TNY274-280
Maximize hatched copper
Safety Spacing
areas (
) for optimum
heatsinking
Y1-
Capacitor
Output
Rectifier
+
Output Filter
Capacitor
HV
-
Input Filter Capacitor
PRI
T
r
a
n
s
f
o
r
m
e
r
SEC
BIAS
PRI
D
S
S
S
S
BP/M
BIAS
TOP VIEW
CBP
EN/UV
Opto-
coupler
DC
-
+
OUT
PI-4368-042506
Figure 15. Recommended Circuit Board Layout for TinySwitch-III with Undervoltage Lock Out Resistor.
the BP/M pin solder pad preventing the design from starting up.
Designs that make use of the undervoltage lockout feature by
connecting a resistor from the high voltage rail to the EN/UV pin
are not affected.
2. Maximum drain current – At maximum ambient temperature,
maximum input voltage and peak output (overload) power,
verify drain current waveforms for any signs of transformer
saturation and excessive leading edge current spikes at
startup. Repeat under steady state conditions and verify that
the leading edge current spike event is below ILIMIT(Min) at the
end of the tLEB(Min). Under all conditions, the maximum drain
current should be below the specified absolute maximum
ratings.
3. Thermal Check – At specified maximum output power,
minimum input voltage and maximum ambient temperature,
verify that the temperature specifications are not exceeded
for TinySwitch-III, transformer, output diode, and output
capacitors. Enough thermal margin should be allowed for
part-to-part variation of the RDS(ON) of TinySwitch-III as
specified in the data sheet. Under low line, maximum power,
a maximum TinySwitch-III SOURCE pin temperature of
ꢀꢀ0 °C is recommended to allow for these variations.
If the contamination levels in the PC board assembly facility are
unknown, the application is open frame or operates in a high
pollution degree environment and the design does not make
use of the undervoltage lockout feature, then an optional
390 kΩ resistor should be added from EN/UV pin to SOURCE
pin to ensure that the parasitic leakage current into the EN/UV
pin is well below ꢀ μA.
Note that typical values for surface insulation resistance (SIR)
where no-clean flux has been applied according to the
suppliers’ guidelines are >>ꢀ0 MΩ and do not cause this issue.
Quick Design Checklist
As with any power supply design, all TinySwitch-III designs
should be verified on the bench to make sure that component
specifications are not exceeded under worst case conditions.
The following minimum set of tests is strongly recommended:
Design Tools
Up-to-date information on design tools is available at the Power
Integrations website: www.powerint.com.
ꢀ. Maximum drain voltage – Verify that VDS does not exceed
610 V at highest input voltage and peak (overload) output
power. The 10 V margin to the 700 V BVDSS specification
gives margin for design variation.
11
www.powerint.com
Rev. I 01/09
POWERINT [ Power Integrations ]
