APPLICATION NOTE

AN4129

5

©2003 Fairchild Semiconductor Corporation

Figure 11. DC Link Voltage and Current Waveforms

The bridge diode conduction time can be calculated by (5)

and the diode RMS current can be calculated by (6).

The calculated value is 1.3A and the selected bridge diode is

KBP206(600V/2A).

2. Transformer Design

Since 2001, the European Commission has been regulating

no load losses for AC adapters, battery chargers and

external power supplies under 75W.

Table 1 shows the regulation target specification.

Vdc_link

Vac

Iac

tc

1

2

π fac

⋅

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

=

arc

Vdc_min

2 Vac_min

⋅

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





cos

×

(5)

IBD RMS

() 2

=

2 Vac_min

⋅

Vdc_min

–

() Cdc

2fac

⋅

3tc

⋅

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

⋅⋅

⋅

(6)

Rated Input Power

No Load Power Consumption

Phase 1

1.1.2001

Phase 2

1.1.2003

Phase 3

1.1.2005

≥ 0.3W and < 15W

1.0W

0.75W

0.30W

≥ 15W and < 50W

1.0W

0.75W

0.50W

≥ 50W and < 75W

1.0W

0.75W

0.75W

At light and no load, the FAN7601 operates in burst mode to

reduce the power loss. To meet the regulation specification

the most important thing is to minimize the number of

MOSFET switchings. The flyback converter transfers energy

during the switch-off time. As the primary inductance of the

flyback transformer increases, the energy transferred to the

secondary side increases during one switching cycle. There-

fore it is better to use a higher inductance transformer, but

inductance is restricted by the size and cost of the trans-

former. In this design example, a 600uH transformer is

selected and the ferrite core is EER2828. The transformer

turns ratio is calculated when the input line voltage is lowest

and the output power is maximum. The maximum duty ratio

must be lower than 0.45 to prevent subharmonic oscillation.

In this design example, the turns ratio must be higher than

0.174 by (7).

Once the minimum turns ratio is determined, then the

numbers of primary and secondary turns is calculated when

input line voltage is highest and the output power is

maximum. If the converter operates in the CCM (Continuous

Conduction Mode) then the turn-on time can be calculated

by (8).

Then the number of primary turns can be obtained as in (9).

Ae is the effective cross sectional area of the core and Bmax

and Bmax is 0.15T. The calculated number of primary turns

is 54. Then the number of secondary turns can be calculated

by (10). The calculated number of secondary turns is 10.

The air gap length can be calculated by (11).

If the primary inductance is not high enough, the converter

can operate in the DCM(Discontinuous Conduction Mode)

when the input line voltage is highest and the output power is

maximum. For the DCM, the turn-on time can be calculated

as in (12).

n

1

–

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

>

+

Vdc_min

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

⋅

(7)

+

n Vdc_max

⋅

++

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

=

1

fsw

---------

⋅

(8)

Np

⋅

Ae Bmax

⋅

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





=

(9)

Ns

Np

=

+

Vdc_min

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

–

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

⋅

(10)

l

g4

π

=10

7

–

Ae

N

2

P

L

----------

⋅⋅

⋅

(11)