APPLICATIONS

Switching transistor selection

The choice of switching transistor has a major impact

on the converter efficiency. For optimum performance,

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:

Sense resistor

A low value sense resistor is required to set the peak

current. Power in this resistor is negligible due to the

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

to

the diode.

Peak current definition

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.

determined by:

R

V

I

SENSE

SENSE

PK

=

ZXSC440

SEMICO NDUC TORS

DRAFT ISSUE F - MAY 2004

6

Manufacturer

Series

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