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Switching HV with IGBT's?

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Marko

Wed Mar 22 2006, 10:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I should mention that I'm looking to run a laser of no more than 50W, and half that would probably 'cut it', so to speak. I'll most likely be cutting things like acrylic and fabric. I don't really have any reason to cut sheet metal, but I suppose I'll think of something if I have that capability.

NST can be used ot power a CO2 laser, output is usually rectified and sometimes doubled for initial 'kicking' pulse.

For fast operation maybe best idea is to use solid state, high frequency supply and turn it on/off rapidly pulsing laser such way.
'CCPS' supply on Steve Ward's site I shown is better than enough.

You can even use SSMOTs or stack of few flybacks for such a small laser.

Switching high voltage side is far more complicated, it is more prone to failures and doesnt have point in this application.

Sulaiman

Thu Mar 23 2006, 12:48AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have no experience with CO2 lasers, a little with HeNe
so if it's similar;
If the laser is operating CW with a (nominaly) constant supply voltage and a resistive ballast, then it's not too difficult.
IF the negative terminal of the laser is grounded,
Imagine 3 kV power supply, laser 2 kV @ 1 mA
so ballast resistor = (3 kV - 2 kV)/1 mA = 1 kOhm
Now disconnect the negative side of the laser from ground and connect it via a 1200V transistor with a 1 kV 1W zener in parallel,
to ground.
By varying the conduction of the transistor the current can be linearly controlled from 0 to 1 mA . tada!
The problem is the 'floating' laser assembly .. safety.

EDIT I'm sorry, I'm under the influence of (prescribed) codeine so my brain is even mushier than usual.....
1 kV / 1 mA = 1 MOhm.
The figures aren't real, I thought I'd simplify it..ha ha ha
I'll try to explain a little clearer another time.

Steve Conner

Thu Mar 23 2006, 10:48AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As far as I know, CO2 lasers are rated by their optical power output. They're not that efficient: To get 50W of optical output usually takes a good few kW of electrical input and serious cooling of the laser to get rid of the waste heat.

I've seen commercial cutting machines that use RF excited lasers. Applying RF power at a few hundred megahertz makes the cavity resonate electrically causing huge voltages to appear inside and ionize the CO2. I believe they do this because it does away with the need for electrodes and all the seals associated with them, and that compensates for the cost and low efficiency of the big RF power amp needed to drive it. PWM control of this kind of laser just consists of pulsing the RF on and off, and the laser unit usually has a control input to do that.

Of course, as usual, Sam's Laser FAQ goes into excruciating detail on the subject, including power supplies.

Marko

Thu Mar 23 2006, 11:58AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

As far as I know, CO2 lasers are rated by their optical power output. They're not that efficient: To get 50W of optical output usually takes a good few kW of electrical input and serious cooling of the laser to get rid of the waste heat.

For CO2 laser efficiency hangs about 10% so 500W supply seems enough, and maybe kilowatt for nicer operation.

Stronger NSTs, HF transformers or stacks of etc. flybacks is actually enough for that small tube.

Sam barros has detailed overwiev of one OC2 laser relatively similar in power, what is used for power supply, etc...

Wolfram

Thu Mar 23 2006, 03:24PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

To me, a tube looks like the best solution. Switching high voltage at low current is just what tubes are good for.

Finding tubes that can switch these voltages might be hard, but far from impossible.

Would this one work? The max voltage rating would be exceeded, but I don't think that would be a big problem. My only concern is the power dissipation. I can't find any data on the internal resistance or voltage drop when it's conducting, what would be normal for a tube of this size?

Marko

Thu Mar 23 2006, 07:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I recomennded tube in few posts before, surely large enough can be found but it is just 'waste of resources' for application that actually doesn't need it.

Tubes could also be used for RF exciting, if it is needed etc...

Would this one work?

That is powerful tube, but I think more voltage and lesser current is needed. It is for 15A and we will use just few tens of mA, so low-current tube for maybe 20kV that could be more easily found.

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