Rev. I 01/09

11

TNY274-280

www.powerint.com

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.

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 1 μA.

Note that typical values for surface insulation resistance (SIR)

where no-clean flux has been applied according to the

suppliers’ guidelines are >>10 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:

Maximum drain voltage – Verify that V

650 V at highest input voltage and peak (overload) output

power. The 50 V margin to the 700 V BV

gives margin for design variation.

1.

Figure 15.

Recommended Circuit Board Layout for TinySwitch-III with Undervoltage Lock Out Resistor.

TOP VIEW

PI-4368-042506

Opto-

coupler

+

-

HV

+

-

DC

OUT

Input Filter Capacitor

Output

Rectifier

Safety Spacing

T

r

a

n

s

f

o

r

m

e

r

PRI

SEC

BIAS

D

Output Filter

Capacitor

Maximize hatched copper

areas (

) for optimum

heatsinking

BP/M

EN/UV

Y1-

Capacitor

S

S

S

S

PRI

C

BP

BIAS

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 I

end of the t

Under all conditions, the maximum drain

current should be below the specified absolute maximum

ratings.

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 R

specified in the data sheet. Under low line, maximum power,

a maximum TinySwitch-III SOURCE pin temperature of

110 °C is recommended to allow for these variations.

Design Tools

Up-to-date information on design tools is available at the Power

Integrations website: www.powerint.com.

2.

3.