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AN1262 APPLICATION NOTE

experience in converters with opto-isolated feedback. This variation is a result of the poor magnetic cou-

pling with the secondary winding. It is then recommended to design the turns ratio of the self-supply

winding so as to get a voltage approximately in the middle of this range (e.g. 11-12 V). This will give

allowance for increasing at heavy load and dropping at zero load.

Secondary diode forward drop. The type of secondary diode will be selected basically depending on the

output voltage. In fact this determines the maximum reverse voltage applied to the diode while the MOS-

FET is switched on. For low output voltages

≤15 V) a Schottky diode can be used and a typical forward

drop of 0.5V can be considered; for higher output voltages an ultrafast PN diode will be used, with a

typical forward drop of 0.8 V.

Bridge Rectifier + EMI filter voltage drop. This drop is subtracted to the peak of the input AC voltage and

affects the peak voltage of the ripple superimposed on top of the DC voltage across the input bulk ca-

pacitor. A typical value can be 3 V.

PRELIMINARY CALCULATIONS (STEP 1)

There are a few quantities that need to be calculated before starting the individual design of each functional

block of the converter. They are summarized in table 3.

Table 3. Preliminary calculations (step 1).

BRIDGE RECTIFIER SELECTION

Due to the limited power range that the device is able to handle, no special considerations are needed to select the

diodes of the bridge rectifier. Any 1A rated standard diodes with 400/600 V reverse voltage are suitable. Some man-

ufacturers make integrated bridge rectifiers housed in small packages. See table 4 for some suggested parts.

Table 4. 1A standard silicon rectifier and bridge selection

Symbol

Parameter

Definition

Converter Input Power

Iout

DC Output Current

VPKmin

Minimum Peak Input Voltage

VPKmin = VACmin ·

VPKmax

Maximum Peak Input Voltage

Type

Part Number

Rated Voltage

Package

Manufacturer(s)

Diode

1N4004

400

DO41

GI, GS, FAGOR, HTA, ON, TSC

Diode

1N4005

600

DO41

GI, GS, FAGOR, HTA, ON, TSC

Bridge

DF04M

400

DIL4

GI,TSC

Bridge

DF06M

600

DIL4

GI,TSC

Bridge

KBP104G

400

SIL4

TSC

Bridge

KBP105G

600

SIL4

TSC

Bridge

DFS04M

400

DIL4 (SMD)

HTA

Bridge

DFS06M

600

DIL4 (SMD)

HTA

outm ax

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

out

outm ax

ou t

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

–

PKm ax

ACm a x

⋅

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