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I am starting to update PCB layout to next version because EMC test result failed. As result of test, the designed Buck IC layout seems have the improvement room and noise is from 2.1MHz harmonic.

The designed Buck IC: LM53625LQRNLTQ1(2.1MHz

Test requirement:CISPR25 level 3

Test Voltage: 24V

Current layout: CE_C Pass, RE fail

Layer 1

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Layer2

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Layer3

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Layer4

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Layer5

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Layer6

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The information from co-worker, they used to pour copper under Buck IC and inductor form layer1 to layer6 and RE has a lot of improvement but CE_C became very bad

I would like to do a little bit modification based on current layout

  1. Remove copper under C32 and R82 from layer1 to layer6

  2. Pour copper under buck IC like layer 2 at layer3

my Question as below:

  1. Why to remove GND copper of layer3 at current layout? do anyone have a kind of experience for this

  2. I read some of buck Layout recommendation for reducing EMI

https://electronics.stackexchange.com/questions/503308/buck-converter-pcb-planes>

https://e2e.ti.com/support/power-management/f/196/t/538740?-DC-DC-Converter-Ground-plane-cut-below-power-inductor>

but there is no article to be discussed with how to pour copper under switching node and Buck IC.

Ethan Ku
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    The first thing to check is where it is radiating from. Is it via the input? Output? Or as part of the interconnect? Unless you determine this, you can keep on spinning boards and going around in circles. – Kartman Mar 03 '21 at 08:14
  • A 2MHz regulator is finnicky for both working and emissions. First of all are you actually sure it is stable? I once lost weeks for an emission issue when it was the control loop coupling noise badly… also your layout seems quite different from the recommended one, I didn't study it a lot but the capacitor loop area seems quite big – Lorenzo Marcantonio Mar 03 '21 at 08:47
  • @Lorenzo Marcantonio yes i will placement input C and output C at next version , it is will as possible as short to Buck IC – Ethan Ku Mar 05 '21 at 08:13
  • @Kartman thank you for your reminding . EMI(2MHz) has been confirmed from buck operation frequency, we use other Buck IC with spread function . this issue has a improvement . due to cost will be up . we try to re-layout and updated current PCB , therefore ,i would like to know if anyone can suggest how to pour copper under switching node and IC – Ethan Ku Mar 05 '21 at 08:13

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