Full Bridge SMPS With High No Load Voltage?

[Jeremy Samuels]

In my attempt to build an inverter-style plasma cutter, I have run into
a problem. How can a full bridge SMPS create a high (270 VDC) near
open circuit voltage, but have a relatively low load voltage (90 vdc at
25+ amps)? The high voltage is used to start the arc, and the lower
voltage is used to maintain the arc. I'm planning to use a CCM PFC
with an output of 220 vdc into a full bridge (current mode, fs=25khz).
See http://www.northerntool.com/downloads/manuals/164686.pdf for
similar power supply requirements.

Thanks.

 

[Robert Baer]

In my attempt to build an inverter-style plasma cutter, I have run into
a problem. How can a full bridge SMPS create a high (270 VDC) near
open circuit voltage, but have a relatively low load voltage (90 vdc at
25+ amps)? The high voltage is used to start the arc, and the lower
voltage is used to maintain the arc. I'm planning to use a CCM PFC
with an output of 220 vdc into a full bridge (current mode, fs=25khz).
See http://www.northerntool.com/downloads/manuals/164686.pdf for
similar power supply requirements.

Thanks.

It has to act like a current source; persumedly you can alter the
feedback to do that.

 

[[email protected]]

In my attempt to build an inverter-style plasma cutter, I have run into
a problem. How can a full bridge SMPS create a high (270 VDC) near
open circuit voltage, but have a relatively low load voltage (90 vdc at
25+ amps)? The high voltage is used to start the arc, and the lower
voltage is used to maintain the arc. I'm planning to use a CCM PFC
with an output of 220 vdc into a full bridge (current mode, fs=25khz).
See http://www.northerntool.com/downloads/manuals/164686.pdf for
similar power supply requirements.

Thanks.

You could put an extra winding/diode on the output choke and feed that
to the output, you would need another diode in the main output to stop
it feeding back.

 

[[email protected]]

You could put an extra winding/diode on the output choke and feed that
to the output, you would need another diode in the main output to stop
it feeding back.

Sounds good. The other option is sense the i_out and switch the
regulation voltage accordingly. You might find you've got almost all
the bits in place to do that already.


NT

 

[Paul Mathews]

In my attempt to build an inverter-style plasma cutter, I have run into
a problem. How can a full bridge SMPS create a high (270 VDC) near
open circuit voltage, but have a relatively low load voltage (90 vdc at
25+ amps)? The high voltage is used to start the arc, and the lower
voltage is used to maintain the arc. I'm planning to use a CCM PFC
with an output of 220 vdc into a full bridge (current mode, fs=25khz).
See http://www.northerntool.com/downloads/manuals/164686.pdf for
similar power supply requirements.

Thanks.

It's not uncommon to use some parallel source of high voltage that has
a comparatively high output impedance. When the heavy arc current
starts to flow, the high voltage effectively collapses to the voltage
provided by the low impedance supply. This is sometimes done with
additional windings on the main transformer, sometimes done as a
completely separate circuit. In either case, you can add circuitry to
actually switch off the HV when the arc starts if you like. It may or
may not be necessary to prevent the HV output from inhibiting LV
circuitry through negative feedback. Many such circuits don't have
negative feedback, but run at a conversion ratio determined by
feedforward (line input) only.
Paul Mathews

 

[budgie]

Sounds good. The other option is sense the i_out and switch the
regulation voltage accordingly. You might find you've got almost all
the bits in place to do that already.

That might be an interesting exercise in stability.