LNK362-364
The
LinkSwitch-XT
is completely self-powered from the DRAIN
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 P
O
≤
2.5 W,
using the LNK362
†
and a V
OR
**
≤
90 V.
2. For designs where P
O
≤
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.
3. For designs where 2
<
P
O
≤
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
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
O
>
2.5 W
Clampless
designs are not
practical and an external RCD or Zener clamp should be
used.
5. Ensure that worst-case high line, peak drain voltage is below
the BV
DSS
specification of the internal MOSFET and 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.
**V
OR
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 V
OR
adds
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
fl
ux 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
capacitance and increased losses that result. Higher
fl
ux densities
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
fi
lm type.
Key Application Considerations
LinkSwitch-XT
Design Considerations
Output Power Table
The data sheet maximum output power table (Table 1) represents
the maximum practical continuous output power level that can
be obtained under the following assumed conditions:
1. The minimum DC input voltage is 90 V or higher for 85 VAC
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
P
>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.
Below a value of 1, K
P
is the ratio of ripple to peak primary
current. Above a value of 1, K
P
is the ratio of primary MOSFET
OFF time to the secondary diode conduction time. Due to
the
fl
ux 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
OR
, 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.
C
12/05
5
POWERINT [ POWER INTEGRATIONS, INC. ]
