4

LNK520

E

2/05

LNK520

LinkSwitch

85-265

VAC

RTN

C3

C4

C2

C1

C

S

D

R3

R2

R5

R4

R1

D2

D1

VR1

D3

PI-3703-030404

U1

T1

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.

Optional Secondary Feedback

Figure 6 shows a typical power supply schematic outline using

LinkSwitch

withoptocouplerfeedbacktoimproveoutputvoltage

regulation. On the primary side, the schematic only differs

from Figure 5 by the addition of optocoupler U1 transistor in

parallel to R1.

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

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.

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