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AN701 Datasheet(PDF) 15 Page - Vishay Siliconix

Part No. AN701
Description  reduce the size of energy storage components
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Maker  VISHAY [Vishay Siliconix]
Homepage  http://www.vishay.com
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AN701 Datasheet(HTML) 15 Page - Vishay Siliconix

 
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AN701
Vishay Siliconix
Document Number: 70575
16-Jan-01
www.vishay.com
15
Appendix A
FORWARD CONVERTER TRANSFORMER
DESIGN
The forward converter transformer is operated as a voltage
transformer. An ac source applies voltage across the primary,
which is then transformed (up or down) by the turns ratio. For
correct operation, the transformer must be correctly sized (in
order to avoid core saturation) and large enough to accept the
number of turns required. Since these two requirements may
often compete with one another, compromise may be
necessary to complete a design.
In operation, the transformer is driven through the magnetic
B/H loop. In a forward converter, the core is only driven in
quadrant 1. Care must be taken not to saturate it, since only a
“minor loop” is used. The core is driven in the +H direction, and
when the MOSFET turns off, the core is allowed to “float back”
to the H = 0 position. As there is no negative drive, the core
cannot be driven into quadrant 3 (as in the case of a push-pull
converter), and so the core always returns to the BREM
(remnant) position.
Figure 28
Bsat
Brem1
Brem2
4
B
1
H
2
3
In some cases, a small air gap is placed in the core, which
causes the B/H curve to be skewed out. The remnant position
can thus be lowered from the Brem1 to the Brem2 position,
allowing a larger area of operation. In all joined cores (such as
RM or EE), this small gap actually exists in the form of the
interface between the two cores and causes the BH curve to
skew over. In high-frequency applications, this gap has a
significant beneficial effect on the operation of the transformer.
The number of turns is already low, and the gap prevents core
saturation.
Other converters (such as half-bridge and push-pull) use all
four quadrants and make better use of the whole core. These,
however, require additional power switching devices and
windings and are therefore not used. At high frequencies, the
core losses of ferrite materials are high, so the used flux must
be reduced. once this is done, the problem becomes
insignificant, very small core excursions are used.
In selecting the core type and size, the transformer losses
need to be divided more or less evenly between core and
copper losses. Most manufacturers will now supply formulas
extracted from core loss tables that allow precise core loss
calculations to be made. It is important to remember that most
ferrite manufacturers
use
peak-to-peak
flux
for their
calculations, while for single quadrant converters, the core
losses will be halved compared to the data published. Most
ferrite manufacturers will likewise recommend levels no higher
than 200 mT for the highest flux swing, to cover the complete
temperature and operating conditions. Most ferrites have the
lowest core losses at between 75 and 95
_C. As the
temperature in the transformer gradually rises, the efficiency of
the transformer will increase.
To design a forward converter transformer, the following data
is necessary:
FSW
Operating Switching Frequency, usually 20 to
500 kHz
dmax
Maximum duty cycle, usually 50% in
Vishay Siliconix products
h
Target efficiency, usually 0.75 to 0.85
Vinmin
Minimum input voltage used
Pout
Total output power
Vout
Output Voltage(s) required
In this case, a design for a 48-V (38- to 60-V), 5-V, 4-A (20-W)
device operating at 50% duty cycle with 500-kHz switching
frequency will be demonstrated. The period of conversion will be
2
ms, and the maximum on-time thus Tonmax = 1 ms.
The EF12.6, or the EPC13, which have similar characteristics,
will be used as the core. The core operating flux has been
selected to 85 mT as a first pass design. The loss in Philips 3F4
material is calculated from (source Philips Components):
Pv = 12x10–21.75@B29@(0.95x10–4@T2–1.1x10–2@T + 1.15)
Where:
PV = power loss in W/m3
B = operating flux density in Tesla
ƒ = operating frequency in Hertz
T = core temperature in
_C
With 500 kHz, 85 mT, and 50
_C, the core loss can be
calculated as:
Pv = 742.5 x 103 W/m3 or 0.742 W/cm3


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