tny274 Power Integrations, Inc., tny274 Datasheet - Page 11

• Manufacturer Part Number: tny274
• Description: Energy Ef?cient, Of?ine Switcher With Enhanced Flexibility And Extended Power Range
• Manufacturer: Power Integrations, Inc.

Primary Clamp Circuit
A clamp is used to limit peak voltage on the DRAIN pin at turn
off. This can be achieved by using an RCD clamp or a Zener
(~200 V) and diode clamp across the primary winding. In all
cases, to minimize EMI, care should be taken to minimize the
circuit path from the clamp components to the transformer and
TinySwitch-III.
Thermal Considerations
The four SOURCE pins are internally connected to the IC lead
frame and provide the main path to remove heat from the device.
Therefore all the SOURCE pins should be connected to a copper
area underneath the TinySwitch-III to act not only as a single
point ground, but also as a heatsink. As this area is connected
to the quiet source node, this area should be maximized for
good heatsinking. Similarly for axial output diodes, maximize
the PCB area connected to the cathode.
Y-Capacitor
The placement of the Y-capacitor should be directly from the
primary input filter capacitor positive terminal to the common/
return terminal of the transformer secondary. Such a placement
will route high magnitude common mode surge currents away
from the TinySwitch-III device. Note – if an input π (C, L, C)
EMI filter is used then the inductor in the filter should be placed
between the negative terminals of the input filter capacitors.
Figure 15. Recommended Circuit Board Layout for TinySwitch-III with Undervoltage Lock Out Resistor.
TOP VIEW
HV
+
-
Input Filter Capacitor
S
S
S
S
EN/UV
BP/M
D
C
BP
BIAS
PRI
BIAS
PRI
Optocoupler
Place the optocoupler physically close to the TinySwitch-III
to minimizing the primary-side trace lengths. Keep the high
current, high voltage drain and clamp traces away from the
optocoupler to prevent noise pick up.
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. In addition, sufficient copper
area should be provided at the anode and cathode terminals
of the diode for heatsinking. A larger area is preferred at the
quiet cathode terminal. A large anode area can increase high
frequency radiated EMI.
PC Board Leakage Currents
TinySwitch-III is designed to optimize energy efficiency across
the power range and particularly in standby/no-load conditions.
Current consumption has therefore been minimized to achieve
this performance. The EN/UV pin undervoltage feature for
example has a low threshold (~1 µA) to detect whether an
undervoltage resistor is present.
Parasitic leakage currents into the EN/UV pin are normally
well below this 1 µA threshold when PC board assembly is in
a well controlled production facility. However, high humidity
conditions together with board and/or package contamination,
Safety Spacing
Capacitor
coupler
Opto-
Y1-
m
T
n
o
a
s
e
r
f
r
r
SEC
Maximize hatched copper
areas (
heatsinking
Rectifier
Output
-
OUT
DC
TNY274-280
+
Output Filter
Capacitor
) for optimum
PI-4368-042506
6/06
G
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