VIPer53 - E

Secondary feedback configuration example

17/36

6

Secondary feedback configuration example

When a more accurate output voltage is needed, the way is to monitor it directly secondary

side, and drive the PWM controller through an optocoupler as shown on Figure 17.

The optocoupler is connected in parallel with the compensation network on the COMP pin.

The design of the auxiliary winding that the VDD voltage is always lower than the internal

15V 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 0.6mA to

the optotransistor. This current does not depend on the compensation voltage, and so it

does not depend on the output load either. Consequently, 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.

Figure 19. Off line power supply with optocoupler feedback

15V

VDD

OSC

DRAIN

SOURCE

COMP

TOVL

U1

VIPer73

R5

R3

R4

D3

D1

C1

T1

C2

F1

R1

D2

C7

C5

C4

C6

T2

D4

C8

C10

L1

C9

R2

C3

AC IN

DC OUT

U2

U3

C12

R6

R7

R8

C11

10nF

R9

1k