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Leakage inductance occurs within a transformer because the internal windings of wire are not perfectly lined up. The phenomenon occurs as a leakage in magnetic flux stores and releases energy, causing the flux to act as an inductor. This causes some voltage to be dropped between the primary and secondary currents. The leakage is usually too small to have much consequence, but in high-power transformers and in a Metal Oxide Field Effect Transistor (MOSFET) it can. Power distribution transformers can use leakage inductance as an advantage, and so can gas discharge lamps, such as those used in neon signs.
When leak inductance is present, a transformer winding shows signs of having self-inductance. Voltage is generated based on how fast the current changes, so any additional inductance has an effect on the power output of a transformer. It becomes difficult to regulate the power supply when the transformer’s core and windings aren’t designed correctly. This becomes more apparent as more electrical load is applied.
A MOSFET’s performance can be severely affected because it switches on and off very quickly. The leakage inductance creates a current that cannot dissipate between cycles. Current flows even when the circuit is in an off state, which can affect applications that require the MOSFET to be on or off at certain times, or which state it is really in can be misread. A relay, on the other hand, can build up the voltage if it is not shut down. Damage to a resistor or switch contact can result if voltages become high enough.
Many times, leakage inductance is used as a design advantage. Some transformers are built to limit current flows this way, without integrating a sophisticated and expensive power dissipation system. It is also critical for gas discharge lamps. In neon signs, the current must be limited so the transformer will still be usable if it shorts out, and the lamp won’t be damaged from high currents. Current in transformers for arc welding systems can be controlled as well, for which variable leakage inductance is a desirable characteristic.
Leakage inductance is calculated mathematically using the capacitance, coupling coefficient, and other electrical wire properties. Graphical measurements allow for it to be visualized by showing the difference in timing between input and output signals. Instant changes in voltage are not possible over a conductor. The result is that increases in leakage inductance will cause larger delays in electrical signal timing.