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I am trying to design a ultra low noise SMPS with 20 W output (85 V to 220 V) and need to make a choice between 4 layers PCB and 6 layers.

On the 4 layers we plan on having components on both sides and two ground layers. Is this a good idea on the high voltage (250 V DC) side? Should I use a component layer, ground layer, power layer, component instead?

On the 5V (secondary side) same question, should I use 2 ground layers and components (top and bottom)?

At last, 6 layers could be made better..1 components, 2 ground, 3 power, 4 hi-speed, 5 ground, 6 components.

What's your advice in order to achieve lowest noise?

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    \$\begingroup\$ I think the noise, especially common mode noise, will be dominated by non-PCB factors. \$\endgroup\$ – Spehro Pefhany Oct 25 '18 at 14:26
  • \$\begingroup\$ Input filters and output filters play an important role on both conducted and radiated noise. Also the transformer construction highly affects the noise performance. \$\endgroup\$ – Rohat Kılıç Oct 25 '18 at 14:34
  • \$\begingroup\$ Define ultra low noise. \$\endgroup\$ – winny Oct 25 '18 at 14:40
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    \$\begingroup\$ Pretty sure there are "ultra-low noise SMPS" with two layers. Which noise are you talking about, by the way? The conducted noise at the input? Radiated EMI noise? Conducted noise at the output? \$\endgroup\$ – dim Oct 25 '18 at 15:20
  • \$\begingroup\$ How slow are you willing to have the 200-volt nodes slew? That sets the Efield coupling to all the other nodes in the Switcher, on both sides of the transformer. \$\endgroup\$ – analogsystemsrf Oct 25 '18 at 17:51
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Henry Ott recommends a 6 layer stackup like this:

Most six-layer boards consist of four signal routing layers and two planes. From an EMC perspective a six-layer board is usually preferred over a four-layer board.

One stack-up NOT to use on a six-layer board is the one shown in Figure 5. The planes provide no shielding for the signal layers, and two of the signal layers (1 and 6) are not adjacent to a plane. The only time this arrangement works even moderately well is if all the high frequency signals are routed on layers 2 and 5 and only very low frequency signals, or better yet no signals at all (just mounting pads), are routed on layers 1 and 6. If used, any unused area on layers 1 and 6 should be provided with "ground fill" and tied into the primary ground plane, with vias, at as many locations as possible.

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Source: http://www.hottconsultants.com/techtips/pcb-stack-up-3.html

I think the goal with an SMPS would be to put the high frequency signals in the middle, you may not need to use 2 planes and may be able to use one for power or another ground. It might be advantageous to sandwich the high speed signals between two grounds as that would reduce cross plane capacitance and crosstalk onto a power plane. The power planes that are switching will also be noisy so it may be good to also sandwich the high speed and switching power planes between two grounds to reduce those planes radiating/EMI.

Remember you have creepage and clearance requirements with high voltages.

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