LNK362-364

B

11/05

5

TheLinkSwitch-XTiscompletelyself-poweredfromtheDRAIN

pin, requiring only a small ceramic capacitor C3 connected to

the BYPASS pin. No auxiliary winding on the transformer is

required.

Key Application Considerations

LinkSwitch-XT

Design Considerations

Output Power Table

Thedatasheetmaximumoutputpowertable(Table1)represents

the maximum practical continuous output power level that can

be obtained under the following assumed conditions:

1. TheminimumDCinputvoltageis90Vorhigherfor85VAC

input, or 240 V or higher for 230 VAC input or 115 VAC

with a voltage doubler. The value of the input capacitance

should be large enough to meet these criteria for AC input

designs.

2. Secondary output of 6 V with a fast PN rectifier diode.

3. Assumed efficiency of 70%.

4. Voltage only output (no secondary-side constant current

circuit).

5. Discontinuous mode operation (K

6. A primary clamp (RCD or Zener) is used.

7. The part is board mounted with SOURCE pins soldered

to a sufficient area of copper to keep the SOURCE pin

temperature at or below 100 °C.

8. Ambient temperature of 50 °C for open frame designs

and an internal enclosure temperature of 60 °C for adapter

designs.

Below a value of 1, K

current.Above a value of 1, K

OFF time to the secondary diode conduction time. Due to

the flux density requirements described below, typically a

LinkSwitch-XT

design will be discontinuous, which also has

the benefits of allowing lower cost fast (instead of ultra-fast)

output diodes and reducing EMI.

Clampless

Designs

Clampless

designs rely solely on the drain node capacitance

to limit the leakage inductance induced peak drain-to-source

voltage. Therefore, the maximum AC input line voltage, the

value of V

leakage inductance and peak primary current, and the primary

winding capacitance determine the peak drain voltage. With no

significant dissipative element present, as is the case with an

external clamp, the longer duration of the leakage inductance

ringing can increase EMI.

The following requirements are recommended for a universal

input or 230 VAC only Clampless design:

1. A Clampless design should only be used for P

2. For designs where P

used to ensure adequate primary intra-winding capacitance

in the range of 25 pF to 50 pF.

3. For designs where 2 < P

added to the transformer using a standard recovery rectifier

diode to act as a clamp. This bias winding may also be used

to externally power the device by connecting a resistor from

the bias-winding capacitor to the BYPASS pin.This inhibits

the internal high voltage current source, reducing device

dissipation and no-load consumption.

4. For designs where P

practical and an external RCD or Zener clamp should be

used.

5. Ensurethatworst-casehighline,peakdrainvoltageisbelow

the BV

≤650 V to allow margin for design variation.

†For 110 VAC only input designs it may be possible to extend

the power range of Clampless designs to include the LNK363.

However, the increased leakage ringing may degrade EMI

performance.

**V

drop that is reflected to the primary via the turns ratio of the

transformer during the diode conduction time. The V

to the DC bus voltage and the leakage spike to determine the

peak drain voltage.

Audible Noise

The cycle skipping mode of operation used in LinkSwitch-XT

can generate audio frequency components in the transformer.

To limit this audible noise generation, the transformer should

be designed such that the peak core flux density is below

1500 Gauss (150 mT). Following this guideline and using the

standard transformer production technique of dip varnishing

practically eliminates audible noise. Vacuum impregnation

of the transformer should not be used due to the high primary

capacitanceandincreasedlossesthatresult.Higherfluxdensities

are possible, however careful evaluation of the audible noise

performance should be made using production transformer

samples before approving the design.

Ceramic capacitors that use dielectrics, such as Z5U, when

used in clamp circuits may also generate audio noise. If this is

the case, try replacing them with a capacitor having a different

dielectric or construction, for example a film type.