VIPER53 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'VIPER53' PDF : 24 Pages View PDF
Figure 15: Off Line Power Supply With Optocoupler Feedback
VIPer53DIP / VIPer53SP
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
DC OUT
C10
R8
U2
C12
R7
U3
R6
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 kΩ, 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
13/24
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