boost / Buck boost

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

Reply to
mrdarrett
Loading thread data ...

Crossposting to SED due to the underwhelming response...

MD

Reply to
mrdarrett

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.

-- Joe

Reply to
J.A. Legris

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

Reply to
Rich Grise

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

Reply to
Jasen

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

Reply to
cabraham01

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

Reply to
mrdarrett

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

Bye. Jasen

Reply to
Jasen

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

Reply to
legg

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.

Reply to
miso

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

--
Bye.
   Jasen
 Click to see the full signature
Reply to
Jasen

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

Reply to
spamfree

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

--
Posted via a free Usenet account from http://www.teranews.com
Reply to
Robert Adsett

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

--
Posted via a free Usenet account from http://www.teranews.com
Reply to
Robert Adsett

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

Reply to
spamfree

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

Reply to
Jasen

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.

--
Posted via a free Usenet account from http://www.teranews.com
Reply to
Robert Adsett

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

--
Posted via a free Usenet account from http://www.teranews.com
Reply to
Robert Adsett

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

Reply to
Jasen

Basically the same way it does in a boost circuit.

energy,

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

--
Posted via a free Usenet account from http://www.teranews.com
Reply to
Robert Adsett

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.