Hi,
I tried to minimize the drain current path, the high current path. Bulk cap, primary winding, mosfet drain to source and shunt resistor and gnd.
You know: No signal goes one way, it always needs a return path. And if the return path is long and high impedance the whole signal will be slow and ringing.
--> Don´t see only the signal from A to B, but also the (HF) path back from B to A.
I am confused about that if the gate current path is short, leakage inductance will be less. And if the Cds and leakage inductance are less, it will be less oscilation on gate signal.
The phrase "leakage" inductance often is used with transformers. I didn´t use this phrase. So I´m not sure whether you talk about the inductance on the PCB or the transformer.
In either case it should be low.
--> here a Ground plane brings a benefit, because the returning current shares the opposite side of the original current path. The current is reversed, the magnetic field is reversed and thus the magnetic field compensates... resulting in way less overall inductance. (and EMI)
Also, most of source says that high current path have to be short. Which approach is more important? Because it can would not have possible everytime to do both of them.
True, the power path is important. But rining often happens because both signals (power and gate) share the same (return) path: from source via shunt to bulk capacitor.
When the controller wants to switch ON, then it rises the gate drive signal (with respect to the driver IC GND)..Then the mosfet becomes conductive, current begins to flow .... through shunt to bulk capacitor .... any inductance in this path will lift the voltage on the MOSFET´s source ... reducing V_GS (since the driver IC GND - and thus the gate_drive_voltage - is not lifted the same ammount than the source voltage). .. --> this is the feedback with it´s phase shift, that enables ringing. (the driver IC wants to increase V_GS where the source path inductance tries to lower V_GS).
This (hopefully) answers "which approach is more important?" --> both are important.
But since the power path carries higher current it will cause higher voltage drop on the common path. But don´t underestimate the gate drive current. It may be several amperes, depending on circuit..
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You refer to (...most of source says...) documents about SMPS. If they are modern ones from reliable sources like MOSFET manufacturers, SMPS controller manufacturers, universities.. don´t they all recommend a solid GND_plane, low impedance shunt resistors, return paths, suitable voltage reference points...? In my eyes they should...
Klaus