How does an 8-Phase Power System work?

[krotchy]

I have been looking several places, and I do not quite understand what an 8 phase power system entails. As background, I have taken course work in power electronics theory, and have physically built a Boost, CCM Flyback, and AC-DC DCM Flyback (emulated resistor) converters. Also I have studied three-phase power converters, and understand how they take an AC input and create torque curve accross things such as an AC motor that never hits zero at any point. I also have read a little on on to 6-phase systems for HVDC transmission, which seem like complex versions of the three phase, and essentially space the power at 60 degree intervals instead of 120 degree. However all of these multi-phase systems all seem to be based on taking an AC signal and create either an AC or DC signal in the end.

My question, doesnt a motherboard take 3.3/5V/12V DC lines from a power supply and drop it down to 1.35 or whatever voltage for your processor. I figure this is likely done with a form of Buck converter, or something more complex and efficient like a Cuk/Flyback/Sepic... whatever (theres tons of switching converters). So what do motherboard manuafacturers refer to when they discuss the use of an 8 phase power system on their motherboards. Is there some AC component to this I am missing, or did I just miss the point entirely. Also if anyone can find one, I would love to see a simplified circuit diagram of a 4,6,8 phase system from a motherboard(simplified as in no twisted pairs,snubbers, bypass caps etc)

 

[Mark R]

Essentially, mobos generally use conventional buck converters, although typically with synchronous active rectifiers (MOSFETs) rather than diodes.

A 2 phase converter is simply 2 buck converters in parallel, but with one feedback loop and the clock to the two bucks 180 degrees out of phase. This way the current is shared between the 2 switch/inductor channels, and because the switching is out of phase - the inductor ripple currents cancel. Additionally, because of the phase difference, the overall effective clock speed is 2x the actual switching frequency of each channel, thus response to transient changes in load is improved.

8 phase, simply means 8 channels with a 45 degree phase difference between them. The load is spread over 8 sets of components, there is improved ripple cancellation (allowing further reduction in capacitor size), and further improved response time.

 

[krotchy]

Originally posted by: Mark R
Essentially, mobos generally use conventional buck converters, although typically with synchronous active rectifiers (MOSFETs) rather than diodes.

A 2 phase converter is simply 2 buck converters in parallel, but with one feedback loop and the clock to the two bucks 180 degrees out of phase. This way the current is shared between the 2 switch/inductor channels, and because the switching is out of phase - the inductor ripple currents cancel. Additionally, because of the phase difference, the overall effective clock speed is 2x the actual switching frequency of each channel, thus response to transient changes in load is improved.

8 phase, simply means 8 channels with a 45 degree phase difference between them. The load is spread over 8 sets of components, there is improved ripple cancellation (allowing further reduction in capacitor size), and further improved response time.

Wow way easier than I was thinking, and it makes perfect sense, lol. So basically in a 2 phase rather than running 1 buck (3.3v*D=1.65v, D=50%) at a 50% duty cycle, you run two at a 25% duty cycle. Since they would be 180 degrees out of phase, you essentially get a simulated double frequency (2 25% duty cycles across 1 period instead of 1 50%). So having 8 buck's in parrallel is like changing from a switching frequency of 100khz to 800khz, but not adding the stress and loss of that extra switching frequency will inflict upon a single buck. You essentially just get the beneficial factors of low switching ripples without the insane heat of super high frequencies.

 

[bobsmith1492]

Huh, that's pretty slick. I'll keep that in mind for future projects...