LNK520
D1
T1
V
OUT
C3
D3
R3
C4
VR1
RTN
85-265
VAC
R4
R2
C2
D2
R5
LinkSwitch
R1
LNK520
D
U1
C
S
C1
PI-3703-030404
Figure 6. Power Supply Schematic Outline with Optocoupler Feedback, Providing Tight CV Regulation.
The characteristics described above provide an approximate
CV/CCpowersupplyoutputwithouttheneedforsecondaryside
voltage or current feedback. The output voltage regulation is
influencedbyhowwellthevoltageacrossC2tracksthereflected
output voltage. This tracking is influenced by the coupling
between transformer output and bias windings. Tight coupling
improvesCVregulationandrequiresonlyalowvalueforresistor
R2.PoorcouplingdegradesCVregulationandrequiresahigher
value for R2 to filter leakage inductance spikes on the bias
winding voltage waveform. 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 even tighter load regulation is required, an optocoupler
configuration can be used while still employing the constant
output current characteristics provided by LinkSwitch.
for U1 current and VR1 slope resistance should be consulted
to determine whether R5 is required.
When the power supply operates in the constant current (CC)
region, for example at start up and when charging a battery,
the output voltage is below the voltage feedback threshold
defined by U1 and VR1 and the optocoupler is fully off. In this
region, the circuit behaves exactly as previously described with
reference to Figure 5 where the voltage across C2 and therefore
thecurrentflowingthroughR1increaseswithincreasingoutput
voltage and the LinkSwitch internal current limit is adjusted to
provide an approximate CC output characteristic.
When the output reaches the voltage feedback threshold set by
U1 and VR1, the optocoupler turns on. Any further increase
in the power supply output voltage results in the U1 transistor
current increasing. The resulting increase in the LinkSwitch
CONTROL current reduces the duty cycle according to
Figure 4 and therefore, maintains the output voltage
regulation.
Optional Secondary Feedback
Figure 6 shows a typical power supply schematic outline using
LinkSwitchwithoptocouplerfeedbacktoimproveoutputvoltage
regulation. On the primary side, the schematic only differs
from Figure 5 by the addition of optocoupler U1 transistor in
parallel to R1.
Figure 7 shows the influence of optocoupler feedback on the
output characteristic. The envelope defined by the dashed lines
represent the worst-case power supply DC output voltage and
currenttolerances(unit-to-unitandovertheinputvoltagerange)
if an optocoupler is not used. A typical example of an inherent
(without optocoupler) output characteristic is shown dotted.
This is the characteristic that would result if U1, R4, R5 and
VR1 were removed. The optocoupler feedback results in the
characteristicshownbythesolidline.Theloadvariationarrowin
Figure7representsthelocusoftheoutputcharacteristicnormally
seen during a battery charging cycle. The two characteristics
are identical as the output voltage rises but then separate as
shown when the voltage feedback threshold is reached. This
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 more accurate references may be employed for
improved output voltage tolerancing and to provide cable drop
compensation,ifrequired.ResistorR4providesbiasingforVR1.
The regulated output voltage is equal to the sum of the VR1
Zener voltage plus the forward voltage drop of the U1 LED.
Resistor R5 is an optional low value resistor to limit U1 LED
peakcurrentduetooutputripple.Manufacturerʼsspecifications
E
2/05
4
POWERINT [ Power Integrations ]
