AND8099 Datasheet, PDF (4/8 Pages) ON Semiconductor – 5.0 V, 2.0 A Flyback Converter

AND8099/D
Feedback Loop
The feedback loop is composed of an optocoupler, a shunt
regulator, a compensation capacitor and a resistor divider.
The optocoupler isolates the AC input from the DC output.
As a shunt regulator, the ON Semiconductor TL431 is used
to regulate the output voltage. This monolithic voltage
reference is programmable from V_ref to 36 V using two
external resistors. It exhibits a wide current range of 1.0 mA
to 100 mA and is an excellent replacement for Zener diodes.
The reference voltage of the TL431 is set at 2.5 V, for a 5.0 V
output voltage, by a resistor divider R5 and R6 (low
tolerance, 2.0 kW resistors). The TL431 monitors the 5.0 V
output voltage and compares the divided down voltage to its
2.5 V internal reference. A small increase in the output
voltage will cause the shunt regulator to start conducting,
thus sinking current through the optocoupler’s LED. In turn,
the optocoupler transistor becomes forward biased and starts
driving current into the control input pin of the NCP1055.
The power switch duty cycle is then adjusted accordingly. A
compensation capacitor C10 of 0.1 mF is placed between the
cathode and the reference pin of the TL431 for improved
stability. The resistor R3 limits the current going through the
optocoupler to a safe level and prevents damage to the
optocoupler.
F1
2.0 A
L1
C1
0.1
10 mH
5.0 V, 2.0 A
D1
D6
L2
T1
C3
220 p
D2
R1
91 k
D3
C4
47 p
+ _C2
D4
33
D5 _R2
2.2 k
5.0 mH
+ C7 + C8 + C9
330 330 330 ^R3
47
R6
2.0 k
R4
1.0 k
IC2
C10
0.1
+ _C11
330
+ _C12
1.0
C6
100 p
IC3
R5
2.0 k
NCP1055P100
C13
X
+ 1n
C5
10
Figure 1. Circuit Diagram
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