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Question about power supplies op amp regulators ?

Discussion in 'Electronic Basics' started by bench, May 18, 2004.

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  1. bench

    bench Guest

    When we use an op-amp to regulate the voltage in the power supply
    we connect one input of the op-amp to a reference voltage e.g. a zener
    diode, and the other input is connected to a sample of the output, usually
    the output is conneced to a resistor divider and the divider is connected
    to the other op-amp input. My questions is, does it make any difference
    if we put the reference from the zener into the minus input or the plus
    input of the op-amp ?
     
  2. Some designs use one and some the other. It depends on how many
    inversions are inside the feedback loop. What matters is that there
    is a net negative feedback around the loop that includes the output
    control device, so that a positive supply output error produces a
    change in the negative direction and a negative output error produces
    a change in the positive direction.

    There also has to be some way (frequency compensation) to make sure
    that this negative feedback is maintained for all frequencies at which
    there is net gain, of the negative feedback will turn into positive
    feedback at some frequency and the regulator will become an
    oscillator.
     
  3. bench

    bench Guest

    yes, thanks, I see now, as long as the feedback is negative. The circuit
    that I saw where the zener was connected to the negative input of
    the op-amp was a shunt regulator, which works differently to the
    series method. B.t.w. except for zener references, which is shunt
    regulation, is it true to say that the main regulation transistor should
    be a series regulation (to save energy)?
     
  4. Series regulation is, ideally a bit more efficient than shunt
    regulation, because shunt regulation requires that the shunt element
    always carry some minimum current. But then, many series regulators
    also have a requirement for a minimum load current to function
    properly, and if the actual load cannot be relied upon to draw this
    minimum, then you have to add a dummy load (shunt resistor) to carry
    it, and the generality gets pretty iffy.

    The really efficient kind of regulation is switching regulation, since
    it contains no intentional disipative elements, but only switches and
    energy storage elements. Its losses are just the imperfections of
    these devices and the control overhead.
     
  5. bench

    bench Guest

    yes but isn't it the case that shunt regulators operate at full load, i.e.
    the
    sum of the shunted current and the load current is always a constant.
    Series regulators do need a minimal load, but by no means do they
    need to be at full load, or have a made a mistake somewhere ?
     
  6. Sounds reasonable. Series regulators often need more excess voltage
    than shunt regulators do, but I guess I can't disagree with anything
    you have said.
     
  7. bench

    bench Guest

    So are we agreed that shunt regulators are only good for small currents, say
    up to a few hundred milliamps, otherwise the electricity bill goes up
    and the room gets too hot ?
     
  8. For regulators fed by a stiff voltage source, I agree. I think there
    are a few exceptions like supplies that are inherently more like a
    current source (some motorcycle alternators that have permanent magnet
    excitation). But for what you have in mind, yes.
     
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