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In electronics, leakage is the gradual transfer of electrical energy across a boundary normally viewed as insulating, such as the spontaneous discharge of a charged capacitor, magnetic coupling of a transformer with other components, or flow of current across a transistor in the "off" state or a reverse-polarized diode.
In capacitors
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Gradual loss of energy from a charged capacitor is primarily caused by electronic devices attached to the capacitors, such as transistors or diodes, which conduct a small amount of current even when they are turned off. Even though this off current is an order of magnitude less than the current through the device when it is on, the current still slowly discharges the capacitor. Another contributor to leakage from a capacitor is from the undesired imperfection of some dielectric materials used in capacitors, also known as dielectric leakage. It is a result of the dielectric material not being a perfect insulator and having some non-zero conductivity, allowing a leakage current to flow, slowly discharging the capacitor.[1]
Another type of leakage occurs when current leaks out of the intended circuit, instead flowing through some alternate path. This sort of leakage is undesirable because the current flowing through the alternate path can cause damage, fires, RF noise, or electrocution.[2] Leakage of this type can be measured by observing that the current flow at some point in the circuit does not match the flow at another. Leakage in a high-voltage system can be fatal to a human in contact with the leak, as when a person accidentally grounds a high-voltage power line.[3]
Between electronic assemblies and circuits
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Leakage may also mean an unwanted transfer of energy from one circuit to another. For example, magnetic lines of flux will not be entirely confined within the core of a power transformer; another circuit may couple to the transformer and receive some leaked energy at the frequency of the electric mains, which will cause audible hum in an audio application.[4]
Leakage current is also any current that flows when the ideal current is zero. Such is the case in electronic assemblies when they are in standby, disabled, or "sleep" mode (standby power). These devices can draw one or two microamperes while in their quiescent state compared to hundreds or thousands of milliamperes while in full operation. These leakage currents are becoming a significant factor to portable device manufacturers because of their undesirable effect on battery run time for the consumer.[5]
When mains filters are used in the power circuits supplying an electrical or electronic assembly, e.g., a variable frequency drive or an AC/DC power converter, leakage currents will flow through the "Y" capacitors that are connected between the live and neutral conductors to the earthing or grounding conductor. The current that flows through these capacitors is due to the capacitors' impedance at power line frequencies.[6][7] Some amount of leakage current is generally considered acceptable, however excessive leakage current, exceeding 30 mA, can create a hazard for users of the equipment. In some applications, e.g. medical devices with patient contact, the acceptable amount of leakage current can be quite low, less than 10 mA.
In semiconductors
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In semiconductor devices, leakage is a quantum phenomenon where mobile charge carriers (electrons or holes) tunnel through an insulating region. Leakage increases exponentially as the thickness of the insulating region decreases. Tunneling leakage can also occur across semiconductor junctions between heavily doped P-type and N-type semiconductors. Other than tunneling via the gate insulator or junctions, carriers can also leak between source and drain terminals of a Metal Oxide Semiconductor (MOS) transistor. This is called subthreshold conduction. The primary source of leakage occurs inside transistors, but electrons can also leak between interconnects. Leakage increases power consumption and if sufficiently large can cause complete circuit failure.
Leakage is currently one of the main factors limiting increased computer processor performance. Efforts to minimize leakage include the use of strained silicon, high-κ dielectrics, and/or stronger dopant levels in the semiconductor. Leakage reduction to continue Moore's law will not only require new material solutions but also proper system design.
Certain types of semiconductor manufacturing defects exhibit themselves as increased leakage. Thus measuring leakage, or Iddq testing, is a quick, inexpensive method for finding defective chips.
Increased leakage is a common failure mode resulting from non-catastrophic overstress of a semiconductor device, when the junction or the gate oxide suffers permanent damage not sufficient to cause a catastrophic failure. Overstressing the gate oxide can lead to stress-induced leakage current.
In bipolar junction transistors, the emitter current is the sum of the collector and base currents. Ie = Ic + Ib. The collector current has two components: minority carriers and majority carriers. The minority current is called the leakage current[clarification needed].
In heterostructure field-effect transistors (HFETs) the gate leakage is usually attributed to the high density of traps residing within the barrier. The gate leakage of GaN HFETs has been so far observed to remain at higher levels compared with the other counterparts such as GaAs.[8]
Leakage current is generally measured in microamperes. For a reverse-biased diode it is temperature sensitive. Leakage current must be carefully examined for applications that work in wide temperature ranges to know the diode characteristics.
See also
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References
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What are leakage currents in electrical installations?
Leakage current is the intensity that circulates through the ground protection conductor in an electrical installation. In the absence of the ground protection conductor, it is the current that could flow from any conductive part or the surface of the non-conductive parts to ground if the circuit were closed through an available conductive path (such as the human body).
How do leakage currents occur?
There are two types of leakage current: AC leakage and DC leakage:
Leakage caused by DC resistance is usually negligible compared to the AC impedance of several parallel capacitances. Capacitance can be intentional (as in electromagnetic interference filter capacitors) or unintentional. Examples of unintentional capacitances include spacings on printed wiring plates, insulation between semiconductors and ground heat sinks, and primary to secondary capacitance of insulation transformers within the power supply.
Where do we find current leaks regularly?
In the day to day we will find that its most usual origin is mainly in the insulation at the electrical level, being the isolation normally high in terms of resistance, this can be affected by damage or by the result of aging. Due to a lower resistance, certain leakage currents can flow, significantly in addition to those longer conductors have a higher capacitance, causing more leakage current.
Another common cause of the appearance of leakage currents is the increase of those electronic equipment that incorporate certain filters of surges or against electrical disturbances, which incorporate certain currents to the installation due to the capacitors at the input adding more capacitance to the entire wiring system and current leaks globally. Due to the increase and its cumulative effect of leakage currents of each of these electronic devices in our homes, it can occur that a leakage current can reach the order of milliamps. This factor in an electrical installation protected by a differential switch (DDR) could consequently trigger the protection at certain times.
Why is it important to monitor leakage currents?
In an electrical installation it usually includes a grounding system to provide protection against a danger of contact with parts in tension if there is a lack of insulation. The grounding system usually consists of a grounding conductor that joins the equipment to the protective (ground) conductor. If there is a catastrophic failure of the insulation between the voltage line (supply) and the accessible conductive parts, the voltage is drifted to the ground. The resulting current flow will cause a DDR differential switch to jump, and can prevent a risk of contact. Obviously, there is a possible risk of contact if the grounding is interrupted, either intentionally or accidentally. The risk of contact may be greater than assumed due to leakage currents. Even if there is no insulation failure, interrupting the leakage currents flowing through the ground conductor could pose a shock hazard to someone touching the unsailed and grounded equipment (or other grounded equipment) at the same time. This possibility is of much more concern in medical applications, where a patient may be the recipient of the shock. A fatal shock could result if the patient is in a weakened or unconscious condition, or if the output current is applied to the internal organs through patient contacts. The double insulation provided in non-grounded equipment provides protection by using two separate layers of insulation. Protection in this case is guaranteed because both layers of insulation are unlikely to fail. However, conditions that produce leakage currents are still present, and must be considered.
How to eliminate or minimize the effects of leakage currents?
The first step in eliminating leakage currents is to quantify the origin of these currents using a leakage current detection clamp and to identify the origin of these currents.
What is a leak detection clamp and what is it for?
This element has a similar appearance to an amperimetric clamp however it differs from this in that it is able to accurately measure small currents of the order of 5mA, as in the case of the MEGGER DCM305E capable of detecting up to a minimum resolution of 0.001mA in the range of 6mA, necessary to determine the existence of leakage currents. This quality distinguishes it from the leak current clamp of amperimetric tweezers since the latter are not able to record such small currents.
Use of leak tweezers?
Active conductors: The leak clamp is placed over the grip around the active conductors (phase/s + neutral). The charge currents of the conductors induce certain magnetic fields that cancel each other out, however, any imbalance or difference in current is a consequence of the leaks that occur by the conductors to the ground or other alternative paths. To measure this current, a leak detection clamp with the capacity to measure currents of less than 0.1 mA will be necessary
Ground protection conductor: The protective conductor passes through the clamp of the amperimetric clamp. In this way, any current that passes through this conductor is recorded and registered.
Bibliography:
Megger DCM305E datasheet
User Manual Leak current detection clamp Megger DCM305E
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