AN4149 datasheet(7/19 Pages) FAIRCHILD

Part #	AN4149

Description	Design Guidelines

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

Direct Link	http://www.fairchildsemi.com

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APPLICATION NOTE

AN4149

7

[STEP-9] Determine the wire diameter for each

winding based on the RMS current of each output.

The RMS current of the n-th secondary winding is obtained

as

where Dmax and Ids

rms are specified in equations (6) and (9),

Vo(n) is the output voltage of the n-th output, VF(n) is the

diode (DR(n)) forward voltage drop, VRO is specified in

STEP-3 and KL(n) is the load occupying factor for n-th

output defined in equation (2).

The current density is typically 5A/mm2 when the wire is

long (>1m). When the wire is short with a small number of

turns, a current density of 6-10 A/mm2 is also acceptable. Do

not use wire with a diameter larger than 1 mm to avoid

severe eddy current losses as well as to make winding easier.

For high current output, it is recommended using parallel

windings with multiple strands of thinner wire to minimize

skin effect.

Check if the winding window area of the core, Aw (refer to

Figure 6) is enough to accommodate the wires. The required

winding window area (Awr) is given by

where Ac is the actual conductor area and KF is the fill factor.

Typically the fill factor is 0.2~0.25 for single output

applications and 0.15~0.2 for multiple output applications.

If the required window (Awr) is larger than the actual window

area (Aw), go back to the STEP-6 and change the core to a

bigger one. Sometimes it is impossible to change the core

due to cost or size constraints. In that case, reduce VRO in

STEP-3 or increase fs

min, which reduces the primary side

inductance (Lm) and the minimum number of turns for the

primary (Np

min) as can be seen in equation (7) and (10).

[STEP-10] Choose the proper rectifier diodes in the

secondary side based on the voltage and current ratings.

The maximum reverse voltage and the rms current of the

rectifier diode (DR(n)) of the n-th output are obtained as

where KL(n), VDC

max, D

max and Ids

rms are specified in

equations (2), (4), (6) and (9), respectively, VRO is specified

in STEP-3, Vo(n) is the output voltage of the n-th output and

VF(n) is the diode (DR(n)) forward voltage drop. The typical

voltage and current margins for the rectifier diode are as

follows

where VRRM is the maximum reverse voltage and IF is the

average forward current of the diode.

A quick selection guide for the Fairchild Semiconductor

rectifier diodes is given in Table 3. In this table, trr is the

maximum reverse recovery time.

Table 3. Fairchild Diode Quick Selection Table

I

n

()

sec

rms

I

ds

rms

1D

max

–

D

max

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

V

RO

K

Ln

()

⋅

V

on

()

V

Fn

()

+

()

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

⋅

=20

()

A

wr

A

c KF

⁄

=

(21)

V

Dn

()

V

on

()

V

DC

max

V

on

()

V

Fn

()

+

()

⋅

V

RO

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

+

=22

()

I

Dn

()

rms

I

ds

rms

1D

max

–

D

max

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

V

ROKLn

()

V

on

()

V

Fn

()

+

()

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

⋅

=23

()

Ultra Fast Recovery Diode

Products

VRRM

IF

trr

Package

EGP10B

100 V

1 A

50 ns

DO-41

UF4002

100 V

1 A

50 ns

DO-41

EGP20B

100 V

2 A

50 ns

DO-15

EGP30B

100 V

3 A

50 ns

DO-210AD

FES16BT

100 V

16 A

35 ns

TO-220AC

EGP10C

150 V

1 A

50 ns

DO-41

EGP20C

150 V

2 A

50 ns

DO-15

EGP30C

150 V

3 A

50 ns

DO-210AD

FES16CT

150 V

16 A

35 ns

TO-220AC

EGP10D

200 V

1 A

50 ns

DO-41

UF4003

200 V

1 A

50 ns

DO-41

EGP20D

200 V

2 A

50 ns

DO-15

EGP30D

200 V

3 A

50 ns

DO-210AD

FES16DT

200 V

16 A

35 ns

TO-220AC

EGP10F

300 V

1 A

50 ns

DO-41

EGP20F

300 V

2 A

50 ns

DO-15

EGP30F

300 V

3 A

50 ns

DO-210AD

EGP10G

400 V

1 A

50 ns

DO-41

UF4004

400 V

1 A

50 ns

DO-41

EGP20G

400 V

2 A

50 ns

DO-15

EGP30G

400 V

3 A

50 ns

DO-210AD

UF4005

600 V

1 A

75 ns

DO-41

EGP10J

600 V

1A

75 ns

DO-41

EGP20J

600 V

2 A

75 ns

DO-15

EGP30J

600 V

3 A

75 ns

DO-210AD

UF4006

800 V

1 A

75 ns

TO-41

UF4007

1000 V

1 A

75 ns

TO-41

V

RRM

1.3 V

Dn

()

⋅

>

(24)

I

F

1.5 I

Dn

()

rms

⋅

>

(25)

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