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Your VRM/PWM questions, please ask away!

post #1 of 105
Thread Starter 
Okay I made this thread because a lot of people have asked me to answer question on VRMs and PWMs, please ask it and i will answer it(with a legit in depth answer), I promise smile.gif.

For instance:
When you hear about Analog and Digital PWMs, what does that really mean? Many of you have heard of analog and digital being used by manufacturers, and they are just marketing terms, as they refer to the way a PWM takes an error(from the voltage output) and compares it to a reference(the voltage you want) to alter the output (voltage) to bring it closer to what the user wants. In a digital PWM this is done by converting the analog error, which is a measurement of the output voltage (voltage=analog), to digital 1s and 0s through an analog to digital converter. Then the 1s and 0s can then be processed by a PID algorithm and then change the PWM outputs. In an analog PWM this is done through a comparator/error amplifier, but the signal is never converted to digital and isn’t processed by any algorithm, it is a hardware based method that works very fast. Digital is more precise, but analog tends to be faster. Digital is able to turn up the duty cycle for a certain period of time that is calculated, while analog will turn it up until it deviates from the reference and then turns it down. However the sampling rate is done much faster than the switching frequency, and normally the lowest switching frequency you see is 200khz, which is 200,000 times per second. The whole process is fast, and digital PWMs are a bit more precise, but a user couldn’t tell the difference between analog or digital. Also Digital PWM technology allows for digital control over the phases(if the drivers used are made by the PWM maker) so that different loading schemes can be used. Digital PWMs also use NVM(non-volatile memory) to allow users to change many settings, and for the manufacture to easily change up the protection levels, LLC, transient schemes, and more through a simple BIOS update. The NVM also allows for the user to control much more than on an analog PWM, which is complicated for many but nicer for some others.

Here's some links:

http://fab.cba.mit.edu/classes/MIT/961.04/topics/pwm.pdf
http://en.wikipedia.org/wiki/Choke_(electronics)
http://en.wikipedia.org/wiki/MOSFET
http://www.smpstech.com/

Cool links:

http://kingpincooling.com/forum/showthread.php?t=980
Edited by Sin0822 - 2/19/12 at 3:35pm
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post #2 of 105
dont digital PWM's put out more power per phase than a analog PWM? say for example a Digital phase would put out a max of 100w per phase were as an analog phase would only put out 80w max per phase?

also digital phases run a lot cooler than analog and tend to put out a cleaner power source?

Also subbed and Rep+, this will be an awesome thread.
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post #3 of 105
Thread Starter 
Quote:
Originally Posted by Kal777 View Post

dont digital PWM's put out more power per phase than a analog PWM? say for example a Digital phase would put out a max of 100w per phase were as an analog phase would only put out 80w max per phase?
also digital phases run a lot cooler than analog and tend to put out a cleaner power source?
Also subbed and Rep+, this will be an awesome thread.
All not true, it totally depends on the MOSFETs and Inductors used as for power output. Temperatures depends on the MOSFET characteristics and switching frequency and such.

Would you like me to go more in depth?

First off i suggest that when you want to talk current per phase you want to switch to talking about current per phase not watts, because watts are dependent on current and voltage.

The top models we have ever seen have the capability to output 40-50A per phase.

1. X58A-OC ran on an analog PWM, ISL6336 but per phase output was 50A at the inductor and continous 35A at the MOSFETs. The board had 12 phases.
2. We have the X58 Classified with the Votlerra Digital PWM VT1185 and 10 phase line up, the MOSFETs could output 45A per phase but the inductors were rated 40A per phase.
3. We have the Rampage 3 Extreme used 28A inductors, and 40A DirectFETs, for a total of 28A per phase, and used Chil digital control.

Volterra has had highest output per phase in the above 3, but anyone with any PWM could top that with some high-end power stages(60A from IR for instance with 50A Inductors for instance, but that prob wont happen) or very high quality MOSFETs. There just is no need. THere are 50A DrMOS that GB could have used, but they didn't feel cost was warranted, in the end the max theoretical is slightly above on the oc board because of the two extra phases. Heat is also a bit lower because of the lower switching frequency used. However the X58 classy performed better on bloomfield while the X58A-OC was built for gulftown, probably b/c of the switching frequency.
Edited by Sin0822 - 2/18/12 at 11:45am
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post #4 of 105
Thread Starter 
This is a simplified electrical diagram i just drew up.
352

The high-side MOSFET connects the load to the input voltage, and the low-side connects the load to the ground to complete the circuit as they switch on and off at different points. When the high-side FET is done providing power, then the low-side turns on and grounds the circuit. This creates AC noise as they switch, and that AC noise is carried through the power output, but the indcutor acts as a low-pass filter, so while you can measure the switching frequency at the leg of the inductor which the MOSFETs are attached too, you only measure 60hz fundamental frequency on the other leg of the inductor facing the CPU. That power form the inductor is decoupled by the capacitors which and power is provided to the CPU.

Inductors can only carry so much current, and that current rating can be defined as saturation current, if the saturation current is surpassed on any inductor then the core of the inductor is flooded with magnetic flux and then basically tries to store energy in the air and causes a short. That short exponentially rises the MOSFETs current until it blows. This happens on the refernce GTX 570 VRM, many users have changed out the inductors and solved it when they want to extreme OC with one.

However some power designs are designed to run at and around the Saturation current of the inductor.
Edited by Sin0822 - 2/20/12 at 11:49am
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post #5 of 105
Sweet. that cleared my understanding. i was under the impression that a digital PWM would be better than an analog PWM. but what your saying is they both have their ups and downs correct?
what do you know about the Chokes? there are a few different types, is there any type of choke that is better performing than another? we see the sealed type iron core chokes on higher end boards and the open topped type in lower end boards, is there a performance difference between the two or are they both the same apart from the epoxy coating to reduce choke whine?

also Rep+ and congrats on hitting 1000
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post #6 of 105
I'm asking a very ignorant question I'm sure redface.gif

I've done the usual googling of VRM and mosfets, but their either two advanced for me to know, or too general.

One thing I've always wondered is the vrm's on a motherboard for a cpu, like a 4 phase setup, well even if they're surface mounted, can I add ummm more VRMs to a motherboard. Like is it possible to do so, can I just solder them on. For example, the setup of the infamous MSI motherboards for AMD, could you replace the VRMs on them and make them safe to use with x6 and overclock as well?

Sorry if this is probably the dumbest thing ever asked, I apologize. I don't really know much about this stuff, and I do not know where to start reading about this. Thanks for this service you're providing Sin:thumb:
     
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post #7 of 105
Thread Starter 
hey thanks, I didn't know i was at 1000 yet, but that is pretty cool. hahaha.

Um well the PWM is just a control chip, the VRM components like the MOSFETs, Chokes, and capacitors have just as much to do with power output and performance.

As far as chokes go there are a lot of types, the most modern are the ferrite core chokes you see. The ones that don't have an exposed core do seem to have higher ratings an perform better, however it is hard to compare chokes on different boards because they are harder to look up and find. Chokes have DCR ratings, Saturation current rating, and ripple current ratings that have huge impacts on performance. There are also difference between the rating in uH. If you see R45 listed on the chokes then its a .45uH rated. 1R0=1.0uH. Also the physical size of the choke can differ too.

What you can do with chokes is hope that the manufacturer picked good rated chokes, on higher end boards they do infact pick higher rated chokes. you just have to trust that they picked ones good enough, usually on motherboard they do, at least on the high-end.
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post #8 of 105
Thanks so much for this Sin, that really helped understand a bit better. I'm hoping once I enter Computer Engineering, and go into advanced circuit design, I might actual understand more about this on something higher than a superficial level. I'll look up all those terms.

Thanks again Sin.
     
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post #9 of 105
Thread Starter 
I think computer engineering only dabbles on advanced SMPS design, I think more what you are looking for is electrical engineering. Computer Engineering focuses more on the CPU and the different components, really a lot of logic circuits, more than power supplies. but i am sure they will teach you the basics anyways.
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post #10 of 105
Thread Starter 
Quote:
Originally Posted by Spykerv View Post

I'm asking a very ignorant question I'm sure redface.gif
I've done the usual googling of VRM and mosfets, but their either two advanced for me to know, or too general.
One thing I've always wondered is the vrm's on a motherboard for a cpu, like a 4 phase setup, well even if they're surface mounted, can I add ummm more VRMs to a motherboard. Like is it possible to do so, can I just solder them on. For example, the setup of the infamous MSI motherboards for AMD, could you replace the VRMs on them and make them safe to use with x6 and overclock as well?
Sorry if this is probably the dumbest thing ever asked, I apologize. I don't really know much about this stuff, and I do not know where to start reading about this. Thanks for this service you're providing Sin:thumb:

EVGA designed their power board, the EVGA untouchables, it is a VRm board, and yea it could be used. however modern CPUs have a few extra issues with using a board like that. most of which is the fact that they use DVID and SVID to communicated with the PWM and set voltages. i am unsure if that was missing whether or not you could work the system. I am sure it might be possible, but it also might not be. I have thought about it as well.

Also you can't really add an extra phase or two on there as the system is pretty much set. you either have to disable what is already on the board or use what is already there.
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