I've got a Q: if caps are removed, what could one expect from adding caps? (I know, it seems obvious).

Hey! Welcome, jimmyswimmy! Since I work in a tangental industry, I get tantalizing glimpses of both sides of the coin. I play with these mobos at home, and I get to work with semiconductor companies and high-end EEs at work. I write software for DC and RF probing of components and on-wafer devices for a company called Cascade Microtech (we produce wafer probers and some RF software that helps calibrate VNAs).

Anyways, why is the ESL so difficult to measure? Is it because the devices vary so much that a representative sample is difficult to achieve? For example, a VNA can measure complex impedance of a device very easily although it is both complex and variant with frequency. The software I'm working on right now has a marker to display inductance (among other things) at a frequency of a smith chart and the ability to show inductance vs. frequency (or vs other things). With a decent modelling program, you should be able to deconstruct a lumped-element model of the device fairly easily and then publish it. No more than a couple days work per device (assuming you have a stable methodology), right?

I'm guessing that it depends very much on the characteristics of cap(s) you add. Unless you know the original design, you could add too much capacitance and throw the control system into instability. Or, adding caps with too much ESL could cause problems when the CPU changes current demand. Or it could be just fine. One of the things I'm learning about with the microwave design stuff I'm working on is that capacitance and inductance both affect the relationship of current and voltage of a signal (in opposite directions though). I think you could end up causing resonance because of newly added capacitance interacting with the control system's feedback loop. For those of you who actually know, please forgive my stumbling attempts here. I get paid for writing software, not EE.

My guess is that adding a bit of low ESR capacitance would help out in most implementations, but that adding bulk capacitance could be BadJuju(tm) in edge cases where current demands change rapidly.

As for having the tools to test a design... I probably come closer than most to having the tools. We have scopes and high-bandwidth VNAs here at work. But I don't know how to use the scopes, to be honest. The only reason I know how to use the VNAs is that I'm writing SW to use them and I blackmailed some EEs to teach me a bit about their theory (and I read "Practical Microwaves" by Thomas S. Laverghetta). How would you test a design modification? Would you model it after doing some component analysis or just throw a mod together and set a triggers on the scope to watch for anomalous events like voltage droop/spike?

awesome comments indeed

so would there be any benefit to adding capacitors in parallel to those existing on say, an NF7-S?

Air cooling in the future...
 

Well, this is all IMHO, but I haven't gotten the idea from the manufacturers I worked with that they wanted to spend more money on ANYTHING. So at least in the near future, next couple years, I would not imagine that you will see systems shipped with anything more than passive or air cooling equipment included.

If you look at the trends of electrical specifications for most processors over the past several years you will see that they are heading to where that will no longer work. Looking at my 6 month old Dell, which has a 2.8GHz P4, I see that there is some ductwork to vent the processor heat outside of the case, and that tells me that there is concern over the amount of heat generated by the CPU.

I can tell you for certain that motherboard manufacturers hate the fact that they have to spend so much on doing something as "boring" as providing power to the CPU, and the rising percentage of cost on the motherboard that entails. I'd be surprised if they weren't demanding of Intel that they do something about it. But Intel is basically fighting the laws of physics...

So I would say that for the next couple years (IMNSHO, I suppose) you're gonna see more inventive approaches to the same old airflow solution, but if there is not a more thorough change to the way processors are made than you might start to see some active cooling approaches. What that entails... I have no idea.

Hope that's helpful!

Hi BigBen!

Well, it gets a bit technical. I sort of vacillate between telling people to add extra caps and telling them that it could be bad. The specs require that the system be stable for something like zero to 30,000 uF of caps, I think, which is pretty ridiculous since at zero it might be stable but it will also destroy your CPU. And there is a spec for ESR as well, which I can't remember. Should be available online. Anyway...

If you change the system too much you can, in theory, get it to where it will no longer be stable. There was one controller vendor that depended on the tiny amount of ripple on the output voltage, and so for that one, if you added too much filtering (more caps, essentially) there would be no ripple and it would lose its mind. For the other vendors' parts, depending on the control mode, you could have the same result.

What do I mean by stable? Well, providing the constant output voltage at the programmed load line, within specified limits. To use an idiotic and simple analogy, I am stable for most inputs; I can remain standing if you give me a little shove. But if you give me a huge push, I will go unstable -- first I'll fall over, then I'll get up and try to return the favor. In a similar way, if you push the controller outside its limits, you could receive an unpleasant surprise.

A well-designed control system will have enough extra margin that you could probably get away with a few little changes. So if you're not too worried about toasting that new motherboard and processor, go for it, see what happens, it will "probably" be all right (standard disclaimer: it might not be all right, too; do not taunt super happy funball; always wear your seat belt; product may contain nuts)

KnightElite is learning the basics of this unpleasant specialty right now. Perhaps he can offer a more intelligent analogy. Technically speaking, in a voltage-mode control topology, the ESR zero will alter your phase margin when you use a Type II (2 poles, 1 zero) or Type III (3 poles, 2 zeros) compensation. If you really lower ESR significantly, the ESR zero will move right, which could change the bandwidth of your power system.

Hope that's at least a little useful. Oh yeah, forgot my "drooly" smileys. I'm telling you, those guys are puking. :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool: :drool:

One of those software guys, eh?
 

Hi Brians,

Neat response. Now I have to think!

Okay, let's see.... first, off sounds like a great job.


Well, it's not impossible to measure by any means. My tech used to do it for me all the time. What used to kill me was that the measurement is so dependent on a good board layout, proper probe placement (say THAT five times fast) and the like. You are basically trying to find V = L * di/dt, where you program di/dt (the current slope) and measure the resulting V to determine L. Since we're talking an L in the nH range, you need either a huuuuuuge di/dt (which is not a great idea in production testing) or the capability to measure small V (back to my point of good layout and probes), not a preferred thing to do in production.

So I'm talking about soldering devices to boards to make this measurement, which means a production sampling program, and those pesky manufacturers never seem to want to do that. It was only a couple years ago that they started specifying a max ESR on datasheets, and that's the kind of information needed to do a proper design.

I used to have a nice HP impedance analyzer, it came in pretty handy a bunch of times. Still couldn't make reproduceable measurements of those nH level inductances though.

ELI the ICEman, huh? I always liked that one. Voltage (called EMF by the old-timers, thus the 'E') leads current (I... no idea why it's an 'I', actually) in an inductive circuit, and vice-versa for a capacitive circuit. Good stuff. Pretty much a good description too.

So, right on both counts. To find the right design in the first place, you do a paper design. Usually more of an Excel design these days, since it's a bunch of mind-numbingly simple calculations that all depend on one another. So you pick everything and figure out where to start, and then build it.

And once it's built, you tweak it to your heart's content, because the paper design never seems to take into account all of the things you forgot or didn't know. :shrug: And then, time permitting, you go back and model it on some variant of SPICE and see how it handles boundary conditions.

Believe it or not, it's significantly faster to build, tweak and test a board than it is to model it. I find modeling most useful when I need to see either gross functionality of something untested, or edge conditions that I simply cannot obtain in real life (where did I put that 3781.22 uF capacitor, anyway???)

So, the guys who do this for a living have a neat little contraption that plugs in the same socket as the real processor. I had that 478 pin mother for a while, and now they've got some even more fancy stuff out with more pins and a pretty crazy hook up; don't know if those have hit the shelves yet. Anyway, all this thing does is turn on and turn off, real fast, and suck out as much power as possible. So that's what they use. It's a neat little computer-controlled device, and it gets damaged way too easily. I'd tell you that you can build your own, but it's pretty tough work, and the parts are expensive.

Hope that was useful. So hey, try to push those engineers to make the UI easy to use. I have a spectrum analyzer at my new job where the buttons are all the same size and grouped together, wait for it, ALPHABETICALLY. A nice idea, but annoying as hell. I can never remember how to use the damn thing, because I can't even rely on muscle memory to help out. I have to get out the book. It's annoying. Personally I love those Tek scopes with tons of different size and shape buttons and knobs... ah, a whole different discussion.

Hope that's interesting...!

I agree about the scope, Tektronix TDS3012A = best scope ever ;). Best I have used, anyway :D.

I hear you on the weirdities of inductors.

Most Mobos use those nice wound core units, they're so forgiving,
they air cool nicely too.

For real fun, try some of the SMT inductors, the Panasonics are
really good, but there are several shell types, and each has different
loss and inductance variation over temp characteristics. But then, I'm
using a 28V Vin and trying to get to CPU core voltage.

The hottest parts in my designs are the inductors, the FETs couple
so well to the PCB with the Power-SO-8 packages that I dont worry
about them any more. Even a 5% duty on the high side isnt bad at 500KHz
amazing new parts out now.

Stabilizing the loop.. UGH
I've gone to current-mode parts from LTC, had such a bad time with the old style parts.

jimmy, thanks for all your posts. It's definitely fascinating to see some of what really goes on and just how practical it might be for us (as nerdy consumers) to restore some of the safety margins to these designs.