Author Topic: Height above ground plane in 4-layer SMPS?  (Read 263 times)

0 Members and 1 Guest are viewing this topic.

Offline incfTopic starter

  • Regular Contributor
  • *
  • Posts: 109
  • Country: us
  • ASCII > UTF8
Height above ground plane in 4-layer SMPS?
« on: January 24, 2025, 12:21:31 am »
I'm laying out a 4-layer PCB for a SMPS. Due to component density the layers in a particular area area are as follows:

  • Layer 1: High power traces switched at ~100kHz + Components and heavily filtered DC signals
    [~0.2mm FR4]
  • Layer 2: Ground
    [~1.2mm FR4]
  • Layer 3: High power traces switched at ~100kHz
    [~0.2mm FR4]
  • Layer 4: Components and heavily filtered DC signals

I've got a question about the height of a PCB trace above a ground plane.
I've got high power traces switched at 100kHz on layers 1 and 3. While I've got a ground plane on layer 2.
The distance of layer 1 to the ground plane on layer 2 is 0.2mm.
The distance of layer 3 to the ground plane on layer 2 is 1.2mm.

Does this difference in height make for a large difference in radiated EMI? Magnetic / capacitive coupling to nearby traces? etc.

Thanks!
« Last Edit: January 24, 2025, 12:26:28 am by incf »
 

Offline CosteC

  • Regular Contributor
  • *
  • Posts: 209
  • Country: pl
Re: Height above ground plane in 4-layer SMPS?
« Reply #1 on: January 24, 2025, 07:33:16 am »
The thinner insulator the higher capacitance. This helps with shielding but also increases common mode currents.
Opposite is also true.

Personally, I would prefer to have lower capacitance from switching nodes to GND in most of places, to avoid parasitic capacitances. This however is layout thing - those nodes shall be geometrically small for many reasons and layerstack may be of secondary importance.

In many practical designs there are holes in GND planes around some nodes to reduce parasitic capacitances.

There is also matter of safety if, it is mains powered SMPS - too thin layer of PCBA insulator may be not sufficient.
 

Offline mtwieg

  • Frequent Contributor
  • **
  • Posts: 409
  • Country: us
Re: Height above ground plane in 4-layer SMPS?
« Reply #2 on: January 24, 2025, 01:31:22 pm »
I would check whether trace capacitance actually contributes significantly to overall capacitance. For the switching node of a power supply I would usually expect the capacitance of semiconductors to dominate.

And also consider that if you are relying on your PCB and power planes to cool the power components, thicker dielectric will make that worse.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf