Question: Where do I connect the grounds of switching regulators?

Answer: How should you proceed with a switching regulator with an analog ground (AGND) and a power ground (PGND)? This is a question asked by many developers designing a switching power supply. Some developers are accustomed to dealing with a digital GND and an analog GND; however, their experience frequently fails them when it comes to the power GND. Designers then often copy the board layout for a selected switching regulator and stop thinking about the problem.

PGND is the ground connection over which higher pulsed currents flow. Depending on the switching regulator topology, this means the currents through a power transistor or the pulsed currents of a power driver stage. This is especially relevant in the case of switching controllers, for example, with external power switches.

AGND, sometimes called SGND (signal ground), is the ground connection that the other, usually very calm, signals use as a reference. This includes the internal voltage reference needed for the regulation of the output voltage. Soft start and enable voltages are also referenced to the AGND connection.

There are two different technical philosophies, and thus different opinions among experts regarding the handling of these two ground connections.

According to one philosophy, the AGND and PGND connections on a switching regulator IC should be joined to each other right next to the respective pins. This keeps the voltage offset between the two pins relatively low. Thus, the switching regulator IC can be protected from disturbances and even destruction.

The other philosophy involves additional separation of AGND and PGND on the board into two separate ground planes connected to each other at one point. Through this connection, interfering signals (voltage offset) remain largely in the PGND region, while the voltage in the AGND region remains very calm and decoupled very well from PGND. However, the disadvantage is that, depending on the transients in the pulsed currents and the current intensities, there may be a significant voltage offset between PGND and AGND at the respective pins. This can lead to improper functioning of, or even damage to, a switching regulator IC.

The grounding question comes down to a trade-off between strong separation with the advantage of separating noise and disturbances and running the risk of generating voltage offsets between the two grounds, and thus causing harm to Silicon and compromising functionality.

Conclusion

The answer to the question of how to deal with the grounds AGND and PGND is not that simple. That’s why this discussion continues.

Engineers use the word “ground” in every electronic circuit to denote some part of a system or structure that is “neutral,” or zero potential. Unfortunately, we often think of circuits and systems, especially those with both analog and digital signals, as having more than one ground. This concept gave rise to a recent discussion on a signal-integrity online community

The kinds of grounds mentioned in this discussion include:

Logic ground
Analog ground
Chassis ground
Safety ground
Earth ground

Methods proposed for connecting these various “grounds” cover a broad range of options including:

Connecting them at only one point.
Cutting the ground plane under a mixed signal component
Connecting them with capacitors.
Segmenting the ground plane in a PCB such that there is only a narrow connection at one place between the analog and digital sides of the design.
Separating the analog and digital grounds.

Figure 1. Digital logic ground symbol seen in most schematic diagrams.

These seemingly conflicting methods for dealing with ground can be a bit confusing.

Digital logic ground is the “reference” terminal of a power supply for your digital logic. For most digital logic systems, it’s the negative terminal of the logic power supply

Analog ground is the reference terminal of the supply that powers an analog circuit.

Chassis ground is the name given to the connection of the safety wire from the AC mains to a product’s case. It gets this name because the case of a product is often called the chassis. This wire is usually the green wire in an extension cord

Sometimes, EMI engineers erroneously refer to this “Chassis ground” (Figure 3) as a place that has some function in the containment of EMI. This statement never has or never will be based on fact because it has no role in this part of an electronic design.

Most power quality problems are due to incorrect connections of an electrical system. Using CTs and leakage current monitors, you can check an electrical system during acceptance testing of the installation and also during maintenance and renovation of the system. Watch out for partial or complete short circuits between neutrals and your grounding network. They’ll often create power quality disturbances

Most power quality problems are due to incorrect connections of an electrical system. Using CTs and leakage current monitors, you can check an electrical system during acceptance testing of the installation and also during maintenance and renovation of the system.

Yes, there’s a certain amount of normal leakage current going from the neutral and the phase conductors to ground in all electrical systems. Usually, the level of this leakage current is from about 10mA to some 100mA, depending on the size of electrical system.

Voltage differences are between different grounding points.

Because of contact between the neutral and ground at Point A, the return current flowing from the load also flows through the grounding network and the grounding circuits in Devices D1 and D2.

This current may disrupt their operation because the voltage difference and resulting current are often quite high.

So, how does ground noise enter a sensitive electronic device? Current flowing in a neutral conductor consists of many kinds of disturbances such as harmonic waves and distortion, high frequency disturbances, transients, etc.

As you can see, a filter or surge suppressor circuit does not prevent disturbances from entering the device. Often sensitive electronic devices are connected to a reference ground. The problem above results from the devices’ grounding conductor connected to the conduit (which is also serving as a ground) rather than to a dedicated reference grounding system tied to the main grounding point.

Sometimes the shield of the communication or data cable connects to conductive parts of the building.

Watch out for magnetic field interference. In a normal electrical system, ground current produces a low frequency (60 Hz) magnetic field. At this frequency, a clean electrical system has a magnetic field between 0.1 mGaus to 45 mGaus. In an industrial working environment the magnetic flux density can be much higher, anywherefrom 20 mGaus to 15 Gaus. Normal sources of stray magnetic fields are transformers, large motors, and various industrial production machines. However, the major significant cause for magnetic stray fields is often a faulty grounding connection or an equipment failure in an electrical system.

Relatively small magnetic fields cause disturbances to sensitive electronic devices. For example, fields higher than 13 mGaus can disturb a computer monitor.

In the U.S., definitive standards on electric magnetic fields aren’t yet established. However, in Europe, the Cenelec (European Committee for Electrotechnical Standardization) EMC Standard EN-50082-1 gives limit values for residential, commercial, and light industry environments. The maximum value of magnetic flux density is 38 mGaus for industrial situations and 13 mGaus for computer monitoring locations. In a facility’s electrical system, you may see very high peaks of magnetic flux density during starting current impulse. These peaks can directly effect the circuits of sensitive devices.

How do you monitor ground leakage current to avoid problems? To prevent electronic noise from disturbing electronic devices, you can use different kinds of noise attenuation circuits, such as filters, isolation transformers, photo-couplers, etc. But one of the most important aspects in preventing electronic noise from disturbing electronic devices is to evaluate the wiring and grounding systems first. This can often be expensive and time consuming. The simplest and most economical method is to continuously monitor leakage current of the whole electrical system or the most critical parts of electrical system.