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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
 
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


Crossposting to SED due to the underwhelming response...

MD
 
J

J.A. Legris

Jan 1, 1970
0
Crossposting to SED due to the underwhelming response...

MD

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.
 
R

Rich Grise

Jan 1, 1970
0
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
 
J

Jasen

Jan 1, 1970
0
polarity on the output.
Crossposting to SED due to the underwhelming response...

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
 
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

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
 
polarity on the output.


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.

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


spam war... gotcha.

Michael
 
J

Jasen

Jan 1, 1970
0
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.

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

Bye.
Jasen
 
L

legg

Jan 1, 1970
0
Crossposting to SED due to the underwhelming response...

MD

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
 
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


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.
 
J

Jasen

Jan 1, 1970
0
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.

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

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
 
R

Robert Adsett

Jan 1, 1970
0
Regenerative braking is not worth a lot of bother unless you have a
large momentum that needs stopping in a controlled fashion.

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.
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

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
 
R

Robert Adsett

Jan 1, 1970
0
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
 
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.

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.
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

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
 
J

Jasen

Jan 1, 1970
0
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.

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
 
R

Robert Adsett

Jan 1, 1970
0
the voltage out of the permanent magnet motor is proportional to the speed
and will be less than the battery voltage,

True so far.
hmm, straight boost is probably
better suited.

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.
 
R

Robert Adsett

Jan 1, 1970
0
And by regenerative braking, I meant putting the braking energy back
into the battery.

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.
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

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
 
J

Jasen

Jan 1, 1970
0
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 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.
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.

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
 
R

Robert Adsett

Jan 1, 1970
0
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.

Basically the same way it does in a boost circuit.
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,

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

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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