AN1262 APPLICATION NOTE

18/42

11

SECONDARY RECTIFIER SELECTION

Although the converter is operated in DCM, it is recommended to use an ultrafast p-n diode or, whenever al-

lowed by the reverse voltage, a Schottky type. The latter, besides optimizing the reverse recovery, minimizes

conduction losses as well.

The voltage rating will be higher than the maximum reverse voltage it experiences:

,

(17)

with a suitable safety margin (usually 20-25%). As to its current rating, it is a common design practice to choose

or through-hole diode and the second one is in SMD package. The sale types in italic are p-n diodes, the others

are Schottky type.

Table 14. Recommended ST parts for secondary rectification.

12

OUTPUT CAPACITOR SELECTION AND POST FILTER

Large, low-ESR electrolytic capacitors usually do the filtering work. The parameters to be considered for their

selection are the working voltage, RMS ripple rating and ESR, the actual capacitance value is of secondary im-

portance.

reliability.

The AC current the output capacitor undergoes causes power dissipation on its ESR and a resulting tempera-

ture rise. This is the major responsible for capacitor degrading. Thus it is important not to operate the capacitor

beyond its AC current ripple rating, otherwise its lifetime will be considerably shortened. This parameter is usu-

ally specified at 85°C or 105°C ambient temperature, depending on capacitor's quality. The value could be de-

(V)

3.3

1N5820

STPS5L25B

STPS5L25B-1

STPS5L25B

STPS10L25D

STPS10L25G

1N58210

STPS3L25S

STPS5L25B-1

STPS5L25B

STPS10L25D

STPS10L25G

5

1N5820

STPS5L25B

1N5820

STPS5L25B

STPS10L25D

STPS10L25G

1N5821

STPS340C

1N5822

STPS340B

STPS640CT

STPS640CB

9

1N5821

STPS2L30A

1N5821

STPS340B

1N5822

STPS340B

–

STPS160U

–

STPS3L60S

STPS5H100-1

STPS3L60S

12

1N5819

STPS1L40A

1N5822

STPS3L60S

–

STPS3L60S

–

STPS1H100U

–

STPS2H100U

STPS5H100B-1

STPS2H100U

15

BYV10-60

STPS160A

BYV10-60

STPS160A

–

STPS3L60S

–

STPS1H100U

–

STPS2H100U

STPS5H100B-1

STPS2H100U

18

BYV10-60

STPS160A

BYV10-60

STPS1H100U

BYW98-100

STPS2H100U

–

STPS1H100U

–

STPS2H100U

STPS5H100B-1

STPS2H100U

24

BAT49

STPS1H100A

–

STPS1H100U

–

STPS1H100U

BYW100-200

STPR120A

BYW100-200

STPR120A

BYW100-200

SMBYW02-200

V

RE V

V

out

1

V

PKm a x

V

R

--------------------

+





⋅

=