3) The main factors influencing the choice of the turns

ratio are the switch breakdown voltage and the duty

cycle. With a smaller turns ratio, the secondary

reflected voltage and the maximum voltage seen by

the switch during flyback are reduced, which is

favorable. On the other hand, a smaller turns ratio

will shorten the duty cycle and increase the primary

RMS current, which can impact efficiency. A good

starting figure is the ratio of the input voltage to the

output voltage, rounding to the nearest integer. To

keep the flyback voltage under control, choose an 8-

to-1 ratio for the 48V to 5V system. The maximum

duty cycle allowed without putting the device in con-

tinuous-conduction mode can be found using the fol-

lowing formula:

where:

For a 48V to 5V system with an 8-to-1 turns ratio, the

maximum duty cycle before putting the device in

36V (minimum input voltage, neglecting drops in the

power switch and in the resistance of the primary

drop). The MAX5003 maximum duty cycle is internal-

ly limited to 75%. Generally this parameter must fall

between 45% to 65% to obtain a balance between

efficiency and flyback voltage while staying out of

continuous conduction. If the value exceeds these

bounds, adjust the turns ratio.

4) Assuming 80% efficiency, a 6.25W input is needed

to produce a 5W output. Set an operating duty cycle

around 12% below the maximum duty cycle to allow

for component variation: 55% - 12% = 43%. Use the

following formula to calculate the primary inductance:

L

DC V

PWR

V

W

kHz

H

PRI

MIN

IN

SW

ƒ

=

()

≅µ

⋅

⋅⋅

⋅

⋅⋅

22

2

043 36

2 6 25

300

65

.

.

DC

V

VN

MAX

MIN

SEC

=









+

⋅

1

1

High-Voltage PWM

Power-Supply Controller

12

______________________________________________________________________________________

10

µF

0.1

µF C

F

390pF

0.1

µF

0.1

µF

0V

470nF

200k

0.1

Ω

51k

62k

V+

INDIV

ES

FREQ

SS

REF

CON

COMP

8

7

6

5

4

3

2

1

9

10

11

12

13

14

15

16

NDRV

PGND

CS

AGND

CMSD4448

XFACOILTRCTX03

MBRS130L

8

2

5

7

9, 10

11, 12

IRFD620S

MAXTON

FB

39k

1M

33

µF

0.1

µF

100

Ω

+5V

1A

+48V

(36V TO 72V)

41.2k

17.4k

22

µF

4.7

µF

65

µH

22

µF

MAX5003

Figure 2. Application Example 1: Nonisolated +48V to +5V Converter