Hi,

I am new to design boards connected to the AC line. As in the attached picture, I have two boards:

1. A POWER BOARD with a flyback converter and some triacs;
2. A MCU BOARD with a microcontroller and some analog and digital input stages.

I am not sure how to connect PE avoiding grounding loops.
I have connected to PE: the Chassis, the output ground of the flyback converter and the reference of the several inputs (analog and digital).
I did this last connection in order to avoid propagation of the incoming noise from the inputs on the GND of the MCU board.
Is it correct this approach? Or is it better to reference the output ground of the flyback converter?

Many thanks in advance,
Riccardo

 

There are various 0V potential points that are labeled as GROUND or GND.

CHASSIS is protective ground. This should be a connection to any metal case, box or enclosure and connected to the mains EARTH. There should be no current flowing in this connection.

GND or COMMON should be signal and power ground. Ideally, this should be a star connection to the power supply 0V reference point.

Digital GND and analog GND should be kept separate but still referenced back to the PSU 0V reference.

If you are mixing a POWER BOARD with a flyback converter and some triacs and a MCU BOARD with a microcontroller and some analog and digital input stages then you should anticipate a worse case scenario with possibility of interference.

I would strongly recommend that you use opto-isolators in your interface.

 

Hi MrChips,
thank you for your reply, yes in the interface I already placed opto-isolators.
My doubt is if it is better to reference the secondary side of the isolate flyback converter to the main EARTH (so also the input reference) or to leave it isolated.
I left in the PCB some 0R to try different configurations and I will try in the LAB which is the best performance.

 

I would tie the converter to ground with a 100kΩ resistor.

 

When you talking ground you are referring to the secondary side ground (GND) or the main EARTH ? Just to don't misunderstand the ground term.

 

When you talking ground you are referring to the secondary side ground (GND) or the main EARTH ? Just to don't misunderstand the ground term.

Sorry, I should have said EARTH.

 

When you talking ground you are referring to the secondary side ground (GND) or the main EARTH ? Just to don't misunderstand the ground term.

It's a N.A. thing.
Everything is GROUND, regardless!
Even the use of the same (Earth) symbol for all various instances.