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SMPS High Open Voltage without PWM
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Terry Given
electronics forum Guru


Joined: 30 Apr 2005
Posts: 868

PostPosted: Sat Jun 03, 2006 5:52 am    Post subject: Re: SMPS High Open Voltage without PWM Reply with quote

Jeremy Samuels wrote:
Quote:
I'm designing a two transistor forward SMPS for a homemade plasma
cutter I'm building. The input is 120vac, open circuit voltage is from
290 to 400 in my power range (~20 amps), and 90-110 vdc under load. I
really have no idea how they do this, I've seen a partial schematic
from a Lincoln procut 25, and it looks like a regular two transistor
forward converter. I don't think PWM can be used, because dividing the
voltage by three through PWM also increase peak current three times,
and duty cycle cannot excede .5 because of the topology (core has to
reset), which would make the peak current requirements 120 amps for
1/6th of the cycle.

A schematic can be found on page 26
http://content.lincolnelectric.com/pdfs/products/navigator/im/IM665.pdf

or page 14
http://www.harborfreight.com/manuals/91000-91999/91814.pdf


You'd better add the two really important bits that are missing from
both diagrams, lest it go bang :)

Finding out which two bits is left as an exercise for the reader :)

Cheers
Terry
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Genome
electronics forum Guru


Joined: 30 Apr 2005
Posts: 789

PostPosted: Wed May 31, 2006 12:29 am    Post subject: Re: SMPS High Open Voltage without PWM Reply with quote

"Jeremy Samuels" <js5301@gmail.com> wrote in message
news:1149033440.021877.146560@y43g2000cwc.googlegroups.com...
Quote:
I'm designing a two transistor forward SMPS for a homemade plasma
cutter I'm building.

No you are not, you are spangling about trying to rip something off. I'm
sure you posted this question before. Do you think that the autoresponders
in this Forum do not have huge amounts of redundant fractally compressed
data available for instant recall?

In the meantime please notice that.

1) The circuits you provide use a voltage doubler configuration on the input
bridge rectifier to get a DC bus at the input of 339V (120VAC).

2) Yes two switch forward converters are limited to 50% duty cycle BUT, with
a 1:1
transformer and zero load on the output the voltage will rise to 339V. On
load it will drop until the filter inductor current becomes continuous at
120V. The output filter requires continuous filter inductor current if it is
to behave as a true averaging filter.

3) There is no three.

4) There might be a four but that assumes that, as a Google user, you work
out how to use the Forum correctly. Search elsewhere in the forum to find
information.


DNA
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Jeremy Samuels
electronics forum beginner


Joined: 01 May 2005
Posts: 7

PostPosted: Tue May 30, 2006 11:57 pm    Post subject: SMPS High Open Voltage without PWM Reply with quote

I'm designing a two transistor forward SMPS for a homemade plasma
cutter I'm building. The input is 120vac, open circuit voltage is from
290 to 400 in my power range (~20 amps), and 90-110 vdc under load. I
really have no idea how they do this, I've seen a partial schematic
from a Lincoln procut 25, and it looks like a regular two transistor
forward converter. I don't think PWM can be used, because dividing the
voltage by three through PWM also increase peak current three times,
and duty cycle cannot excede .5 because of the topology (core has to
reset), which would make the peak current requirements 120 amps for
1/6th of the cycle.

A schematic can be found on page 26
http://content.lincolnelectric.com/pdfs/products/navigator/im/IM665.pdf

or page 14
http://www.harborfreight.com/manuals/91000-91999/91814.pdf
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Google

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