SWITCHING POWER SUPPLIES, OR I’M NEVER TURNING MY COMPUTER ON AGAIN
Nostalgia
Remember the TV ad that went something like,
"The worst thing you can do is start your car"? Every time you push
your computer’s power button, something akin to starting your car takes place.
In the case of the combustion engine, cold, thick oil sitting at the bottom
of the drain pan can’t suddenly shoot into the engine block and properly lubricate
all internal parts. The time lag when waiting for the pressure to build does
increase wear in the cylinder walls.
Those cheap, nasty switching power supplies,
some 235 watt, some 300 watt or more have a similar effect on your motherboard
(MB) every time the power button is pushed. If they were of the "linear"
configuration, the poor old MB wouldn’t get such a shock of 5, -5, +12, and
–12 (and others) volts. You see, switching power supplies are some of the
weakest components, if not the weakest, in that precious little box
that is such a source of frustration and entertainment.
If you’re old enough to remember the days of
the 286 machine, you may recall the huge, heavy power supply that resided
within (and the real ball bearing fan) and an unusually well
constructed case. If it ever fell on your foot, an emergency room had better
be close by. Many of those old bomb computers came with linear power supplies.
Some of which still work after 15 or more years, while Joe Blow’s 250 watt,
3 year old generic unit "popped" and caused a frazzled user to run
down to the nearest retail store to inquire about another small metallic 250
watt wonder.
I Don’t Get It
I still can’t understand why we will shell out
500 smackers for the latest processor or even 250 George Washington’s for
the most recently touted "Gee-Whiz-Force" video cards, but when
it comes to the power supply, a very crucial part, we can’t bring ourselves
to fork over much more than ten lousy bucks. That’s right, about 10 dollars
is what we seem to commonly spend on supplies in the 200-250 watt range, when
purchased with a corresponding case. And we are amazed that
AMD has a recommended list for their Athlon processors.
Realistically, no significant competition exists
in the market for computer power supplies other than what we have become used
to. You think that the sub-$500 buyer would spring for that 15 year warranted
supply in a system that he/she will throw away in three years? I think not.
But why don’t we have high quality linear units for servers and other demanding
workstation/gaming environments? Two reasons: (1) zero demand, and (2) high
costs. A small, lightweight (read: less shipping charges) 300 watt unit may
cost $10 of the $50 total cost, when purchasing any number of AT or ATX style
cases. If a similar capacity linear unit would be substituted, the previous
figure would jump to around $30-$50. But, you wouldn’t believe the advantages
a class A, linear power supply would have for your beloved box.
More History
Bulky, linear units were the standard until around
30 years ago, when engineers figured out a way to dramatically reduce the
weight and heat dissipation of the current crop of supplies. In the race to
make electronic products smaller and smaller, physics appeared to be limiting
just how tiny any device could shrink. By accepting a huge amount of garbage
in and frantically trying to filter out the AC (alternating current) ripple
at the output, heat dissipation and weight could be reduced by as much as
70-90%.
What’s AC ripple? Take the amount of "dirty"
voltage, which will be in an AC form and divide that into the absolute value
of the DC (direct current) voltage. That result represents a percentage or
ratio of the bad stuff (AC) and the good stuff (DC). For example, say the
AC hash measures 0.030 volts (30mV) and the DC component is 12 volts. Dividing
0.03 by 12 gives 0.0025 or 0.25% of the output is garbage (dirty). More importantly,
this AC garbage has a frequency, sometimes in the worst case scenarios, several
frequencies. Ever hear a whine when listening to your car radio that follows
the engine rpm? That is usually an indication that your voltage regulators
or filter capacitors/inductors are on vacation. Now imagine you’ve just spent
$500 on RDRAM and you just can’t seem to get your system stable at 700Mhz.
Couldn’t be the power supply, nah, I’ll RMA the motherboard…
Principles Of A Switching Supply
No matter what country where you live, AC comes
out of the wall socket. Maybe it’s at 100 volts or 120 volts or 240 volts,
but it’s AC. Here’s a crappy hand drawn picture of what AC looks like on an
oscilloscope.
Actually, the quality or smoothness of your AC
depends on your supplier. Some countries are well known for their "dirty"
power. A DC curve isn’t really a curve, just a flat line.
Some engineer, probably with a mathematics background,
figured out that if you chop or slice a very small piece of that swoopy AC
line, and fiddle with it, a rough approximation of DC begins to take shape.
As demand for greater current (amperage) increases, it becomes more difficult
to grab a small piece of relatively flat AC, so the snippet nabbed becomes
even smaller. Sounds like the frequency of chopping the AC, must be increased,
which is exactly what happens. This means that there exists, inside your power
supply, a low frequency generator. These modern switching supplies may not
be all that low as their frequencies can top out in the Megahertz range. Back
to the radio analogy. Try turning on an AM radio near your box and see what
happens. You will probably hear a squeal or squawking that makes listening
unbearable. Moving the antenna away from the computer reduces the noise. During
high current periods, the obnoxious noises can vary as much as an octave or
more.
Principles Of A Linear Supply
Linear implies that for every change, a proportional
change occurs. A disproportional change may be likened to spilling Hawaiian
Punch on the carpet, then shooting the dog for sniffing it.Linear
power supplies are much more reasonable or "forgiving" as the case
may be. Linear supplies don’t necessarily have to include the dreaded feedback
loop, which is the main culprit that causes squealing in the output. One
example of a class A, non-feedback linear power supply (my favorite to build)
is shown below.
Seems so simple, right? To contrast, take a look
inside the switching wonder next time you pull your case cover off. Remember
to disconnect any power to the supply! What isn’t shown here is the large
and heavy power transformer (which just two inductors) and the full wave rectifier
made with 4 diodes. The switching unit has the advantage of not using a large
transformer or huge filter capacitors, as C would denote.
From left to right, the raw rectified AC enters
and is filtered with C. The value of C for a 300 watt supply
could easily be in the 10,000 to 33,000 micro Farad (uF) range and that would
be a rather large and expensive piece. Next, a small fraction of the current,
say 1/100th of the total, flows through the resistor R.
This current feeds a Zener Diode, denoted by D. The Zener Diode sets
up a constant voltage (i.e. 12 volts) and also rejects various AC ripple components.
This base voltage feeds the base input of the NPN pass transistor Q
and thusly, a regulated voltage spits out of the node labeled DC OUT.
Notice that there is no possibly way to take a small portion of the output
and feed it back into the input. Hence, this circuit has zero feedback, and
cannot squeal. Unlike the switching unit, when this circuit gets loaded with
a strong power drain, the ripple merely increases, not a switching frequency
in the megahertz range. Lastly, when this circuit powers up, there exists
a delay (short, but it’s there) instead of WHAM! with the switching
supply.
Nifty, So Now What Can I Do?
When purchasing a computer power supply, pay
particular attention to the overall power rating. Switching supplies don’t
like to be run anywhere near their maximum limits, so leave plenty of room.
I would venture to guess that most custom builders should use no less than
a 235 watt supply. I recently opened up a Hewlett Packard Pavilion series
and was shocked to find a wimpy 110 watt unit struggling along inside its
incredibly small case. If the owner ever decides to add another hard drive,
CD-R or a real AGP video card (instead of 2MB shared RAM type), he’s gonna
be out of luck. If you’re the SCSI maniac type and have a hot-rod 3D machine,
no less than 300 watts is the order of the day. The last time I checked the
price difference between 235 watts and 300 watts was a measly 10 bucks. If
you’re already spending $1000 or more, don’t be cheap with the power supply!
Bill’s done with his rant now, get some sleep.
