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Why are power supplies so complex?

D

Don A. Gilmore

Jan 1, 1970
0
Just a silly question from an ME.

Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.

Don
Kansas City
 
J

Jonathan Kirwan

Jan 1, 1970
0
Just a silly question from an ME.

Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.

That process would be open-loop. Your voltage at the output of the
transformer would vary depending on the input source voltage (which
should be able to tolerate some +/-10% to +/-15% variation, I suspect)
and on loading. Rectifying and filtering would yield something. But
what? And how would the voltage vary versus various time-varying load
situations?

Closed-loop control helps to hold a precise voltage over a wide range
of currents, provide protection against all-too-easily applied short
circuits, guard against over-voltage to protect the attached circuit,
deal with large capacitive or especially inductive loads like a motor,
accept a relatively large range of line voltages, etc. It may also
need to start up the entire system with all those protections without
accidentally tripping them (for example, protecting against
over-current/short circuit while also properly powering up your own
internal system, when current loading is momentarily high as you
provide the initial charging of capacitors.)

And, of course, there is _efficiency_. If you plan for the worst case
(say, a -15% low input source voltage), then when you have it hooked
up to a line with +10% high source your output would nominally be too
high and you will need to burn off the excess, unless you apply a
switching system.

It does depend on what you are powering, though. If you are powering
a train set and have a human to adjust a knob to get the speed they
want, it can be a very simple system and not much different from what
you point out above.

But I'm a hobbyist and not a professional designer in electronics.

Jon
 
C

Chris

Jan 1, 1970
0
Don said:
Just a silly question from an ME.

Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.

Don
Kansas City

Hi, Don. Unregulated DC power supplies do exactly what you're saying,
and they're exactly that simple (except for a fuse). Transformer,
bridge rectifier, filter cap, voilla. This is similar to the large
open frame unregulated DC power supplies, and also the small
unregulated DC wall warts.

If you want a fairly small regulated power supply, you can do it
without too much complexity. If you want a regulated 5VDC at less than
an amp, take your 10VDC wall wart output, and just add an LM7805 3-pin
IC in a TO-220 package with a good heat sink, and a small (10uF) cap at
the output. Simple, straightforward, fairly large and heavy, and
probably less than 50% efficient (meaning it dissipates more energy as
heat than it delivers to the load).

It's when you need more power, and need to reduce the size and increase
the efficiency of the regulated power supply, that things start getting
complicated. It's a tradeoff.

Good luck
Chris
 
J

John Larkin

Jan 1, 1970
0
Just a silly question from an ME.

Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.

Don
Kansas City

All electronics is complex. That's why it's fun, but also why it's
hard to learn and understand. I could ask you "why is a car engine so
complicated?"

John
 
D

Don Bowey

Jan 1, 1970
0
Just a silly question from an ME.

Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.

Don
Kansas City

I believe the "simple" power supply you describe is what you would find in
most equipment that is more than 20 years old, and it's still a viable
design today. I think now-days it is called a linear power supply. The
newer power supply design is a "switching" power supply.

Most components in a switching supply are much smaller than those in a
linear supply, primarily because of the frequencies at which voltages are
transformed in the two types of supply. In a linear supply the frequency is
50 or 60 Hz from the Mains. But in a switching supply the Mains voltage is
rectified, filtered and then used to power an oscillator at a high-frequency
(maybe 50 kHz) which drives the power transformer. The more sophisticated
switching supplies use regulators that permit the supply to work over a wide
range of input voltages, for example 90 to 220VAC.

Don
 
J

John Popelish

Jan 1, 1970
0
Don said:
Just a silly question from an ME.

Why are ordinary DC power supplies so complicated? What's going on in
there that seems to take so many components and makes them so big and
expensive? It seems to me that you would just need to transform the
voltage down, rectify it, and filter it to DC. What does all the rest
of that stuff do? Thanks for any replies.

Regulate the voltage.
Limit the current during overload.
Correct the line current power factor.
 
S

spudnuty

Jan 1, 1970
0
In the '70s I was shooting with Ikagami HL79 video cameras. Originally
the AC power supply used a transformer like you describe. The thing was
a real bear since it was so heavy. Later in the model line they
developed a switchmode power supply. We loved it since it weighed
1/10th as much as the original. A EE told me that a main reason that so
many things are switchmode these days is the cost of making huge
multitapped transformers. When I repaired these things replacing those
transformers would easily equal the original cost of the equipment.
Richard
 
R

Ryan

Jan 1, 1970
0
Why are ordinary DC power supplies so complicated? What's going on in
All electronics is complex. That's why it's fun, but also why it's
hard to learn and understand. I could ask you "why is a car engine so
complicated?"


Then I would explain part of the complexity. There is a lubrication
system, and this needs a pump, and to be cooled. There is a cooling
system, which needs to circulate and needs a pump and a whole lot more
cooling. There is a timing system, which must be precise and many
things exist to make this happen. And so forth.


If I knew the equivalent for a power supply, I'd post about that.
 
J

John Larkin

Jan 1, 1970
0
Then I would explain part of the complexity. There is a lubrication
system, and this needs a pump, and to be cooled. There is a cooling
system, which needs to circulate and needs a pump and a whole lot more
cooling. There is a timing system, which must be precise and many
things exist to make this happen. And so forth.


If I knew the equivalent for a power supply, I'd post about that.


For a simple linear supply, there's

AC inlet, fuse, switch, EMI filtering

Transformer for isolation and voltage stepdown

Rectifier for ac-to-dc

Filter capacitors

Regulator, an IC (complex inside) or a discrete circuit with
reference, error amp, pass element, current limiter

Output capacitor

Bleeder resistor

Maybe remote sense circuits

Maybe overvoltage crowbar

Heatsinks, maybe a fan

PC board, metal chassis, in/out terminals

Switchers are more complex.

John
 
J

Joel Kolstad

Jan 1, 1970
0
Ryan said:
If I knew the equivalent for a power supply, I'd post about that.

There is a good comparison between engines and power supplies in that
"environmental friendliness" drives some of the added complexity -- all the
EGR components, catalytic converter stuff, etc. in an engine syste, all the
power factor correction stuff in a power supply.
 
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