Precautions for EMI design of switching power supply circuit in layout process

Later on about PCB EMC design of will become more and more important .
Conducted emission (CE) And radiation emission (RE) yes EMC Two important items in the test , It is also a project that is difficult to rectify .
Usually PCB stay EMI The problems on the will focus on the switching power supply and clock , This paper mainly introduces the design of switching power supply , Relevant matters needing attention .
At present, there are no more than three topologies of switching power supply on the market ：buck,boost,buck-boost.
BUCK circuit analysis
topology （ Everyone is too familiar , To use , It's easy to explain ）

The red arrow in the figure indicates the path of current when the switch is disconnected , The blue arrow indicates the path of current when the switch is closed .
On the way , It's obvious that , There is current change at both ends of the diode at the moment when the switch is closed and opened , Switches are generally used mos, Its switching time is generally 10ns To 50ns, The wiring at both ends of the diode will have a high di/dt, The wiring itself will have parasitic inductance , According to the formula U=Lx(di/dt) You know , The wiring at both ends of the diode will produce a short rise time , Peak voltage with high amplitude . According to Fourier transform , The spike voltage contains a large number of high-frequency components , Will lead directly to EMI The test exceeds the standard
, Again , Switch tube , The wiring at both ends of the input capacitor will produce peak voltage with high amplitude .
So I want to rectify emi adopt , Peak voltage needs to be eliminated or reduced .
U=Lx(di/dt)
di Determined by the output current , Non modifiable ,dt from mos Management decision , Increasing the conduction time will increase mos power , Reduce dcdc Circuit conversion efficiency , It will burn seriously MOS tube .
Therefore, the parasitic inductance of the wiring can only be reduced , The cost is low , The effect is good .
Parasitic inductance of wiring L

l by PCB Routing length ,w by PCB Routing width . Reduce the routing length , Increasing the wiring width can significantly reduce the parasitic inductance of the wiring , Thus reducing the peak voltage .
So ask for , Input capacitance , Switch tube （ If it's external ）, The wiring at both ends of the three components of the diode shall be as short and wide as possible .

BOOST circuit analysis
topology

Similarly, the red arrow indicates the path of current when the switch is disconnected , The blue arrow indicates the path of current when the switch is closed .
It can be seen from the picture that , Switch tube , The wiring at both ends of diode and output capacitor should be as short and wide as possible

BUCK-BOOST circuit analysis
topology

Similarly, the red arrow indicates the path of current when the switch is disconnected , The blue arrow indicates the path of current when the switch is closed .
It can be seen from the picture that , Switch tube , diode , The wiring at both ends of input capacitance and output capacitance should be as short and wide as possible .