where:

DC = Duty cycle. Set to calculated minimum duty

mately 65µH.

5) The other parameter that defines the transformer is

peak current. This is given by:

The peak secondary current is the peak primary cur-

rent multiplied by the turns ratio, or 0.8A · 8 = 6.4A.

Calculating the minimum duty cycle:

With these numbers, the transformer manufacturer

can choose a core.

6) For this application, the MAX5003 must be pro-

grammed for a maximum duty cycle of 55% at 36V.

The MAX5003 will automatically scale the limit with

the reciprocal of the input voltage as it changes. The

duty-cycle limit for an input voltage of 72V will be

27% (half of 55%). The duty cycle needed to stay out

of continuous conduction at 72V is 37%, so there is a

10% margin. The maximum duty time scales with the

voltage at INDIV is set by selecting the power line

undervoltage lockout trip point. The trip point for this

system, running from 36V to 72V, is 32V. Then INDIV

must be connected to the center point of a divider

with a ratio of 32/1.25, connected between the

R

V

V

kHz

DC

V

k

V

V

kHz

kHz

kk

MAXTON

MIN

UVL

SW

MAX

MIN

=









ƒ









()













= 























=

%

%

%

100

75

200

36

32

100

300

55

75

200

55

Ω

ΩΩ

DC

DC

V

V

V

V

MIN

MAX

IN MAX

IN MIN

()

(

)

()

()

%

. %

==

=

⋅⋅

43

36

72

21 5

I

2 PWR

L

2 6.25W

65 H 300kHz

0.8A

PRI

IN

PRI

SW

=

ƒ

==

⋅

⋅

⋅

⋅

µ

High-Voltage PWM

Power-Supply Controller

______________________________________________________________________________________

13

10

µF

0.1

µF

51k

1.3k

0.1

Ω

0.1

µF

0.1

µF

470nF

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

MDC217

IRFD620S

65

µH

MAXTON

FB

R2

39k

R1

1M

33

µF

0.1

µF

100

Ω

+5V

1A

-

51

Ω

680

Ω

240k

6

7

5

2

1

22

µF

0.01

µF

3900pF

24.9k

24.9k

TL431

4.7

µF

22

µF

MAX5003

0V

XFACOILTRCTX03

CMSD4448

MBRS130L

8

2

5

7

9, 10

11, 12

-48V

-36V TO -72V

Figure 3. Application Example 2: Isolated -48V to +5V Converter