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boost / Buck boost

Discussion in 'Electronic Basics' started by [email protected], May 15, 2007.

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  1. Guest

    Apart from the inductor and rectifier simply being in different
    positions, what is the difference between the boost and Buck-boost DC-
    DC converter?

    Is one more reliable than the other? Is one more energy-efficient?

    Thanks,

    Michael
     
  2. Guest


    Crossposting to SED due to the underwhelming response...

    MD
     
  3. J.A. Legris

    J.A. Legris Guest

    The main difference is that the buck-boost gives a negative output
    voltage, which can vary right down to zero. The buck-boost also
    requires semiconductors with higher voltage ratings than the boost.
     
  4. Rich Grise

    Rich Grise Guest

    The "buck" regulator outputs a voltage lower than the supply. A "boost"
    regulator outputs a voltage higher than the supply. A "buck-boost"
    regulator can output either, depending on operating circumstances and
    stuff.

    Since you're already at google, why not search for "buck-boost regulator"?

    Hope This Helps!
    Rich
     
  5. Jasen

    Jasen Guest

    polarity on the output.
    there's a spam war going on and many may have battened the hatches too
    tightly, also usenet takes time, give it 48 hours so so before you
    decide everyone is ignoring you.

    Bye.
    Jasen
     
  6. Guest

    A boost converter provides an output voltage which is greater than the
    input voltage, and is non-inverting. It requires one power switch
    (usually a MOSFET), and one rectifier. A buck-boost converter
    provides an output which can be less than or greater than the input
    voltage. The inverting buck-boost, or IBB, outputs a negative voltage
    with a positive input. It utilizes one power switch, and one
    rectifier. The non-inverting buck-boost, or NIBB, outputs a positive
    voltage with a positive input. It requires two power switches, and
    two rectifiers. Efficiency is highest for the boost, followed by the
    IBB, and lowest for the NIBB, due to twice the switches and rectifiers
    and their associated losses.

    All three are reliable, but the boost is not inherently short circuit
    protected. In a boost, the power switch is not in line with the
    input, but shunted to ground. Should the output get shorted, turning
    off the power switch does not break the fault current. The user must
    provide additional means to protect the output. The buck-boost, both
    IBB and NIBB, have the power switch in line with the input. A direct
    short on the output is broken by turning off the power switch. Thus
    the IBB and NIBB are short circuit protected. I hope this helps.

    Claude
     
  7. Guest


    Oh is that all then... thanks.

    I googled it before posting, and it seemed that the preference is buck-
    boost for electric bicycles for some reason. I personally only built
    a (tiny, prototype) boost converter, and was wondering what I was
    missing out on.


    spam war... gotcha.

    Michael
     
  8. Jasen

    Jasen Guest

    buck-boost can also reduce or increase the magnitude of the voltage,
    boost can only match, or increase it.

    Bye.
    Jasen
     
  9. legg

    legg Guest

    Besides the characteristics noted in other responses, keep in mind
    that actual energy processed in the boost converter is less for the
    same output power. In effect, the source is present during energy
    transfer to the load for the boost, where as in buck-boost the source
    is disconnected from the load during transfer.

    So energy processing and transfer efficiency does come into it.

    RL
     
  10. Guest


    Buck/Boost is good for use with batteries that dramatically change
    their voltage as they are used For instance, alkaline cells vary 3 to
    1 as they are discharged. That is, you start at 1.5V, and the cells
    are dead at 0.5V.

    Seems to me you would power an electric bike with lead acid or nicad.
    These batteries do not change their voltage greatly (or at least as
    much as compared to alkalines), so I would go for a buck converter.
     
  11. Jasen

    Jasen Guest

    for powering ther wheels I'd use PWM, but a buck-boost converter could
    be useful for regenerative braking.
     
  12. Guest

    Regenerative braking is not worth a lot of bother unless you have a
    large momentum that needs stopping in a controlled fashion. I'm
    thinking of suburban rail cars that accelerate to high speed, and then
    decelerate before the next stop. In the days of plenty, they used to
    use resistive braking by slowing the train down with electric
    generation fed into a great resistance that glowed red hot. These days
    that waste can be redirected back into the grid.
    For cars, RB is a moot point. For the few percent of saving, not much
    expense should be made. For smaller vehicles, like wheelchairs and old
    farts' scooters like mine :) it is a total waste of complexity. jack
     
  13. There are very few vehicles that I ride that I'd be happy with them
    stopping in an uncontrolled fashion. Actually I cannot think of any off
    hand.
    It also saves on brakes (and in some cases motors). I'd also say there
    was a very good chance that both the wheelchairs and scooters you
    mention have regenerative braking (less certain on the scooters). They
    probably use PM motors (vehicles that small usually do) and regenerative
    braking on a PM motor is a trivial addition to the proportional control,
    it's actually harder to prevent it than to use it.

    Robert
     
  14. Umm, why would a buck-boost be useful for regenerative braking? Anything
    other than a straight PWM seems overkill for most vehicle motor driving
    applications.

    Robert
     
  15. Guest

    What I meant by controlled, was not at the whim of any pedestrian, or
    random traffic light or other vehicle that might force you to jam on
    your service brakes. Knowing that at a certain point on the track, a
    certain slowing is required to come to a stop at a station a certain
    distance ahead is what I meant by "controlled". Sorry I was not
    clearer.
    And by regenerative braking, I meant putting the braking energy back
    into the battery. Of course, electric motors can be set up to save the
    service brakes by dynamic braking (if that is the term) without the
    regenerative element. jack
     
  16. Jasen

    Jasen Guest

    the voltage out of the permanent magnet motor is proportional to the speed
    and will be less than the battery voltage, hmm, straight boost is probably
    better suited.

    Bye.
    Jasen
     
  17. True so far.
    We may be running into a terminology issue here as well. Motors in
    electric vehicles are usually controlled with some form of PWM, for a PM
    or BLDC in a vehicle of this size this would be a MOSFET based
    controller (1/2H, full H or multi-phase). Now it's obvious how that
    acts to buck down the voltage to drive the motor. What's less obvious
    is this also automatically provides regen. If the motor's speed is
    greater than that provided by the PWM'd voltage (the back emf is greater
    than the PWM) then you will generate a current in the motor and this
    current will feed back to the DC bus. This is used by commercial PM and
    BLDC vehicle controllers at least down to the wheelchair class size. I
    would be surprised if anyone went to the effort and cost to remove it
    from smaller controllers.

    You could consider the regen operation a boost converter but I don't
    think that's what you meant and I usually don't think of it as such.

    The regen happens because
    - the motor acts as a generator
    - The motor windings are an inductor

    The latter point means that when the PWM turns off the voltage rises
    until current can continue to flow giving the boost action. Note that
    gives rise to a very real failure mode, if the battery is disconnected
    from the DC bus during regen the voltage will quickly rise high enough
    to blow up the controller power section.

    The difference between a PM or BLDC motor controller with regen and one
    with out is the level the the regen current limit is set to(1). A
    robust controller also has trips on the DC bus voltage.

    No external boost or buck required. Unless we consider the PWM/motor
    combination to be a buck/boost convertor. Probably technically true but
    not what I usually think of.

    Teranews seems to be dropping/delaying posts so this reply is a bit
    delayed.

    Robert

    (1) I have seen controllers with a diode on the DC bus to prevent regen.
     
  18. So did I (or at least the DC bus). See my longer reply to Jasen. It's
    harder to avoid regenning a PM motor when using PWM than it is to
    implement it.

    Regenning with a PM motor to the battery is just a matter of allowing
    regen current to flow to the battery.
    That's one term. Although that's also used to describe braking using
    regen to the DC bus and then using a resistor load to preven the DC bus
    from rising. A method used commonly on industrial drives.

    I think I've left the quoting properly intact.


    Robert
     
  19. Jasen

    Jasen Guest

    the probelem I have with this is that the back EMF of the motor will
    always be less than the supply EMF, and it's back EMF that does the
    regen... so how do you get regen without boosting.
    but how does the voltage get that high when the PWM is stopped.
    I can see the flyback voltage producing regen current when the
    pulse ends, but it seem to me that that's just returning some of the pulse energy,

    I just can't picture what you describe, how would I go about modeling
    it?

    should I treat the motor as an AC voltage source with less than battery voltage
    in series with an inductor?

    Bye.
    Jasen
     
  20. Basically the same way it does in a boost circuit.
    First I'll take as a given that the motor responds to the mean applied
    voltage. The PWM is fast enough that current ripple is minimal. In
    practice minimal current ripple is easily achieved, except perhaps for
    some very low inductance motors. That being the case the motor back emf
    can easily be larger than the applied voltage and the motoe will be
    generating current.

    Now consider a MOSFET switching on the B- side of the motor being PWMed
    at 50%, and the motor running at the equivalent of 100% applied voltage.

    - When the MOSFET is on the battery voltage applied across the
    motor will act to decrease the current and reduce the torque, increasing
    the speed of the motor. Due to inductance this change in current is
    small, at high enough frequencies it's insignificant.
    - When the MOSFET is off the voltage will rise since there is now
    no longer a complete circuit path but the inductor will 'want' the
    current to remain constant. The voltage will continue to rise until
    it's high enough to complete the path through the integral body diode of
    the MOSFET. Note that without the integral body diode or similar the
    voltage will continue to rise until something conducts.

    Now if you just shut off the PWM to zero duty cycle then you do just get
    a short pulse. It's the PWM establishing the operating point that
    provides the reference, without that the motor is just floating.
    An inductor with a voltage source in series, a flyback diode a across
    the two of them and a MOSFET switching the lowside. Maybe another
    voltage source to act as an ideal battery. I've never tried it, there
    never seemed to be much point.

    Robert
     
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