Optocoupled Feedback

Isolated voltage feedback is achieved by using an opto-

coupler and a shunt regulator as shown in Figure 2. The

output voltage set point accuracy is a function of the

accuracy of the shunt regulator and feedback resistor-

divider tolerance.

Internal Regulators

The internal regulators of the MAX5014/MAX5015

enable initial startup without a lossy startup resistor and

regulate the voltage at the output of a tertiary (bias)

winding to provide power for the IC. At startup V+ is

tiary winding to only have a small filter capacitor at its

output thus eliminating the additional cost of a filter

inductor.

When designing the tertiary winding calculate the num-

ber of turns so the minimum reflected voltage is always

higher than 12.7V. The maximum reflected voltage

must be less than 36V.

Current-Mode PWM Controllers with Integrated

Startup Circuit for Isolated Power Supplies

8

_______________________________________________________________________________________

MAX5015

V+

SS_SHDN

NDRV

CS

GND

OPTO

(36V TO 72V)

OPTOCOUPLER

47

µF

0.1

µF

3k

Ω

4.75k

Ω

10

µF

100

Ω

100m

Ω

20

Ω

25.5k

Ω

8.25k

Ω

3

× 560µF

0.1

µF

3

× 0.47µF

L1

4.7

µH

0.1

µF

240k

Ω

M1

IRF640N

4.7nF

250VAC

TLV431

1N4148

6

CMHD2003

14

5

1nF

SBL204OCT

5V/10A

220

Ω

14

Figure 2. Forward Converter