Useful Information | Why does the switching power supply need to be grounded?
Latest update time：2019-09-16
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In power supply design, safety is often the first priority, and this is also true for switching power supplies. Grounding can protect the personal safety of users and ensure the normal operation of power equipment. So what is the appropriate grounding method in a switching power supply?
What are the common grounding symbols?
This article will introduce the grounding issues in switching power supplies.
What is the definition of grounding?
In modern grounding concepts, for line engineers, the term usually means "the reference point for the line voltage."
For system designers, it is often the cabinet or rack. For
electrical engineers, it means the green safety ground wire or connection to the earth.
A more common definition is "ground is a low-impedance path for current to return to its source."
Note the requirements are "low impedance" and "pathway."
What are the common grounding symbols?
PE, PGND, FG - protective ground or casing;
BGND or DC-RETURN - DC -48V (+24V) power supply (battery) return;
GND - working ground;
DGND - digital ground;
AGND - analog ground;
LGND - lightning protection ground.
What is the appropriate grounding method?
There are many ways to ground, including single-point grounding, multi-point grounding and mixed types of grounding.
Single-point grounding is divided into series single-point grounding and parallel single-point grounding.
Generally speaking, single-point grounding is used for simple circuits, grounding distinction between different functional modules, and low-frequency (f10MHz) circuits require multi-point grounding or multi-layer boards (complete ground plane layers).
Why does the switching power supply need to be grounded?
The introduction of grounding technology was originally a protective measure to prevent electrical or electronic equipment from being struck by lightning. The purpose was to introduce the lightning current generated by lightning into the earth through the lightning rod, thereby protecting the building.
At the same time, grounding is also an effective means to protect personal safety. When the phase line caused by some reason (such as poor insulation of the wire, aging of the line, etc.) touches the equipment shell, a dangerous voltage will be generated on the equipment shell, and the fault current generated will flow through the PE line to the earth, thereby playing a protective role.
With the development of electronic communications and other digital fields, only considering lightning protection and safety in the grounding system is far from meeting the requirements.
For example, signals between devices need a "ground" as the reference ground for the signal.
And as electronic equipment becomes more complex, signal frequencies are getting higher and higher. Therefore, in grounding design, electromagnetic compatibility issues such as mutual interference between signals must be given special attention. Otherwise, improper grounding will seriously affect the reliability and stability of system operation.
Recently, the concept of "ground" has also been introduced in the signal return technology of high-speed signals.
Why should we separate analog ground and digital ground, and how to separate them?
Both analog and digital signals need to return to the ground, because digital signals change quickly, which will cause a lot of noise on the digital ground, and analog signals need a clean ground reference to work.
If the analog and digital grounds are mixed together, the noise will affect the analog signal.
Generally speaking, analog ground and digital ground should be processed separately, and then connected together through thin traces, or connected at a single point.
The general idea is to try to block the noise on the digital ground from reaching the analog ground.
Of course, this is not a very strict requirement that the analog ground and digital ground must be separated. If the digital ground near the analog part is still very clean, they can be combined.
How are the signals on the board grounded?
In general, it is best to use nearby grounding when designing, and after a complete multi-layer board design, it is very easy to ground ordinary signals. The basic principle is to ensure the continuity of the routing, reduce the number of vias, and be close to the ground plane or power plane, etc.
How are the interface devices of a single board grounded?
Some boards have external input and output interfaces, such as serial port connectors, network port RJ45 connectors, etc. If their grounding is not done well, it will affect normal operation, such as bit errors and packet loss in network port interconnection, and will become an external electromagnetic interference source, sending the noise inside the board to the outside.
Generally speaking, an independent interface ground will be separated out, and the connection with the signal ground is connected by a thin trace, and a 0 ohm or small resistance can be connected in series. The
thin trace can be used to block the noise on the signal ground from passing to the interface ground.
Similarly, the filtering of the interface ground and interface power supply should also be carefully considered.
How to ground the shield in a shielded cable?
The shielding layer of the shielded cable should be connected to the interface ground of the single board, rather than the signal ground. This is because there are various noises on the signal ground. If the shielding layer is connected to the signal ground, the noise voltage will drive the common-mode current to interfere outward along the shielding layer. Therefore, improperly designed cables are generally the largest noise output source of electromagnetic interference.
Modern people's lives are increasingly inseparable from electronic devices, and electronic devices must rely on stable and safe power supplies.
Switching power supply technology has penetrated into various electronic equipment fields. I hope everyone can find the problems they are currently confused about in this article and get the answers smoothly.
Source: Network compilation
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