Datasheet LT1249 (Linear Technology) - 10

LT1249
U U W U APPLICATIONS INFORMATION
voltages above 8V and the 12V limit is only for surge resistor, the standard LT1249 application will not be conditions. affected because the chip is not yet powered. Problems are only created if the V In normal operation, the voltage into M CC pin is powered from some external OUT does not housekeeping supply that remains powered when bridge exceed 1.1V, but under surge conditions, the voltage power is switched off. could temporarily go higher. To date, no field failures due to surges have been reported for normal LT1249 configu- A huge line voltage surge, beyond the normal worst-case rations, but if the possibility exists for extremely large limits, can also create a large current surge. The peak of current surges, please read the following discussion. the line voltage must significantly exceed the storage capacitor voltage (typically 380V) for this to occur, so peak Offline switching power supplies can create large current line voltage would probably have to exceed 450V. Such surges because of the high value storage capacitor used. excessive surges might occur if a very large mains load The surge can be the result of closing the line switch near was suddenly removed, with a resulting line “kickback”. If the peak of the AC line voltage, or because of a large the surge results in voltage at the M transient in the line itself. These surges are well known in OUT pin greater than 6V, it must also last more than 30µs (three switch cycles) the power supply business, and are normally controlled to cause FET problems. with a negative temperature coefficient thermistor in series with the rectifier bridge. When power is switched
External Clamp
on, the thermistor is cold (high resistance) and surges are limited. Current flow in the thermistor causes it to heat and The external clamp shown in Figure 6 will protect the resistance drops to the point where overall efficiency loss LT1249 MOUT pin against extremely large line current in the resistor is acceptable. surges (see above). Protection is provided for all VCC power methods. The 100Ω resistor and three diodes limit This basic protection mechanism can be partially defeated the peak negative voltage into M if the power supply is switched off for a few seconds, then OUT to less than 3V. Current sense gain is attenuated by only 100Ω/4000Ω = turned back on. The thermistor has not had time to cool significantly and if the subsequent turn-on catches the AC 2.5%. Three diodes are used because the peak negative line near its peak, the resulting surge is much higher than voltage into MOUT in normal operation could go as high as normal. Even if this surge current generates a voltage –1.1V and the diodes should not conduct more than a few greater than 6V (but less than 12V) across the sense microamps under this condition. THERMISTOR + + BRIDGE STORAGE SURGE PATH CAPACITOR RS – 100Ω MOUT LT1249
Figure 6. Protecting MOUT from Extremely High Current Surges
10