What is FG (frame ground)?


Switching power supply has an FG (frame ground) terminal. The symbol is also different for FG and ground.
I think that many people think that it has the same meaning as earth, so I will explain it in an easy-to-understand manner.



What is frame ground (FG)?

Ground (GND) is the voltage reference of the circuit. It means a reference point when considering the voltage and current in various places in the power supply circuit. Since the current flowing through the circuit (loop) plays a leading role in the operation of the circuit, the voltage reference at each point can be used anywhere. However, the stable place is the “metal chassis” part, which is generally the ground. So the FG terminal is connected to the metal chassis of the power supply.

Also, since the frame ground (FG) is not necessarily connected to the earth’s earth, it may not always have the same voltage as the earth. If you touch a device that uses a gland, current can flow through your body between the frame gland and the ground. To prevent this electric shock accident, grounding is performed and the frame voltage is adjusted to the ground. With a switching power supply, input feedback noise, radiation noise, and output noise will increase unless the frame is grounded.

So what about the power supply for a single board without a chassis?
Power supplies without a chassis (frame) do not have an FG terminal. Instead, there is a ground terminal (ground terminal), which is connected to the metal ring in the mounting hole of the board so that either one can be grounded.




Grounding is a connection to the ground. Ground (GND) can also be mistakenly used to mean a connection to the ground. The potential that is the reference for circuit operation is ground, and the connection to the ground is ground.