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SMPS High Open Voltage without PWM

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

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or page 14

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Reply to
Jeremy Samuels
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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

Reply to
Genome

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

Reply to
Terry Given

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