TNY254 Power Integrations Inc., TNY254 Datasheet - Page 9

• Manufacturer Part Number: TNY254
• Description: TinySwitch Family Energy Efficient/ Low Power Off-line Switchers
• Manufacturer: Power Integrations Inc.

accommodated by rating the output diode to handle the short
circuit current. The short circuit current can be minimized by
choosing the smallest (lowest current limit) TinySwitch for the
required power.
Layout
Single Point Grounding
Use a single point ground connection at the SOURCE pin for the
BYPASS pin capacitor and the Input Filter Capacitor (see
Figure 13).
Primary Loop Area
The area of the primary loop that connects the input filter
capacitor, transformer primary and TinySwitch together, should
be kept as small as possible.
Primary Clamp Circuit
A clamp or snubber circuit is used to minimize peak voltage and
ringing on the DRAIN pin at turn-off. This can be achieved by
using an RC snubber for less than 3 W or an RCD clamp as
shown in Figure 13 for higher power. A Zener and diode clamp
across the primary or a single 550V Zener clamp from DRAIN
to SOURCE can also be used. In all cases care should be taken
to minimize the circuit path from the snubber/clamp components
to the transformer and TinySwitch.
Thermal Considerations
Copper underneath the TinySwitch acts not only as a single point
ground, but also as a heatsink. The hatched area shown in
Figure13 should be maximized for good heat-sinking of
TinySwitch and output diode.
Figure 13. Recommended PC Layout for the TinySwitch.
TOP VIEW
HV
+
–
Input Filter Capacitor
C
BP
BP
TinySwitch
S
S
Transformer
EN
D
PRI
Safety Spacing
Capacitor
coupler
Opto-
Y1-
SEC
Y-Capacitor
The placement of the Y-capacitor should be directly from the
primary single point ground to the common/return terminal on
the secondary side. Such placement will maximize the EMI
benefit of the Y-capacitor.
Optocoupler
It is important to maintain the minimum circuit path from the
optocoupler transistor to the TinySwitch ENABLE and SOURCE
pins to minimize noise coupling.
Output Diode
For best performance, the area of the loop connecting the
secondary winding, the Output Diode and the Output Filter
Capacitor, should be minimized. See Figure13 for optimized
layout. In addition, sufficient copper area should be provided
at the anode and cathode terminals of the diode to adequately
heatsink the diode under output short circuit conditions.
Input and Output Filter Capacitors
There are constrictions in the traces connected to the input and
output filter capacitors. These constrictions are present for two
reasons. The first is to force all the high frequency currents to
flow through the capacitor (if the trace were wide then it could
flow around the capacitor). Secondly, the constrictions minimize
the heat transferred from the TinySwitch to the input filter
capacitor and from the secondary diode to the output filter
capacitor. The common/return (the negative output terminal in
Figure13) terminal of the output filter capacitor should be
connected with a short, low resistance path to the secondary
winding. In addition, the common/return output connection
should be taken directly from the secondary winding pin and
not from the Y-capacitor connection point.
Output Filter Capacitor
–
Out
DC
+
Maximize hatched copper
areas (
heat sinking
TNY253/254/255
) for optimum
PI-2176-071398
7/01
C
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