Using X2Y caps

Go To Last Post
6 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I'm attempting to design a PCB with a low enough EMI profile that it will pass FCC part 15 class B in a plastic case.

To this end I've been advised to use X2Y caps (amongst other things, see original thread: https://www.avrfreaks.net/index.p...).

Never having used them I would really appreciate it if you guys could look at the attached schematic.

X2Y documents indicate (http://www.x2y.com/filters/TechDay09kr_hpa_Track2_1_Precision_Analog_Designs_Demand_GoodPCBLayouts%20_JohnWu.pdf page 31) that bypass capacitors can be replaced by a x2y caps. As you can see in the schematic below I've replaced the normal 100nF bypass caps by two x2y caps.

* Does this make sense or is this overkill?
* Does this also remove the need for a ferrite bead in series with the fuse? (http://www.x2y.com/filters/TechDay09kr_hpa_Track2_1_Precision_Analog_Designs_Demand_GoodPCBLayouts%20_JohnWu.pdf)

If this is the way to go then I'll replace most other bypass caps by x2y's.

Attachment(s): 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

What sort of load are u driving? Just saying a 8Mhz MCU is a little low on details.

In short decouple the input with a LC filter, have local LC filter for MCU supply. Make sure all the MCU power pins are decoupled with capacitors and also make sure they are located very close the the mcu.

Look at the return paths for your high frequency areas (including crystal oscillator areas).

You should look at some USB peripherals to see how they do it. Often you will see ferrite supression on the inputs to the USB peripheral.

Try your best and go for a precompliance scan- you should watch and try to take part in the scanning process. Probably will be a fair chance you fail, but at least by going and watching the process u will understand how the tests are conducted and how improvements could be made. You might also get lucky and be able to work side by side with the test engineer who might just offer some advice.

oddbudman

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The MCU is an atmega644 at 8Mhz with an max3232, an enc28j60, 4 leds and 4 5v reed relays.

I already found a lab in LA. They seem very helpful. They asked if I could be present for the test and offered to have someone on hand that knows how to deal with this stuff.

Anyone any thoughts on the X2Y components?

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You might like to have a look at this comparison between x2y caps and 0402:-
http://www.speedingedge.com/PDF-...

The small advantages do not seem to outweigh the cost.

Jim

Your message here - reasonable rates.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Rereading this document:

http://www.x2y.com/appnotes/emi/...

It would appear that they might have take some liberties. On first sight it looks like the x2y solution is _VASTLY_ superior to the ferrite bead.

However on close inspection it appears that the comparison is almost apples and oranges. In the X2Y test the case is grounded, through the G terminals, but in the ferrite bead test it is not, connected at all. Probably leading to the emissions that are shown.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Ron, for your application I would place an X2Y as close to the power entry connector as possible, and also put down pads for an optional series ferrite bead inboard of the X2Y cap. You should not need to use ferrite beads to divide up your internal power. It is easy to get into trouble misapplying ferrites. If you want tips on how to do it correctly:
http://www.ipblox.com/pubs/Ferrite_beads/Understanding%20Ferrite%20Beads%20and%20Applications.pdf

X2Y caps work great, but you need to mount them properly. That is something that SpeedingEdge did not do. If you look at Figure 4 in their report, the layout for the X2Y cap has absurd skinny traces between the capacitor pads and the vias. They knew better, which is why they put the vias right up agains the 0402s. This paper http://www.x2y.com/bypass/mount/get_the_most.pdf Shows how to do it correctly. There are other problems with that report, but the tragically bad X2Y mounting is the biggie.

The single most important thing that you can do to insure your success is to plan your stack-up correctly. This paper http://www.x2y.com/bypass/mount/impact_of_pcb.pdf explains how to minimize the number of capacitors you need by arranging the stack-up properly.

Good luck with your project.

Disclosure: I consult for X2Y.

Steve.