LNK500
Figure 5 by the addition of R3, C3 and optocoupler U1. R3
forms a potential divider with R1 to limit the U1 collector
emitter voltage.
The characteristics described above provide an approximate
CV/CC power supply output without the need for secondary
side voltage or current feedback. The output voltage regulation
is influenced by how well the voltage across C2 tracks the
reflected output voltage. This tracking is influenced by the
value of the transformer leakage inductance which introduces
an error. Resistor R2 and capacitor C2 partially filter the
leakage inductance voltage spike reducing this error. This
circuitry, used with standard transformer construction
techniques provides much better output load regulation than a
linear transformer, making this an ideal power supply solution
in many low power applications. If tighter load regulation is
required, an optocoupler configuration can be used while still
employing the constant output current characteristics provided
by LinkSwitch.
On the secondary side, the addition of voltage sense circuit
componentsR4,VR1andU1LEDprovidethevoltagefeedback
signal. In the example shown, a simple Zener (VR1) reference
is used though a precision TL431 reference is typically needed
to provide 5% output voltage tolerancing and cable drop
compensation, if required. R4 provides biasing for VR1. The
regulated output voltage is equal to the sum of the VR1 Zener
voltage plus the forward voltage drop of the U1 LED. R5 is an
optional low value resistor to limit U1 LED peak current due to
output ripple. Manufacturer’s specifications for U1 current and
VR1sloperesistanceshouldbeconsultedtodeterminewhether
R5 is required.
Optional Secondary Feedback
U1 is arranged with collector connected to primary ground and
emitter to the anode of D1. This connection keeps the opto in an
electrically “quiet” position in the circuit. If the opto was
Figure 6 shows a typical power supply outline schematic using
LinkSwitchwithoptocouplerfeedbacktoimproveoutputvoltage
regulation. On the primary side, the schematic differs from
LinkSwitch
D
S
V
OUT
LNK500
C
C1
R5
C2
R1
U1
R4
85-265
VAC
R2
D1
VR1
U1
R3
C3
RTN
PI-3418-111802
Figure 6. Power Supply Outline Schematic with Optocoupler Feedback.
Output Voltage
Inherent
CC to CV
transition
point
Tolerance envelope
without optocoupler
Typical inherent
characteristic without
optocoupler
Voltage
feedback
threshold
Characteristic with
optocoupler
Load variation
during battery
charging
Output Current
PI-2788-092101
Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic.
B
3/03
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POWERINT [ Power Integrations ]
