pollution degree environment, then an optional 390 kΩ resistor
should be added from EN pin to SOURCE pin to ensure that
the parasitic leakage current into the EN pin is low.
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-LT 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:
1. Maximum drain voltage – Verify that the worst case VDS
does not exceed 650 V at highest input voltage and peak
(overload) output power.
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 t . LEB(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-LT, transformer, output diode, and output
capacitors. Enough thermal margin should be allowed for
part-to-part variation of the RDS(ON) of TinySwitch-LT as
specified in the data sheet. Under low line, maximum power,
a maximum TinySwitch-LT SOURCE pin temperature of
110 °C is recommended to allow for these variations.
TNY174-180
11 A
08/07