VIPer53 - E Secondary feedback configuration example
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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
SOURCECOMPTOVL
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
