10

LT1249

voltages above 8V and the 12V limit is only for surge

conditions.

exceed 1.1V, but under surge conditions, the voltage

could temporarily go higher. To date, no field failures due

to surges have been reported for normal LT1249 configu-

rations, but if the possibility exists for extremely large

current surges, please read the following discussion.

Offline switching power supplies can create large current

surges because of the high value storage capacitor used.

The surge can be the result of closing the line switch near

the peak of the AC line voltage, or because of a large

transient in the line itself. These surges are well known in

the power supply business, and are normally controlled

with a negative temperature coefficient thermistor in

series with the rectifier bridge. When power is switched

on, the thermistor is cold (high resistance) and surges are

limited. Current flow in the thermistor causes it to heat and

resistance drops to the point where overall efficiency loss

in the resistor is acceptable.

This basic protection mechanism can be partially defeated

if the power supply is switched off for a few seconds, then

turned back on. The thermistor has not had time to cool

significantly and if the subsequent turn-on catches the AC

line near its peak, the resulting surge is much higher than

normal. Even if this surge current generates a voltage

greater than 6V (but less than 12V) across the sense

resistor, the standard LT1249 application will not be

affected because the chip is not yet powered. Problems are

housekeeping supply that remains powered when bridge

power is switched off.

A huge line voltage surge,

beyond the normal worst-case

limits, can also create a large current surge. The peak of

the line voltage must significantly exceed the storage

capacitor voltage (typically 380V) for this to occur, so peak

line voltage would probably have to exceed 450V. Such

excessive surges might occur if a very large mains load

was suddenly removed, with a resulting line “kickback”. If

6V, it must also last more than 30

µs (three switch cycles)

to cause FET problems.

External Clamp

The external clamp shown in Figure 6 will protect the

power methods. The 100

Ω resistor and three diodes limit

Current sense gain is attenuated by only 100

Ω/4000Ω =

2.5%. Three diodes are used because the peak negative

–1.1V and the diodes should not conduct more than a few

microamps under this condition.

THERMISTOR

100

Ω

SURGE PATH

STORAGE

CAPACITOR

LT1249

+

–

BRIDGE

+