AN701 datasheet(10/19 Pages) VISHAY

10 / 19 page

AN701

Vishay Siliconix

www.vishay.com

Document Number: 70575

16-Jan-01

Figure 17. Marcon 10

mF 25 V, Capacitance Versus

Temperature

Temperature (

_C)

Figure 18. Marcon 10

mF 25 V, Capacitance Change

with Voltage

Voltage (V)

DESIGN EXAMPLE: A 15W, 500KHZ, 48V/5V DCTODC CONVERTER

Resonant Reset Forward Converter

Most forward converters are designed using a clamp circuit.

While at low frequencies this technique may be acceptable, at

high frequencies it becomes unnecessary: the parasitic

elements of the circuit will reset the transformer flux

automatically, provided a few precautions are taken.

It has been shown that [1] the resonant reset concept is

dominated by the parasitic capacitance of the MOSFET and

the magnetizing inductance of the transformer. Yet the

capacitance of the output diode should also be considered.

The

correct

equivalent

circuit

the

converter

the

approximates to Figure 19.

During the off time, D2 is conducting and CD1 appears

connected across the primary of the transformer, in parallel

with LMAG. The leakage inductance has a small and

insignificant effect on the waveform—as the primary current

has ceased flowing—and the only remaining current is the

current that is charging COUT.

Figure 19 Resonant Reset Forward Converter

Lout

Cout

Vds

Cin

CAP

Lmag

N = 1

CD1

In effect, the magnetizing inductance of the transformer forms

a parallel tuned circuit across the transformer and resonates

at a frequency determined by the parasitic elements. The reset

period needs to be short enough to allow full reset of the core,

before the next switching interval occurs. This will be governed

by the selection of the MOSFET and the Schottky diode.

Component Selection

The following information is supplied in order to help designer

select correct components for use with the Si9114A.

Vishay Siliconix does not necessarily recommend or approve

these components for specific applications. Designers should

contact manufacturers directly to obtain correct and current

data sheets.

Capacitor Selection

As stated previously, ceramic capacitors are a good choice

when operating at high frequency, due to the extremely low

ESR, and high reliability, and long operating lifetimes.

In the design example, the required size of capacitors was

defined as follows:

Input capacitor:

A 15-W output converter with 85% efficiency will require

15/0

@85 = 17.65 W of input power. Assuming that operation

at nominal conditions is 48 V, with duty cycle of

δ=0.376

(measured), the switching current will be governed by the

size of the output inductor (Figure 20).

Figure 20

lDC

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