Re: Building computer - power supplies
On 9/28/2013 3:18 PM, Doug wrote:
> On 09/28/2013 03:23 PM, Stan Hoeppner wrote:
>> On 9/27/2013 6:37 PM, Joel Rees wrote:
>>> On Fri, Sep 27, 2013 at 9:36 PM, Stan Hoeppner <stan@hardwarefreak.com> wrote:
>>>> A point I forgot to make. This is something everyone should know.
>>>>
>>>> Subject: The marketing myth of multiple +12V rails
>>>> [...]
>>>
>>> What I want to know is why Intel CPUs still need the +12V.
>>
>> They don't now and they never did. The 8088 through 80486 and the first
>
> /snip/
>
>>
>> Installing two voltage regulators next to the CPU socket and using
>> standard ~22 gauge copper wires from the 12V rail of the PSU solves the
>> problem cheaply. The 12V rail was chosen because 3.6x less current is
>> required vs using the 3.3V rail as was used previously, which means much
>> smaller wires are needed.
> /snip/
>
> Do I understand correctly that there are two switching power supply
> chips at the input to the CPU to produce 3.3Volts? Obviously, a linera
> regulator cannot produce and gain in current. Iin = Iout for linear
> regulators.
No, the switching in the PSU is done on the AC current before
rectification, increasing the cycles from 60 Hz to 30-50 KHz. The high
frequency AC, among other things, facilitates the conversion to DC, and
allows for a much smaller transformer, though the output has ripple,
unlike a battery, which provides true DC current.
I'm thinking my attempt at a simple explanation of PSU design in my
single vs dual rail rant was probably confusing, as it was technically
inaccurate. Shame on me.
The cost difference I was referring to is in the components that lay
after the rectifier (which lay after the switching MOSFETs). This is
mainly the regulator, which is also a MOSFET. With a single 12V rail
design, the regulator FET is much larger and more expensive than the
combined smaller FETs in a multi-rail design.
However, another cost saving aspect to this I didn't mention previously
is component commonality. If a vendor sizes it PSUs optimally, most if
not every PSU in the line can use the same regulator FETs. For
instance, a 25A regulator can be used for a single 12V rail for any PSU
up to about 400W. For PSUs between ~400-700W there will be two 25A
rails with two regulators. For PSUs above ~700W there would be three
25A rails, and so on. Since the same regulator is used in all models,
this regulator can be acquired in much larger volume with a greater
discount per unit.
With single rail designs one would need to acquire 5 or more sizes of
regulators which are progressively costlier per unit, and there is
little or no volume discount relative to above. This is the major cost
driver for the multi-rail designs, not simply the per unit cost of
larger FETs.
--
Stan
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