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Switching Regulator Efficiency vs. Load Current

Discussion in 'Electronic Design' started by Navraj, Jun 18, 2008.

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  1. Navraj

    Navraj Guest

    I'm trying to understand why the efficiency of a switching regulator
    reduces dramatically as the load current reduces, while it stays high
    and relative flat when load current is large. The initial reason I
    cooked up was this:

    efficiency ~ Pout/Pin.
    Pout = Vout*Iout, Pin = Vin*Iin
    If Vout and Vin are relatively constant, and average input current is
    also relatively constant (Its actually pulsing I guess, but I'm
    ignoring that), then the efficiency should linearly increase with
    Iout.

    The problem is this - the efficiency doesn't depend linearly at all on
    the load current. Most graphs I've seen seem to look like a log
    function on linear axes (i.e. growing rather quickly as load current
    increases from zero, and then flattening out for higher currents).

    So can anyone suggest any reasons why the curve looks like this? Here
    is an explanation that is brewing currently in my head - I believe the
    switching losses in the FET are largely dependent on the switching
    frequency and rather independent of the load current. I've also heard
    that this switching loss comprises most of the regulator loss. Is that
    true? If that's the case, then I would assume the other losses become
    more and more significant as the output power reduces, so for higher
    output power, the switching loss being dominant makes the curve flat,
    which for lower and lower output power, the other losses start
    becoming dominant. Have I hit the nail on the head here? If not, can
    someone please clarify this issue...Thanks!
     
  2. DJ Delorie

    DJ Delorie Guest

    The problem with your argument is that the average input current
    depends on the average output current. It is not "relatively
    constant".
     
  3. Tom Bruhns

    Tom Bruhns Guest

    Consider that whatever controller you use will draw nearly constant
    power, if the switching frequency stays constant. That overhead
    exists even when the output power goes to zero. That's pretty close
    to what you've said in your closing paragraph. There's significant
    power used in just driving the gate of the FET, and as you note,
    there's power associated with the drain circuit switching and with the
    transformer core cycling through its hysteresis loop and with some
    other similar things. V*I in the FET during its on time is typically
    reduced, because with lower output, either the FET current and voltage
    drop are lower or the duty cycle is lower, or both.

    Bring up LTSpice/Switcher Cad III, and you can explore some of these
    things through simulation. LTSpice lets you get a report on just
    where the power is going, if you want, and all the Linear Technology
    switcher chips have included models.

    Note that some controllers go to lower frequency (skip cycles) at low
    output power, so the power-hungry cycles that aren't necessary to
    maintain the output voltage at low load are eliminated. If they've
    done other things right, the efficiency will hold up better at low
    load than with other controllers that keep cycling unnecessarily.

    Cheers,
    Tom
     
  4. These may be controllers that go into a "burst mode" operation at low
    output levels, which works well when output regulation and ripple are not
    highly critical. I found that some switching regulators do this, perhaps
    unintentionally, with certain combinations of output capacitance and
    compensation capacitance. Burst mode works best when the controller and
    MOSFET driver can be put into a low power mode during the off times.

    Some power supplies require a minimum load, and capacitors need some sort
    of bleeder resistor, so these may also contribute to less efficiency at low
    loads.

    But I think switching losses are the big item, which is a fairly constant
    power at all duty cycles, while conduction losses are what increase with
    load, and limit the efficiency at the high end.

    Paul
     
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