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To achieve this goal, the minimum voltage feedback threshold

should be set at V

CC to CV transition point of the inherent characteristic will

alwaysoccurbelowthevoltagefeedbackthreshold.However,the

outputvoltagetoleranceisthenincreased,sincetheinherentCV

characteristic tolerance below V

of the optocoupler feedback circuit.

The LNK520 can also be used in the high-side configuration as

shown in Figure 9(a). This configuration provides a very low

component count solution with an approximate CV/CC power

supply output characteristic. A typical output characteristic

envelope is shown in Figure 9(b).

This configuration is ideal for very low cost charger and adapter

applications where output CC tolerance is loose or unspecified.

Typical applications include low cost chargers and adapters

where direct replacement for a linear transformer is required.

In applications with a high voltage DC input voltage, the circuit

is further simplified with the removal of input rectifiers, EMI

filter choke and input capacitors. Typical applications of this

type include auxiliary supplies in domestic appliances and

industrial applications.

In the high-side configuration, the CONTROL pin receives

feedback current through R1 generated by the voltage across

C2. To a first order, this voltage is proportional to V

V

the off time of the LNK520 switching cycle. The output CV

regulationisthereforedeterminedbyhowwellthevoltageacross

C2 tracks the output voltage. This tracking is influenced by the

value of the transformer leakage inductance, which introduces

an error. This error, which is partially filtered by R2 and C2,

causes a slope in the output CV regulation characteristic.

The LNK520 is optimized for use with a bias winding where

tracking of feedback voltage and output voltage is typically

better than it is in the high-side configuration of Figure 9 (a).

As a consequence, the increased leakage error in the high-side

configuration causes the output current to increase with falling

output voltage, as indicated by the output CC characteristic

envelope in Figure 9 (b).

In this high-side configuration, the SOURCE pins and circuit

board traces form a switching node. Extra care should be taken

to optimize EMI performance. The LNK520 internal MOSFET

switching characteristics have been designed to significantly

reduce EMI, particularly in the radiated spectrum (>30 MHz).

However, the SOURCE trace area should be minimized and

EMI filter components should be distanced from the SOURCE

node whenever possible. In embedded applications where a

high voltage DC input voltage is available, system level EMI

filtering is typically located away from the power supply and

circuit board layout is less critical.

Applications Example

The circuit shown in Figure 10 shows a typical implementation

of an approximate constant voltage / constant current (CV/CC)

charger using LinkSwitch in the low-side configuration. This

design delivers 2.75 W with nominal peak power point voltage

of 5.5 V and a current of 500 mA (Figure 11). Efficiency is

greater than 65% over an input range of 85 VAC to 265 VAC.

Thebridgerectifier,D1-D4,rectifiestheACinput. Therectified

ACissmoothedbyC1andC2,withinductorL1formingapi-filter

to filter differential mode conducted EMI. Resistor RF1 is a

fusible,flameprooftypeprovidingprotectionfromprimary-side

shortcircuitsandlinesurgesandprovidesadditionaldifferential

EMI filtering. The switching frequency of 42 kHz allows such

a simple EMI filter to be used without the need for aYcapacitor

while still meeting international EMI standards.

When power is applied, high voltage DC appears at the DRAIN

pin of LinkSwitch (U1). The CONTROL pin capacitor C5 is

then charged through a switched high voltage current source

connected internally between the DRAIN and CONTROL

pins. When the CONTROL pin reaches approximately

5.6 V relative to the SOURCE pin, the internal current source

is turned off. The internal control circuitry is activated and the

high voltage MOSFET starts to switch, using the energy in C5

to power the IC.

Figure 9. High-side Configuration Using LNK520: (a) Schematic Outline; (b) Typical Output Characteristic Envelope.

Typical

Characteristic

D2

C4

(a)

(b)

LNK520

C2

R1

R2

C1

D1

LinkSwitch

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S

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