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AN701

Designing High-Frequency DC-to-DC Converters With the Si9114A Switchmode Controller

Vishay Semiconductors 

AN701

Vishay Siliconix

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

Document Number: 70575

16-Jan-01

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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