AN-9719

APPLICATION NOTE

© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 11/2/10

7

saturation flux density as 0.3T, the minimum number of

turns for the primary-side is obtained as:

·

·

10

540 10

·0.8

0.3 · 19.2

10

75

[STEP-7] Determine the Number of Turns for each

Winding

Figure 12 shows a simplified diagram of the transformer.

First calculate the turn ratio (n) between the primary-side

and the secondary-side from the reflected output voltage

(13)

supply is determined as:

·

(14)

voltage protection condition during the peak-load operation.

Figure 12. Simplified Transformer Diagram

(Design

Example)

Assuming the diode forward-

voltage drop is 0.85V, the turn ratio is obtained as:

74

12

0.85

5.8

13,

·

75

Setting

as 12V, the number of turns for the

auxiliary winding is obtained as:

·

12

0.85

12

0.5

·13

13

[STEP-8] Determine the Wire Diameter for Each Winding

Based on the RMS Current of Winding

The maximum RMS current of the secondary winding is

obtained as:

·

1

(15)

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

using wire with a diameter larger than 1mm to avoid severe

eddy current or ac losses. For high-current output, it is better

to use parallel windings with multiple strands of thinner

wire to minimize skin effect.

(Design Example)

The RMS current of primary-side

winding is obtained from Step-4 as 0.31A. The RMS

current of secondary-side winding is calculated as:

·

1

5.8 · 0.31

1

0.48

0.48

1.87

are selected for primary and secondary windings,

respectively.

[STEP-9] Choose the Rectifier Diode in the Secondary-

Side Based on the Voltage and Current Ratings

The maximum reverse voltage and the RMS current of the

rectifier diode are obtained as:

(16)

·

1

(17)