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PCB Tech
Regarding the return current path of the PCB circuit board
PCB Tech
Regarding the return current path of the PCB circuit board

Regarding the return current path of the PCB circuit board


To know that the return current of the PCB circuit board will use the plane closest to the signal trace. Then 2 is based on these two specific conditions

1) When we say 4 layers, layer 1 2 3 is the signal layer, and the continuous ground plane is on the 4th layer. For all 3 layer signals, the return current path will be on the 4th plane because there is no other plane.

2) Now for the second case, the 4-layer superposition is the signal ground plane signal. Suppose there are signal traces on layer 3, but I also injected copper into some vcc tracks on some areas (incomplete) of layer 3. Then, if I draw the signal trace on layer 4, the return current will be injected on the ground plane of layer 2 or part of the copper on layer 3, so that the trace will pass under the part of the copper casting and will not be on the part of the copper casting Next pass.

I hope I am clear.

It will be clearer if you post some photos. In addition: What is the PCB used for high-speed signals? If not, remember that the return current (actually any current) always follows the path of least impedance. -

For DC and low frequency filling (switching edges several hundred ns long, don't worry about too much RF magic), the ground plane will be the return path. After entering the VHF range, the return path is usually the path with the least impedance. If your VCC power plane has a lot of ceramic decouplings, then the return current will flow along the nearest laminar (ie either have a good capacitive coupling to the ground or the ground)

pcb board

The PCB capacitor has an effect on the return current because the return path is a huge road and the capacitor is located near the pin. The only thing I know about dec. cap. They have frequency. Therefore, by connecting some of them in parallel, we obtain low impedance, and they provide the more constant voltage level we want.

So in this case, since continuous grounding may have the lowest impedance, it will be our PCB circuit board return current path, because the partial copper pouring on any layer cannot have a lower impedance than a fully pouring ground layer-

 , But how to reach each plane (via) will have a greater impact on the impedance of copper dumping on the plane.

Ceramic decoupling capacitors are very close to short circuits for high-frequency signals, which is why they allow return current to pass between the layers at the higher end of the frequency range. DC / LF may pass through the ground plane, but if there are close decoupling capacitors, then multi-frequency things will want to keep as close as possible to the signal trace, so that HF can run closer to the signal trace along the VCC layer, and then That's it. If nothing is running at a few megabits of frequency and nothing produces a switching edge below 100ns, then I am not worried.

 But there is no actual contact. What I mean is that if we connect a trace from the source to the sink, for example from the MCU pin to the i2c sensor pin, then the electromagnetic field will accomplish this, and the current will return along the same path but on a plane. In fact, it seems to me that we are not physically connected to any place. I'm confused:) - 

 Therefore, it is actually important to summarize all these details such as the impedance ground plane layer stacking at high speed or high frequency. I don't need to worry too much at low frequencies. Circuits such as mcu sensors-

The current will separate all possible ways according to its impedance, so the shortest and more conductive way will get more current instead of being longer and less conductive. But you can consider the combination of each wire/via/connector/equal resistance and (usually small) capacitance and inductance. Then you have to solve the resulting network to get the current and voltage values everywhere. Usually this is not important, because there is a wide path, almost all currents are like this, and there is such a small impedance in the path, the voltage is almost zero anyway.

Why do you care about the return current? Are you concerned about radiated emissions or signal integrity? Or something else? In any case, you should always route the fastest edge signal to the location next to the continuous plane. If there are any breaks in the aircraft, please do not route the fast edge signal at the break of the PCB circuit board. This applies to both stacking options. Clocks and any signals with fast rise and fall times should be placed close to GND. If you put GND on the internal layer, you can place two tracking layers next to GND. Things to consider.