LNK362-364
TheLinkSwitch-XTiscompletelyself-poweredfromtheDRAIN
pin, requiring only a small ceramic capacitor C3 connected to
the BYPASS pin. No auxiliary winding on the transformer is
required.
The following requirements are recommended for a universal
input or 230 VAC only Clampless design:
1. A Clampless design should only be used for PO ≤ 2.5 W,
using the LNK362† and a VOR** ≤ 90 V.
2. For designs where PO ≤ 2 W, a two-layer primary should be
used to ensure adequate primary intra-winding capacitance
in the range of 25 pF to 50 pF.
Key Application Considerations
LinkSwitch-XT Design Considerations
3. For designs where 2 < PO ≤ 2.5 W, a bias winding should be
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
thebias-windingcapacitortotheBYPASSpin.Thisinhibits
the internal high voltage current source, reducing device
dissipation and no-load consumption.
4. For designs where PO > 2.5 W Clampless designs are not
practical and an external RCD or Zener clamp should be
used.
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 (KP >1).
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.
5. Ensurethatworst-casehighline,peakdrainvoltageisbelow
the BVDSS specification of the internal MOSFETand ideally
≤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.
**VOR isthesecondaryoutputplusoutputdiodeforwardvoltage
drop that is reflected to the primary via the turns ratio of the
transformer during the diode conduction time. The VOR adds
to the DC bus voltage and the leakage spike to determine the
peak drain voltage.
Below a value of 1, KP is the ratio of ripple to peak primary
current.Above a value of 1, KP is the ratio of primary MOSFET
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.
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.
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 VOR, the leakage inductance energy, a function of
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.
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.
C
12/05
5
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