LNK501

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.

Optional Secondary Feedback

Figure 6 shows a typical power supply outline schematic using

LinkSwitch

with optocoupler feedback to improve output

voltage regulation. On the primary side, the schematic differs

from Figure 5 by the addition of R3, C3 and optocoupler U1.

Resistor R3 forms a potential divider with R1 to limit the U1

collector emitter voltage.

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,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.

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

Output Voltage

Tolerance envelope

without optocoupler

Inherent

CC to CV

transition

point

Load variation

during battery

charging

Voltage

feedback

threshold

Characteristic with

optocoupler

Typical inherent

characteristic without

optocoupler

PI-2788-092101

Output Current

Figure 6. Power Supply Outline Schematic with Optocoupler Feedback.

Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic.

LNK501

LinkSwitch

85-265

VAC

RTN

S

D

C

C1

C3

C2

U1

R4

R5

VR1

R1

R2

D1

R3

PI-2787-112102

U1