Question about DC- DC boost converter

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Could some electronics guru please clarify the following ?
The following is the basic structure of a boost converter:

DC_in -> inductor --> diode --> capacitor --> output
                               |                         |
                               |                         |
                           switch                    |
                               |                         |
                            GND                  GND
The diode is forward biased in the sense that when
the switch is closed, the diode is reverse-biased and
prevents the capacitor from discharging. The question
is:
How is the maximum output voltage set, i.e., how is
maximum output voltage set ? Is it done by the reverse
breakdown voltage of the diode, since once that is
exceeded, the capacitor will try to discharge, when
the switch is on.

Re: Question about DC- DC boost converter
On Sat, 1 Oct 2011 09:14:41 -0700 (PDT), Daku

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I'm not a guru and not even a designer.  So the following is
what has helped me, only.  I've read some of your other posts
and from them I am assured you have LTspice.  Have you set
this up in LTspice, yet?  You should.  It supports a
voltage-controlled switch, which you can use to try out
different switch periods and duty cycles.  Make sure you test
a variety of cases and see if you can make predictions.

You will need to keep in mind some limitations on inductors
that aren't air-core (Bmax at low-freq or eddy current losses
at high-freq) when considering core area, volt-seconds,
turns, and possible gapping or distributed gap materials
(ferrite), if you actually try and build something and expect
it to work according to some idealized LTspice schematic.

There are some excellent PDF files available on the web, if
you google for them.  Look for 'boost,' and 'continuous,' and
'discontinuous,' and 'switcher,' as some example keywords.
And in particular, look at the Wiki that comes up.  TI writes
a few good things (Unitrode series comes to mind.)

http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF
http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401001
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM405006
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM406008
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM408003
http://www.smps.us/Unitrode.html

Jon

Re: Question about DC- DC boost converter

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Lets see, it's saturday, looks basic 101 electronics
class work to find the error!

Jamie



Re: Question about DC- DC boost converter

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Check the position of your diode.  I think you'll find that
it needs to be moved.

boB



Re: Question about DC- DC boost converter
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If you dont add any further bits than you've drawn, there is no
voltage control. Voltage control is achieved by not switching the
switch on when sufficient V exists in the capacitor.

Note to get adequate V control you need a capacitor that stores far
more energy than the coil does. If, for experimental reasons, you were
to use a small cap and a big coil, one single switching cycle would
put a large voltage increase into the cap, making regulation
unachievable.

We all had to learn some time ya know.


NT

Re: Question about DC- DC boost converter
ischarge, when
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I am afraid the simple diagram I had in my initial post was lifted
from the Maxim Web site. My own design is a lot complicated.

Re: Question about DC- DC boost converter
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The output voltage is set by the duty cycle, D, of switch which is
defined as Ton/(Ton+Toff) where Ton is the switch ON time and Toff is
the switch OFF time. This follows from the fact that in steady state
the average voltage across an ideal inductance is zero. During the ON
time the voltage across the inductor is DC_in, and for the OFF time
the voltage is DC_in-Vout (making the simplification that diode
forward drop is 0), so that the average is 03D%DC_in x Ton + (DC_in-
Vout) x Toff. Solving for Vout yields Vout3D% DC_in x (Ton+Toff)/Toff 3D%
DC_in/(1-D).

Re: Question about DC- DC boost converter
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This explains things a little, might be of help.

http://ww1.microchip.com/downloads/en/AppNotes/00980a.pdf

Re: Question about DC- DC boost converter

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Just google it and you'll find a formula which says that the
input/ouput voltage ratio is determined by the duty cycle. This goes
as long as the minimum load current is satisfied.

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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