Protection
A resistor in series with the line can be used to limit the current charging input capacitors. However, this approach is not very efficient, especially in high power devices, since the resistor will have a voltage drop and dissipate some power.
Inrush current can also be reduced by inrush current limiters. Negative temperature coefficient (NTC) thermistors are commonly used in switching power supplies, motor drives and audio equipment to prevent damage caused by inrush current. A thermistor is a thermally-sensitive resistor with a resistance that changes significantly and predictably as a result of temperature changes. The resistance of an NTC thermistor decreases as its temperature increases .
As the inrush current limiter self-heats, the current begins to flow through it and warm it. Its resistance begins to drop and a relatively small current flow charges the input capacitors. After the capacitors in the power supply become charged, the self heated inrush current limiter offers little resistance in the circuit, with a low voltage drop with respect to the total voltage drop of the circuit. A disadvantage is that immediately after the device is switched off, the NTC resistor is still hot and has a low resistance. It cannot limit the inrush current unless it cools for more than 1 minute to get a higher resistance. Another disadvantage is that the NTC thermistor is not short circuit proof.
Another way to avoid the transformer inrush current is a "transformer switching relay". This does not need time for cool down. It can deal also with power line half-wave voltage-dips and is short circuit proof. This technique is Important for IEC 61000-4-11 tests.
Another option, particularly for high voltage circuits, is to use a pre-charge circuit. The circuit would support a current limited precharge mode during the charging of capacitors, and then switch to an unlimited mode for normal operation when the voltage on the load is 90% of full charge.
Read more about this topic: Inrush Current
Famous quotes containing the word protection:
“... actresses require protection in their art from blind abuse, from savage criticism. Their work is their religion, if they are seeking the best in their art, and to abuse that faith is to rob them, to dishonor them.”
—Nance O’Neil (1874–1965)
“Freedom of religion, freedom of the press, and freedom of person under the protection of habeas corpus, and trial by juries impartially selected. These principles form the bright constellation which has gone before us, and guided our steps through an age of revolution and reformation.”
—Thomas Jefferson (1743–1826)
“The protection of a ten-year-old girl from her father’s advances is a necessary condition of social order, but the protection of the father from temptation is a necessary condition of his continued social adjustment. The protections that are built up in the child against desire for the parent become the essential counterpart to the attitudes in the parent that protect the child.”
—Margaret Mead (1901–1978)