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Why are power supplies so complex?

Discussion in 'Electronic Basics' started by Don A. Gilmore, Mar 3, 2006.

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  1. 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
     
  2. 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
     
  3. Chris

    Chris Guest

    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
     
  4. John Larkin

    John Larkin Guest

    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
     
  5. Don Bowey

    Don Bowey Guest

    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
     
  6. Regulate the voltage.
    Limit the current during overload.
    Correct the line current power factor.
     
  7. spudnuty

    spudnuty Guest

    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
     
  8. Ryan

    Ryan Guest

    Why are ordinary DC power supplies so complicated? What's going on in

    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.
     
  9. John Larkin

    John Larkin Guest


    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
     
  10. Joel Kolstad

    Joel Kolstad Guest

    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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