Ground plane
As a ground plane ( English ground plane ) is in the electronics , especially in the printed circuit board design , one with the ground potential connected large copper area on printed circuit boards , respectively.
In addition to ground areas, other larger areas with non- ground- related potential are also used on printed circuit boards . These can be used, for example, to supply power to modules on the circuit board and thus have supply voltage potential.
motivation
Noise reduction
Each line (conductor track) on a circuit board has a certain impedance . If current flows through one of these lines , it inevitably causes a voltage drop . In most cases, a circuit is closed via a ground line , which also causes a voltage drop at this connection. The ground connection therefore has a different potential at each point. Since integrated circuits on the printed circuit board, such as measuring amplifiers or analog-to-digital converters , relate the voltage of their inputs or outputs to the ground potential, these signals are thus falsified because the reference potential - the ground line - is not the same for every circuit . In order to counteract this, the ground line can be designed as a surface.
Common ground
The simplest implementation of a ground plane for one-sided printed circuit boards is to fill free space (no conductor tracks) with ground-connected copper surfaces. Every component that is to be connected to the ground is contacted with this surface. Since the ground plane is “cut into” by the other signal and supply services, the advantage of a low-resistance and low- inductance ground connection is rather limited.
In the case of printed circuit boards with several layers , one layer can be reserved as a ground layer on which a ground plane is located. All components of the circuit board can thus fall back on a solid and low-resistance ground connection. Potential differences between the ground points can thus be significantly reduced.
Shared mass
In some cases it can make sense not to use a common ground plane but to use several separate ground planes and to bring them together at a certain point. If different components ( analog , digital and power components ) are used on a printed circuit board , the mass can be divided between components that belong together. If, for example, three different ground planes ( analog ground , digital ground and supply ground ) are used, potential differences will still occur along the ground due to higher currents, but components that require high currents no longer affect the entire ground, but only their own. If a switching regulator on the circuit board causes disturbances on the supply ground due to high high- frequency currents, these are not noticeable on the separate analog ground. The analog part of the circuit board thus has an interference-free reference potential with the separate analog ground . The same applies to the digital part, which can not be disturbed by the switching regulator due to its own digital ground and cannot disturb the analog part itself.
Of course, all ground planes must be connected to one another at one point. This is preferably done at the voltage supply terminal of the circuit board. Each ground plane is thus directly connected to the absolute reference potential (ground of the supply terminal) of the circuit board. However, depending on the application, the ground planes can also be brought together at other points on the board. A kind of tree structure of the mass can thus also form.
Electromagnetic compatibility
Each current-carrying conductor generates a magnetic field that radiates from the circuit board and affects electromagnetic compatibility . In order to reduce this magnetic field, ground planes can be used in which an alternating magnetic field induces eddy currents and thus dissipates energy . Especially in the case of coils on a circuit board, which usually always have a leakage flux , an underlying ground plane has a noticeable shielding effect. However, this effect has significant disadvantages in switching regulators , since here the energy-transmitting component (coil) is withdrawn by the occurrence of eddy currents and thus the efficiency is reduced. For this reason, ground planes under coils are usually exposed.
Electric fields on circuit boards are also shielded by ground planes.
Impedance controlled printed circuit boards
For high-frequency applications or very high-clocked digital circuits, it is necessary that a conductor track has a certain wave impedance in order to be able to match components to one another. In order to implement impedance-controlled conductor paths, signal lines on circuit boards are routed with several layers between and next to two ground planes at a certain distance. Similar to a coaxial cable , the signal line is surrounded by a ground screen and has a certain wave impedance.
cooling
In order to minimize the heat development of SMD components ( transistors ), the cooling pads of the components are soldered on copper surfaces (if possible on ground planes) . The copper surface on the circuit board acts as a heat sink and releases the heat to the environment. Even wired components such as diodes , require copper pads on the printed circuit board, since they can release the heat developed in any other way to a sufficient extent.
capacitor
In the case of printed circuit boards with several layers, two superimposed layers can be designed as copper surfaces. If one of these copper surfaces is connected to the ground potential and the second to the supply potential, the result is a plate capacitor distributed over the entire surface of the printed circuit board , which is equivalent to a backup capacitor . However, this cannot replace actual backup capacitors, at most complement them.
Mechanical stability
In order to increase the mechanical stability of mechanical SMD components such as switches or sockets , the housing of these can be soldered to a ground plane (if electrically possible). This increases the mechanical strength of the component, since a large copper surface is less prone to detachment from the circuit board than a single pad.
PCB manufacturing
If a printed circuit board is manufactured by means of an etching process, those areas where no copper should be present are removed by the etching agent . If a ground plane is used on the circuit board, only the points between the conductor tracks have to be removed, which means that the etchant is significantly less saturated. Especially when manufacturing printed circuit boards yourself, it is advisable to use ground planes to protect the etchant.
literature
- Joachim Franz: EMV - Failure-free construction of electronic circuits 3rd edition, Vieweg + Teubner Verlag / GWV Fachverlage GmbH, Wiesbaden 2008, ISBN 978-3-8351-0236-1 .
- Adolf J. Schwab, Wolfgang Kürner: Electromagnetic Compatibility 5th Edition, Springer, Berlin / Heidelberg / New York 2007, ISBN 978-3-540-42004-0 .