VIPER53SP datasheet(13/24 Pages) STMICROELECTRONICS

13 / 24 page

VIPer53DIP / VIPer53SP

13/24

Figure 15: Off Line Power Supply With Optocoupler Feedback

SECONDARY FEEDBACK CONFIGURATION

EXAMPLE

When a more accurate output voltage is needed,

the definitive way is to monitor it directly on

secondary side, and to drive the PWM controller

through an optocoupler as shown on figure 15.

The optocoupler is connected in parallel with the

compensation network on the COMP pin. The

design of the auxiliary winding will be made in such

a way that the VDD voltage is always lower than the

internal 15 V reference. The internal error amplifier

will therefore be saturated in the high state, and

because of its transconductance nature, will

deliver a constant biasing current of 0.6 mA to the

optotransistor. This current doesn’t depend on the

compensation voltage, and so it doesn’t depend on

the output load either. The gain of the optocoupler

ensures consequently a constant biasing of the

TL431 device (U3) which is in charge of secondary

regulation. If the optocoupler gain is sufficiently

low, no additional components are required to

ensure a minimum current biasing of U3. Also, the

low biasing current value avoid any ageing of the

optocoupler.

The constant current biasing can be used to

simplify the secondary circuit: Instead of a TL431,

a simple zener and resistance network in series

with the optocoupler diode can insure a good

secondary regulation. As the current flowing in this

branch remains constant for the same reason as

above, typical load regulation of 1% can be

achieved from zero to full output current with this

simple configuration.

Since the dynamic characteristics of the converter

are set on the secondary side through components

associated to U3, the compensation network has

only a role of gain stabilization for the optocoupler,

and its value can be freely chosen. R5 can be set

to a fixed value of 1 k

Ω, offering the possibility of

using C7 as a soft start capacitor: When starting up

the converter, the VIPer53 device delivers a

constant current of 0.6 mA on the COMP pin,

creating a constant voltage of 0.6 V in R5 and a

rising slope across C7. This voltage shape

together with the operating range of 0.5 V to 4.5 V

15V

VDD

OSC

DRAIN

SOURCE

COMP

TOVL

VIPer73

C10

AC IN

DC OUT

C12

C11

10nF

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