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AN701 Datasheet(PDF) 9 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) 9 Page - Vishay Siliconix

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AN701
Vishay Siliconix
Document Number: 70575
16-Jan-01
www.vishay.com
9
CHOOSING THE TOPOLOGY
The choice of topology is usually based on the designer’s
previous experience. The two best candidates for the Si9114A
are the forward and flyback types, although other types, such
as Cuk, are also possible.
In general, forward converters are best for higher-power
applications, and flyback converters are best for lower-power
applications. Both topologies have their merits, and the
designer will have to select the one most suited to his or her
own application. See appendices A and B for brief descriptions
of topologies and magnetic design equations.
SELECTING THE SEMICONDUCTORS
For power switching, the recommended device is the
Si9420DY. The Si9420DY is a 200-V, 1-
W MOSFET housed in
an industry-standard SO-8 package. Since the die is mounted
on a copper header, cooling can be accomplished using the
PCB area directly below the Drain pins. The combined
performance of the Si9420DY’s features makes it the best
low-profile device available on the market. It is suitable for
designing power supplies ranging from 10 to 25 W. Other such
single and dual LITTLE FOOT devices are available in both n-
and p-channel versions with voltages starting from 12 V.
Rectification for low-voltage outputs (< 5 V) is accomplished
using Schottky diodes. In this case, the rectifier selected
exhibited forward voltage drops of 0.4 V at 4 A. A 5-V output will
require a rectifier with a 40-V reverse voltage rating. Where
lower voltages, such as 3 V, are required, devices with lower
reverse blocking should be used, since these will have lower
forward voltage drops. Designers should avoid using an
oversized Schottky diode, since all such devices have
parasitic capacitances that need to be charged and
discharged to the applied voltages. Driving and commutating
oversize devices will not necessarily yield better efficiency,
especially at higher frequencies.
Rectification
for
voltages
above
12
V
is
generally
accomplished using fast or ultra-fast rectifiers. Look for
devices that have recovery times below 50 ns. An excellent
example is Telefunken Semiconductors’ BYG22B rated for
100 V and 2 A with 25 ns recovery, and Forward voltage of
0.7 V for 0.5 A current. This device is available in a DO-214
surface-mount package.
Opto-isolators are now available in SO-8 packages with
3000 Vrms isolation rating. These are by far the least
expensive and simplest isolated feedback devices now
available. Their reliability, once considered questionable, has
been greatly improved, and manufacturers now have quality
data demonstrating their suitability under the correct operating
conditions. A typical device would be the Telefunken
Semiconductors’ TCMT1020.
CHOOSING FERRITE MATERIALS
Ferrites suitable for operation at high frequencies have
recently been introduced to the market. Two such offerings are
the Philips 3F3 and 3F4, designed for operation up to 500 kHz
and 2 MHz respectively. Many different geometries and good
supporting data are now available. Appropriate choices for
low-profile and surface-mount capability include devices in the
EFD series, which have been extended down to 10 mm. It is
better to choose core geometries with shallow and wide
bobbins, since these permit good coupling from winding to
winding when using high frequencies.
CHOOSING CERAMIC CAPACITORS
High-frequency operation allows the use of very low-value
capacitances not generally associated with switchmode
power supply output stages. As substantially lower energy
storage is required, multilayer ceramic capacitors can be used,
and suppliers have made good advances in quality and
manufacturing to supply low-cost, high-performance designs.
In the sub-25-
mF area, a number of good dielectric devices are
now available, such as X7R, Z5U, and Y5T.
From manufacturers’ data sheets, the following observations
were made:
S Z5U has the lowest cost, highest unit capacity, and worst
dynamic variations
S X7R has the highest cost, lowest unit capacity, and best
dynamic stability
S Y5T has an average of each of the above.
The recently introduced Marcon TCCR series uses the Y5T
dielectric, which offers good all-around volumetric, cost, and
high-frequency impedance performance, and is available in a
surface-mount package with values such as 10
mF at 25 V and
3.3
mF at 100 V. For input and output energy storage, two of
each of these devices were selected with the following
considerations:
S Realistic market price.
S Voltage variation with applied dc voltage and temperature.
Most ceramic capacitors suffer from a drop of capacitance
with applied voltage and with temperature. The device
needs to be selected so that at the extremes of operation
the minimum energy storage is present.
S Equivalent Series Resistance (ESR). ESR will determine
the output ripple voltage, and the heating of the device.
This should be selected on the basis of the value of output
choke, insofar as its design sets the ripple current present
in the output capacitor.
The following data was obtained from commercially obtained
samples:
At 70oC, the 10-
mF device has dropped to 75% of its nominal
value. With 5 V applied, the same device has retained 110%
of its nominal value.
Care should be taken in selecting these devices to consider
worst case requirements and minimum/ maximum operating
conditions.


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