# PCB return currents at low frequency and High Frequency

#### hoyyoth

Joined Mar 21, 2020
74
Dear Team,
I am new to pcb design.I the PCB learning material I found that for

Low Frequency Return Currents Take Path of Least Resistance and High Frequency Return Currents Take Path of Least Inductance
--Could you please tell me why and how it is happening ,

Regards
HARI

#### crutschow

Joined Mar 14, 2008
27,243
All traces or wires have a small inductance.
The inductive reactance goes up with frequency.
At low frequencies this reactance it less than the trace resistance so the current follows the least resistive path.
When the frequency gets high enough that the trace inductive reactance exceeds the resistance than the current will follow the least inductive path.

#### hoyyoth

Joined Mar 21, 2020
74
All traces or wires have a small inductance.
The inductive reactance goes up with frequency.
At low frequencies this reactance it less than the trace resistance so the current follows the least resistive path.
When the frequency gets high enough that the trace inductive reactance exceeds the resistance than the current will follow the least inductive path.
Thank you.
In a PCB how will we identify least resistance path and least inductive path.
Let us take a two layer PCB with L1-Signal and L2 GND.
When the frequencies become high frequency

#### andrewmm

Joined Feb 25, 2011
1,496
Thank you.
In a PCB how will we identify least resistance path and least inductive path.
Let us take a two layer PCB with L1-Signal and L2 GND.
When the frequencies become high frequency
HI @hoyyoth
The easy answer is two fold

a) Its possible to do a full field 3D simulation that will give you within a few percent accuracy the numbers,
BUT , the tools are horrendous expensive, need a fair machine to run, and a fair skill set ( garbage in garbage out )

b) most of the time, you will learn by experience / your mentors as to when to worry

this might help