Parasitic element (electrical networks)
Encyclopedia
In electrical network
s, a parasitic element is a circuit element
(resistance
, inductance
or capacitance
) that is possessed by an electrical component but which it is not desirable for it to have for its intended purpose. For instance, a resistor
is designed to possess resistance, but will also possess unwanted parasitic capacitance
.
Parasitic elements are unavoidable. All conductors possess resistance and inductance and the principles of duality ensure that where there is inductance, there will also be capacitance. Component designers will strive to minimise parasitic elements but are unable to completely eliminate them.
The most commonly seen manifestations of parasitic elements in components is in the parasitic inductance and resistance of the component leads and the parasitic capacitance of the component packaging. For wound components such as inductor
s and transformer
s, there is additionally the important parasitic effect of capacitance that exists between the individual turns of the windings. This winding parasitic capacitance will cause the inductor to act as a resonant circuit at some frequency, at which point (and all frequencies above) the component is useless as an inductor.
Parasitic elements are often modelled as lumped components
in equivalent circuits but this is not always adequate. For instance, the inter-winding capacitance mentioned above is really a distributed element
along the whole length of the winding and not a capacitor
in one particular place. Designers sometimes take advantage of parasitic effects to achieve a function in a component, see for instance helical resonator
.
Non-linear parasitic elements can also arise. The term is commonly used to describe parasitic structure
s formed on an integrated circuit
whereby an unwanted semiconductor device is formed from p-n junction
s which belong to two or more intended devices or functions.
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
s, a parasitic element is a circuit element
Electrical element
Electrical elements are conceptual abstractions representing idealized electrical components, such as resistors, capacitors, and inductors, used in the analysis of electrical networks...
(resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
, inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
or capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
) that is possessed by an electrical component but which it is not desirable for it to have for its intended purpose. For instance, a resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
is designed to possess resistance, but will also possess unwanted parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
.
Parasitic elements are unavoidable. All conductors possess resistance and inductance and the principles of duality ensure that where there is inductance, there will also be capacitance. Component designers will strive to minimise parasitic elements but are unable to completely eliminate them.
The most commonly seen manifestations of parasitic elements in components is in the parasitic inductance and resistance of the component leads and the parasitic capacitance of the component packaging. For wound components such as inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, there is additionally the important parasitic effect of capacitance that exists between the individual turns of the windings. This winding parasitic capacitance will cause the inductor to act as a resonant circuit at some frequency, at which point (and all frequencies above) the component is useless as an inductor.
Parasitic elements are often modelled as lumped components
Lumped element model
The lumped element model simplifies the description of the behaviour of spatially distributed physical systems into a topology consisting of discrete entities that approximate the behaviour of the distributed system under certain assumptions...
in equivalent circuits but this is not always adequate. For instance, the inter-winding capacitance mentioned above is really a distributed element
Distributed element model
In electrical engineering, the distributed element model or transmission line model of electrical circuits assumes that the attributes of the circuit are distributed continuously throughout the material of the circuit...
along the whole length of the winding and not a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
in one particular place. Designers sometimes take advantage of parasitic effects to achieve a function in a component, see for instance helical resonator
Helical resonator
A helical resonator is a passive electrical component that can be used as a filter. Physically, a helical resonator is a wire helix surrounded by a square or cylindrical conductive shield. Like cavity resonators, helical resonators can achieve Q factors in the 1000s...
.
Non-linear parasitic elements can also arise. The term is commonly used to describe parasitic structure
Parasitic structure
In a semiconductor device, a parasitic structure is a portion of the device that resembles in structure some other, simpler semiconductor device, and causes the device to enter an unintended mode of operation when subjected to conditions outside of its normal range...
s formed on an integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
whereby an unwanted semiconductor device is formed from p-n junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
s which belong to two or more intended devices or functions.