LNK562-564
The following requirements are recommended for a universal
input or 230 VAC only Clampless design:
Key Application Considerations
Output Power Table
Thedatasheetmaximumoutputpowertable(Table1)represents
the maximum practical continuous output power level that can
be obtained under the following assumed conditions:
1. Clampless designs should only be used for PO ≤ 2.5Wusing
a VOR of ≤90 V
2. For designs with PO ≤ 2 W, a two-layer primary must be
used to ensure adequate primary intra-winding capacitance
in the range of 25 pF to 50 pF.
3. Fordesignswith2<PO ≤2.5W,abiaswindingmustbeadded
to the transformer using a standard recovery rectifier diode
(1N4003–1N4007)toactasaclamp.Thisbiaswindingmay
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 with PO > 2.5 W, Clampless designs are not
practical and an external RCD or Zener clamp should be
used.
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 Schottky rectifier diode.
3. Assumed efficiency of 70%.
4. Voltage only output (no secondary-side constant current
circuit).
5. Discontinuous mode operation (KP > 1).
6. Asuitably sized core to allow a practical transformer design
(see Table 2).
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.
5. Ensurethatworst-case,highline,peakdrainvoltageisbelow
the BVDSS specification of the internal MOSFETand ideally
≤ 650 V to allow margin for design variation.
8. Ambient temperature of 50 °C for open frame designs
and an internal enclosure temperature of 60 °C for adapter
designs.
VOR (Reflected Output Voltage), is the secondary output plus
output diode forward voltage 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.
Lin k S w it c h -LP Device
Core Size
EE13
LNK562
1.1 W
1.3 W
1.9 W
LNK563
1.4 W
1.7 W
2.5 W
LNK564
1.7 W
2 W
Audible Noise
The cycle skipping mode of operation used in LinkSwitch-LP
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 is not recommended, as it does not provide
any better reduction of audible noise than dip varnishing. And
although vacuum impregnation has the benefit of increased
transformer capacitance (which helps in Clampless designs),
it can also upset the mechanical design of the transformer,
especially if shield windings are used. Higher flux densities are
possible, increasing the power capability of the transformers
above what is shown in Table 2. However careful evaluation of
theaudiblenoiseperformanceshouldbemadeusingproduction
transformer samples before approving the design.
EE16
EE19
3 W
Table 2. Estimate of Transformer Power Capability vs.
LinkSwitch-LP Device and Core Size at a Flux Density of
1500 Gauss (150 mT).
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-LP design will be discontinuous, which also has
the benefit of allowing lower-cost fast (vs. 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 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.
Bias Winding Feedback
To give the best output regulation in bias winding designs, a
slow diode such as the 1N400x series should be used as the
F
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