Power Switching and Current Limiting

[Renheowein]

Hello, I am a senior engineering student at Virginia Tech. I'm
currently working on my design project, trying to simplify the
power supply to a plasma torch. Currently we are using a welding
power supply and high frequency starter to light the torch, but
my faculty adviser wants to try something different.

His idea: Light the torch with the power supply and HF starter,
but then switch over to a battery supply to keep it lit for a
short period of time. The torch needs about 30V and 20A to stay
running, which I know is a high power rating for batteries. But I
was wondering if anyone had any recommendations. My 2 problems:
1. Switching from the supply to batteries. It needs to be safe,
and I'd rather not blow up any batteries while I'm at it.
2. Controlling the current. If the torch is allowed to **** as
much current as it can, it will drain the batteries very quickly.


I'm pretty new to electronics, in particular power electronics, but I have access to some resources. But try not to use any jargon (ie, "Just use a CCT, and hook it into the AsM.") cause you'll probably just lose me.

If you have any suggestions, I am definitely open to them. Thanks
for all your help!

---------- Post added at 09:35 ---------- Previous post was at 09:33 ----------

Replace the **** with "drain."

 

[mtwieg]

Car batteries would handle that no problem. Your wires would likely melt their insulation before the battery failed.

I have to ask, what's the motivation for using a battery if you still need the welding supply to get the plasma started? How fast does the switching need to be so the plasma doesn't die out?

 

[Renheowein]

Eventually, we'd like to move the system to a point where all we needed was the batteries without the high frequency starter. We've found that some combination of Flyback and Buck converters, we can accomplish this.

But we'd like to take baby steps here, particularly as the semester is winding down, and just be able to switch over to the batteries after the torch is lit.

Are you saying that car batteries can handle that power, or that the car batteries could handle the switching?

Thanks!

 

[Renheowein]

PS: I don't know fast it needs to switch, but probably pretty fast.

 

[mtwieg]

Eventually, we'd like to move the system to a point where all we needed was the batteries without the high frequency starter. We've found that some combination of Flyback and Buck converters, we can accomplish this.

But we'd like to take baby steps here, particularly as the semester is winding down, and just be able to switch over to the batteries after the torch is lit.

Are you saying that car batteries can handle that power, or that the car batteries could handle the switching?

Thanks!

SLA car batteries can generally supply hundreds of amps for brief periods, and probably at least 50A continuous (might get warm though). They're just really robust, so I wouldn't worry too much about destroying them, so long as you use a good fuse.

As for doing the switching, you should probably first decide on a supply topology for converting the 12V from the battery to your output voltage/current. But I'm betting that you could just parallel the two supplies without much trouble (so long as they are both current regulated).

 

[Renheowein]

As for doing the switching, you should probably first decide on a supply topology for converting the 12V from the battery to your output voltage/current. But I'm betting that you could just parallel the two supplies without much trouble (so long as they are both current regulated).

That sounds like good advice. For the battery supply, I was just planning on stacking them in series to get the necessary voltage. But since the plasma torch acts like a negative impedance, don't I have to worry about the current backrushing into the batteries? And what do you think is the best way to regulate that current?

 

[mtwieg]

That sounds like good advice. For the battery supply, I was just planning on stacking them in series to get the necessary voltage. But since the plasma torch acts like a negative impedance, don't I have to worry about the current backrushing into the batteries?

Do you mean the plasma arc itself is a negative impedance, or the input to the power supply is a negative impedance?

And what do you think is the best way to regulate that current?

If you care at all about efficiency, then you'll want a switching power supply with current feedback on the output, and a output voltage limit. The topology of the converter will depend on your desired load/line ranges.

 

[Renheowein]

Do you mean the plasma arc itself is a negative impedance, or the input to the power supply is a negative impedance?

The plasma arc itself is a negative impedance.

If you care at all about efficiency, then you'll want a switching power supply with current feedback on the output, and a output voltage limit. The topology of the converter will depend on your desired load/line ranges.

I guess that makes sense....care to point me in the direction of a design?

 

[FvM]

I don't think that voltage limiting is needed for this application. Just a basic buck converter with constant current output. What's the required ignition voltage & current?