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power decoupling myths

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ehughes:

--- Quote ---Ever since I put a number of layouts into actual EM solvers I realized that everyone on the internet who says 'you have to do it this way' is wrong.
--- End quote ---

What field solver do you use?

SiliconWizard:

--- Quote from: CatalinaWOW on July 20, 2020, 11:10:16 pm ---I object to this use of of the word, myths, which is found widely in Facebook clickbait.

The articles are completely correct, new packaging techniques have changed the rules for design, but the old rules were not anti-factual.  Just limited applicability.

It is like calling turn of the twentieth century engineering handbooks myths because they have tables of strength of various wood species that cannot be duplicated today.  The tables were correct for the old growth wood being used almost totally at the time.  They don't apply to wood grown in rapid growth tree farms which constitute much of the wood on the market today.

Engineers in all fields have to understand what they are doing.  If they don't they are really technicians.  Or amateurs.

--- End quote ---

Yup. Obviously rules that made sense when using TH capacitors are not to be translated directly when using modern SMD packages.
Still, they have the merit of making engineers AWARE of the question of frequency response for capacitors. May sound obvious for experienced engineers, but still not so much for beginners; and this kind of articles, along with the fact things have indeed evolved a lot with modern packaging, doesn't necessarily help: sure this is food for thought for the experienced, but may give a false impression for young engineers that capacitors' frequency response doesn't matter at all. And yes, beyond caps, PCB design can matter a lot too.

You may object that you don't have inexperienced engineers design high-frequency stuff anyway, but reality is sometimes a different story. When dealing with digital circuits these days, a given design has a good chance of being in the high-frequency realm.

TheUnnamedNewbie:

--- Quote from: ehughes on July 21, 2020, 03:36:52 pm ---
--- Quote ---Ever since I put a number of layouts into actual EM solvers I realized that everyone on the internet who says 'you have to do it this way' is wrong.
--- End quote ---

What field solver do you use?

--- End quote ---

Keysight ADS/Momentum and EMPro/HFSS for 3D simulations. I realize that those tools are outside of the average engineers budget, but you can also get very capable simulators like Sonnet and similar for lower cost, especially if you just want to do smaller simulations (like, say, some individual decoupling capacitors).

David Hess:

--- Quote from: OwO on July 21, 2020, 10:28:35 am ---
--- Quote from: David Hess on July 20, 2020, 10:33:11 pm ---I had nearly perfect application to make a test case for three different values versus three of the same value.  Initially 12 surface mount capacitors of the same large value were placed in parallel *in a radially symmetrical transmission line structure*, and it completely failed up to 1.2 GHz.  Replacing the capacitors with a set using three decades of values worked from 50 MHz to 1.2 GHz.
--- End quote ---

That is very strange, because in my experience it's very hard to fuck up decoupling below 3GHz. Even just one or two capacitors of 470nF or 1uF gives low impedance up to 6GHz without any effort. If you are having problems at 1.2GHz the layout must be at fault.
--- End quote ---

The problem was that the layout was much better than anything possible on a board; the capacitors were literally and physically part of a coaxial transmission line, and everything was symmetrical.  So there wasn't anything to adjust or improve except maybe fabrication accuracy.

A similar coplanar waveguide would have been limited to 4 capacitors and I was up against power limits so the radial structure was used instead allowing 12, or even more had I made it larger, capacitors in parallel.  And if it was a large signal effect from high power, then why would the lower value capacitors improve the situation?

Lacking suitable RF test equipment, I declared victory and moved on.  Had it not worked, I would have replaced the common C0G ceramic capacitors with RF capacitors and tried again.

OwO:

--- Quote from: TheUnnamedNewbie on July 21, 2020, 03:08:48 pm ---Another thing I learned: Discrete components are pointless to do any decoupling past a few hundred MHz. Even if you ignore any layout effects, the interal ESL of a 008004 cap makes it's self-resonance frequency be around the 100 MHz range. You just can't get around that without using special bonded capacitors meant for RF/MMIC applications. And to be frank, even if you could, your bond wires will just ruin that too. Just trust that the IC guys know their stuff and take care of decap past 100 MHz.

--- End quote ---
Or actually you know, make real life measurements that will tell you a typical 0402 1uF capacitor has good decoupling up to 6GHz. Or if you don't have the TE maybe manufacturer's graphs will convince you? http://weblib.samsungsem.com/mlcc/mlcc-ec-data-sheet.do?partNumber=CL05A105KA5NQN

You are already halfway to being enlightened. The first half is knowing that "best practices" are BS, the second half is knowing simulations are BS.

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