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'[OT] High-current boost converter?'
2006\09\24@075524 by PicDude

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Hi all,

I need to know how to design high current boost DC-DC converters.  For a
regular DC-DC converter, I would usually use National's webench tool or
similar to do the design for me, but how would I modify those to handle say
50 amps?

I expect it's not just a matter of putting on a higher-current switch and
passives rated for the higher currents, correct?

Anyone have references/links to articles, books, tutorials, etc on this?

Cheers,
-Neil.

2006\09\24@114646 by Xiaofan Chen

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On 9/24/06, PicDude <spam_OUTpicdudeTakeThisOuTspamnarwani.net> wrote:
> Hi all,
>
> I need to know how to design high current boost DC-DC converters.  For a
> regular DC-DC converter, I would usually use National's webench tool or
> similar to do the design for me, but how would I modify those to handle say
> 50 amps?
>
> I expect it's not just a matter of putting on a higher-current switch and
> passives rated for the higher currents, correct?
>
> Anyone have references/links to articles, books, tutorials, etc on this?
>

What is the requirement for the boost converter? For example,
the input and output voltage, the efficiency requirement. Depending on
the requirement, the design will be totally different.

50A is not small current. 50A @3V will be 150W. 50A@24V will be
1.2kW and the design approach will be very different.

The design which fits 1A@3V will not fit for the design of 50A@3V
since the loss on the diode will be too much and you may have to use a
synchronous boost design.

2006\09\24@120105 by PicDude

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On Sunday 24 September 2006 10:46, Xiaofan Chen wrote:
> What is the requirement for the boost converter? For example,
> the input and output voltage, the efficiency requirement. Depending on
> the requirement, the design will be totally different.

Input is automotive voltage -- say 10V to 14V.  Output is approx 20V, and the
higher current will be used to power some motors.  Efficiency is "as
efficient as possible, within reasonable cost".  It's not a portable
battery-powered device, so it does not have to be 99% efficient, yet, given
the current requirement, I'd like it to be as efficient as possible so that
there's less heat to be dissipated.  I know that's very shady, but I'm not
familiar with the high-current design, so I would evaluate various
efficiencies vs. cost ... that is, IF I have choices.


> 50A is not small current. 50A @3V will be 150W. 50A@24V will be
> 1.2kW and the design approach will be very different.
>
> The design which fits 1A@3V will not fit for the design of 50A@3V
> since the loss on the diode will be too much and you may have to use a
> synchronous boost design.

2006\09\24@123408 by Harold Hallikainen

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I think the most efficient approach would be to use motors that are rated
for the battery voltage. Failing that, I suspect a bridge switcher driving
a transformer is the most efficient way to get a relatively high power to
a higher voltage. A typical boost converter (which I consider a form of a
"flyback converter") becomes impractical above 100 or 200 watts.

Harold

{Quote hidden}

> -

2006\09\24@145435 by Dave Lag

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PicDude wrote:
> Input is automotive voltage -- say 10V to 14V.  Output is approx 20V, and the
> higher current will be used to power some motors.  Efficiency is "as
> efficient as possible, within reasonable cost".  It's not a portable
> battery-powered device, so it does not have to be 99% efficient, yet, given
> the current requirement, I'd like it to be as efficient as possible so that
> there's less heat to be dissipated.  I know that's very shady, but I'm not
> familiar with the high-current design, so I would evaluate various
> efficiencies vs. cost ... that is, IF I have choices.
>
Starts to have some similarities to big car amps I suspect. Maybe
another place to look.

2006\09\24@165032 by Xiaofan Chen

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On 9/24/06, Harold Hallikainen <.....haroldKILLspamspam@spam@hallikainen.org> wrote:
> I think the most efficient approach would be to use motors that are rated
> for the battery voltage. Failing that, I suspect a bridge switcher driving
> a transformer is the most efficient way to get a relatively high power to
> a higher voltage. A typical boost converter (which I consider a form of a
> "flyback converter") becomes impractical above 100 or 200 watts.
>

Flyback converter is not really derived from boost converter. But you are
right that flyback converters are more suitable for power level below 200Watts.

Boost converters are very often used as the front end of PFC (power
factor correction) circuits and they are okay even at power levels more
than 1kW.

But again I think you are right it might be better to use a transformer
isolated full-bridge converter for this kind of application.

Eg: TI UCC3895
http://focus.ti.com/docs/prod/folders/print/ucc3895.html

Regards,
Xiaofan

2006\09\24@223015 by Harold Hallikainen

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>
> Flyback converter is not really derived from boost converter. But you are
> right that flyback converters are more suitable for power level below
> 200Watts.


I see a bit of similarity between the flyback and boost converter. The
difference is that during the collapse of the magnetic field, the output
of a boost converter is pulled from the "bottom" of the "primary" (and
only) winding, while in a flyback converter, the output is pulled from the
"bottom" of the secondary winding. In the boost converter, the converter
output is in series with the incoming DC supply (boosting the voltage). In
a flyback converter, the output is generally ground referenced and
isolated from the primary.

So... I see similarities, but sometimes I see stuf that isn't there.

Harold

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2006\09\24@230952 by Xiaofan Chen

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On 9/24/06, Harold Hallikainen <haroldspamKILLspamhallikainen.org> wrote:
>
> I see a bit of similarity between the flyback and boost converter. The
> difference is that during the collapse of the magnetic field, the output
> of a boost converter is pulled from the "bottom" of the "primary" (and
> only) winding, while in a flyback converter, the output is pulled from the
> "bottom" of the secondary winding. In the boost converter, the converter
> output is in series with the incoming DC supply (boosting the voltage). In
> a flyback converter, the output is generally ground referenced and
> isolated from the primary.
>

ece-www.colorado.edu/~ecen4517/labs/flyback/flyback.pdf
The flyback converter is based on the buck-boost converter.

2006\09\24@232126 by Harold Hallikainen

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> On 9/24/06, Harold Hallikainen <.....haroldKILLspamspam.....hallikainen.org> wrote:
>>
>> I see a bit of similarity between the flyback and boost converter. The
>> difference is that during the collapse of the magnetic field, the output
>> of a boost converter is pulled from the "bottom" of the "primary" (and
>> only) winding, while in a flyback converter, the output is pulled from
>> the
>> "bottom" of the secondary winding. In the boost converter, the converter
>> output is in series with the incoming DC supply (boosting the voltage).
>> In
>> a flyback converter, the output is generally ground referenced and
>> isolated from the primary.
>>
>
> ece-www.colorado.edu/~ecen4517/labs/flyback/flyback.pdf
> The flyback converter is based on the buck-boost converter.

Interesting! If you look at figure 1(d), flip over the secondary circuit,
then connect the primary and secondary together, it looks like a boost
converter to me!

Harold


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opportunities available!

2006\09\25@044644 by Alan B. Pearce

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>I need to know how to design high current boost DC-DC converters.

How high current, and what in/out voltages? Check out the Linear technology
Application notes. One device that comes to mind is the LT1170, but you may
be asking for higher currents.

2006\09\25@071544 by Russell McMahon

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> The flyback converter is based on the buck-boost converter.

I don't know what the link said BUT the flyback topology came well
before buck boost was thought of AFAIR.
Not that it really matters.
I see buck boost as a clever combination of just what its name
suggests. Someone decided they could make a buck boost as well by
switching both ends at once.

Note that a pure boost converter is fundamentally MORE efficient than
a buck boost in boost mode.
This is because the boost converter output "stands on" the input rail
and that part of the input is 100% efficient*. A buck-boost converter
"stand on" ground and so it's whole output is affected by the
efficiency of the energy converter. A boost converter efficiency
approaches 100% as its output approaches the input voltage, subject to
some losses. A buck-boost in boost mode does not have this same
fundamental property.

* One of the very few things in engineering that can be claimed to be
literally 100% efficient :-).



       Russell

2006\09\26@141600 by PicDude

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On Sunday 24 September 2006 13:56, Dave Lag wrote:
> Starts to have some similarities to big car amps I suspect. Maybe
> another place to look.

Not familiar with how much voltage and current car amps put out, but I always
thought the voltage was relatively low, but the current was much higher.

Alternatively, I've been thinking that vehicle power inverters (12VDC to
120VAC) could be a source of "inspiration".  AFAIK, those as basically
oscillators with step-up transformers.  With a different-sized transformer
and some rectification & filtering, I should be able to get this done.

Cheers,
-Neil.

2006\09\26@141720 by PicDude

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Changing motors is not an easy option, especially since there will be complex
mechanical changes.  What does "bridge switcher" mean?

I also just mentioned a thought about vehicle power inverters on another email
a minute ago.

But from this and other sources, I see that it's not such an easy task, so
what if I scale back a bit...  Rather than running multiple motors off a
single boost converter, if I simplified it as one boost converter for each
motor, I'd need 18V-20V at 10A-12A.  We're still up around the 200W mark, so
my next thought would be to parallel 2 or more boost converters, if that's
allowed.  I guess that may be dependent on the specific regulators.

Cheers,
-Neil.


On Sunday 24 September 2006 11:34, Harold Hallikainen wrote:
{Quote hidden}

2006\09\26@143834 by PicDude

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Well that explains a lot.  I also googled and found some other docs on this.
Relatively simple actually, and though I usually avoid transformers (cost,
space, etc), I don't mind so much since this is a one-off (or few-off)
application for my car, so I can wind some transformers for this.  Guess I'm
off to do some reading...

Cheers,
-Neil.



On Sunday 24 September 2006 15:50, Xiaofan Chen wrote:
{Quote hidden}

2006\09\26@150039 by Harold Hallikainen

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> Changing motors is not an easy option, especially since there will be
> complex
> mechanical changes.  What does "bridge switcher" mean?
>

That's just what I'm calling a circuit that has a transformer driven by a
half bridge or full bridge circuit. It could also be a classic "push pull"
circuit where the center of the transformer goes to plus supply and the
two ends go to a couple transistors.

{Quote hidden}

Driving individual motors might make it a bit easier. Perhaps you could go
to a multiphase boost converter which is two or more boost converters with
the pulses interleaved. This increase the ripple frequency making the use
of smaller components possible. This could be especially appropriate if
the boost converter can also be used to control the motor. However, the
boost converter output cannot go below the input, so that's pretty
limiting. It may still be simpler to just have a big transformer based
inverter to get the higher voltage, then use PWM transistors between the
low side of the motor and ground to control the motors.

Harold


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2006\09\26@153320 by Herbert Graf

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On Tue, 2006-09-26 at 14:18 -0500, PicDude wrote:
> On Sunday 24 September 2006 13:56, Dave Lag wrote:
> > Starts to have some similarities to big car amps I suspect. Maybe
> > another place to look.
>
> Not familiar with how much voltage and current car amps put out, but I always
> thought the voltage was relatively low, but the current was much higher.

Depends on the amp. The cheapest amps basically put out 12Vp-p. Slightly
more expensive ones put out 24p-p (they use an "analog" h-bridge).
Beyond that you have to start stepping up the voltage to get more
volume.

This is the reason "car" speakers often are much lower in impedance then
"home" speakers, it's harder to boost the voltage above 12V then it is
to build a lower impedance speaker.

TTYL

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