Mobo designer speaks out

[Brians256]

I saw these on Slashdot, so who knows if any of this is true. However, I know enough to say it sounds plausible.

This is a mostly one guy talking about how to design the CPU power supply on the motherboard (and the same applies to the other DC-DC units on the motherboard).

Originally Posted by jimmyswimmy

I used to design motherboard power system components, and the author spends a good bit of time talking about that. That is actually the most complicated part of the board design, as it is not at all automated. Most component vendors try to sell a complete solution to the motherboard vendor, easing their job somewhat while helping the sales of the component vendor.

One particularly interesting item of note: all those capacitors the author describes are absolutely crucial, and together form one of the largest cost items on the board. The system is tested using a processor-vendor-supplied "load tool" which simulates the worst case load transients one can ever expect to see. Most of that testing is done by the power system component vendor and then provided as a block to the motherboard vendor. Most motherboard vendors have no idea what they are doing there.

In fact, a lot of the foreign manufacturers (no names) cost-reduce their designs by simply pulling out caps until the system blue screens. Then they put the last one back in and sell it. Intel is the only manufacturer I am aware of that actually sells the worst-case performing design.

Note that I am only aware of products related to Intel-type motherboards. I never worked on the othe stuff.

Originally Posted by Anonymous

I always leave 3 or 4 no stuff capacitors (Bulk and HF)on the motherboards I work on (and I AM a power engineer). That way when I do buy one of these systems I can add the extra caps fairly easily and I'll get a system that I can actually trust.

I find that the industry is only now starting to appreciate how difficutly power supply design is.

-Coward cause I don't want my boss to hear (Empty Caps locations means harder buss routing because the vias go alllll the way through the MB)

Originally Posted by Anonymous

Something you should be aware of concerning inductors is that if they are ferrites, their optimal temperature is usually 100 C. That's where they have the least losses. So the inductor will quickly heat up to 60 C or more, but once there is will actually have lower losses. (The difference can be a factor of two for some materials and frequencies.)

You don't actually want to design your inductors to run at 100 C though, since losses increase above that, which can lead to thermal runaway (mmmm, fried computer!).

Originally Posted by jimmyswimmy

In the design of a CPU motherboard power supply, I suppose my two biggest concerns were heat and transient response. In dealing with the heat issue, you have to select the "best" components to make the system work. In other words, the cheapest set that will work. That's not trivial at all. First you decide how many phases to work with, considering that the norm is about 25-30 A per phase maximum. Then you pick a frequency. The operating frequency of the multiphase converter determines the inductors you use, and those get HOT at high frequency, due to core loss effects. Then the MOSFETs, and you parallel however many as are necessary to keep the heat down while also keeping the cost down -- usually an empirical selection process (hours upon hours in the lab learning what works and what doesn't).

On the other side you're dealing with transient response. That is, the CPU is working hard, decoding your DVD's and playing MP3's and all that other sht. And then, suddenly, it's not doing much at all. The load goes from, say, 110 A to 10 A, or at least that's how the design might go. Now you have to select capacitors to deal with that. These capacitors are selected based on some specified parameters, such as raw, bulk capacitance and ESR, as well as some unspecified ones, such as ESL and whether or not you have any in the lab. And they are tested and tested to death. It's amazing how much component placement and orientation can matter.

In the field, you do not have the tools to test the resulting design, so it's not a great idea to dick around with the decoupling solution. But for fun, if you had a motherboard to mess around with, you could try replacing whatever existing bulks with the smallest Sanyo OS-CON's you can find, or some of those sweet Fujitsu yellow caps, I never found a part number. If the total C is at least the same as what you had, it "might" work.

As far as MOSFETs go, to replace them you're mostly worried about heat. For the high side switches, you worry about gate charge Qg and thetaJA, the lower the better. For the low side switches, RDSON and thetaJA, the lower the better. The high side switches aren't on very much (only 1.5/12 of the time) and depending on the switching speed they will generate a lot of heat. So we want good switching capability out of them. The lowside switches are on 10.5/12 of the time so we want them to conduct well, with low resistance.

But if you replace the MOSFETs, and you happen to find low side switches with too high gate charge, the existing drivers may be unable to deal with them, and so you are screwed. Drivers can be tough, depending on whether or not they are standard. There is a fairly standard SO-8 format for driver pinouts that is ignored as often as it is accepted.

In short, there's not a lot you can do. Replacing inductors is too hard to do right, and same with the power design. Best bet, go with the Intel design (and no, I don't and didn't work for them, and I don't own stock in them, I just understand their methodology). If not, then buy the motherboard with the most phases (most big inductors around the CPU). That means the lowest current per phase and, if well designed, the best future CPU upgrading capability.

All quotes taken from this forum thread. It was a thread talking about MBReview's article on motherboard design (fairly basic, trying to cover too broad of an audience by having both beginner and advanced concepts in the same text).

[pHaestus]

Really interesting quotes and thread Brian

[Brians256]

What I find most astonishing (and frightening) is that the mobo designers have such a limited amount of time to design that they simply try out different components in a generic design to see it it will work.

In other words, slap in a couple of these caps here and use these MOSFETs and inductors that are mostly the same as the reference design and see if it works! The worst case scenarios might need more capacitance, but hey! it runs WinDVD, so it must be OK. Ship it!

Sounds like the easiest way to "improve" the on-board DC power supplies is to add some very low ESR filter caps and keep the MOSFETs cool. In other words, not much we can easily do.

[pHaestus]

It puts the rationale for never buying a new via chipset-based mobo without an "A" at the end though doesn't it?

I also remember clearly the voltage regulation issues on a few mobos I have owned when you pushed them. Puts that into perspective a bit too.

[bigben2k]

It all sounds like straight forward talk.

The advice about swapping caps is one I'd ignore, unless one knows precisely what one is doing.

Otherwise I know that Asus is one of the rare board designer that's at the top level for development with Intel. Beats me why they still use the same old phase scheme though.

I've had a chance to see many ads in electronic magazines for plug in power modules. There's an astounding number of them out there. I guess that's the future?!?

[Groth]

Originally Posted by pHaestus

It puts the rationale for never buying a new via chipset-based mobo without an "A" at the end though doesn't it?

You could easily have a well designed board with a crappy chipset, or vice versa.

Originally Posted by bigben2k

Beats me why they still use the same old phase scheme though.

Huh?

Not a bad article, but needs editing for spelling/grammar/clarity.

[nicozeg]

exellent read. I only recomend cutting the couple Intel adds in between!

[bigben2k]

Originally Posted by Groth

...
Huh?

...

Check this out:
http://www.overclockers.com.au/article.php?id=76562
(a classic, linked many times here).

There's a perception out there that Asus' scheme of using only 2 phase is a bad thing.

I'm not sure where they stand on that today, but I haven't heard of any changes.

Oops, here's a change:
http://www.xbitlabs.com/articles/mai...p4p800s_2.html

Asus is now on three phase, at least for Intel. Looks like they still use a 2 phase system on the A7N8X-X:
http://www.devhardware.com/c/a/Mothe...sus-A7N8X-X/4/

[SysCrusher]

Stupid question. I know nothing of electronics or electrical other than my house. What about changing out the caps for bigger better ones? Maybe faster switching mosfets?

[bigben2k]

The problem is that it adds to the load, on startup. Otherwise it won't have any beneficial effects, since they wouldn't filter out anything, really: they're just there to pickup the transients on the phase switches (from my limited understanding).

[Brians256]

Originally Posted by SysCrusher

Stupid question. I know nothing of electronics or electrical other than my house. What about changing out the caps for bigger better ones? Maybe faster switching mosfets?

Well, this is not my field (fascinating as it is), but I believe that you cannot replace these caps with bulk capacitance units and get good results.

One factor that I know of is ESR. ESR is equivalent series resistance. It is the amount of resistance current encounters going in and out of the capacitor. So, at high frequencies, it means that slows down the filling and emptying of the capacitor. When a capacitor is used as a filter (to keep voltage steady instead of choppy), it lets high-frequency waves exist instead of smoothing them out. High capacitance caps usually have high ESR, so they are good at delivering large quantities of current, but not instantaneously. They handle low frequency high magnitude ripples.

Also, adding bulk capacitance could interfere with startup. Large capacitors act like a low resistance (ESR) short to ground until they are "filled". So, excessive capacitance can actually burn out or at least stress the circuitry leading up to them. Modern DC-DC controllers probably have a soft start algorithm though I'm not sure if this would completely take care of this issue.

This is just scratching the surface. The output filter is actually designed as an LC filter, and it is tuned for the given components. The output filter should be modelled to see what values *should* be there. Increasing C might be good if you change the inductance value at the same time. Go up or down? You know the answer to that? I'd have to look it up and then trust that I'm reading the book right. A more effective filter might actually be more complicated than an LC filter. How many nodes do you want to involve in the filter?

My hope is that we'll convince someone smart like Groth to add some tidbits here and there, although I think he's on record as saying we're too uneducated to actually make a positive difference on this area of a mobo without getting lucky.

Originally Posted by Groth-Clone

You have no idea what you are doing. Buy a quality board to begin with or get an EE degree.

Get an EE degree

[nicozeg]

thats the thing that needs to be reduced. switch transients increase a lot at high power demand, and can make a noise big enough to mess cpu signals.

[jaydee]

Originally Posted by SysCrusher

Stupid question. I know nothing of electronics or electrical other than my house. What about changing out the caps for bigger better ones? Maybe faster switching mosfets?

Not sure but faster switching mosfets may **** up the drivers. I think I read that in the quotes.

But if you replace the MOSFETs, and you happen to find low side switches with too high gate charge, the existing drivers may be unable to deal with them, and so you are screwed. Drivers can be tough, depending on whether or not they are standard. There is a fairly standard SO-8 format for driver pinouts that is ignored as often as it is accepted.

Maybe that dosn't have anything to do with faster switching mosfets though.

Looks like Brian covered the caps.

Interesting thread.

[Groth]

Originally Posted by Brians256

Hey! I can come up with stupid and insulting things to say all on my own!

I'm no expert by any means, but...

Like the guy Brian quoted earlier, changing the MOSFETs may save you little heat, but it won't appreciably affect Vcore quality. The only effect that might be noticable would be a decrease in ground float at high currents, sort of a small scale droop-mod.

Forget changing the inductors. Not only are they a total bitch to remove (yes, I've done it) but their value is largely determined by the switching freqency (which you can't change). If you want to change 'em anyway, you've got a compromise: lower values will reduce the transients caused by changes in power demand, but store less energy so you get more ripple during constant demand. Higer values give smoother power during constant demand, but bigger transients when demand changes.

The active-droop that Intel specs call for (and people mod to remove) is to help deal with these inductor transients. Reduce the Vcore under load so the high voltage transient from going to idle doesn't kill, increase Vcore at idle so the low voltage transient from going to load doesn't cause crashes.

For the caps, not only the capacitance value and ESR need watching, but the ESL (effective series inductance). Electrolytic caps are rolled, so they always have some inductance, but it's harder than hell to get numbers. Like ESR, ESL will limit the caps ability to absorb a transient, and also allow noise at a specific frequency through, noise than needs to be killed (with a ceramic cap near the CPU, usually).

If you want to play with the caps, add more in parallel instead of replacing (kinda like the engineer who talked about leaving unstuffed holes he could fill at home ). Paralleling increases capacitance and decreases ESR and ESL, so you're not likely to cause problems (if you keep the leads short).

Realistically, you're unlikely to make any positive changes.

[freecableguy]

Forget changing the inductors. Not only are they a total bitch to remove (yes, I've done it) but their value is largely determined by the switching freqency (which you can't change).

Untrue! I can show you EXACTLY how to change the switching frequency! It involves changing ONE SMT resistor for the PWM controller. Couldn't be easier. Now, would I suggest doing this....no, not really...

[nicozeg]

Originally Posted by freecableguy

Untrue! I can show you EXACTLY how to change the switching frequency! It involves changing ONE SMT resistor for the PWM controller. Couldn't be easier. Now, would I suggest doing this....no, not really...

That's interesting. It would not be the firt time i solder an smd for a volt mod

now the question is wich way is usefull to go when in an amp hungry overclocked situation? Higher or lower frecuency?

[Groth]

Okay, yeah some VRM controllers use external caps and/or resistors to set the frequency.

Lower frequencies will mean cooler MOSFETs and inductors, but more ripple and transients. Higher will mean more heat and a smoother output. But if your inductors get too hot, thermal runaway and everything dies.

[zoson]

Reading this, my 8k7a+ came to mind... :O 2 phase power... It's a small wonder the board blew up so many times, and it's a miracle that it ran so fast.

You know what? Everyone bitches about keeping the mofets cool at high frequency. Yet we already see MANY motherboard manufacturers adding cooling solutions to them. This brings up an interesting question: Are the power designs going to get even worse simply because we're treating symptoms with brute force cooling? "hey! we can save money by making a two phase solution again and cooling it with an obnoxious 'overclocking' feature!"

This also opens up an interesting new DIY idea for those who have the experience. Can we get schematics for these power supplies, and can we effectively analyze and FIX the problem ourselves? Personally, when I was taking VLSI I could barely follow the examples our professor gave. I can't imagine having to look at a design I know nothing about and fix it.

[freecableguy]

Information for the PWM controller and buck/boost controllers for the MOSFETs are amazingly simple to follow. The best thing I have found to keep the MOSFETs cool has been the installation of a TEC to my system. Since they are located so close to the CPU the board actually keeps them at 20C or below. Amazingly simple....plus I have used AS epoxy and heatsinks on them (Al) with a big 12cm fan providing about 80CFM to them...

[DrMemory]

Originally Posted by Groth

Okay, yeah some VRM controllers use external caps and/or resistors to set the frequency.

Lower frequencies will mean cooler MOSFETs and inductors, but more ripple and transients. Higher will mean more heat and a smoother output. But if your inductors get too hot, thermal runaway and everything dies.

Resistance in the inductor windings increases with temperature. When the resistance increases, current decreases and temperature goes down. They can't go into thermal runaway. MOSFETS are also immune to thermal runaway. However higher MOSFET temperatures mean less current from the power supply and a decrease in output voltage if the CPU needs more current can be supplied.

Another way to decrease MOSFET temperatures is to replace the ones on the motherboard with ones with lowest RSDON. These cost more which is why the best ones may not be used on motherboards. If all the other MOSFET parameters are the same, this should cause no problems.

[Groth]

The permitivity of ferrites changes with temperature. Get it too hot and more of the energy that is intended to be stored magnetically will be lost as heat, temperatures rises, more is lost to heat, etc.

[DrMemory]

Interesting. I've never heard of thermal runaway in magnetic circuits. But then I've done very little research or work with magnetics in power circuits.

[Groth]

I'm no magnetics expert either, those stupid B's and H's and how they interact really messes with my brain.

[KnightElite]

No kidding. I'm taking a Power Systems class and a Magnetic circuits class right now . Power Electronics is coming up either next term or next year, can't remember.

Fun fun fun... .

Nice thread you found Brian, interesting discussion thus far.

[zoson]

What I was getting at freecableguy is the possibility of adding more phases to the power supply via a secondary PCB attached creatively to the existing power supply on the mainboard. We did see ASUS do something similar to this for THG and their 100 Amp modification for their a7n8x deluxe.