will be 0.093
mm (or approximately 0.1 mm). Ideally, a conductor with a
diameter just over twice the skin depth is recommended. In this
case, therefore, 0.2 mm or thereabouts will suffice.
LEAKAGE INDUCTANCE
In PWM converters, the coupling factor between windings
should be optimized to minimize the leakage inductance.
Leakage inductance is a parasitic element, storing energy that
will need to be dissipated. This leakage is a measure of the
quality of the coupling; the lower it is, the better the transformer
and its performance. The leakage inductance can be
measured by shorting out the 5-V winding and measuring the
primary inductance. Ideally, this should be zero, but in reality
values of less than 10% of the primary inductance are typical.
This value can be minimized by splitting the primary winding in
two halves, and by sandwiching all secondaries in between the
two primary halves.
In this case, the transformer was wound as follows:
Winding
Order
Winding
Name
#
Turns
Wire Size
Start
Pin
End
Pin
1
Half
Primary
17
1x0.3 mm
1
10
2
Bias
Winding
11
3x0.3 mm
3
9
3
Secondary
11
3x0 3 mm
4,5
7 6
4
Half
Primary
18
1x0.3 mm
10
2
Copper wire of 0.3 mm was used to obtain single layer fill
across the surface of the bobbin.
The wound transformer had the following characteristics:
Winding
Inductance
DC Resistance
Primary
883
µH
151 m
W
Secondary
87
µH
25 m
W
The leakage inductance was measured at 2.25
mH, which
represents less than 1/4% of the primary inductance.


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