ZXSC440 datasheet(6/16 Pages) ZETEX

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APPLICATIONS

Switching transistor selection

The choice of switching transistor has a major impact

on the converter efficiency. For optimum performance,

a bipolar transistor with low VCE(SAT) and high gain is

required. The VCEO of the switching transistor is also an

important parameter as this sees typically three times

the input voltage when the transistor is switched off.

Zetex SuperSOT

™ transistors are an ideal choice for

this application. At input voltages above 4V, suitable

Zetex MOSFET transistors will give almost the same

performance with a simpler drive circuit, omitting the

ZXTD6717 pre-drive stage.

Using a MOSFET, the

Schottky diode may be omitted, as the body diode of

the MOSFET will perform the same function, with just a

small loss of efficiency.

Output rectifier diode selection

The diode should have a fast recovery, as any time

spent in reverse conduction removes energy from the

reservoir capacitor and dumps it, via the transformer,

into the protection diode across the output transistor.

This seriously reduces efficiency. Two BAS21 diodes

in series have been used, bearing in mind that the

reverse voltage across the diode is the sum of the

output voltage together with the input voltage

multiplied by the step-up ratio of the transformer:

VR(DIODE) = VOUT(MAX) + (VIN x TURNSRATIO)

Sense resistor

A low value sense resistor is required to set the peak

current. Power in this resistor is negligible due to the

low sense voltage threshold, VSENSE. Below is a table of

recommended sense resistors:

Using a 22m

sense resistor results in a peak current of

just over 1.2A.

Therefore, with a 300V output, a supply of 8 volts and a

1:12 step-up transformer, there will be a 396V across

the diode. This occurs during the current ramp-up in

the primary, as it transforms the input voltage up by the

turns ratio and the polarity at the secondary is such as

add to the output voltage already being held off by

the diode.

Peak current definition

In general, the IPK value must be chosen to ensure that

the switching transistor, Q1, is in full saturation with

maximum output power conditions, assuming

worse-case input voltage and transistor gain under all

operating temperature extremes.

Once IPK is decided the value of RSENSE can be

determined by:

SENSE

ZXSC440

SEMICO NDUC TORS

ISSUE 1 - JANUARY 2005

Manufacturer

Series

RDC( ) Range

Size

Tolerance

URL

Cyntec

RL1220

0.022 - 10

0805

±5%

http://www.cyntec.com

IRC

LR1206

0.010 - 1.0

1206

±5%

http://www.irctt.com

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