VIPER53-E View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'VIPER53-E' PDF : 36 Pages View PDF
VIPer53 - E
Secondary feedback configuration example
6
Secondary feedback configuration example
Figure 19. Off line power supply with optocoupler feedback
F1
AC IN
C1
T1
R1
D1
C2
U1
VIPer73
R3
VDD
OSC
15V
C4
TOVL
R4
DRAIN
COMP SOURCE
C5
R9
1k
C11
10nF
R5
C6
C7
R2
C3
T2
D2
L1
D4
D3
C8
C9
C10
R8
U2
C12
R7
U3
R6
DC OUT
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.
17/36
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